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We perform a detailed comparison of 2D Auger coefficients in narrow-gap type-I and type-II quantum wells (T1QWs and T2QWs), and the relative effects of Auger non-radiative decay on midwave infrared lasers employing both types of gain media. Comparison is also made to 3D Auger coefficients in bulk mid-IR materials, by defining a “normalization length” that accounts for the spatial extent of the electron and hole wavefunctions in the quantum wells. The comparisons confirm that Auger recombination in both types of QW is substantially suppressed relative to bulk, due primarily to the effects of compressive strain on the valence subband dispersions. We find that the 2D Auger coefficients in T1QWs remain substantially lower than those in T2QWs out to wavelengths beyond $3.5~\mu \text{m}$ . However, this does not necessarily imply a lower lasing threshold because a substantial fraction of the holes injected into T1QWs occupy lower subbands that do not contribute gain, so more must be injected to reach the lasing threshold. When all of the relevant considerations are combined, the thresholds for the best T1QW and T2QW lasers cross over near $\lambda ~\approx ~3.0~\mu \text{m}$ . Other characteristics governing the relative T1QW and T2QW laser performances above threshold, such as maximum output power and wallplug efficiency, are also considered.

We describe the framework for a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical components such as spectral multiplexers and demultiplexers on the native GaSb substrate of an interband cascade laser (ICL) structure.1,2 These building blocks may be used, for example, to fabricate a laser spectroscopy PIC for on-chip chemical sensing.3

Mid-infrared semiconductor lasers have emerged as indispensable compact coherent sources for military and commercial applications. While much of the historical emphasis has been on maximizing the output power and/or spectral purity, a recent new focus has been on engineering these lasers to operate as optical frequency combs (OFCs) for broadband real-time spectroscopy. In particular, the combination of low-drive-power and broad gain bandwidth has made interband cascade laser (ICL) OFCs an attractive complement to quantum cascade laser OFCs operating at longer wavelengths. Moreover, ICL combs can potentially be incorporated into fully-integrated dual-comb spectrometers that employ fast, room-temperature IC photodetectors processed on the same chip. However, the high refractive index of the ICL’s GaSb substrate poses some challenges to the optical waveguiding. Because the modal index is considerably lower than that of the substrate, the optical field can penetrate the bottom cladding layer and leak into the GaSb, inducing wavelength-dependent interference that modifies the gain and group velocity dispersion (GVD) profiles. Even when the effect on lasing threshold is small, the comb properties can be adversely affected. Using the sub-threshold Fourier transform technique, we studied ICL combs with various ridge widths, substrate thicknesses, and center wavelengths. This allowed us to evaluate the effects of modal leakage on the GVD. We find that the resonant nature of the substrate modes induces oscillations, which affect both the spectral bandwidth and the phase-locking properties above threshold. Strategies to mitigate the GVD’s undesired and unpredictable spectral variation will be presented.

We describe how a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical elements may be constructed on the native GaSb substrate of an interband cascade laser (ICL) structure. The active and passive building blocks may be used, for example, to fabricate an on-chip chemical detection system with a passive sensing waveguide that evanescently couples to an ambient sample gas. A variety of highly compact architectures are described, some of which incorporate both the sensing waveguide and detector into a laser cavity defined by two high-reflectivity cleaved facets. We also describe an edge-emitting laser configuration that optimizes stability by minimizing parasitic feedback from external optical elements, and which can potentially operate with lower drive power than any mid-IR laser now available. While ICL-based PICs processed on GaSb serve to illustrate the various configurations, many of the proposed concepts apply equally to quantum-cascade-laser (QCL)-based PICs processed on InP, and PICs that integrate III-V lasers and detectors on silicon. With mature processing, it should become possible to mass-produce hundreds of individual PICs on the same chip which, when singulated, will realize chemical sensing by an extremely compact and inexpensive package.

We demonstrate a mid-IR mode-resolved Vernier optical frequency comb spectrometer with an interband cascade laser source. The free-running system provides 35 meters of effective path length for monitoring 1-THz-broad spectra at 3.63 µm.

We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time.

By exploiting the bi-functional operation capability of interband cascade laser (ICL) frequency combs, we have utilized the laser medium not only for comb generation, but also as a room-temperature photodetector with near-GHz bandwidth for multi-heterodyne beating of the comb lines. Our self-contained platform consuming less than 2 W of electrical power enables free-running room-temperature broadband dual-comb spectroscopy of 1,1 difluoroethane with ~3% standard deviation in 2 ms over 600 GHz of optical bandwidth around 3.6 µm. We will discuss progress toward the optimization of ICL combs for realizing compact, low-power chemical sensors operating in the MWIR.

Interband cascade laser (ICL) optical frequency combs are promising midwave infrared sources for dual comb spectroscopy probing the strong fundamental absorption lines of numerous chemical and biological agents. In this work, a 4-mm-long ICL frequency comb emitting at 3.3 μm was operated by single-cavity optical self-injection. The experiments employing a free-space optical self-injection length of 1.1m with fine-delay control demonstrated a tuning range of 119MHz around the free-running intermode beat frequency of 9.58 GHz. For resonant fine-delay the line width of the intermode beat frequency was reduced to 390 kHz, what is an improvement by a factor of 40 in comparison to the solitary laser line width.

We report a preliminary investigation of ion bombardment (IB) effects on interband cascade laser (ICL) properties. Under some conditions, IB almost completely suppresses the vertical transport through a broad-area laser, although other times only a partial or negligible suppression is observed. To elucidate the mechanism that induces the suppression and in what part of the structure it occurs, we investigated the effects of IB on samples containing only ICL sub-regions. While IB increased the resistivity of a lightly-n-doped GaSb layer such as that used as a top or bottom separate confinement layer in an ICL, that layer was still much too conductive to strongly suppress the current flowing through a full device. The voltage drop was larger following IB of an InAs-AlSb superlattice such as that used in the top and bottom optical cladding layers in an ICL, although the effect was not large enough to fully account for the strong net suppression. And finally, the resistivity of an interband cascade LED containing the same active stages as an ICL was actually found to decrease following IB. Despite the inconclusive and sometime inconsistent findings of this study, it is nonetheless clear that if the effects can be controlled reproducibly, IB may provide a valuable tool for enhancing such ICL device configurations as weakly-index-guided narrow ridges and interband cascade vertical-cavity surface-emitting laser mesas that inject current and emit light only within a small central aperture.

Mid-infrared photonic integrated circuits (PICs) that combine on-chip light sources with other optical components constitute a key enabler for applications such as chemical sensing, light detection, ranging, and free-space communications. In this paper, we report the monolithic integration of interband cascade lasers emitting at 3.24 µm with passive, high-index-contrast waveguides made of chalcogenide glasses. Output from the chalcogenide waveguides exhibits pulsed peak power up to 150 mW (without roll-over), threshold current density 280 A/cm2, and slope efficiency 100 mW/A at 300 K, with a lower bound of 38% efficiency for coupling between the two waveguides. These results represent an important step toward the realization of fully integrated mid-infrared PICs.

Optical frequency combs have revolutionized the field of high resolution real-time molecular spectroscopy. Here, we demonstrate an electrically-driven optical frequency comb whose sub-picosecond pulses span more than 1 THz of spectral bandwidth centered near 3.3 mm. This is achieved by passively mode locking an interband cascade laser in a multi-contact architecture with gain and saturable absorber sections monolithically integrated on the same chip.

Midwave infrared interband cascade light-emitting devices (ICLEDs) with split active stages positioned at antinodes of the optical electric field are shown to generate up to 2.9 mW of output power for CW operation at 25 °C.

We have experimentally investigated the effects of sidewall corrugations on the beam quality and brightness of narrow-ridge interband cascade lasers (ICLs) emitting at λ ≈ 3.3 μm. We find that at this wavelength a corrugation period of 10 µm provides greater suppression of higher-order lateral modes than a shorter period of 2-4 µm. While the power and efficiency decrease modestly for the longer corrugation period, there is a net increase of the brightness defined as the output power divided by the beam quality factor M2 . However, the brightness degrades when the corrugation amplitude is increased from 2 µm to 3.5 µm, since lower output power offsets a relatively small improvement of the beam quality

Midwave infrared interband-cascade light-emitting devices (ICLEDs) have the potential to improve the selectivity, stability, and sensitivity of low-cost gas sensors. We demonstrate a broadband direct absorption CH4 sensor with an ICLED coupled to a plastic hollow-core fiber (1 m length, 1500 µm inner diameter). The sensor achieves a 1σ noise equivalent absorption of approximately 0.2 ppmv CH4 at 1 Hz, while operating at a low drive power of 0.5 mW. A low-cost sub-ppmv CH4 sensor would make monitoring emissions more affordable and more accessible for many relevant industries, such as the petroleum, agriculture, and waste industries.

Interband cascade (IC) laser structures offer attractive potential for operation at room temperature as both broadband coherent sources of mid-infrared light and fast photodetectors. This makes the realization of extremely compact spectrometers on a monolithic platform possible, and even dual-comb spectroscopy (DCS) configurations. IC comb devices are perfect candidates for this configuration, since they develop near-THz-wide optical frequency comb spectra from a millimeter-sized cavity, using a multi-stage structure that can also function as a very fast photodetector. In this work, we leverage IC photodetectors with a gigahertz bandwidth to demonstrate a self-contained, free-running, room-temperature DCS system in the mid-infrared. The DCS system used detection by the same bi-functional IC device structure to measure 1,1-difluoroethane over ∼600 GHz of optical coverage around 3.6 µm. These results show that the IC platform is suitable for full integration as a broadband, high-resolution on-chip spectrometer in a future chemical sensing system.

We demonstrate interband cascade lasers (ICLs) that are heterogeneously integrated on silicon. These lasers emit λ ≈ 3.6 μm light into silicon-on-insulator waveguides and operate up to 40 °C.

We demonstrate an electrically-driven frequency comb whose sub-picosecond pulses span more than 1 THz of spectral bandwidth centered near 3.6 μm. This is achieved by passively mode locking an interband cascade laser with gain and saturable absorber sections monolithically integrated on the same chip.

We demonstrate a strategy for increasing the operating temperatures of nBn midwave infrared (MWIR) focal plane arrays, based on the use of two-dimensional plasmonic gratings to enhance the quantum efficiency (QE) of structures with very thin absorbers. Reducing the absorber volume correspondingly reduces the dark current in a diffusion-limited photodiode, while light trapping mediated by the plasmonic grating increases the net absorbance to maintain high QE. The plasmonically enhanced nBn MWIR sensors with absorber thicknesses of only 0.5 μm exhibit peak internal QEs as high as 57%, which enables a 5-fold reduction in dark current. Numerical simulations indicate the potential for further improvement.

We are developing midwave infrared (mid-IR) quantum cascade lasers (QCLs) and interband cascade lasers (ICLs) bonded to silicon. The heterogeneous integration of mid-IR photonic devices with silicon promises to enable low-cost, compact sensing and detection capabilities that are compatible with existing silicon photonic and electronic technologies. The first Fabry-Perot QCLs on silicon were bonded to pre-patterned silicon-on-nitride-on-insulator (SONOI) substrates. Lateral tapers in the III-V mesas transferred the optical mode from the hybrid III-V/Si active region into the passive silicon waveguides, with feedback provided by reflections from both the III-V tapers and the polished passive silicon facets. Lasing was observed at   4.8 m with threshold current densities as low as 1.6 kA/cm2 when operated in pulsed mode at T = 20 oC. The first mid-IR DFB lasers integrated on silicon employed gratings patterned into the silicon waveguides before bonding. Over 200 mW of pulsed power was generated at room temperature, and operated to 100 °C with T0 = 199 K. Threshold current densities were measured below 1 kA/cm2.The grating imposed considerable wavelength selectivity and 22 nm of thermal tuning, even though the emission was not spectrally pure. Ongoing research focuses on flip-chip bonding to improve heat sinking for continuous-wave operation, and arrayed waveguide gratings for beam combining. ICLs have also been bonded to silicon and the GaSb substrate has been chemically removed with an InAsSb etch-stop layer. Tapered ICL ridges designed for lasing in a hybrid III-V/Si mode have been processed above passive silicon waveguides patterned on SOI. A goal is to combine the power generated by arrays of QCLs and ICLs residing on the same chip into a single, high-quality output beam.

We report interband cascade light-emitting devices (ICLEDs) emitting at λ ≈ 3.1mm, which produce generate higher radiance, output power, and efficiency than any other mid-infrared LEDs operating in continuous wave (cw) mode near and above room temperature. This is achieved in part by splitting the 22 LED stages into four groups placed at the antinodes of the near-normal optical field when the device is mounted epitaxial-side-down on a reflective metal contact. At an applied bias of 9.4 V and injection current of 0.6 A, an ICLED with mesa diameter 400 mm produces 2.9 mW of cw output power at T = 25°C, which corresponds to a radiance of 0.73 W/cm2/sr. The wall-plug efficiency ranges from 0.4% at low powers to 0.05% at the maximum output power.

We demonstrate methane and hydrogen chloride measurements around 3.6 μm using dual-comb spectroscopy with free running interband cascade laser frequency combs and a 76 m astigmatic Herriott multipass cell.

We demonstrate dual comb spectroscopy using free running passively mode-locked interband cascade laser frequency combs by measuring methane around 3.6 μm with 100 μs acquisition time and an instantaneous optical bandwidth of 570 GHz.

Interband cascade lasers (ICLs) are a promising light source for the mid-infrared (mid-IR) spectral range. However, for certain applications such as spectroscopic techniques for chemical sensing and non-invasive disease diagnostics, a broadband incoherent radiation source such as an LED may be more desirable. Here we investigate both ICLs and interband cascade light emitting devices (ICLEDs). The ICLEDs follow the example of ICLs by cascading multiple active stages in series to improve efficiency and increase output power, but without an optical cavity to provide feedback. In this work we will present studies of these devices using high hydrostatic pressure techniques to determine the key efficiency limiting processes so that they might be mitigated. The application of hydrostatic pressure causes reversible changes to the band structure, increasing the energy of the conduction band gamma point and moving other key points in the band structure. This makes it a useful technique to probe recombination processes that depend on band gap and offsets, independently of temperature. For a laser dominated by CHCC Auger recombination, as is typical in narrow band gap devices for the mid-IR, one would expect a decrease in threshold current with increasing pressure, as the Auger process decreases with increasing band gap. However, the lasers studied here exhibit an increase in threshold current with pressure, indicating that other processes also play a significant role. We will discuss the relative contributions from Auger recombination and other processes such as defect-related recombination and carrier leakage in these devices, with respect to relevant modelling.

Interband cascade lasers (ICLs) have proven to be efficient semiconductor sources of coherent mid -infrared (mid-IR) radiation. Single mode distributed-feedback (DFB) ICLs are excellent high-resolution spectroscopic sources for targeting important molecular species in the mid-IR fingerprint region, but are limited to a narrow spectral tuning range. Recent developments in multi-heterodyne spectroscopy with multi-mode Fabry-Perot (FP) lasers have enabled significant progress towards broadband high-resolution spectroscopic sensing applications in the mid-infrared. Here, we characterize the mode structure and tuning properties of multi-mode FP-ICLs for the purpose of evaluating the feasibility of ICL-based multiheterodyne spectroscopy.

While much of the previous work on interband cascade lasers (ICLs) has been limited to the 3-4 μm spectral range, it was recently demonstrated at NRL and elsewhere that the low threshold current and power densities characteristic of ICLs can be extended to longer wavelengths. Here we report on the performance of ICLs operating in the 4.6-6.1 μm spectral range. The pulsed threshold current density at room temperature for an ICL emitting at λ = 4.8 μm is 220 A/cm2, the lowest ever reported for a semiconductor laser at such a long emission wavelength. Broad-area devices emitting in the 4.6-4.9 μm range are observed to maintain pulsed external differential quantum efficiencies (EDQEs) of 11-17% when operating at 375 K. An ICL emitting at λ = 5.7 μm exhibits a threshold current density of 450 A/cm2 and EDQE of 27% at room temperature. The Auger coefficients extracted from these thresholds indicate a systematic increase with wavelength, with the value at 6 μm being 3-4 times higher than that at λ = 3.5 μm.

Midwave infrared ICLEDs with split active stages positioned at antinodes of the optical electric field are shown to generate up to 1.86 mW of output power and 0.3% wallplug efficiency in CW operation at 25 °C.

While mid-infrared radiation can be used to identify and quantify numerous chemical species, contemporary broadband mid-IR spectroscopic systems are often hindered by large footprints, moving parts and high power consumption. In this work, we demonstrate multiheterodyne spectroscopy using interband cascade lasers, which combines broadband spectral coverage with high spectral resolution and energy-efficient operation. The lasers generate up to 30 mW of continuous wave optical power while consuming less than 0.5 W of electrical power. A computational phase and timing correction algorithm is used to obtain kHz linewidths of the multiheterodyne beat notes and up to 30 dB improvement in signal-to-noise ratio. The versatility of the multiheterodyne technique is demonstrated by performing both rapidly swept absorption and dispersion spectroscopic assessments of low-pressure ethylene (C$_2$H$_4$) acquired by extracting a single beat note from the multiheterodyne signal, as well as broadband multiheterodyne spectroscopy of methane (CH$_4$) acquired with all available beat notes with microsecond temporal resolution and an instantaneous optical bandwidth of 240 GHz. The technology shows excellent potential for portable and high-resolution solid state spectroscopic chemical sensors operating in the mid-infrared.

A multiheterodyne spectroscopy system based on Fabry-Perot interband cascade lasers is demonstrated for broadband spectroscopic assessments of gaseous methane. The spectrometer is capable of ∼240 GHz of spectral coverage around 3.21 μm.

The interband cascade laser (ICL) is an ideal candidate for low-power mid-infrared frequency comb spectroscopy. In this work, we demonstrate that its intracavity second-order optical nonlinearity induces a coherent up-conversion of the generated mid-infrared light to the near-infrared through second-harmonic and sum-frequency generation. At 1.8 µm, 10 mW of light at 3.6 µm convert into sub-nanowatt levels of optical power, spread across 30 nm of spectral coverage. The observed linear-to-nonlinear conversion efficiency exceeds ${3\;{\unicode{x00B5} {\rm W/W}}^2}$3µW/W2 in continuous wave operation. We use a dual-band ICL frequency comb source to characterize water vapor absorption in both spectral bands.

Chalcogenide glasses are a promising group of materials for remote sensing applications. Two compositions from the Ge-Sb-Se/S system are investigated as core and cladding glasses in a step index fiber (SIF). Following thermomechanical and refractive index measurements, mid-infrared (MIR) light guiding is demonstrated through an 8 m length of SIF with a Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding. Using a single distillation procedure, Ge20Sb10Se70 at. % glass fibers are shown to have low optical loss across the 2 to 10 µm wavelength range with the lowest baseline loss shown as 0.44 dB/m at 6.4 µm.

We report on an investigation of dark current contributions from common microscale crystallographic defects in type-II InAs/GaInSb superlattice infrared PIN type photodiode structures grown on (100) GaSb substrates and have identified three general classifications. Defects on several wafers of varying design were examined from multiple perspectives to correlate electrical activity with structural properties, to develop the identification and classification scheme. Active defects were first identified by current density vs voltage (J-V) measurements and electron beam induced current (EBIC) scans of individual diodes with micrometer resolution. The EBIC scans were then correlated with plan-view optical and atomic force microscopy images, both before and after anisotropic etch-pit analysis using a newly developed etchant. The atomic scale structure of active and inactive defects was then compared using cross-sectional transmission electron microscopy (TEM) on vertical slices of defects extracted using focused ion beam milling. Analysis of the TEM images yielded important clues as to the structure and root causes of benign and active defects, in which only significant disruptions at the epi-substrate interface appear to play a key role in producing microscale defects that efficiently promote dark current.

Two semiconductor optical frequency combs, consuming less than 1 W of electrical power, are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3–4 μm spectral region. The devices are 4 mm long by 4 μm wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with optical multi-pass cell sensitivity enhancement. The system provides a spectral coverage of 33 cm−1 (1 THz), 0.32 cm−1 (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ∼100 at 100 μs integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.

The application of an interband cascade laser, ICL, to multi-mode absorption spectroscopy, MUMAS, in the mid-infrared region is reported. Measurements of individual mode linewidths of the ICL, derived from the pressure dependence of lineshapes in MUMAS signatures of single, isolated, lines in the spectrum of HCl, were found to be in the range 10–80 MHz. Multi-line spectra of methane were recorded using spectrally limited bandwidths, of approximate width 27 cm−1, defined by an interference filter, and consist of approximately 80 modes at spectral locations spanning the 100 cm−1 bandwidth of the ICL output. Calibration of the methane pressures derived from MUMAS data using a capacitance manometer provided measurements with an uncertainty of 1.1 %. Multi-species sensing is demonstrated by the simultaneous detection of methane, acetylene and formaldehyde in a gas mixture. Individual partial pressures of the three gases are derived from best fits of model MUMAS signatures to the data with an experimental error of 10 %. Using an ICL, with an inter-mode interval of ~10 GHz, MUMAS spectra were recorded at pressures in the range 1–10 mbar, and, based on the data, a potential minimum detection limit of the order of 100 ppmv is estimated for MUMAS at atmospheric pressure using an inter-mode interval of 80 GHz.

We report the single-mode operation of mid-infrared distributed-feedback (DFB) interband cascade lasers (ICLs) with contacts that cover only a fraction of the top surface of the laser ridge. This reduces the optical loss from the metal for the GaSb-relevant device configuration in which the grating is fabricated in the top layer of the DFB laser. Continuous wave (cw) room-temperature operation in a single spectral mode is observed for contact duty cycles as small as 14% when the width of the contact is fixed at 10 μm. The reduced contact duty cycle results in a factor of 2 decrease in the threshold current. The highest slope efficiency is observed for a contact duty cycle of 33%, for which the cw single-mode output power is as high as 6.8 mW.

We report cw wallplug efficiencies (WPEs) for mid-infrared interband cascade lasers (ICLs) that are comparable to those of state-of-the-art quantum cascade lasers at temperatures ranging from the cryogenic regime to room temperature. The continuous wave (cw) WPE for 10-stage broad-area devices remains above 40% for temperatures up to 125 K, and is still

A volume Bragg grating recorded in photo-thermo-refractive glass was used to spectrally lock the emission from an 18-μm-wide interband cascade laser ridge to a wavelength of 3.12 μm. The spectral width of emission into the resonant mode is narrowed by more than 300 times, and the thermal wavelength shift is reduced by 60 times. While the power loss penalty is about 30%, the spectral brightness increases by 200 times.

Threshold current densities for single-mode cw operation at room temperature were lowest and slope efficiencies highest for DFB interband cascade lasers with top contacts covering only a fraction of the ridge surface.

Multi-mode absorption spectroscopy, MUMAS,1,2,3 is a technique that allows simultaneous multi-species gas sensing with compact, robust, simple and relatively inexpensive laser sources. It provides wider spectral coverage than is afforded by traditional TDLAS, (tunable diode laser absorption spectroscopy), is simpler and less expensive than fs-comb methods since it does not require complex laser systems nor high resolution dispersive optics or complex electronics required for heterodyning methods. A MUMAS signal is generated by measuring the temporal variation in total transmitted intensity as the modes are scanned in frequency over the inter-mode frequency interval, Δν mode . A change in transmitted intensity, relative to the incident intensity, is recorded whenever any of the modes, during their scan across Δν mode , comes into resonance with an absorption line in the spectrum of the gas. The signal, therefore, consists of a superposition of single mode scans, one for each mode lying within the spectral range of the laser output, Δν band . The resulting signal is characteristic of the absorbing species probed by the particular multi-mode laser used. The signal can also be modelled if the spectral locations of the absorption lines are available from a suitable database such as HITRAN,4 and the laser mode parameters are also known. By fitting the modelled MUMAS signature to the experimental data, using the gas concentrations as fit parameters, the absolute and relative concentrations can be determined.

Simultaneous sensing of methane, acetylene and formaldehyde in a mixture by multi-mode absorption spectroscopy, MUMAS, using an interband cascade laser, ICL, at 3.7 microns. is reported. Concentrations of pure gases were measured with ~1% uncertainty.

The cw threshold current at room temperature is lowest and the slope efficiency highest for DFB interband cascade lasers with reduced duty cycle of the top electrical contact. All DFBs operated in a single spectral mode.

The interband cascade laser (ICL) concept provides robust and efficient emission in the midwave infrared spectral band. While the geometry is somewhat analogous to that of a quantum cascade laser employing intersubband transitions, the ICL implementation exploits the type-II band alignment of the GaSb-based material system. A semimetallic band overlap at the boundary between the electron and hole injector regions automatically generates carriers with densities tunable by quantum confinement. Electrical injection then replenishes the carriers already present rather than creating the population inversion. In this paper, we describe and analyze the physical principles governing ICL operation, and discuss specific modifications to the active region, electron injector, hole injector, and waveguide designs that demonstrably improve the performance. The pulsed I- V and L-I characteristics of devices processed from over 50 wafers provide a statistically meaningful confirmation of the established trends.

The effects of 2 MeV proton irradiation on a set of four long-wave infrared type II superlattice photodiodes with various structures were studied. Changes were monitored in operating bias, quantum efficiency (QE), and dark current. Shifts in operating bias indicate that irradiation causes the superlattices to become more p-type, and decreases in QE are found to be consistent with a reduction in carrier lifetime. Leakage currents remain lower in graded-gap diodes at all fluences.

Shallow-etch mesa isolation (SEMI) of graded-bandgap “W”-structured type II superlattice (GGW) infrared photodiodes provides a powerful means for reducing excess dark currents due to surface and bulk junction related processes, and it is particularly well suited for focal-plane array fabrication. In the n-on-p GGW photodiode structure the energy gap is increased in a series of steps from that of the lightly p-type infrared-absorbing region to a value typically two to three times larger. The wider gap levels off about 10 nm short of the doping-defined junction, and continues for another 0.25 μm into the heavily n-doped cathode before the structure is terminated by an n +-doped InAs top cap layer. The increased bandgap in the high-field region near the junction helps to strongly suppress both bulk tunneling and generation–recombination (G–R) current by imposing a much larger tunneling barrier and exponentially lowering the intrinsic carrier concentration. The SEMI approach takes further advantage of the graded structure by exposing only the widest-gap layers on etched surfaces. This lowers surface recombination and trap-assisted tunneling in much the same way as the GGW suppresses these processes in the bulk. Using SEMI, individual photodiodes are defined using a shallow etch that typically terminates only 10 nm to 20 nm past the junction, which is sufficient to isolate neighboring pixels while leaving the narrow-gap absorber layer buried 100 nm to 200 nm below the surface. This provides for separate optimization of the photodiode’s electrical and optical area. The area of the junction can be reduced to a fraction of that of the pixel, lowering bulk junction current, while maintaining 100% optical fill factor with the undisturbed absorber layer. Finally, with the elimination of deep, high-aspect-ratio trenches, SEMI simplifies array fabrication. We report herein results from SEMI-processed GGW devices, including large-area discrete photodiodes, mini-arrays, and a focal-plane array. Current–voltage data show strong suppression of side-wall leakage relative to that for more deeply etched devices, as well as scaling of dark current with junction area without loss of quantum efficiency.

Limiting the defect-mediated dark currents in type-II superlattice (T2SL) IR photodiodes remains the key challenge to focal plane arrays (FPAs) based on this material system. In spite of its larger effective mass to suppress tunneling and more than an order of magnitude longer Auger lifetime, the T2SL photodiode performance still lags behind that of the incumbent HgCdTe-based technology. The tunneling and generation–recombination currents can be strongly suppressed by employing a “W” T2SL structure and gradually increasing the energy gap in the depletion region. For maximum quantum efficiency, this graded-gap geometry is combined in a hybrid structure with two-constituent T2SL absorbers that exhibit roughly twice the diffusion length of the “W” structure. Finally, if the etch used to isolate neighboring pixels is stopped just beyond the junction in the graded-gap device, narrow-gap regions are not exposed and the total sidewall area is reduced by a factor of 20. We have combined all of these approaches to produce a 10.5 μm cutoff FPA with diffusion-limited performance and noise-equivalent differential temperature (NEDT) of 35 mK at 70 K.

We report a detailed experimental investigation of five interband cascade lasers with five active stages each and emitting at wavelengths between 3.2 μm and 4.2 μm at room temperature. Pulsed threshold current densities as low as 394 A/cm2 and voltage efficiencies as high as 76% are obtained at 300 K. The low pulsed threshold power densities (0.9–1.6 kW/cm2 at 300 K) imply that ambient-temperature cw operation should be possible over the entire spectral band once optimized narrow ridges can be fabricated.

We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral range, i.e., 2.9 μm to 5.2 μm, near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm2 at 78 K (λ = 3.6 μm) and 590 A/cm2 at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6 μm to 2.9 μm) was even lower, 450 A/cm2. A␣cavity-length study of the lasers emitting at 3.6 μm to 4.1 μm yielded an internal loss varying from 7.8 cm−1 at 78 K to 24 cm−1 at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%.

The recent development of graded-gap W-structured superlattices (GG-Ws) has led to a substantial improvement in the dark current performance of infrared photodiodes implemented with type-II superlattices (T2SLs). A study of ten GG-W photodiode wafers with 50% power responsivity cutoffs from 9 μm to 13.4 μm is presented. Dark current performance has increased by a factor of 10 over that of previous type-II structures, without degrading quantum efficiency. In relation to HgCdTe (MCT) based photodiodes, several samples in the study show effective dynamic-resistance-area products close to the MCT trend line for diffusion-limited R 0 A.