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Lipid nanoparticles (LNPs) have become a critical tool in modern medicine, particularly in diagnostics and therapy. Various synthesis techniques, including ultrasonication, high-pressure homogenization, solvent evaporation, and microemulsion, have been employed to produce LNPs with desired properties. Modification techniques such as surface functionalization, polymer coating, PEGylation, and ligand conjugation further enhance their efficacy and biocompatibility. This study highlights the recent advances in the application of LNPs for improving diagnostic accuracy and therapeutic outcomes. In diagnostics, LNPs enhance visualization, bioimaging, and analytical methods, especially through genomic, proteomic, and immunoassay-based techniques, as well as plasmonic biosensors. In therapy, LNPs are utilized in innovative treatments such as plasmonic photothermal and photodynamic therapies, antibacterial applications, and drug delivery systems. These LNPs serve as effective carriers for drugs, genes, and antibodies, improving targeted delivery and therapeutic efficacy. Despite their promising potential, challenges related to safety, scalability, and long-term impacts remain, and this research outlines future perspectives on overcoming these barriers to further integrate LNPs into clinical applications.

We have performed magneto-absorption measurements in p-Ge under intense crossed electric and magnetic fields. The results are interpreted with the coupled-band (Pidgeon and Brown) model. By comparison with the parabolic (Luttinger) model we show that non-parabolic effects, caused by the influence of the spin-orbit split-off band and the conduction band on the valence bands, have to be taken into account to explain the observed spectra.

The refractive indices of short-period binary (AlAs)m(GaAs)n superlattices, where m = n, were investigated by measuring the beam divergences of optical waveguides using these superlattices as the core material. It is demonstrated that the refractive index depends on the period of the superlattice and not simply on the average composition. The refractive index is shown to depend in a systematic way on the direct bandgap of the superlattice, although the relationship may not be quite the same as that for a random alloy.

Picosecond excitation-probe measurements using a far-infrared free-electron laser (CLIO) have revealed large nonlinearities for indium antimonide at 4.7 mu m. A theoretical model is described to determine the cw third-order nonlinear susceptibility and the interband relaxation time of the semiconductor which were found to be -8.6X10(-11) m(2) V-2 and 0.3 ns, respectively. Furthermore, the observation of the associated coherent transient grating effects allows us to obtain a coherence time of the laser system (2.5 ps) and the chi((3)) of the transient grating which was found to be -7.2X10(-13) m(2) V-2. The measurements were performed at room temperature on undoped bulk InSb. (C) 1995 American Institute of Physics.

We have performed the first optical experiment using the laser output of the CLIO free electron laser. In a transmission experiment we have observed strong power limiting at wavelengths longer than the absorption edge at 7.5 μm associated with induced free carrier absorption produced by direct interband two-photon transitions in InSb. We have estimated the two-photon adsorption (TPA) coefficient ( b) of InSb at 8.9 μm to be 2 cm/MW, vy fitting the power-limiting effect with a simple theoretical model. An important feature of this result is that we are utilising the broad tunability of the FEL to explore the TPA theory to shorter wavelengths than hitherto available with the CO 2 laser. The result is in broad agreement with the longer wavelength measurements of Sheik-Bahae et al. [J. Opt. Soc. Am. B4 (1987) 1964], and distinguishes between different theoretical models for the process. Applications that make use of the intrinsic nonlinearity of the interaction include the measurement of picosecond optical pulse lengths using an autocorrelation method, direct measurement of the optical pulse intensity and intensity stabilisation of the FEL source using the power limiting capabilities of the semiconductor.

In a previous paper measurements were reported of two-photon absorption (TPA) in the narrow gap semiconductors InAs and InSb utilising the mid-infrared tunability of CLIO. An assumption had to be made concerning the lifetime of the excited carriers in order to interpret the results. We have now made a direct measurement of the lifetime of two-photon-induced free holes in InAs and InSb using an excite-probe technique. We have measured the TPA coefficient over the entire range from one-photon to two-photon threshold, showing good agreement with a non-parabolic band model for the process.

To date the role of streaming motion in hot-hole p-Ge lasers has been inferred only through theoretical considerations; in this paper we present experimental proof of the importance of streaming motion for p-Ge lasers. We have performed magnetoabsorption measurements in p-Ge under intense crossed electric and magnetic fields. Observation of the change of a magneto-optical resonance from loss (absorption) to gain with increasing electric field has enabled us to identify directly the high-field lasing transition for the first time. It is confirmed to be the light-hole b-set n = 1-0 transition in the coupled band model. By analysing emission data obtained with stressed laser crystals we suggest impurity-related cyclotron resonance transitions to be involved in the lasing process for high-concentration samples.

We have performed the first experiment on the free-electron laser (CLIO) at the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique (LURE). In a transmission experiment we observed strong power limiting at wavelengths longer than the absorption edge associated with induced free carrier absorption produced by direct interband two-photon transitions in InSb. We have estimated the two-photon absorption (TPA) coefficient (beta) of InSb at 8.9 mum to be 2 cm MW-1, by fitting the power-limiting effect with a simple theoretical model. An important feature of this result is that we are utilizing the broad tunability of the FEL to explore the TPA theory to shorter wavelengths than hitherto available with CO2 lasers. The result is in broad agreement with the longer-wavelength measurements of Sheik-Bahae et al.

Low-temperature interband magnetoabsorption spectra have been obtained for a range of MBE-grown epitaxial InAs1-xSbx alloy layers. The field dependence of the spectral minima has been fitted within the Pidgeon and Brown model to determine a full set of band parameters. In particular a direct determination is made of the energy gap E(g), and the valence-conduction band interaction matrix element E(p). Good agreement is obtained with recent cyclotron resonance measurements of the conduction band mass. The alloy ordering effects result in a pronounced drop in mobility in the mid-alloy range together with a large bowing in the bandgap. The momentum matrix elements also show bowing across the alloy range. Fair agreement is obtained with alloy disorder models.

We have made direct pump-probe measurements of spin lifetimes in long wavelength narrow-gap semiconductors at wavelengths between 4 and 10 μm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes, τ ∼300 ps, even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that τ is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of τ in non-degenerate lightly n-type Hg Cd Te of approximately the same band gap as InSb, and find that τ varies from 356 ps at 150 K to 24 ps at 300 K. Our results, both in magnitude and temperature dependence of τ , imply that the Elliott-Yafet model dominates in these materials. © 2003 Elsevier B.V. All rights reserved.

Optically detected magnetic resonance (ODMR) of heavily nitrogen-doped ZnSe at 24 GHz reveals an anisotropic spectrum which can be assigned to a deep donor comprised of a Se vacancy paired with an impurity. The newly resolved spectrum is trigonal along [111] with g(parallel-to) = 2.0072(2) and g(perpendicular-to) =2.0013(2). Comparison of this g tensor with previous work leads to the assignment of the spectrum to V(Se)-X, where X is most probably Cu or Ag. Combining this result with previous photoluminescence and ODMR work shows that ZnSe:N has three distinct donors which include both impurities and intrinsic defects.

We demonstrate THz photon echoes from silicon doped with phosphorus donors. We provide experimental evidence showing that the echo emerges at the predicted angle and that it arrives at our detector at the predicted time. We use this to demonstrate coherent control of the silicon donor states.

We describe the observation of the circular and linear photogalvanic effects in HgTe/CdHgTe quantum wells. The interband absorption of mid-infrared radiation as well as the intrasubband absorption of terahertz radiation in the quantun well structures is shown to cause a dc electric current due to these effects. The photocurrent magnitude and direction varies with the radiation polarization state and crystallographic orientation of the substrate in a simple way that can be understood from a phenomenological theory. The observed dependences of the photocurrent on the radiation wavelength and temperature are discussed in terms of a microscopic model.

Using plasma-assisted molecular beam epitaxy (MBE), GaN x Sb 1− x films have been grown onto GaSb and GaAs substrates over a range of growth temperatures (310–460 °C). The films showed excellent crystalline quality and a nitrogen incorporation of 0–1.75% measured by X-ray diffraction. These materials should enable access to wavelengths in the 2–4 μm range.

We present two-colour pump–probe measurements of the intraband absorption in low growth rate, self-assembled InAs/GaAs quantum dots. The far-infrared resonance observed is unambiguously associated with the dots, and not related to the surrounding material. The results also imply that the interband photoluminescence lines, which appear under high excitation, are from conduction band levels with successively increasing in-plane quantum number.

The first FIRM-PL measurements in InSb/GaSb quantum dots have been performed. At low power densities the FIR absorption causes a transfer of carriers between dots and effectively cools the system, preferentially populating large, low energy dots and increasing the photoluminescence (PL) intensity. At higher powers the carrier temperature increases and the PL intensity falls. The spectral dependence of the FIRM-PL signal measures the energy spectrum of the quantum dots, showing a peak at 14.5 meV corresponding to transitions between the first two energy levels of the quantum dot distribution.

Optically addressable spin impurities in crystals along with device engineering provide an attractive route to realizing quantum technologies in the solid state, but reconciling disparate emitter and host material constraints for a given target application is often challenging. Rare-earth ions in two-dimensional (2D) materials could mitigate this problem given the atomic-like transitions of the emitters and the versatile nature of van der Waals systems. Here we combine ion implantation, confocal microscopy, and ab initio calculations to examine the photon emission of Er-doped WS2 flakes. Optical spectroscopy reveals narrow, long-lived photoluminescence lines in the telecom band, which we activate after low-temperature thermal annealing. Spectroscopic and polarization-selective measurements show a uniform response across the ensemble, while the fluorescence brightness remains mostly unchanged with temperature, suggesting nonradiative relaxation channels are inefficient. Our results create opportunities for novel solid state devices coupling 2D-hosted, telecom-band emitters to photonic heterostructures separately optimized for photon manipulation.

Summary form only given. Shallow donor impurities in silicon, once frozen out at low temperature, share many properties in common with free hydrogen atoms [1]. They have long been the subject of spectroscopic investigation, but it is only very recently [2,3] that it has been possible to investigate the time-domain dynamics of orbital excitations such as the 1s to 2p, due to the difficulty of obtaining short, intense pulses in the relevant wavelength range, around 10THz. These new techniques make shallow donors (and also acceptors [4]) attractive for studying atomic physics effects, and for applications in quantum information. We have measured the population dynamics of electrons orbiting around phosphorus impurities in commercially-available silicon, using a free electron laser as the THz source, and shown that the lattice relaxation lifetime is about 200ps, only 1 order of magnitude shorter than the radiative lifetime of free hydrogen. Coherent oscillation, where many particles cycle in phase, is responsible for classical phenomena like the emission of strong radio waves by many individual electrons in an antenna. At the quantum scale, coherent superposition of spin polarisations are central to Magnetic Resonance Imaging and its analogues, in which coherence is excited and then reappears later producing a delayed radio pulse (the spin "echo"). Quantum computer logic will also rely on coherence, and spins are often chosen as "qubits" because they are only weakly connected to, and disturbed by, the environment. Paradoxically, connection with the outside world is crucial for control, making charge (i.e. orbital) oscillations in semiconductors attractive. We have shown that silicon donor electrons can be put into a coherent superposition of orbital states that lasts for nearly as long as the lattice relaxation time [3,5]. The result of sequences of coherent control can also be read out simply using a voltmeter. Our results pave the way for new devices where information is stored in single electron orbits ("coherent orbitronics") in silicon, the material that has dominated the classical computing industry for half a century.

Recent developments in terahertz quantum cascade lasers are reviewed. Structures operating from a wavelength of λ = 66 μm down to λ = 87 μm are demonstrated. These devices used either a three-quantum-well chirped-superlattice active region or an active region based on a bound-to-continuum transition. The comparison between structures grown in a waveguide based on a single interface plasmon and a buried contact and (non-lasing) structures using a double plasmon waveguide demonstrates the importance of waveguide design on the operation of such devices. Continuous-wave operation up to a maximum temperature of 55 K with up to 15 mW output power at 10 K was demonstrated.

Strong (300 mW) narrow-band stimulated cyclotron resonance emission has been observed from the light holes in germanium in crossed electric and magnetic fields. We have achieved the widest tuning range yet reported for a single sample concentration, in the absence of external stress. A numerical analysis, including non-parabolic and 'quantum' effects, has enabled us to evaluate a previous perturbation treatment of the cross-field laser problem.

In the present work we report the first measurement of intersubband lifetimes in Si/Si Ge quantum well samples. We have determined T by a time resolved pump and probe experiment using the far infrared picosecond free electron laser source FELIX at Rijnhuizen, the Netherlands. In a sample with a well width of 50 Å and a sheet density of 2.1 × 10 cm we find a lifetime of 30 ps while 20 ps is observed for a density of 1.1 × 10 cm and a well width of 75 Å. We discuss acoustic phonon, as well as optical phonon intersubband scattering as possible limiting processes for the observed lifetimes in Si/SiGe and GaAs/AlGaAs quantum wells.

The intensity dependent intersubband absorption in GaAs/AlGaAs quantum wells with a subband separation smaller than the optical phonon energy has been measured with a pulsed far infrared free electron laser (FELIX). Complete bleaching of the absorption is observed at I = 100 kW/cm 2. When fitted with a two-level system, the saturation intensity is found to be 10 kW/cm 2, which corresponds to a characteristic time constant of 1–2 ps. Possible interpretations are discussed in the situation of finite FEL pulse width.

Indium antimonide (InSb) has the smallest energy gap of any of the binary III-V materials, leading to a cut-off wavelength of 7 μm at 300 K. The addition of small proportions of nitrogen to InSb offers the prospect of extending the response wavelength into the 8-12 μm range, which is important for thermal imaging in that atmospheric transmission window and because it encompasses the absorption lines of several environmentally important gases and can therefore be used for monitoring the gases. We report on the growth, by a combination of molecular beam epitaxy and a nitrogen plasma source, of InSb N with up to 10% nitrogen. Structural characterisation techniques of TEM, AFM and SIMS have enabled some optimisation of material quality to be demonstrated by biasing the sample during growth. Measurements on light emitting diodes comprising a superlattice of InSb N /InSb show an emission wavelength of 10.5 μm, which is confirmed by free electron laser assessment. Comparison with first principles band-structure calculations indicate that approximately 10% of the nitrogen is active. Hall effect measurements of 1 μm thick bulk layers indicate an increasing n-type behaviour, the degeneracy effects of which mean, however, that this is only a lower limit. Crown Copyright © 2002 Published by Elsevier Science Ltd. All rights reserved.

We present far-infrared (FIR) modulated photoluminescence measurements of self-assembled InAs/GaAs quantum dots. The sample was annealed to remove any indium profile roughness from within the quantum dots and thus from the inhomogeneous broadening. The annealed sample shows resonant FIR excitations just as the unannealed sample, but exhibits spectral hole burning unlike the unannealed sample. The results indicate that indium roughness is one of the primary inhomogeneous broadening mechanisms in self-assembled quantum dots.

The problem of intersubband relaxation in GaAs/GaAs quantum wells, where the energy separation of the two lowest subbands is smaller than the optical phonon energy, is considered. Time resolved pump-and-probe measurements are performed with a far-infrared free-electron laser on two multiquantum well samples with similar thicknesses (≈300 Å), but different doping and mobilities. The measured lifetimes are shorter than could be explained by acoustic phonon emission alone. Monte-Carlo calculations show the importance of electron–electron scattering for thermalization of the hot electron distribution function and subsequent optical phonon emission from the long thermal tail.

The short pulsed, tunable output from the Dutch free electron laser FELIX, coupled with a 45 T pulsed magnet has been used to measure the non-radiative decay rates between Landau levels in n-type InAs/GaSb quantum wells. The decay rate is 2 ps −1 when the cyclotron resonance energy, E CR= E LO=30 meV, and only slightly less, 0.8 ps −1 when E CR=2 E LO when the principal relaxation pathway is the emission of multiple longitudinal optical (LO) phonons.

Semiconductor emitters in the mid-infrared (mid-IR) fall broadly into two categories; interband and intersubband devices. The former are based on transitions from conduction to valence bands, whereas the latter use one-dimensionally quantum confined states within the conduction band. The problems which limit light-emitting diode (LED) and laser operation by keeping efficiencies low and laser thresholds high are quite different in these two cases, but both stem from high competing transition rates due to non-radiative processes. In interband devices Auger scattering dominates, and in intersubband devices, though Auger scattering is still fast, phonon emission is even faster. Working intersubband devices are only made possible by the fact that the phonon scattering is strongly resonant in energy, so three-level laser schemes may be made with a dynamic equilibrium that is inverted. We shall review our own work on suppression of these non-radiative processes by band-structure engineering, and we present new results on Auger suppression in 11 μm bandgap InSbN alloys and predictions for phonon suppression in narrow-gap intersubband devices.

We present a quantitative study of the efficiency of degenerate THz transient four-wave mixing in germanium doped by shallow impurities using the Dutch free electron laser FELIX. We derive the third order nonlinear sheet susceptibility and find values at low temperature that exceed those of any other material and wavelength range reported to date.

We studied InSb quantum well devices using Landau level tunneling spectroscopy through a three-terminal differential conductance technique. This method is similar to filled state scanning tunneling microscopy but uses a stationary contact instead of a mobile tip to analyze the two-dimensional electron system. Applying magnetic fields up to 15 T, we identified clear peaks in the differential current-voltage profiles, indicative of Landau level formation. By examining deviations from the expected Landau fan diagram, we extract an absolute value for the exchange-induced energy shift. Through an empirical analysis, we derive a formula describing the exchange shift as a function of both magnetic field strength and electron filling. Our findings indicate that the emptying of the í µí¼ˆ = 2 and í µí¼ˆ = 3 Landau levels causes an exchange interaction energy shift in the í µí¼ˆ = 1 level. Unlike prior studies that infer level energies relative to one another and report oscillatory g-factor behavior, our method references the energy of the Landau levels above the filled states of the contact under a bias voltage, revealing that only the ground state Landau level experiences a measurable exchange shift.

Single impurities in insulators are now often used for quantum sensors and single photon sources, while nanoscale semiconductor doping features are being constructed for electrical contacts in quantum technology devices, implying that new methods for sensitive, non-destructive imaging of single- or few-atom structures are needed. X-ray fluorescence (XRF) can provide nanoscale imaging with chemical specificity, and features comprising as few as 100 000 atoms have been detected without any need for specialized or destructive sample preparation. Presently, the ultimate limits of sensitivity of XRF are unknown - here, gallium dopants in silicon are investigated using a high brilliance, synchrotron source collimated to a small spot. It is demonstrated that with a single-pixel integration time of 1 s, the sensitivity is sufficient to identify a single isolated feature of only 3000 Ga impurities (a mass of just 350 zg). With increased integration (25 s), 650 impurities can be detected. The results are quantified using a calibration sample consisting of precisely controlled numbers of implanted atoms in nanometer-sized structures. The results show that such features can now be mapped quantitatively when calibration samples are used, and suggest that, in the near future, planned upgrades to XRF facilities might achieve single-atom sensitivity.

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.

We have used time resolved spectroscopy to measure the relaxation of spin polarization in InSb/AlInSb quantum wells (QWs) as a function of temperature and mobility. The results are consistent with the D'yakonov - Perel (DP) mechanism for high mobility samples over the temperature range from 50 to 300 K. For low mobility samples at high temperature the Elliott - Yafet and DP mechanisms become comparable. We show that the mobility can in certain circumstances determine which mechanism is dominant, and that above 1 m(2) V-1 s(-1) in 20 nm wide InSb QWs it is the DP mechanism. We also give a criterion for the maximum spin lifetime in terms of mobility and temperature, and show that for our 20 nm wide QWs this corresponds to 0.5 ps at 300 K and mobility 1 m(2) V-1 s(-1).

Featured Application Knowledge of temperature dependence of refractive indices of Ge and Si at THz frequencies is required in many applications where these materials are used for THz devices and optical components. Refractive indices of high resistivity Si and Ge were measured at temperatures between 4-296 K and at frequencies between 4.2-7.7 THz using a Fourier-transform spectrometer (FTS) in transmission mode. A phenomenological model of the temperature dependence of the refractive index is proposed.

In this perspective article, we discuss the application of ion implantation to manipulate strain (by either neutralizing or inducing compressive or tensile states) in suspended thin films. Emphasizing the pressing need for a high-mobility silicon-compatible transistor or a direct bandgap group-IV semiconductor that is compatible with complementary metal-oxide-semiconductor technology, we underscore the distinctive features of different methods of ion beam-induced alteration of material morphology. The article examines the precautions needed during experimental procedures and data analysis and explores routes for potential scalable adoption by the semiconductor industry. Finally, we briefly discuss how this highly controllable strain-inducing technique can facilitate enhanced manipulation of impurity-based spin quantum bits (qubits).

We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si: P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the mJ=±3/2 heavy hole is found to be larger than that of the mJ=±1/2 light hole.

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser

We present what we believe to be the first ever high-pressure and spontaneous emission measurements on quantum dash lasers. The results show that temperature sensitivity of these lasers is caused by nonradiative processes, which depend on the lasing wavelength.

Erbium implanted silicon is promising for both photonic and quantum technology platforms, since it possesses both telecommunications and integrated circuit processing compatibility. However, several different Er centres are generated during the implantation and annealing process, the presence of which could hinder the development of these applications. When Si is co-implanted with 1017 cm-3 Er and 1020 cm-3 O ions, and the appropriate annealing process is used, one of these centres, which is present at higher Er concentrations, can be eliminated. Characterisation of samples with Er concentrations

Excited states of a single donor in bulk silicon have previously been studied extensively based on effective mass theory. However, proper theoretical descriptions of the excited states of a donor cluster are still scarce. Here we study the excitations of lines of defects within a single-valley spherical band approximation, thus mapping the problem to a scaled hydrogen atom array. A series of detailed full configuration-interaction, time-dependent Hartree-Fock and time-dependent hybrid density-functional theory calculations have been performed to understand linear clusters of up to 10 donors. Our studies illustrate the generic features of their excited states, addressing the competition between formation of inter-donor ionic states and intra-donor atomic excited states. At short interdonor distances, excited states of donor molecules are dominant, at intermediate distances ionic states play an important role, and at long distances the intra-donor excitations are predominant as expected. The calculations presented here emphasise the importance of correlations between donor electrons, and are thus complementary to other recent approaches that include effective mass anisotropy and multi-valley effects. The exchange splittings between relevant excited states have also been estimated for a donor pair and for three-donor arrays; the splittings are much larger than those in the ground state in the range of donor separations between 10 and 20 nm. This establishes a solid theoretical basis for the use of excited-state exchange interactions for controllable quantum gate operations in silicon.

Dilute nitrogen alloys of InSb exhibit extremely strong band gap bowing with nitrogen composition that has been associated with anticrossing between the localized resonant states of the nitrogen within the conduction band and the extended states of the conduction band itself. This also results in the conduction band dispersion having an enhanced nonparabolicity. We have measured the electron effective mass near the anticrossing by cyclotron resonance in InNxSb1–x alloys with absorption edge near 15 µm, using pulsed fields up to 150 T. The results directly demonstrate the band anticrossing and quantitatively confirm the increase of effective mass versus x predicted for InNxSb1–x by a tight binding calculation for low nitrogen concentration (x

We report investigation of the spin relaxation in InAs films grown on GaAs at a temperature range from 77 K to 290 K. InAs is known to have a surface accumulation layer and the depth profile of the concentration and mobility is strongly nonuniform. We have correlated the spin relaxation with a multilayer analysis of the transport properties and find that the surface and the interface with the GaAs substrate both have subpicosecond lifetimes (due to the high carrier concentration), whereas the central semiconducting layer has a lifetime of an order of 10 ps. Even for the thickest film studied (1 micro-m, the semiconducting layer only carried 30% of the total current (with 10% through the interface layer and 60% through the surface accumulation layer). Designs for spintronic devices that utilize InAs, which is attractive due to its narrow gap and strong Rashba effect, will need to include strategies for minimizing the effects of the surface.

Photoluminescence (PL) has been used as a means of unambiguously observing band gap reduction in InNAs epilayers grown by molecular beam epitaxy. The observed redshift in room temperature emission as a function of nitrogen concentration is in agreement with the predictions of the band anticrossing (BAC) model, as implemented with model parameters derived from tight-binding calculations. The temperature dependence of the emission from certain samples exhibits a signature non-Varshni-like behavior indicative of electron trapping in nitrogen-related localized states below the conduction-band edge, as predicted by the linear combination of isolated nitrogen states (LCINS) model. This non-Varshni-like behavior tends to grow more pronounced with increasing nitrogen content, but for the highest nitrogen concentration studied, the more familiar Varshni-like behavior is recovered. Although unexpected, this observation is found to be consistent with the BAC and LCINS models. With consideration given to the effects of conduction-band nonparabolicity on the emission line shapes, the BAC model parameters extracted from the measured PL transition energies are found to be in excellent agreement with the predictions of the aforementioned tight-binding calculations.

We investigate the spin relaxation under conditions of optical excitation between the Rydberg orbital states of phosphorus donor impurities in silicon. Here we show that the spin relaxation is less than a few percent, even after multiple excitation/relaxation cycles. The observed high level of spin preservation may be useful for readout cycling or in quantum information schemes where coupling of neighbor qubits is via orbital excitation.

Implicit summation is a technique for conversion of sums over intermediate states in multiphoton absorption and the high-order susceptibility in hydrogen into simple integrals. Here we derive the equivalent technique for hydrogenic impurities in multi-valley semiconductors. While the absorption has useful applications it is primarily a loss process, conversely the non-linear susceptibility is a crucial parameter for active photonic devices. For Si:P we predict the hyperpolarizability ranges from X(3)⁄n3D = 2:9 to 580x10-38m5V2 depending on the frequency even while avoiding resonance. Using samples of reasonable density n3D and thickness L to produce third harmonic generation at 9 THz, a frequency that is difficult to produce with existing solid state sources, we predict that X(3) should exceed that of bulk InSb and X(3)L should exceed that of graphene and resonantly enhanced quantum wells.

We report the direct determination of nonradiative lifetimes in Si/SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k center dot p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height >= 1 A), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs/AlxGa1-xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV materials.

We have used time-resolved spectroscopy to measure the relaxation of spin polarizations in the narrow gap semiconductor material n-InAs as a function of temperature, doping, and pump wavelength. The results are consistent with the D'Yakonov-Perel mechanism for temperatures between 77 and 300 K. However, the data suggest that electron-electron scattering should be taken into account in determining the dependence of the spin lifetime on the carrier concentration in the range 5.2×1016–8.8×1017 cm–3. For a sample with doping of 1.22×1017 cm–3 the spin lifetime was 24 ps at room temperature. By applying a magnetic field in the sample plane we also observed coherent precession of the spins in the time domain, with a g>/i> factor g*=–13, also at room temperature.

We present theoretical results on the nonlinear optics of semiconductor quantum wells in intense THz electric fields (the dynamic Franz-Keldysh effect or DFKE). The absorption spectra show a rich variety of behavior, including THz replicas of the 2p exciton and THz sidebands of the 1s exciton. We calculate the dependence of these features on the phase and intensity of the THz field using the extended semiconductor Bloch equations, and discuss the relevance of our results to future experiments. The 1s-sideband absorption feature shows a strong dependence on the phase of the THz field, and phase averages to zero. We also discuss the relative advantages and disadvantages of reflectivity and absorption spectroscopies for probing the DFKE.

We have made direct pump-probe measurements of spin lifetimes in intrinsic and degenerate n-InAs at 300 K. In particular, we measure remarkably long spin lifetimes (tau(s)similar to1.6 ns) for near-degenerate epilayers of n-InAs. For intrinsic material, we determine tau(s)similar to20 ps, in agreement with other workers. There are two main models that have been invoked for describing spin relaxation in narrow-gap semiconductors: the D'yakonov-Perel (DP) model and the Elliott-Yafet (EY) model. For intrinsic material, the DP model is believed to dominate in III-V materials above 77 K, in agreement with our results. We show that in the presence of strong n-type doping, the DP relaxation is suppressed both by the degeneracy condition and by electron-electron scattering, and that the EY model then dominates for the n-type material. We show that this same process is also responsible for a hitherto unexplained lengthening of tau(s) with n-type doping in our earlier measurements of n-InSb.

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230 cm–1 from the amplified spontaneous emission. This high value makes the authors' waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.

Superposition of orbital eigenstates is crucial to quantum technology utilising atoms, such as atomic clocks and quantum computers, and control over the interaction between atoms and their neighbours is an essential ingredient for both gating and readout. The simplest coherent wavefunction control uses a 2-eigenstate admixture, but more control over the spatial distribution of the wavefunction can be obained by increasing the number of states in the wavepacket. Here we demonstrate THz laser pulse control of Si:P orbitals using multiple orbital state admixtures, observing beat patterns produced by Zeeman splitting. The beats are an observable signature of the ability to control the path of the electron, which implies we can now control the strength and duration of the interaction of the atom with different neighbours. This could simplify surface code networks which require spatially controlled interaction between atoms, and we propose an architecture that might take advantage of this.

By measuring THz absorption and time-resolved photoluminescence on the same GaAs quantum well sample we confirm the recent prediction of Kira that PL at the exciton frequency does not require a population of bound excitons.

THz optical properties of lithium borate (LBO) crystals were measured using time-domain spectroscopy (TDS). The LBO crystal samples were of high optical quality and were cut and polished along the h100i, h010i and h001i axes. Two independent measurements were performed in order to con rm the reproducibility and consistency of results. The contradictions in the previously published data on the THz optical properties of LBO were clari ed. It was shown that the birefringence order at THz frequencies is nz < nx < ny, whereas at optical frequencies it is known to be nx < ny < nz. It was seen that nz, which has the highest value in the visible, has the lowest value at THz. This is explained in terms of ionic polarizability and is consistent with the fact that the THz absorption coe cient for a wave polarized along the Z-axis is more than an order of magnitude lower than for the X and Y axes. Absorption as low as 0.2 cm 1 was found at frequencies up to 0.5 THz for a wave polarized parallel to the Z-axis. A set of new dispersion equations was designed for the entire transparency range.

We present time-resolved measurements of the relaxation between the orbital states of the shallow acceptor boron in silicon. The silicon host was enriched Si-28, which exhibits life-time broadened absorption lines. We observed a wide range of T1 lifetimes from 6ps to 130ps depending on the excited state and the pump intensity. The fastest transients have not been observed previously in the time domain, and they are caused by the phonon relaxation responsible for the small-signal frequency domain line-width. We identify the slower components with an ionisation/recombination/cascade pathway.

Dilute nitrogen alloys of InSb exhibit strong band gap bowing with increasing nitrogen composition, shifting the absorption edge to longer wavelengths. The conduction band dispersion also has an enhanced nonparabolicity, which suppresses Auger recombination. We have measured Auger lifetimes in alloys with 11 and 15 µm absorption edges using a time-resolved pump-probe technique. We find the lifetimes to be longer at room temperature than equivalent band gap Hg1–yCdyTe alloys at the same quasi-Fermi level separation. The results are explained using a modified k·p Hamiltonian which explicitly includes interactions between the conduction band and a higher lying nitrogen-related resonant band.

A brief review is given of lifetime and line broadening studies with the free electron laser at FOM-Rijnhuizen (FELIX), emphasising work on far infrared (FIR) modulated photoluminescence (PL) of bound-to-bound transitions in InAs/GaAs quantum dots and mid-infrared (MIR) four wave mixing experiments on localised modes of H- ions in calcium fluoride crystals. We have made new far/near infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The FIR resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots, and analysis of the FIR double resonance linewidth enables us to show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation. In addition we have made time resolved studies of local modes in ionic crystals, where nonradiative decay plays an important role in the optical pumping cycle of laser gain media. We have made pump-probe studies of the local modes created upon the introduction of a light impurity, in particular the H - ion, in CaF2 in the spectral region 700 to 1200 cm-1. We have also measured the free induction decay of this mode using a noncollinear degenerate four wave mixing geometry. The observed coherent transient is striking in having large quantum beat modulations at negative time which are associated with vibrational ladder climbing.

We observe third-harmonic generation (THG) in gallium-doped germanium ( Ge:Ga) upon pumping with picosecond sub-THz (0.6-0.7 THz) pulses from the THz free-electron laser FLARE with a peak electric field strength of up to 4 kV/cm (30kW/cm2). The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 3 as expected.

This study reports on high energy bismuth ion implantation into silicon with a particular emphasis on the effect that annealing conditions have on the observed hyperfine structure of the Si:Bi donor state. A suppression of donor bound exciton, D0X, photoluminescence is observed in implanted samples which have been annealed at 700 °C relating to the presence of a dense layer of lattice defects that is formed during the implantation process. Hall measurments at 10 K show that this implant damage manifests itself at low temperatures as an abundance of p‐type charge carriers, the density of which is observed to have a strong dependence on annealing temperature. Using resonant D0X photoconductivity, we are able to identify the presence of a hyperfine structure in samples annealed at a minimum temperature of 800 °C; however, higher temperatures are required to eliminate effects of implantation strain.

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230 cm(-1) from the amplified spontaneous emission. This high value makes the authors' waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.

Electron spin relaxation times have been measured in InSb and InAs quantum wells and epi-layers in a moderate (

A detailed study of the ability of pyridine-2,6-dicarboxylic acid (1) and its 4-mono- and 3,4,5-tri-substituted analogues to sensitize emission from Pr3+, Nd3+, Gd3+, Dy3+ and Er3+ is presented. Sensitization of Ln3+ emission was demonstrated via the ligands in all complexes, excluding Gd3+, with emission covering the spectral range from 500 nm to 1850 nm obtained with variation of the Ln3+ ion. From the study of the ligand-based photoluminescence obtained from Gd3+-complexes, and the relative ligand and Ln3+ emission obtained from the other complexes, the singlet and triplet state energies of complexes of (1) are estimated to be at 3.1 eV and 2.6 eV respectively whilst for the 3,5-dibromo-substituted complexes (4) they are at 2.9 eV and 2.3 eV. Hypersensitivity of the Er3+4I15/2 → 2H11/2 and 4I15/2 → 4G11/2 intra-atomic transitions is also observed in the 4-chloro-substituted (3) complex. Enhanced sensitization of Nd3+ (ca. 5-fold) and Er3+ (ca. 2-fold) near-infrared emission is demonstrated for complexes of (3) and (4) respectively in comparison with those of (1). •Sensitized luminescence from dipicolinate lanthanide complexes from 500 to 1850 nm.•Enhanced sensitization of near infrared emission from Nd3+ and Er3+ complexes.•Hypersensitivity of Er3+ intra-atomic transitions observed.•Study of ligand-state interactions with Pr3+, Nd3+, Gd3+, Dy3+ and Er3+ ions.

We report Larmor precession in bulk InSb observed in the time domain from 77 to 300 K. The optically oriented polarization precesses coherently even at 300 K. The inferred Zeeman spin splitting is strongly nonparabolic, and the electron g factor (g*) is in good agreement with k·p theory (provided we take only the dilational contribution to the change in energy gap with temperature). We also show here that correct application of the 14-band k·p model agrees with apparently anomalous trends previously reported for GaAs and confirm that the most widely quoted formula for g* in GaAs is incomplete.

The spin-orbit (SO) coupling parameters for the lowest conduction subband due to structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are calculated for a range of carrier densities in [001]-grown delta-doped n-type InSb/In1-xAlxSb quantum wells using the established eight-band k center dot p formalism [J. Deng , Phys. Rev. B 59, R5312 (1999)]. We present calculations for conditions of zero bias at 10 K. It is shown that both the SIA and BIA parameters scale approximately linearly with carrier density, and exhibit a marked dependence on well width when alloy composition is adjusted to allow maximum upper barrier height for a given well width. In contrast to other material systems, the BIA contribution to spin splitting is found to be of significant and comparable value to the SIA mechanism in these structures. We calculate the spin lifetime tau(s[1 (1) over bar0]) for spins oriented along [1 (1) over bar0] based on D'yakonov-Perel' mechanism using both the theory of Averkiev [J. Phys.: Condens. Matter 14, R271 (2002)] and also directly the rate of precession of spins about the effective magnetic field, taking into account all three SO couplings, which show good agreement. tau(s[1 (1) over bar0]) is largest in the narrowest wells over the range of moderate carrier densities considered, which is attributed to the reduced magnitude of the k-cubic BIA parameter in narrow wells. The inherently large BIA induced SO coupling in these systems is shown to have considerable effect on tau(s[1 (1) over bar0]), which exhibits significant reduction in the maximum spin lifetime compared to previous studies that consider systems with relatively weak BIA induced SO coupling. The relaxation rate of spins oriented in the [001] direction is found to be dominated by the k-linear SIA and BIA coupling parameters and at least an order of magnitude greater than in the [1 (1) over bar0] direction.

Third order non-linearities are important because they allow control over light pulses in ubiquitous high quality centro-symmetric materials like silicon and silica. Degenerate four-wave mixing provides a direct measure of the third order non-linear sheet susceptibility (3)L (where L represents the material thick- ness) as well as technological possibilities such as optically gated detection and emission of photons. Using picosecond pulses from a free electron laser, we show that silicon doped with P or Bi, has a value of (3)L in the THz domain that is higher than that reported for any other material in any wavelength band. The immediate implication of our results is the ecient generation of intense coherent THz light via up-conversion (also a (3) process), and they open the door to exploitation of non-degenerate mixing and optical nonlinearities beyond the perturbative regime.

The theoretical error rates in deterministic ion implantation when using an ion beam governed by a Poisson point process with a detector that counts the impacts are investigated. It is concluded that if the error rates are small, then for spots with nominally one implanted ion the probability of failure to implant the correct number is ≈ 𝜅/𝜆+𝜂⎯⎯+𝜆/2 for a synchronous (i.e., pulsed) system or 𝐾/𝐿+𝜂⎯⎯+𝐿𝑡s for an asynchronous (i.e., continuous beam) system, where 𝜂⎯⎯ is the probability that the detector misses an ion impact, and 𝐿(𝐾) and 𝜆(𝜅) are the number of ions (dark counts) per unit time and per pulse, respectively. ts is the system reaction time for an asynchronous system. This approximation allows easy identification of the greatest need for engineering effort. Some experimental efforts to measure these parameters and their uncertainties are examined.

We have used time-resolved spectroscopy to measure the relaxation of spin polarizations in the narrow gap semiconductor material n-InAs as a function of temperature, doping, and pump wavelength. The results are consistent with the D'Yakonov-Perel mechanism for temperatures between 77 and 300 K. However, the data suggest that electron-electron scattering should be taken into account in determining the dependence of the spin lifetime on the carrier concentration in the range 5.2x10(16)-8.8x10(17) cm(-3). For a sample with doping of 1.22x10(17) cm(-3) the spin lifetime was 24 ps at room temperature. By applying a magnetic field in the sample plane we also observed coherent precession of the spins in the time domain, with a g factor g(*)=-13, also at room temperature.

The spontaneous electroluminescence emission of InAs light-emitting diodes (LEDs) operating at 3.3 μm was studied as a function of applied hydrostatic pressure. An enhancement of a factor of almost four in radiative efficiency at room temperature was observed in the range 0 to 10 kbar. Analysis of the dependence of electroluminescence emission intensity on hydrostatic pressure at constant current reveals that nonradiative Auger recombination dominates the quantum efficiency of these LEDs

We report X-ray fluorescence (XRF) imaging of nanoscale inclusions of impurities for quantum technology. A very bright diffraction-limited focus of the X-ray beam produces very high sensitivity and resolution. We investigated gallium (Ga) dopants in silicon (Si) produced by a focused ion beam (FIB). These dopants might provide 3/2-spin qubits or p-type electrical contacts and quantum dots. We find that the ion beam spot is somewhat larger than expected, and the technique provides a useful calibration for the resolution of FIBs. Enticingly, we demonstrate that with a single shot detection of 1 second integration time, the sensitivity of the XRF would be sufficient to find amongst background a single isolated inclusion of unknown location comprising only 3000 Ga impurities (a mass of just 350 zg) without any need for specialized nm-thickness lamellae, and down from >105 atoms in previous reports of similar work. With increased integration we were able to detect 650 impurities. The results show that planned facility upgrades might achieve single atom sensitivity with a generally applicable, non-destructive technique in the near future.

Ion Implantation In article number 2000237 by Nathan Cassidy, Roger Webb, David Cox, and co‐workers, preliminary results are presented from the first commercially produced implanter specifically designed for rapid and precise positioning of single ions into device structures for solid state quantum technology applications. Specifically the implantation of single bismuth ions into Si, Ge, Cu, and Au are reported, and the counting detection efficiency for single ion implants and the factors that affect such detection efficiencies are determined.

This study reports the effect of an increasing ion dose on both the electrical activation yield and the characteristic properties of implanted bismuth donors in silicon. A strong dependence of implant fluence is observed on both the yield of bismuth donors and the measured impurity diffusion. This is such that higher ion concentrations result in both a decrease in activation and an enhancement in donor migration through interactions with mobile silicon lattice vacancies and interstitials. Furthermore, the effect of implant fluence on the properties of the Si:Bi donor bound exciton, D0X, is also explored using photoluminescence (PL) measurements. In the highest density sample, centers corresponding to the PL of bismuth D0Xs within both the high density region and the lower concentration diffused tail of the implanted donor profile are identifiable.

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser.

We describe the observation of the circular and linear photogalvanic effects in HgTe/CdHgTe quantum wells. The interband absorption of mid-infrared radiation as well as the intrasubband absorption of terahertz (THz) radiation in the QWs structures is shown to cause a dc electric current due to these effects. The photocurrent magnitude and direction varies with the radiation polarization state and crystallographic orientation of the substrate in a simple way that can be understood from a phenomenological theory. The observed dependences of the photocurrent on the radiation wavelength and temperature are discussed.

Low‐dimensional microwave interconnects have important applications for nanoscale electronics, from complementary metal–oxide‐semiconductor (CMOS) to silicon quantum technologies. Graphene is naturally nanoscale and has already demonstrated attractive electronic properties, however its application to electronics is limited by available fabrication techniques and CMOS incompatibility. Here, the characteristics of transmission lines made from silicon doped with phosphorus are investigated using phosphine monolayer doping. S‐parameter measurements are performed between 4–26 GHz from room temperature down to 4.5 K. At 20 GHz, the measured monolayer transmission line characteristics consist of an attenuation constant of 40 dB mm−1 and a characteristic impedance of 600 Ω. The results indicate that Si:P monolayers are a viable candidate for microwave transmission and that they have a.c. properties similar to graphene, with the additional benefit of extremely precise, reliable, stable, and inherently CMOS compatible fabrication. Low‐dimensional microwave interconnects have important applications for nanoscale electronics, from complementary metal–oxide‐semiconductor (CMOS) to silicon quantum technologies. In this study, monolayer transmission lines made from silicon delta‐doped with phosphorus are characterized and it is shown that they have a.c. properties similar to graphene, with the additional benefit of extremely precise, reliable, stable, and inherently CMOS compatible fabrication.

We present far-/near-infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The far-infrared resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots. The results show that the interband photoluminescence (PL) lines originate from conduction levels with successively increasing in-plane quantum numbers. We determine the confinement energies for both electrons and holes in the same dots. Furthermore, we show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation.

The structural and optoelectronic properties in GaNxSb1−x alloys (0≤x

Under oblique incidence of circularly polarized infrared radiation the spin-galvanic effect (SGE) has been unambiguously observed in (001)-grown n-type GaAs quantum well structures in the absence of any external magnetic field. Resonant intersubband transitions have been obtained making use of the tunability of the free-electron laser FELIX. A microscopic theory of the SGE for intersubband transitions has been developed, which is in good agreement with experimental findings.

We report on the pressure dependence of the threshold current in 1.3 mum InGaAsP and 1.5 mum InGaAs quantum-well lasers measured at low temperatures similar to100 K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at similar to100 K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from similar to100 to 300 K.

The spontaneous electroluminescence emission of InAs light-emitting diodes (LEDs) operating at 3.3 µm was studied as a function of applied hydrostatic pressure. An enhancement of a factor of almost four in radiative efficiency at room temperature was observed in the range 0 to 10 kbar. Analysis of the dependence of electroluminescence emission intensity on hydrostatic pressure at constant current reveals that nonradiative Auger recombination dominates the quantum efficiency of these LEDs.

Photoluminescence (PL) has been used as a means of unambiguously observing band gap reduction in InNAs epilayers grown by molecular beam epitaxy. The observed redshift in room temperature emission as a function of nitrogen concentration is in agreement with the predictions of the band anticrossing (BAC) model, as implemented with model parameters derived from tight-binding calculations. The temperature dependence of the emission from certain samples exhibits a signature non-Varshni-like behavior indicative of electron trapping in nitrogen-related localized states below the conduction-band edge, as predicted by the linear combination of isolated nitrogen states (LCINS) model. This non-Varshni-like behavior tends to grow more pronounced with increasing nitrogen content, but for the highest nitrogen concentration studied, the more familiar Varshni-like behavior is recovered. Although unexpected, this observation is found to be consistent with the BAC and LCINS models. With consideration given to the effects of conduction-band nonparabolicity on the emission line shapes, the BAC model parameters extracted from the measured PL transition energies are found to be in excellent agreement with the predictions of the aforementioned tight-binding calculations.

The ordinary Fano effect occurs in many-electron atoms and requires an autoionizing state. With such a state, photo-ionization may proceed via pathways that interfere, and the characteristic asymmetric resonance structures appear in the continuum. Here we demonstrate that Fano structure may also be induced without need of auto-ionization, by dressing the continuum with an ordinary bound state in any atom by a coupling laser. Using multi-photon processes gives complete, ultra-fast control over the interference. We show that a line-shape index q near unity (maximum asymmetry) may be produced in hydrogenic silicon donors with a relatively weak beam. Since the Fano lineshape has both constructive and destructive interference, the laser control opens the possibility of state-selective detection with enhancement on one side of resonance and invisibility on the other. We discuss a variety of atomic and molecular spectroscopies, and in the case of silicon donors we provide a calculation for a qubit readout application. Fano resonances occur in many platforms that have auto-ionizing states. Here the authors show that auto-ionizing states are not required for multi-photon Fano resonance in a Si:P system with significant screening by using a pump-probe method.

The authors have measured the output spectrum and the threshold current in 9.2 mu m wavelength GaAs/Al0.45Ga0.55As quantum-cascade lasers at 115 K as a function of hydrostatic pressure up to 7.3 kbars. By extrapolation back to ambient pressure, thermally activated escape of electrons from the upper lasing state up to delocalized states of the Gamma valley is shown to be an important contribution to the threshold current. On the other hand leakage into the X valley, although it has a very high density of states and is nearly degenerate with the Gamma band edge in the barrier, is insignificant at ambient pressure.

Doping of silicon via phosphine exposures alternating with molecular beam epitaxy overgrowth is a path to Si:P substrates for conventional microelectronics and quantum information technologies. The technique also provides a new and well-controlled material for systematic studies of two-dimensional lattices with a half-filled band. We show here that for a dense (ns = 2.8 × 1014 cm−2 ) disordered two-dimensional array of P atoms, the full field angle-dependent magnetostransport is remarkably well described by classic weak localization theory with no corrections due to interaction effects. The two- to three-dimensional cross-over seen upon warming can also be interpreted using scaling concepts, developed for anistropic three-dimensional materials, which work remarkably except when the applied fields are nearly parallel to the conducting planes.

We report the quantitative and direct determination of hole intersubband relaxation times in a voltage biased SiGe heterostructure using density matrix calculations applied to a four-level system in order to interpret photocurrent (PC) pump–pump experiments. One consistent set of parameters allows the simulation of two kinds of experiments, namely pump–pump photocurrent experiments at a free electron laser (wavelength 7.9 μm) and the laser-power dependence of the PC signal. This strongly confirms the high reliability of these parameter values, of which the most interesting in respect to Si based quantum cascade laser development is the extracted heavy-hole relaxation time. The simulations show that this relaxation time directly determines the experimentally observed decay of the pump–pump photocurrent signal as a function of the delay time. For a heavy hole intersubband spacing of 160 meV, a value of 550 fs was obtained. The experimental method was further applied to determine the LH1–HH1 relaxation time of a second sample with a transition energy below the optical phonon energy. The observed relaxation time of 16 ps is consistent with the value found for the same structure by transmission pump–probe experiments.

Photogalvanic effects are observed and investigated in wurtzite (0001)-oriented GaN/AlGaN low-dimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions.

The introduction of strain into semiconductors offers a well-known route to modify their band structure. Here, we show a single-step procedure for generating such strains smoothly and deterministically, over a very wide range, using a simple, easily available, highly scalable, ion implantation technique to control the degree of amorphization in and around single-crystal membranes. The amorphization controls the density of the material and thus the tension in the neighboring crystalline regions. We have demonstrated up to 3.1% biaxial tensile strain and 8.5% uniaxial strain in silicon, based on micro-Raman spectroscopy. This method achieves strain levels never previously reached in mesoscopic defect-free, crystalline silicon structures. The flexible, gently controllable, single-step process points toward very high mobility complementary metal-oxide-semiconductor devices and easy fabrication of direct-bandgap germanium for silicon-compatible optoelectronics.

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.

The temperature dependence of the threshold current of InGaAsSb/AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T0 values.

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9 mu m). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained. (c) 2006 American Institute of Physics.

We present what we believe to be the first ever high-pressure and spontaneous emission measurements on quantum dash lasers. The results show that temperature sensitivity of these lasers is caused by nonradiative processes, which depend on the lasing wavelength.

We model theoretically the dependence of excitonic absorption spectra of semiconductor quantum wells in intense THz electric fields on the phase and intensity of those fields, and discuss the implications of our results for experiment.

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9 µm). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained.

Measurements of the THz absorption and the time-resolved photoluminescence have been performed on the same GaAs quantum well sample. The strength of the absorption at the internal 1s-2p exciton transition frequency is used as a measure of the density of excitons in the sample. When the interband pump laser is resonant with the 1s exciton frequency, induced absorption at the s-2p frequency is clearly seen. If the same density of carriers is created pumping in the continuum, no significant 1s-2p absorption is seen in a time window of 450 ps. Complementary time-resolved photoluminescence experiments, detecting the emission at the exciton energy under the same pump conditions, show the PL intensity in resonant and nonresonant cases to be similar. The counter-intuitive existence of luminescence at the exciton energy simultaneously with the absence of the s-2p absorption is consistent with the recent theoretical predictions of Kira et al., Phys. Rev. Lett. 81, 3263 (1998).

We report the quantitative and direct determination of hole intersubband relaxation times in a voltage biased SiGe heterostructure using density matrix calculations applied to a four-level system in order to interpret photocurrent (PC) pump-pump experiments. One consistent set of parameters allows the simulation of two kinds of experiments, namely pump-pump photocurrent experiments at a free electron laser (wavelength 7.9 mu m) and the laser-power dependence of the PC signal. This strongly confirms the high reliability of these parameter values, of which the most interesting in respect to Si based quantum cascade laser development is the extracted heavy-hole relaxation time. The simulations show that this relaxation time directly determines the experimentally observed decay of the pump-pump photocurrent signal as a function of the delay time. For a heavy hole intersubband spacing of 160 meV, a value of 550 fs was obtained. The experimental method was further applied to determine the LH1-HH1 relaxation time of a second sample with a transition energy below the optical phonon energy. The observed relaxation time of 16 ps is consistent with the value found for the same structure by transmission pump-probe experiments.

We present results for the lifetime of the orbital transitions of Bi donors in Si, measured using both frequency domain and time-domain techniques, allowing us to distinguish between homogeneous and inhomogeneous processes. The proximity of the energy of the optically allowed transitions to the optical phonon energy means that there is an unusually wide variation in the lifetimes and broadening mechanisms for this impurity, from fully homogeneous lifetime-broadened transitions to fully inhomogeneously broadened lines. The relaxation lifetime (T1) of the states range from the low 10’s to 100’s of ps, and we find that there is little extra dephasing (so that T1 is of the order of T2/2) in each case.

The spin relaxation in undoped InSb films grown on GaAs has been investigated in the temperature range from 77 to 290 K. Two distinct lifetime values have been extracted, 1 and 2.5 ps, dependent on film thickness. Comparison of this data with a multilayer transport analysis of the films suggests that the longer time (similar to 2.5 ps at 290 K) is associated with the central intrinsic region of the film, while the shorter time (similar to 1 ps) is related to the highly dislocated accumulation region at the film-substrate interface. Whereas previous work on InAs films grown on GaAs showed that the native surface defect resulted in an additional charge accumulation layer with high conductivity but very short spin lifetime, in InSb layers the surface states introduce a depletion region. We infer that InSb could be a more attractive candidate for spintronic applications than InAs.

Photoluminescence (PL) has been observed from dilute InNxAs1-x epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a (5x5) k center dot p Hamiltonian.

Pump-probe transmission experiments have been performed on PbSe above the fundamental absorption edge near 4 μm in the temperature range 30 to 300 K, using the Dutch ps free-electron laser. For temperatures below 200 K and carrier densities above the threshold for stimulated emission, stimulated recombination represents the most efficient recombination mechanism with relatively fast kinetics in the 50–100-ps regime, in good agreement with earlier reports of photoluminescent emission. Above this temperature Auger recombination dominates, and the Auger coefficient C is determined from the pump-probe decay curves. In the low-temperature regime the Auger coefficient is determined from the decay curves at times beyond 100 ps. The Auger coefficient is approximately constant (with a value of about 8×10-28 cm6 s-1) between 300 and 70 K, and then drops a value of about 1×10-28 cm6 s-1 at 30 K, in good agreement with the theory for nonparabolic near-mirror bands and nondegenerate statistics. It is found that C for PbSe is between one and two orders of magnitude lower than for Hg1-xCdxTe of comparable band gap.

The structural and optoelectronic properties in GaNxSb1-x alloys (0 <= x < 0.02) grown by molecular-beam epitaxy on both GaSb substrates and AlSb buffer layers on GaAs substrates are investigated. High-resolution x-ray diffraction (XRD) and reciprocal space mapping indicate that the GaNxSb1-x epilayers are of high crystalline quality and the alloy composition is found to be independent of substrate, for identical growth conditions. The band gap of the GaNSb alloys is found to decrease with increasing nitrogen content from absorption spectroscopy. Strain-induced band-gap shifts, Moss-Burstein effects, and band renormalization were ruled out by XRD and Hall measurements. The band-gap reduction is solely due to the substitution of dilute amounts of highly electronegative nitrogen for antimony, and is greater than observed in GaNAs with the same N content. (C) 2005 American Institute of Physics.

We report the direct determination of nonradiative lifetimes in Si/SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k·p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height 1 greater than or equal to Å), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs/AlxGa1−xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV materials.

We have performed a high field magneto-absorption spectroscopy on silicon doped with a variety of single and double donor species. The magnetic field provides access to an experimental magnetic length, and the quadratic Zeeman effect in particular may~be used to extract the wavefunction radius without reliance on previously determined effective mass parameters. We were therefore able to determine the limits of validity for the standard one-band anisotropic effective mass model. We also provide improved parameters and use them for an independent check on the accuracy of effective mass theory. Finally, we show that the optically accessible excited state wavefunctions have the attractive property that interactions with neighbours are far more forgiving of position errors than (say) the ground state.

Frequency domain spectroscopy allows an experimenter to establish optical properties of solids in a wide frequency band including the technically challenging 3-10 THz region, and in other bands enables metrological comparison between competing techniques. We advance a method for extracting the optical properties of high-index solids using only transmission-mode frequency domain spectroscopy of plane-parallel Fabry-Perot optical flats. We show that different data processing techniques yield different kinds of systematic error, and that some commonly used techniques have inherent systematic errors which are underappreciated. We use model datasets to cross-compare algorithms in isolation from experimental errors, and propose a new algorithm which has qualitatively different systematic errors to its competitors. We show that our proposal is more robust to experimental non-idealities such as noise or apodization, and extract the complex refractive index spectrum of crystalline silicon as a practical example. Finally, we advance the idea that algorithms are complementary rather than competitive, and should be used as part of a toolbox for better metrology.

The spin relaxation in undoped InSb films grown on GaAs has been investigated in the temperature range from 77 to 290 K. Two distinct lifetime values have been extracted, 1 and 2.5 ps, dependent on film thickness. Comparison of this data with a multilayer transport analysis of the films suggests that the longer time (~2.5 ps at 290 K) is associated with the central intrinsic region of the film, while the shorter time (~1 ps) is related to the highly dislocated accumulation region at the film-substrate interface. Whereas previous work on InAs films grown on GaAs showed that the native surface defect resulted in an additional charge accumulation layer with high conductivity but very short spin lifetime, in InSb layers the surface states introduce a depletion region. We infer that InSb could be a more attractive candidate for spintronic applications than InAs.

We have used time resolved spectroscopy to measure the relaxation of spin polarization in InSb/AlInSb quantum wells (QWs) as a function of temperature and mobility. The results are consistent with the D'yakonov–Perel (DP) mechanism for high mobility samples over the temperature range from 50 to 300 K. For low mobility samples at high temperature the Elliott–Yafet and DP mechanisms become comparable. We show that the mobility can in certain circumstances determine which mechanism is dominant, and that above 1 m2 V-1 s-1 in 20 nm wide InSb QWs it is the DP mechanism. We also give a criterion for the maximum spin lifetime in terms of mobility and temperature, and show that for our 20 nm wide QWs this corresponds to 0.5 ps at 300 K and mobility 1 m2 V-1 s-1.

We report on the pressure dependence of the threshold current in 1.3 µm InGaAsP and 1.5 µm InGaAs quantum-well lasers measured at low temperatures ~100 K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at ~100 K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from ~100 to 300 K

The Poisson distribution of event-to-ith-nearest-event radial distances is well known for homogeneous processes that do not depend on location or time. Here we investigate the case of a non-homogeneous point process where the event probability (and hence the neighbour configuration) depends on location within the event space. The particular non-homogeneous scenario of interest to us is ion implantation into a semiconductor for the purposes of studying interactions between the implanted impurities. We calculate the probability of a simple cluster based on nearest neighbour distances, and specialise to a particular two-species cluster of interest for qubit gates. We show that if the two species are implanted at different depths there is a maximum in the cluster probability and an optimum density profile.

We report investigation of the spin relaxation in InAs films grown on GaAs at a temperature range from 77 K to 290 K. InAs is known to have a surface accumulation layer and the depth profile of the concentration and mobility is strongly nonuniform. We have correlated the spin relaxation with a multilayer analysis of the transport properties and find that the surface and the interface with the GaAs substrate both have subpicosecond lifetimes (due to the high carrier concentration), whereas the central semiconducting layer has a lifetime of an order of 10 ps. Even for the thickest film studied (1 mu m), the semiconducting layer only carried 30% of the total current (with 10% through the interface layer and 60% through the surface accumulation layer). Designs for spintronic devices that utilize InAs, which is attractive due to its narrow gap and strong Rashba effect, will need to include strategies for minimizing the effects of the surface.

Pump-probe spectroscopy is the most common time-resolved technique for investigation of electronic dynamics, and the results provide the incoherent population decay time T1. Here we use a modified pump-probe experiment to investigate coherent dynamics, and we demonstrate this with a measurement of the inhomogeneous dephasing time T2* for phosphorus impurities in silicon. The pulse sequence produces the same information as previous coherent all-optical (photon-echo-based) techniques but is simpler. The probe signal strength is first order in pulse area but its effect on the target state is only second order, meaning that it does not demolish the quantum information. We propose simple extensions to the technique to measure the homogeneous dephasing time T2, or to perform tomography of the target qubit.

Laser cooling and electromagnetic traps have led to a revolution in atomic physics, yielding dramatic discoveries ranging from Bose-Einstein condensation to the quantum control of single atoms(1). Of particular interest, because they can be used in the quantum control of one atom by another, are excited Rydberg states(2-4), where wavefunctions are expanded from their ground-state extents of less than 0.1 nm to several nanometres and even beyond; this allows atoms far enough apart to be non-interacting in their ground states to strongly interact in their excited states. For eventual application of such states(5), a solid-state implementation is very desirable. Here we demonstrate the coherent control of impurity wavefunctions in the most ubiquitous donor in a semiconductor, namely phosphorus-doped silicon. In our experiments, we use a free-electron laser to stimulate and observe photon echoes(6,7), the orbital analogue of the Hahn spin echo(8), and Rabi oscillations familiar from magnetic resonance spectroscopy. As well as extending atomic physicists' explorations(1-3,9) of quantum phenomena to the solid state, our work adds coherent terahertz radiation, as a particularly precise regulator of orbitals in solids, to the list of controls, such as pressure and chemical composition, already familiar to materials scientists(10).

The electrically detected orbital spectrum of a mesoscopic silicon device containing a small number of donors has been investigated. The device was fabricated on silicon-on-insulator with an optically active channel containing 6 x 105 substitutional bismuth centers introduced by ion implantation. The 1s(A₁) → 2p± orbital transition at the energy associated with isolated bismuth donors was detected via a change in photocurrent when illuminated by THz light from a free electron laser. The spectral dependence on bias, temperature, and laser intensity is explored to determine optimum conditions for detecting orbital transitions in smaller devices with fewer donors. These results suggest that photo-induced impact ionization can offer a route for the spectroscopic detection of few impurities providing a useful tool for the development of solid-state quantum technologies.

Measurements of the THz absorption and the time-resolved photoluminescence have been performed on the same GaAs quantum well sample. The strength of the absorption at the internal 1s-2p exciton transition frequency is used as a measure of the density of excitons in the sample. When the interband pump laser is resonant with the Is exciton frequency, induced absorption at the 1s-2p frequency is clearly seen. If the same density of carriers is created pumping in the continuum, no significant 1s-2p absorption is seen in a time window of 450 ps. Complementary time-resolved photoluminescence experiments, detecting the emission at the exciton energy under the same pump conditions, show the PL intensity in resonant and nonresonant cases to be similar. The counter-intuitive existence of luminescence at the exciton energy simultaneously with the absence of the 1s-2p absorption is consistent with the recent theoretical predictions of Kira et al., Phys. Rev. Lett. 81, 3263 (1998).

Picosecond time-resolved far-infrared measurements are presented of the scattering between conduction-band states in a doped quasi quantum dot. These states are created by the application of a magnetic field along the growth direction of an InAs/AlSb quantum well. A clear suppression of the cooling rate is seen, from 1012 s-1 when the level spacing is equal to the phonon energy, to 1010 s-1 away from this resonance, and thus the results provide unambiguous evidence for the phonon bottleneck. Furthermore, the lifetimes had only weak dependence on temperature between 4 and 80 K.

Photoluminescence (PL) has been observed from dilute InNxAs1–x epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a (5×5) k·p Hamiltonian.

The absorption of multiple photons when there is no resonant intermediate state is a well-known nonlinear process in atomic vapours, dyes and semiconductors. The N-photon absorption (NPA) rate for donors in semiconductors scales proportionally from hydrogenic atoms in vacuum with the dielectric constant and inversely with the effective mass, factors that carry exponents 6N and 4N, respectively, suggesting that extremely large enhancements are possible. We observed 1PA, 2PA and 3PA in Si:P with a terahertz free-electron laser. The 2PA coefficient for 1s–2s at 4.25 THz was 400,000,000 GM (=4 × 10−42 cm4 s), many orders of magnitude larger than is available in other systems. Such high cross-sections allow us to enter a regime where the NPA cross-section exceeds that of 1PA—that is, when the intensity approaches the binding energy per Bohr radius squared divided by the uncertainty time (only 3.84 MW cm−2 in silicon)—and will enable new kinds of terahertz quantum control.

The temperature dependence of the threshold current of InGaAsSb/AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T-0 values.

The first far-infrared modulated photoluminescence (FIRM-PL) measurements in InSb/GaSb quantum dots have been performed. Far-infrared absorption is found to both enhance and suppress the quantum dot PL depending on the FIR intensity. This behavior is attributed to the nonthermal distribution of carriers amongst the quantum dots. The spectral dependence of the FIRM-PL signal measures the energy spectrum of the quantum dots, showing a peak at 14.5 meV corresponding to transitions between the first two energy levels of the dot distribution.