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In order to identify the performance limitations of InGaAs/AlAs(Sb) quantum cascade lasers, experimental investigations of the temperature and pressure dependencies of the threshold current (I) were undertaken. Using the theoretical optical phonon current (I) and carrier leakage (I) to fit the measured threshold current at various pressures, we show that the electron scattering from the top lasing level to the upper L-minima gives rise to the increase in I with pressure and temperature. It was found that this carrier leakage path accounts for approximately 3% of I at RT and is negligible at 100K. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Infrared spectroscopy is a highly attractive read-out technology for compositional analysis of biomedical specimens because of its unique combination of high molecular sensitivity without the need for exogenous labels. Traditional techniques such as FTIR and Raman have suffered from comparatively low speed and sensitivity however recent innovations are challenging this situation. Direct mid-IR spectroscopy is being speeded up by innovations such as MEMS-based FTIR instruments with very high mirror speeds and supercontinuum sources producing very high sample irradiation levels. Here we explore another possible method – external cavity quantum cascade lasers (EC-QCL’s) with high cavity tuning speeds (mid-IR swept lasers). Swept lasers have been heavily developed in the near-infrared where they are used for non-destructive low-coherence imaging (OCT). We adapt these concepts in two ways. Firstly by combining mid-IR quantum cascade gain chips with external cavity designs adapted from OCT we achieve spectral acquisition rates approaching 1 kHz and demonstrate potential to reach 100 kHz. Secondly we show that mid-IR swept lasers share a fundamental sensitivity advantage with near-IR OCT swept lasers. This makes them potentially able to achieve the same spectral SNR as an FTIR instrument in a time x N shorter (N being the number of spectral points) under otherwise matched conditions. This effect is demonstrated using measurements of a PDMS sample. The combination of potentially very high spectral acquisition rates, fundamental SNR advantage and the use of low-cost detector systems could make mid-IR swept lasers a powerful technology for high-throughput biomedical spectroscopy.

FTIR spectroscopy using a thermal light source has been the dominant method for obtaining infrared spectra since the 1950’s. Unfortunately the limited surface brightness and low spatial coherence of black-body radiators limits the spectral SNR in microspectroscopy and stand-off detection. Two recent innovations are addressing this problem a) FTIR instruments illuminated by high-spatial coherence broad-band supercontinuum sources and b) high spatial coherence narrow-band EC-QCL’s. Here we ask whether these two approaches offer equivalent sensitivity. By noting an analogy with near-infrared optical coherence tomography we rigorously show that the high temporal coherence of the EC-QCL brings an additional, very large SNR advantage over an FTIR instrument illuminated by a supercontinuum source under otherwise matched conditions. Specifically if a spectrum containing N points is recorded by both instruments using the same illumination intensity and the same detector noise level, then the EC-QCL can deliver a given spectral SNR in a time xN shorter than the FTIR instrument. This factor can reach x100, potentially even x1000, in realistic applications. We exploit the analogy with OCT further by developing a mid-infrared “swept laser”, using commercially available components, in which the tuning rate is much higher than in commercial EC-QCL devices. We use this swept laser to demonstrate the SNR advantage experimentally, using a custom-made EC-QCL spectrometer and PDMS polymer samples. We explore the potential upper limits on spectral acquisition rates, both from the fundamental kinetics of gain build-up in the external cavity and from likely mechanical limits on cavity tuning rates.

We review the recent development of high-performance short-wavelength (λ ~ 3.05-3.8 μm) strain-compensated InGaAs/AlAs(Sb)/InP quantum cascade lasers (QCLs). The lasers are demonstrated in which wavelengths as low as 3.05 μm are obtained at temperatures up to 295 K. We also verify that strain-compensated In0.7Ga0.3As/AlAs(Sb) QCLs with AlAs barriers in the active region operate with much better performance compared with the lasers having identical design but with AlAsSb barriers throughout the whole core region. λ ~ 3.3-3.7 μm laser emission is observed at temperatures up to at least 400 K and up to 20 W of output optical power at 285 K for the QCLs with various core region designs. Room temperature distributed feedback InGaAs/AlAs(Sb) QCLs with buried third-order gratings have been also developed, displaying single-mode operation in the wavelength range of 3.358-3.380 μm for temperatures between 270 and 360 K.

The study of the electron distribution and intersubband transitions in quantum cascade (QC) lasers provides fundamental insights into the laser operation. Measurements on intersubband transitions are often performed on QC lasers under nonoperating conditions, i.e., at zero electric field. In this study, a single-pass transmission technique that allows for probing of QC lasers under operating conditions was used. The measurements and analysis were done on QC lasers with vertical and diagonal transition active region designs operating at lambda ~ 10 mum. The single-pass modal gain coefficients, extracted for the vertical transition design, were 24.04 cm/kA at 100 K, 21.36 cm/kA at 125 K, 14.25 cm/kA at 150 K, and 5.26 cm/kA at 175 K.

The understanding of charge carrier distributions is fundamental to our knowledge of laser systems. In semiconductor lasers, because of the propensity of charge carriers to undergo extremely fast momentum relaxation1,2, they accumulate at band extrema—that is, they have a small wavevector close to k ≈ 0 in direct-gap semiconductors. Conventional understanding suggests that the device-level physics occurs at these band extrema, including population inversion for lasing. This behaviour is universal in diode lasers3,4, interband cascade lasers5 and quantum cascade lasers6,7. Here, we report on a quantum cascade laser with an energy configuration able to establish local population inversion high in k-space. We observe dual-wavelength emission from two discrete optical transitions. Temperature-dependent performance attributes show that the two transitions are highly coupled; competition for charge carriers is apparent from the anti correlated behaviour. The two optical transitions represent a conventional quantum cascade laser transition at k ≈ 0 and another laser transition from non-thermal electrons near k ≈ 3.6 × 108 m−1. Scientists report a dual-wavelength quantum cascade laser that lases at wave factors k ≈ 0 and k ≈ 3.6 × 108 m−1. The finding may change the conventional idea that population inversion of lasing occurs only at k ≈ 0 and give ways on designing intersub-band devices with high k-space.

We present experimental and theoretical investigations of the bias-dependent spectral shift of the photoresponse in InAs/InxGa1?xAs quantum-dots-in-a-well structures. Experimental results show that the wavelength response of the transition from the quantum dot ground state to quantum well states can be Stark-shifted by ~15% by changing the applied bias between ?1?V and +1?V. A theoretical model based on the 8-band k???p method fits our experimental data well using realistic dot parameters. We also demonstrate an increase in the operating wavelength and a reduced bias-dependent spectral shift for samples containing dots formed by depositing less InAs during growth.

We present details of the fabrication and operating characteristics of quantum cascade distributed feedback lasers with lateral gratings. These devices emit light with a wavelength of ~10 µm and operate with pulsed drive current above room temperature. InP-based material offers significant advantages over the GaAs system for mid-infrared quantum cascade lasers. High performance, single-mode lasers are achieved using InP-based material grown by metal organic vapor phase epitaxy and utilising double-sided lateral gratings. The deeply etched gratings were made possible by the development of a high aspect ratio, multi-stage, inductively coupled plasma (ICP) etch process, using Cl2/Ar and SiCl4/Ar gas mixtures. Threshold current density was measured to be ~5.5 kA/cm2 at a temperature of 293 K. Side mode suppression ratios >20 dB and a tuning coefficient of -0.067 cm-1 K-1 were observed.

Interest in mid-infrared spectroscopy instrumentation beyond classical FTIR using a thermal light source has increased dramatically in recent years. Synchrotron, supercontinuum, and external-cavity quantum cascade laser light sources are emerging as viable alternatives to the traditional thermal black-body emitter (Globar), especially for remote interrogation of samples (“stand-off” detection) and for hyperspectral imaging at diffraction-limited spatial resolution (“microspectroscopy”). It is thus timely to rigorously consider the relative merits of these different light sources for such applications. We study the theoretical maximum achievable signal-to-noise ratio (SNR) of FTIR using synchrotron or supercontinuum light vs. that of a tunable quantum cascade laser, by reinterpreting an important result that is well known in near-infrared optical coherence tomography imaging. We rigorously show that mid-infrared spectra can be acquired up to 1000 times faster—using the same detected light intensity, the same detector noise level, and without loss of SNR—using the tunable quantum cascade laser as compared with the FTIR approach using synchrotron or supercontinuum light. We experimentally demonstrate the effect using a novel, rapidly tunable quantum cascade laser that acquires spectra at rates of up to 400 per second. We also estimate the maximum potential spectral acquisition rate of our prototype system to be 100,000 per second.

Near-infrared external cavity lasers with high tuning rates (“swept lasers”) have come to dominate the field of nearinfrared low-coherence imaging of biological tissues. Compared with time-domain OCT, swept-source OCT a) replaces slow mechanical scanning of a bulky reference mirror with much faster tuning of a laser cavity filter element and b) provides a × N (N being the number of axial pixels per A-scan) speed advantage with no loss of SNR. We will argue that this striking speed advantage has not yet been fully exploited within biophotonics but will next make its effects felt in the mid-infrared. This transformation is likely to be driven by recent advances in external cavity quantum cascade lasers, which are the mid-IR counterpart to the OCT swept-source. These mid-IR sources are rapidly emerging in the area of infrared spectroscopy. By noting a direct analogy between time-domain OCT and Fourier Transform Infrared (FTIR) spectroscopy we show analytically and via simulations that the mid-IR swept laser can acquire an infrared spectrum × N (N being the number of spectral data points) faster than an FTIR instrument, using identical detected flux levels and identical receiver noise. A prototype external cavity mid-IR swept laser is demonstrated, offering a comparatively low sweep rate of 400 Hz over 60 cm -1 with 2 cm- 1 linewidth, but which provides evidence that sweep rates of over a 100 kHz should be readily achievable simply by speeding up the cavity tuning element. Translating the knowledge and experience gained in near-IR OCT into mid-IR source development may result in sources offering significant benefits in certain spectroscopic applications.

An external cavity quantum cascade laser based on Fabry-Perot etalon design is demonstrated for the first time. The Fabry-Perot reflector was adjusted to produce single mode emission across the whole gain region.

Focussed ion beam (FIB) processing has been applied to the fabrication of novel InP-based cleaved coupled cavity (CCC) quantum cascade lasers (QCL). Gas assisted etching using XeF2 has been shown to significantly reduce the redeposition of sputtered material onto the mirror surfaces during final milling. For the unprocessed laser a broad spread of lasing peaks are observed between 9.72µm to 9.78µm at a current of 380mA (1kA/cm−2). After FIB processing, substantial side mode suppression is observed on applying a current of 20mA (100A/cm−2) to the short section and the main lasing peak is observed at 9.77µm.

We present a comprehensive study of the intraband transitions in n-type InAs/GaAs quantum dots (QDs) with a filling varying from 0.5 to 4 electrons per dot, using both polarization-dependent absorption and photocurrent spectroscopy. Applying these complementary mid- and far-infrared spectroscopies over a wide energy range allows us to obtain a detailed picture of the intraband transitions and energy levels in self-assembled QDs.

Polaron relaxation processes in a series of n-type InAs quantum dots (QDS) have been investigated using energy-dependent far-infrared pump–probe spectroscopy. For energies up to ∼53 meV, polarons decay to 2 longitudinal acoustic phonons; above this energy additional decay channels open resulting in a reduction of the decay time. Inter-state transfer has been observed between closely spaced p-like excited states, with the measured transfer times in good agreement with calculations assuming acoustic phonon assisted transfer. Finally, for QDs containing ⩾2 electrons we observe evidence of a spin-flip process resulting in long (∼700 ps) relaxation times.

We report entirely MOVPE-grown QCL lasers operating around 9 μm. High-resolution X-ray diffraction analysis and TEM data confirm precise thickness and composition control for the individual layers. Consequently, these MOVPE-grown QCLs demonstrate a pulsed mode performance comparable to similarly designed MBE-grown devices. QCLs having a conventional three-well active region design exhibit a threshold current density of ∼1.5 kA cm −2 at 12 K and continue to lase up to room temperature. Furthermore, QCLs designed with a four-QW double-phonon active region show a threshold current density as low as 880 A cm −2 at 12 K and 3.75 kA cm −2 at 300 K with peak powers in excess of 1 W.

The AlAs-like TO-phonon decay in Al0.7Ga0.3As has been investigated using an infrared four wave mixing technique with picosecond resolution. The low temperature TO-phonon lifetime is found to be ∼5 ps and is significantly shorter than ∼40 ps recently predicted for the TO-phonon in AlAs [A. Debernardi, Phys. Rev. B 57, 12847 (1998)]. In alloy materials Al and Ga atoms are randomly distributed on the Group III sublattice of the zincblende structure. This disorder considerably reduces the lifetime of AlAs phonon mode due to scattering of the TO phonon with Ga atoms which results in an additional decay channel: AlAs-TO GaAs-L(T)O + LA in AlGaAs alloy materials. The new decay channel also leads to stronger temperature dependence of the TO-phonon lifetime. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using energy and temperature dependent far-infrared pump-probe spectrsocopy we demonstrate that polarons in InAs/GaAs quantum dots decay to two equal energy phonons. An additional decay process involving polaron transfer between closely spaced (∼5meV) excited states has also been observed, for which we find very good agreement between our experimental results and an acoustic phonon assisted transfer model. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polaron decay in n-type InAs quantum dots has been investigated using energy dependent, mid-infrared pump-probe spectroscopy. By studying samples with differing ground state to first excited state energy separations the relaxation time has been measured between similar to40 and 60 meV. The low-temperature decay time increases with increasing detuning between the pump energy and the optical phonon energy and is maximum (55 ps) at 56 meV. From the experimentally determined decay times we are able to extract a low-temperature optical phonon lifetime of 13 ps for InAs QDs. We find that the polaron decay time decreases by a factor of similar to2 at room temperature due to the reduction of the optical phonon lifetime. (C) 2003 Elsevier B.V. All rights reserved.

We report the metal-organic vapour phase epitaxy growth of quantum cascade lasers (QCLs) in both the GaAs–AlGaAs and InGaAs–AlInAs–InP materials systems. The GaAs based sample shows an increased emission wavelength relative to an equivalent design grown by molecular beam epitaxy, which we attribute to interfacial grading in the metal-organic vapour phase epitaxy device. The InP-based QCL (λ≈8.5 μm ) operates in pulsed mode at room temperature, with low temperature threshold current density ∼1500 A / cm 2 . This performance is comparable with that previously reported for similar structures grown by molecular beam epitaxy. The high quality interfaces necessary for successful QCL operation are achieved by using individually purged vent/run valves in the gas-handling section of the reactor, as well as a low growth rate for the active region of the structure.

We report on distributed feedback quantum cascade lasers at a wavelength of 3.58 μm operating at room temperature. Single-mode emission with a side-mode suppression ratio of 30 dB is achieved by manufacturing single-sided third-order lateral gratings. The devices exhibit watt level peak powers with a threshold current density of ~ 4.3 kA/cm2 at room temperature and remain in single-mode operation over the temperature range of 280-420 K.

We report two novel methods for reducing the emission wavelength of GaAs–AlAs quantum cascade lasers. We demonstrate that for lasing to occur electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm ) is overcome by utilising a novel injection barrier design to achieve lasing at λ=7.2 μm . In addition, we have deposited InAs monolayers in the active regions of a GaAs–AlAs quantum cascade laser to decrease the lasing transition wavelength. The InAs monolayers have a minimal effect on the upper laser level but decrease the confinement energy of the lower laser level. Thus a significantly reduced emission wavelength ( 8.3 μm compared with 11.2 μm ) is achieved whilst maintaining very similar laser performance.

We demonstrate λ ~ 9 μm GaAs/Al0.45Ga0.55As quantum cascade lasers (QCLs) operating up to 320 K. Metal- organic vapor phase epitaxy has been used throughout for the growth of the devices. Detailed comparison has been carried out for the QCLs with various waveguides and grown on (100) GaAs substrates with different miscut angles towards (111)A. Introduction of InGaP cladding layers into the optical waveguide significantly improves the QCL performance due to a better optical confinement and lower waveguide losses compared with the GaAs-based waveguide. A 20- μm-wide 4-mm-long device with high reflectivity coating on the laser back facet demonstrates room-temperature pulsed output power of 220 mW and a lowered threshold current density of 6.5 kA/cm2.

We report the development of room-temperature distributed-feedback quantum-cascade lasers operating in a single mode in the 3.34 to 3.35 μm wavelength range. First-order lateral gratings with high aspect ratio (the ratio between the grating etch depth and its period) were formed using inductively coupled plasma etching. The as-cleaved lasers emit in pulsed regime with a sidemode suppression ratio of up to 24 dB and a peak single-mode output power of 130 mW from a single facet.

Mid-infrared spectroscopic imaging is a powerful technique for cytology, histology and pathology. However, the fourier transfor infrared (FTIR) microscopes commonly used are limited by the large, low coherence Globar light source since the power per unit bandwidth focused to the diffraction limit at the sample is extremely low. A quantum cascade laser[1] (QCL) can provide many orders of magnitude improvement in focused power. To record a typical 1000-point spectrum using a tuneable QCL with a similar radiant flux on the detector as an FTIR system, the tuneable laser could either increase the SNR by or increase the measurement speed by . Advantages over Globar based FTIRs have been shown through the use of a laser frequency comb[2] or even a multiple beam synchrotron[3]. However, the external cavity QCL (EC-QCL) is potentially the most practical and adaptable light source but presently the tuning speed is the main limitation. Therefore, a rapid-tuning swept QCL is required to achieve full-field images at video rate which is necessary for many applications such as ‘digital staining’. The majority of tuneable EC-QCLs are in a Littrow configuration for high power emission for gas sensing applications. In this paper we describe the design and realization of a swept-source EC-QCL. Since the moving mass of a galvanometer based mirror is less than that of a diffraction grating, a Littman external cavity was constructed to enable rapid sweep rates. We demonstrate a 400Hz repetition rate with a tuning speed of 45,000cm -1 /s. Both figures of merit are higher than those achieved in conventional Littrow configuration EC-QCLs. The tuning speed is not limited by the cavity design or gain chip but by the inertia of the moving mirror used. We go on to assess the fundamental limit to the sweep rate of our swept-source. A key factor is the number of round trips of the cavity that are required to reach saturation. This photon build up time is dependent on the cavity length, the round trip gain, the cavity linewidth, and the ASE power within the spectral window of the cavity [4]. To the best of our knowledge, this is the first assessment of QCL gain chips for rapid swept-source applications. We demonstrate that >100kHz repetition rate should be possible from sweptsource EC-QCLs. The grating resolution and collimation beam size was chosen to give an instantaneous linewidth of ~2cm -1 . Due to the external cavity, this linewidth consisted of a packet of many longitudinal cavity modes. This mitigates the effect of discrete mode-hopping by enabling smooth average tuning of the packet. In order to test this ‘smooth tuning’, we measure the interference curve of a fixed interferometer as the QCL is swept in 0.05cm -1 steps. This curve is shown in figure 1 with the inset showing a FTIR spectrum of one instantaneous ECQCL output. Whilst this shows that the tuning has been smoothed, there are still small hops as the mode packet is not perfectly smooth, probably as a result of residual facet modulation. However, these small jumps (

Discrete-mode quantum-cascade lasers have been developed in the InGaAs-AlAsSb-InP materials system. For an uncoated 10-μ m-wide ridge waveguide and 3000-μm-long cavity, the laser had a threshold current density Jth of 4.2 kA·cm-2 (Ith=1.5 A) at 300 K with a slope efficiency of 80 mW/A. A stable single-mode emission near 3.3 μ m with a sidemode suppression ratio of nearly 25 dB was observed and a tuning coefficient of 0.22 nm/K was obtained in the temperature range of 253 K

We report the results of transmission experiments performed on hybridized plasmonic metamaterial quantum cascade lasers. This device was formed by etching an array of split ring resonators (SRRs) onto the gold coated facet of a laser. Broadband midinfrared transmission experiments confirm the resonant nature of the SRR structure, in good agreement with finite-difference time-domain calculations and results from structures fabricated on bare GaAs. The long fundamental resonance was observed at lambda ~ 6.3 μm. These devices have potential uses in near-field vibrational microscopy of chemical and biological samples, nonlinear studies, and beam shaping.

We report the development of high-peak-power λ ~ 3.6 μm strain-compensated In0.7Ga0.3As-AlAs(Sb) quantum cascade lasers (QCLs). The QCLs operate up to at least 400 K with high wall-plug efficiency (~9% at 300 K) in the pulsed regime. The lasers are based on a vertical transition bound-to-continuum design. Devices of 30-μm width and 4-mm length, with high reflectivity coating on the rear facet, deliver more than 20-W peak optical power at 285 K with a threshold current density of 3.1 kA/cm2. Over 2 W of peak power is obtained at 400 K.

Complementary intersubband and interband optical measurements have been employed in order to study GaAs–AlGaAs quantum cascade light-emitting diode and laser structures. Using these techniques, we have measured the redistribution of electrons throughout the bridging regions and upper states in the active regions of the diode device with increasing bias. The high quality of the sample gives very narrow line widths in the optical spectra, permitting the resolution of transitions involving closely spaced energy levels. This has allowed the direct observation of level alignment at the onset of current flow through the device. In addition, stimulated emission at λ=9.7 μm has been observed from a GaAs–AlGaAs laser structure under pulsed operation. A threshold current density of 6.5 kA / cm 2 and peak power ∼300 mW are measured at 10 K and lasing operation is observed up to 200 K.

We present a comparison of the lasing characteristics for two nearly identical GaAs–AlGaAs quantum cascade lasers with an optimised InGaAs–AlInAs device operating at a similar wavelength. The effects on the laser performance resulting from modifications to the upper lasing state lifetime and optical transition matrix element for the GaAs–AlGaAs devices are investigated by selectively altering the aluminium composition of a single barrier within the active region. The sample containing an Al 0.4 Ga 0.6 As barrier exhibits a lower-threshold current density (3.75 kA/cm 2 ) and increased slope efficiency (280 mW/A) at 10 K relative to the sample with an Al 0.33 Ga 0.67 As barrier, consistent with calculations.

We report an electrical transport and electroluminescence (EL) spectroscopy study of single-barrier GaAs-AlAs-GaAs $p\ensuremath{-}i\ensuremath{-}n$ tunnel structures with a barrier thickness in the range 4.5--8.0 nm. The results permit us to determine directly the relative roles of nonresonant \ensuremath{\Gamma}-\ensuremath{\Gamma} tunneling and resonant $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ intervalley transfer in the transport through the indirect-gap tunnel barriers. The $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ transport is shown to take place predominantly without conservation of transverse wave vector ${(k}_{\ensuremath{\parallel}}),$ with ${k}_{\ensuremath{\parallel}}$-conserving scattering via ${X}_{z}$ states only significantly close to the onset of $\ensuremath{\Gamma}\ensuremath{-}X$ intervalley transfer. By detecting extremely weak EL arising from excited ${X}_{z}$ states we show that the complete $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ transport process is very strongly sequential and determine, quantitatively, the comparative time scales for $X\ensuremath{-}\ensuremath{\Gamma}$ and inter-X-level scattering. The bias-dependent $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ and \ensuremath{\Gamma}-\ensuremath{\Gamma} transport times are determined for AlAs barrier widths in the range 3.0--10 nm. The intervalley $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ transport model yields results in good agreement with experiment and demonstrates that, providing intervalley $\ensuremath{\Gamma}\ensuremath{-}X$ transfer is energetically possible, nonresonant \ensuremath{\Gamma}-\ensuremath{\Gamma} tunneling only contributes significantly to the transport characteristics for barrier widths of \ensuremath{\sim}3 nm or less.

We have used interband photoluminescence (PL) spectroscopy to demonstrate the occurrence of an intersubband population inversion in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ triple barrier structure containing asymmetric coupled quantum wells. The relative populations of the $n=1$ $(E1)$ and $n=2$ $(E2)$ electron subbands of the wider quantum well (QW1) are deduced from the relative intensities of the PL peaks arising from recombination of the $E1$ and $E2$ electrons. A significant population inversion is obtained between $E2$ and $E1$ when the structure is biased so that $E1$ is in resonance with the $n=1$ ${(E1}^{*})$ level of the narrower quantum well (QW2). The key importance of the interwell resonance in achieving population inversion is confirmed by comparison with PL results from structures in which ${E1\ensuremath{-}E1}^{*}$ alignment does not occur.

We report the observation of resonant tunneling and magneto-tunneling between states of different effective mass derived from zone centre (Γ) and zone edge ( X ) points of the Brillouin zone in single AlAs barrier diodes. The nature of the X states involved (longitudinal X z or transverse X x y ) is deduced from the observed resonances in the conductance versus bias characteristics at zero magnetic field ( B ). At finite B , the σ– V curves exhibit resonant magneto-tunneling with X z Landau levels (LL), whilst no evidence of resonances with X x y LLs is found. Clear observation of both LL index (in-plane momentum) conserving and non-conserving tunneling to X z allows the transverse effective mass in AlAs to be determined. As a consequence of the different effective masses, momentum-conserving tunneling is inhibited at B = 0, but is restored when high B is applied.

We report the use of high sensitivity electroluminescence (EL) techniques to study intervalley scattering processes in single barrier p–i–n GaAs/AlGaAs/GaAs heterostructures. The EL experiments provide spectroscopic information on the nature of the intervalley scattering mechanisms and on their relative strengths, as well as providing estimates of the Γ–L and Γ–X intervalley scattering times. In addition, we observe clear evidence for quasi-ballistic transport of holes injected into the spin-orbit split-off valence band within the n-type regions of the structures.

We have carried out an investigation by electroluminescence (EL) spectroscopy of hot-electron and -hole transport in a p-i-n GaAs/${\mathrm{Al}}_{0.35}$${\mathrm{Ga}}_{0.65}$As single-barrier tunneling structure at high electric fields (\ensuremath{\sim}150 kV ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). We observe EL arising from hot electrons in the p-type collector which reenter the \ensuremath{\Gamma} conduction-band valley from the L minimum. Analysis of the EL spectra provides an estimate of the relative \ensuremath{\Gamma}-L scattering rates by longitudinal and transverse phonon emission. In addition we observe EL due to recombination of hot holes injected into the spin-orbit split-off valence band of the n-type emitter region. Measurement of the pressure dependence of the EL spectra permits unambiguous identification of these spectral features. \textcopyright{} 1996 The American Physical Society.

We present an electroluminescence (EL) and transport study of the tunnelling mechanisms in GaAs/AlAs/GaAs single barrier p-i-n tunnelling structures. We show that both Г-X and XГ tunnelling processes arise from either elastic transfer involving the longitudinal X z barrier states, or momentum conserving phonon assisted transfer involving transverse X xy states. The tunnelling process is shown to be very strongly sequential.

Photoluminescence, measured as a function of hydrostatic pressure, has been used to determine the band offsets of two tensile-strained ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$P-(${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${)}_{0.52}$${\mathrm{In}}_{0.48}$P quantum-well structures. Unlike other techniques commonly used to determine band offsets, this method has the advantage that a detailed knowledge of the material parameters is not required. Conduction-band offsets of \ensuremath{\Delta}${\mathit{E}}_{\mathit{c}}$=(0.79\ifmmode\pm\else\textpm\fi{}0.07)\ensuremath{\Delta}${\mathit{E}}_{\mathit{G}}^{\mathrm{HH}}$ and \ensuremath{\Delta}${\mathit{E}}_{\mathit{c}}$=(0.74\ifmmode\pm\else\textpm\fi{}0.10)\ensuremath{\Delta}${\mathit{E}}_{\mathit{G}}^{\mathrm{HH}}$, where \ensuremath{\Delta}${\mathit{E}}_{\mathit{G}}^{\mathrm{HH}}$ is the total heavy-hole-related band-gap discontinuity, are obtained for structures having strains of +0.56 and +0.71% and barrier Al compositions of 0.7 and 0.55, respectively. Alternatively the band offsets expressed in terms of the light-hole band-gap discontinuity are \ensuremath{\Delta}${\mathit{E}}_{\mathit{c}}$: \ensuremath{\Delta}${\mathit{E}}_{\mathit{G}}^{\mathrm{LH}}$=0.70:0.30 and 0.61:0.39 [corresponding to absolute light-hole valence-band offsets of \ensuremath{\Delta}${\mathit{E}}_{\mathit{V}}^{\mathrm{LH}}$ =(110\ifmmode\pm\else\textpm\fi{}12) meV and (113\ifmmode\pm\else\textpm\fi{}15) meV] for the +0.56 and +0.71% strained samples, respectively. At high pressures the structures become type II, and the observed indirect real and k-space transition exhibits a blueshift with increasing incident laser power density. An analysis of this blueshift allows both the density and lifetime of the spatially separated photoexcited carriers to be determined. \textcopyright{} 1996 The American Physical Society.

We report the operation of distributed feedback quantum cascade lasers, grown by metalorganic vapor phase epitaxy. Single-mode laser emission at λ∼10.3μm and λ∼7.8μm is observed from two different samples, with 300 K threshold current densities of Jth∼3 and ∼2.4kAcm−2, respectively. Structural investigation by x-ray diffraction and transmission electron microscopy, and the close correlation between the predicted and observed emission wavelengths indicate exceptional control of the layer thicknesses, including ultrathin (∼8A) barrier layers in the active region. These results confirm metalorganic vapor phase epitaxy as a viable technology for the growth of high-performance quantum cascade lasers.

The dynamics of interband transitions in InGaAs/AlGaAs heterostructures with funnel-shaped quantum wells, when a narrow and deep well is positioned asymmetrically in wide and shallow ones, has been studied experimentally. These structures have been proposed as a dual-colour laser in mid- and near-IR range simultaneously. The lifetime of electrons in the ground and excited states in the quantum wells has been determined by the time of the photoluminescence intensity decline. The obtained lifetime values allow precise identification of peaks in the photoluminescence spectra and a deep insight into the process of high electron states populating. The experimentally found electron lifetime for the interband transitions corresponds on the order of magnitude with the interband transitions lifetime in GaAs. Analysis of the photoluminescence spectra as well as the time dependence of the photoluminescence intensity decline indicates a possibility of population inversion achievement in these structures under high excitation.

The In0.53Ga0.47As–AlAs0.56Sb0.44 materials system, lattice matched to InP, is an attractive candidate for short wavelength quantum cascade lasers due to the very large conduction band discontinuity (∼1.6 eV) and compatibility with well established quantum cascade laser waveguide design and fabrication technology. In this letter we report the operation of In0.53Ga0.47As–AlAs0.56Sb0.44 quantum cascade structures emitting in the wavelength range λ∼4–5.3 μm. Clear intersubband electroluminescence peaks are observed close to the design wavelengths, with full widths at half maximum in the range of ∼30–40 meV.

A high-resolution electroluminescence study of the hot-electron energy distribution, and of the nature of the tunneling processes, in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As single-barrier p-i-n diodes is reported. Application of quantizing magnetic fields permits electron injection from narrow, well-defined electron states into the collector region, even at high emitter density. As a result ballistic-electron peaks, accompanied by their LO-phonon cascade, are clearly resolved. Cross-barrier recombination is also observed. The energy distribution of the ballistic electrons is shown to reflect exactly that of the emitter electrons, even though the density is \ensuremath{\sim}${10}^{5}$ times lower, thus demonstrating that the two-dimensional to three-dimensional tunneling process is elastic and independent of the in-plane motion. The energy relaxation of the ballistic electrons is shown to be dominated by LO-phonon emission, with energy randomization by, e.g., carrier-carrier scattering playing no significant role.

We report the growth of GaAs-based quantum cascade lasers using atmospheric pressure metalorganic vapor phase epitaxy. The necessary control of interface abruptness and layer thickness uniformity throughout the structure has been achieved using a horizontal reactor in combination with individually purged vent/run valves. A low-temperature threshold current density of 10 kA/cm2 and maximum operating temperature of 140 K have been measured. These performance levels are comparable with early GaAs-based devices grown using molecular-beam epitaxy. The measured emission wavelength (λ∼11.8 μm) is approximately 3-μm longer than the calculated transition wavelength, which we explain using a model incorporating compositional grading of the active region barriers.

We report the shortest-wavelength (λ∼8.5 μm) room-temperature laser operation so far achieved for GaAs-based quantum-cascade (QC) lasers. By depositing InAs monolayers in the device active regions during growth, we are able to both reduce the emission wavelength and minimize thermally activated carrier leakage into quasicontinuum states. This approach results in laser operation up to 305 K, with a peak optical power of ∼10 mW at room temperature. A reduced temperature sensitivity of the threshold current compared with similar GaAs–Al0.45Ga0.55As QC lasers is also observed.

We present a midinfrared absorption study of an n–i–n structure containing two planes of strongly coupled In0.5Ga0.5As self-assembled quantum dots. We find that the dominant absorption occurs for light polarized in the growth plane (E//xy), contrasting with uncoupled dots which exhibit stronger absorption for light polarized in the growth direction (E//z) in the same energy range. Results from the coupled dot sample indicate that the confinement length in the growth direction is increased, lowering the energy of the s-like ground state and hybridizing px,y-like excited states and wetting layer states. A significantly increased normal incidence photocurrent signal is measured for the sample containing coupled dots, relative to samples containing up to 30 layers of uncoupled dots, confirming the enhancement of the normal incidence absorption.

Temperature dependent transparency current values have directly been extracted from the transmission spectra for the mid infrared quantum cascade laser. This current is found to contribute more than 65% to the threshold at room temperature.

We report an investigation of ballistic electron transport in GaAs/AlGaAs p-i-n single barrier structures with magnetic fields of up to 14T applied parallel to the tunneling direction (B//z). The energy distribution and relaxation processes of the non-equilibrium electron population injected into the p-doped collector from the Landau levels of the emitter accumulation layer are studied by means of electroluminescence (EL) spectroscopy. The observation of emitter Landau level structure in the ballistic electron EL spectra shows that the 2D to 3D tunneling process is elastic. In addition to the ballistic electron EL, cross-barrier recombination between the electron and hole accumulation layers is observed. This allows a precise determination of the initial energy distribution of the injected electrons.

In this paper we present an electroluminescence study of electron populations in the quasiconfined excited states of two GaAs-AlGaAs double-barrier resonant tunnelling structures. We investigate the dependence of the electron population on both electron collector barrier width, which controls the electron tunnelling-out time, and on the confined level energy spacing, which controls the intersubband scattering time. We show that the observed excited state populations are in good agreement with the predictions of a rate equation analysis.

We present results of photoluminescence and electroluminescence spectroscopy carried out on GaAs/ Al0.33Ga0.67As double-barrier resonant-tunnelling structures having quantum wells of either 200 or 250 A width. At high bias, luminescence recombination is observed from electrons in highly excited (n > 2) quantum-well (QW) states of the structures. Analysis of the luminescence intensities shows that when the structures are biased at the fourth electron resonance, a population inversion occurs between the n = 4 and n = 3 confined electron levels. In addition, when the 250 A QW structure is biased at the fifth resonance, a population inversion occurs between the n = 5 and n = 4 confined levels. We show that such population inversions are explained by a rate-equation analysis of the excited-state populations.

We have investigated the importance of intervalley (Γ–Χ) electron transfer between Γ-point quantum well states and X-point barrier states in GaAs-based quantum cascade lasers with indirect band gap AlAs barriers. A series of samples has been studied in which the energy separation between the coupled injector/upper laser levels and the lowest confined X state in the injection barrier is varied. We demonstrate that for lasing to occur, electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm) for the present laser design is overcome by utilizing a double injection barrier to achieve lasing at λ=7.2 μm.