Search

We report a continuous wave room temperature quantum cascade laser operating in an external cavity in the Littrow configuration with a 10-facet polygon mirror rotating at 24,000 RPM. The quantum cascade laser emission is swept across ∼1520 - 1625 cm−1 wavenumber range in less than ∼45 µs with a sweep repetition rate of 4 kHz. The measured maximum output power at the laser gain maximum, 15°C and 0.86 A driving current is ∼90 mW; the estimated average output power across the 45 µs wavenumber sweep is ∼50 mW. Through its sweep, the laser emits on the sequential Fabry-Perot longitudinal modes of the laser chip cavity with the mode separation of ∼0.5 cm−1. The linewidth of the emitting modes is less than ∼0.05 cm−1. Spectral measurements of the infrared absorption features of a 10 µm thick layer of acetophenone and water vapor in the air have demonstrated the capability of obtaining spectral data in less than 45 µs.

Mid-infrared optical coherence tomography (MIR OCT) has shown promise in the last few years in applications such as spectroscopy and non-destructive testing. Previously, we have successfully demonstrated a MIR swept-source OCT and measured its noise from three main sources: quantization noise from the ADC, shot noise from the detectors and relative intensity noise (RIN) of the laser. Of these sources, RIN places an upper limit on the SNR of swept-source OCT systems. We attempt to characterize RIN in greater depth and determine whether it can be reduced through normalization. The pulsed laser used (Block Engineering Lasertune) is tunable within the wavelength range of 5.4-12.8μm. The laser output was held at a fixed wavelength, repetition rate and pulse width. Each laser pulse was integrated to find its average power along the pulse duration. A Fourier transform of the result was used to calculate the ratio of the AC power to DC power, giving a value for RIN. By using a beamsplitter and aspheric lenses to carefully focus the beam onto two detectors (Vigo System’s PVMI-4TE), the two pulse trains can be normalized. Through normalization, RIN was reduced from -74dB/Hz to -92dB/Hz. Increasing the repetition rate and pulse width leads to a decrease in RIN, but an upper limit on the laser duty cycle constrains improvements to RIN via this method. As the swept laser has four integrated quantum cascade laser (QCL) chips, we also examine the effect of different emission wavelengths on RIN.

A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed profile of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe force microscopy (KPFM) across the cleaved facets for two mid-infrared quantum cascade laser structures. An InGaAs/InAlAs quantum cascade device with InP claddings demonstrates a linear potential distribution across the laser core region with constant voltage drop across the doped claddings. By contrast, a GaAs/AlGaAs device with AlInP claddings has very uneven potential distribution with more than half of the voltage falling across the claddings and interfaces around the laser core, greatly increasing the overall voltage value necessary to achieve the lasing threshold. Thus, KPFM can be used to highlight design and fabrication flaws of QCLs.

A swept-source optical coherence tomography (OCT) system is demonstrated in the mid-infrared region. A Michelson interferometric setup is illuminated by an external cavity quantum cascade laser (QCL), with a scanning frequency of 1 Hz. A-scans were collected using three different samples: a mirror, CaF2 coated with germanium on both of its surfaces, and CaF2 coated with germanium on the back side of the sample. These depth-profiles were used to mimic a tissue sample with multiple reflective boundaries. Fourier transformation of these interference fringes clearly showed the expected depths of reflection, allowing for the signal to noise ratio of the system to be determined.

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.

A quantum cascade laser based on a strain-compensated Ga0.4In0.6As/Al0.58In0.42As heteropair is developed, which operates in a pulse mode in the wavelength range of 5.5–5.6 µm at a temperature of up to 350 K. Such characteristics are obtained due to increased quantum well depth and a two-phonon depopulation mechanism for the lower laser level. The laser epitaxial heterostructure was grown by the MOVPE method. Investigation by the high-resolution X-ray diffraction technique confirmed a high quality of the heterostructure. The threshold current density is 1.6 kA/cm2 at 300 К. The characteristic temperature is T0 = 161 K for the temperature range of 200–350 K. For a laser of size 20 µm × 3 mm with cleaved mirrors, the maximum pulse power is 1.1 W at 80 K and 130 mW at 300 K. Keywords: quantum cascade laser, GaInAs/AlInAs heteropair, MOVPE, the middle IR spectrum

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.

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.

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 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 (

It is established that doping (by means of ion implantation) of SiO2 films with Si nanoinclusions (NI) (SiO2:Si) by phosphorus results in an enhancement of the photoluminescence (PL) peak at ∼800 nm without a significant shift of it. This peak is believed to be linked with silicon NI serving as quantum dots (QD). Three mechanisms of PL enhancement are suggested. It is shown experimentally that the most probable mechanisms are: the passivation of broken bonds by phosphorus; the increase of donor centers in the NI. Theoretical investigation of the energy spectra and the energy of radiative transition in a QD with and without one donor center is provided. It is shown that the energy of radiative transition does not affected by the presence of a donor center.

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.

The light transmittance modulation by an electric field near the fundamental edge absorption has been measured in a p-type In0.21Ga0.79As/GaAs quantum well (QW) heterostructure at 4.2 K. The hole distribution function modulations were calculated using the light transmittance modulation data. In calculations we took into account fluctuations of the QW parameters.

Modulation of the fundamental absorption edge by a high lateral electric field in a p-type In0.21Ga0.79As/GaAs heterostructure with quantum wells was studied at 4.2 K and electric fields as high as 1.9 kV/cm. The field-induced change in the symmetric part of the hole distribution function was measured.

Hot holes in strained MQW In x Ga 1− x As/GaAs heterostructures excited at lateral charge transport are probed by far-infrared emission and band gap photoluminescence. Highly nonequilibrium phenomena discovered are shown to result from the real space transfer. New mechanism for the population inversion between continuum and shallow bound states in quantum wells in high lateral electric fields is put forward.

Results of electrical and optical studies of GaAs/InxGa1−xAs heterostructures are reported. The aim of these studies was to identify the quantum dots and develop a technology of their growth by spontaneous transformation of an InxGa1−xAs layer. The surface charge at the depth of the quantum dots and their surface density as a function of the deposition time of this narrow-band material are estimated by C-V profiling. A photoluminescence study of the quantum dots revealed peculiarities of the filling of their electron states at various excitation levels. The influence of Coulomb interactions on the optical properties of the quantum dots is discussed.

The infrared radiation from hot holes in InxGa1−x As/GaAs heterostructures with strained quantum wells during lateral transport is investigated experimentally. It is found that the infrared radiation intensities are nonmonotonic functions of the electric field. This behavior is due to the escape of hot holes from quantum wells in the GaAs barrier layers. A new mechanism for producing a population inversion in these structures is proposed.

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.

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.

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.

The principles of control of the luminescent properties of the Si + (150 keV) ion-synthesized system, composed of silicon nanoinclusions in a SiO 2 matrix, are considered. The influence of the ion dose, annealing regime and additional doping with phosphorus is investigated. It is shown that the intensity of the red/infrared photoluminescence peak (at ∼800 nm) non-monotonously changes with the dose. The optimum dose is shifted to smaller values with increasing annealing temperature (from 1000 to 1100 °C for an annealing time of 2 h). The highest intensity reached by the dose variation is nearly the same for both temperatures. Phosphorus doping increases the luminescence intensity for 1000°C, the effect of doping is lower for higher Si + doses. The physical nature of the features observed is discussed.

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.

The 3–4μm wavelength range is of key technological importance for a wide range of applications. As a consequence of the strong fundamental C-H stretch mode that occurs at around 3.3μm, the detection of many important hydrocarbon species has maximum sensitivity (potentially ∼ parts per trillion) in this range. This leads to many important applications in various areas, for example, clinical diagnostics, process monitoring and remote detection of oil and gas deposits. This short wavelength IR region is also of interest for several defence and security applications as well as free-space “last mile” telecommunications. The unsuitability of the “conventional” InGaAs/AlInAs materials system for high performance QCL sources in this short wavelength range has led to significant interest in new QCL materials with very high conduction band offsets (ΔE c ) based on III-V antimonides such as InAs/AlSb/InAs (ΔE c ∼ 2 eV) and InGaAs/AlAs(Sb)/InP (ΔE c ≤ 1.6 eV). Very recently, significant progress has been made in the development of these “short wavelength” QCLs operating in the close vicinity of λ ∼ 3 μm. In our work, we have focussed on the InGaAs/AlAsSb system, which provides both a very high ΔE c that can comfortably accommodate the high energy intersubband transitions required, and lattice-matched compatibility with InP. This latter feature is extremely important as it greatly simplifies optical waveguide development and facilitates advanced device fabrication. Recent highlights of our research on InGaAs/AlAs(Sb) QCLs have included strain compensated QCL growth, the shortest wavelength room temperature InP-based QCL operation at around 3.1μm [1] and novel high performance structures at λ ∼ 4.1 μm with selectively incorporated AlAs barriers in the active regions.

Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions in quantum wells [1]. Semiconductor lasers in ring format do not need cleaved facets or gratings to achieve optical feedback required for lasing and could support either bidirectional or unidirectional operation[2].

We review the development of high performance, short wavelength (3 μm < λ < 3.8 μm) quantum cascade lasers (QCLs) based on the deep quantum well InGaAs/AlAsSb/InP materials system. Use of this system has enabled us to demonstrate room temperature operation at λ ~ 3.1 μm, the shortest room temperature lasing wavelength yet observed for InP-based QCLs. We demonstrate that significant performance improvements can be made by using strain compensated material with selective incorporation of AlAs barriers in the QCL active region. This approach provides reduction in threshold current density and increases the maximum optical power. In such devices, room-temperature peak output powers of up to 20 W can be achieved at λ ~ 3.6 μm, with high peak powers of around 4 W still achievable as wavelength decreases to 3.3 μm.

We report the development of strain compensated InGaAs/AlAs(Sb)/InP quantum cascade lasers emitting in the wavelength range of 3.3–3.8µm with high peak optical power (up to 17W at 300K), and high temperature operation (up to 400K).

The Si+-dose dependence of the photoluminescence (PL) intensity at λ≈800 nm for SiO2 with Si nanoinclusions (NIs) produced by ion implantation is studied. It is shown that this dependence is determined by the increase of the surface density of NIs, the mean size of NIs being constant until they overlap. Doping by phosphorus enhances the PL due to the joint action of two mechanisms: (i) the supply of additional electrons from the donors to the conduction band of quantum dots and (ii) the passivation of dangling bonds by phosphorus atoms.

We report the first room temperature strain compensated InGaAs/AlAs(Sb)/InP quantum cascade lasers operating down to 3.15µm. The lasers with selective incorporation of AlAs barriers in the active regions emit hundreds of milliwatts peak optical power.

Sensitive single-pass transmission measurements for probing the electron distribution of Quantum Cascade lasers under an applied bias are used to extract the intersubband nonparabolicity and single-pass gain of a lambda~10 mum Quantum Cascade laser.

We report the first realization of short wavelength (λ ~ 3.05 - 3.6 μm) lattice matched In 0.53 Ga 0.47 As/AlAs 0.56 Sb 0.44 /InP quantum cascade lasers (QCLs). The highest-performance device (λ ~ 3.6μm) displays pulsed laser action for temperatures up to 300 K. The shortest wavelength QCL (λ ≈ 3.05 μm) operates in pulsed mode at temperatures only up to 110 K. The first feasibility study of the strain compensated InGaAs/AlAsSb/InP QCLs (λ ~ 4.1 μm) proves that the lasers with increased indium fractions in the InGaAs quantum wells of 60 and 70% display no degradation compared with the lattice matched devices having identical design. This strain compensated system, being of particular interest for QCLs at λ

We show that mid infrared transmission spectroscopy of a quantum cascade laser provides clear cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for the $\lambda \sim$ 7.4 $\mu$m InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes in hand with a decrease of discrete intersubband absorption peaks and an increase of broad high-energy absorption towards the continuum delocalised states above the barriers., Comment: 6 pages, 5 figures