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This Conference Presentation, High-brightness GaSb-based quantum-well lasers with an unstable resonator was recorded at Photonics West 2020 held in San Francisco, California, United States.

We investigate the requirements for the pulse width and intensity of the pump beam, for the Fe:ZnSe lasing medium to reach threshold. The rate equation is solved for the time dependence of the population density, N2 of the 5T2 manifold which includes the upper level for the lasing transition.

Achieving high brightness, i.e., the combination of high optical power and good beam quality, in quantum cascade lasers (QCLs) puts this technology in position to supplant other laser sources of midwave infrared (MWIR, 3–5 μm) and long-wave infrared (LWIR, 8–12 μm) light in various current and emerging applications. The mode quality implied by the term “high brightness” ensures that a minimal amount of divergence will be experienced by a well-collimated beam that also exhibits superior coherence by operating with high power near the fundamental transverse mode. Various waveguide designs are presented as techniques to suppress higher order transverse modes while preserving high optical power of the resulting laser beam.

In-situ desorption mass spectrometry (DMS) tool is used to report monolayer oscillations of the Sb desorption rate during molecular beam epitaxy (MBE) growth of GaSb and GaInSb. Oscillations in the Sb desorption rate appear to be correlated to the variation in the surface-step-density that periodically changes the additional uptake and expulsion of Sb beyond the fixed rate of consumption. This notion is corroborated by comparing the Sb DMS signal to the time derivative of the specular RHEED beam intensity.

An 80-nm continuous, mode-hop-free, wavelength tuning range centered at $3.1~\mu \text{m}$ has been demonstrated for a large area, optically pumped mid-infrared type-II semiconductor distributed feedback (DFB) laser with a spectral linewidth of 1.2 nm at $2.5\times $ threshold and 820 mW single-facet output power operated CW at 80 K, suitable for atmospheric pressure molecular spectroscopy and remote sensing. Wavelength tuning of this DFB laser is achieved by translating the optical pump stripe across the device patterned with a hyperbolically chirped, location-dependent-period grating. The impact of chirp along the gain stripe is clarified and optimized to allow continuous tuning.

We have modeled broad-area ridge-waveguide quantum cascade lasers (QCLs) using COMSOL and generated a rank-ordering of high-order transverse lateral modes as a function of cavity width on the basis of a figure-of-merit. In particular, we corroborate experimental results previously obtained using the distributed sidewall loss (DSL) method to suppress high-order transverse lateral modes and extract fundamental mode in broad-area QCLs. COMSOL modeling shows that waveguide losses preferentially incurred by the high-order modes, due to the direct sidewall contact with metal as prescribed by the DSL method, can be used to obtain single-lobed emission with enhanced brightness in broad-area QCLs.

Incorporation of bismuth into GaSb is of interest because the alloy bandgap can be substantially reduced while avoiding strain relaxation. Thus, the GaSb1 − xBix alloy is potentially useful in extending the wavelength in optoelectronic devices. However, the epitaxial growth of this alloy via molecular beam epitaxy is challenging because the parameter space is relatively small. Additionally, a precise relationship between Bi incorporation and growth parameters is not well understood. In this study, the authors have utilized the in situ desorption mass spectrometry (DMS) technique to make relative measurements of Sb and Bi desorption during GaSb1 − xBix growth. Employed in this material system for the first time, the authors were able to identify critical growth parameters and correlate these parameters with Bi incorporation as well as accumulation at the growth surface. Specifically, the authors used the ability of DMS to precisely adjust the Sb/Ga flux ratio in order to determine the sensitivity of Bi incorporation to this flux ratio, in particular, as it approaches unity. Additionally, the authors used DMS to provide information on the surface accumulation of Bi and how this population can be purged from the surface using an incident Sb flux. The authors found that DMS can be very useful in rapidly locating the appropriate growth parameter window for this alloy as well as identifying relationships between Bi incorporation and other parameters such as substrate temperature and incident fluxes within that window.

A GaSb-based type-I diode laser heterostructure was grown with a highly mismatched GaAs top clad using MBE. A direct comparison of laser diode characteristics with a conventional structure indicates that despite the fully relaxed clad layer, high peak power >300 mW is generated by these devices. The small increase in the threshold current density and ∼27% reduction in the slope efficiency is likely due to increase in the internal loss due to defects.

We describe two distinct methods that provide preferential losses to the high order transverse modes present in broad-ridge quantum cascade lasers. The first of these incorporates intracavity lateral constrictions that are localized near the back facet. In the second method, metal is placed in contact with the sidewalls of the cavity to provide distributed losses along the ridge length. When applied properly, both of these techniques suppress high order transverse modes and result in fundamental or near-fundamental mode operation in broad-ridge devices. Despite relatively small loss in efficiency, increased beam quality produces increased brightness.

We apply the desorption mass spectrometry (DMS) technique and analyze the desorbed Sb species in-situ during MBE growth of mixed As/Sb heterostructures. We demonstrate how DMS is useful in pre-growth calibration of the V/III ratio, the group-III ratio, as well as the Sb-content in quaternary or quinary mixed As/Sb alloys. We also apply DMS to the digital alloy growth method. For demonstration purposes, we start with an un-calibrated MBE system, use the DMS technique to calibrate all of the previously undetermined MBE parameters and grow a ~3.3 µm diode laser heterostructure in only one attempt. The results demonstrate that the DMS technique will allow the MBE to quickly converge toward a set of acceptable growth parameters without the need for ex-situ calibration of alloy composition.

We describe the growth of GaSb-based diode lasers with quinary AlInGaAsSb alloy waveguide/barriers and highly strained InGaAsSb type-I quantum wells, designed to emit at ~3.2 µm at room temperature. To increase the compressive strain in the QWs above the pseudomorphic limit, we employ strain compensation in which the lower portion of the quinary alloy waveguide is slightly arsenic-rich as compared to the lattice-matched composition. Using this scheme, we fabricated a set of laser devices with incrementally increasing compressive strain in the quantum wells. With compressive strain reaching as high as ~2.4%, the device performance corroborates the notion that adding compressive strain in the quantum wells helps improve hole confinement and suppresses carrier leakage during operation near room temperature.

Focused ion beam milling was used to fabricate on-chip unstable resonator cavity quantum well laser devices. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Perot cavity device, the unstable resonator cavity device exhibits a 2x diffraction limited beam. The preliminary results demonstrate that a much higher brightness can be reached in this class of broad area devices.

We demonstrate GaSb-based diode lasers emitting near 2.7 μm that are fabricated as on-chip unstable resonator cavities using the focused ion beam tool. A near-diffraction-limited lateral beam is observed in broad-area devices where the reflection from the cylindrical mirror at the back facet overrides direct counterpropagation in the cavity to suppress the formation of filaments. We identify a geometric parameter that can be used as a general guide to optimizing the on-chip unstable resonator cavity for maximum brightness. This parameter is the aperture within the unstable resonator cavity, a measure of the angular divergence contained within the device, and is shown to correlate with the onset of filament suppression.

Mode creation and temporal response of broad-area quantum cascade lasers (BA-QCL) placed within an external feedback cavity are described in this publication. The critical feedback parameter becomes the mirror angle relative to the BA-QCL facet. With judicious angle choices, a plethora of curious modes can be created, each with their particular threshold and slope efficiency. These range from a nearly single far-field intensity peak to highly multimode emission similar to their diode counterparts. Dynamics are strongly dominated by transverse mode competition ranging for less than 20MHz to greater than 100MHz. When the mirror is parallel to the facet, higher frequency external cavity oscillations become undamped.

International audience; The growth by molecular-beam epitaxy of novel electrically pumped type-II multi-quantum well (MQW) Sb-based laser diodes in which only the holes are quantum confined was studied. These laser structures were fabricated on (0 0 1) GaSb substrates. In the MQW region, radiative recombinations originate from InGaSb hole wells embedded in InGaAsSb barriers lattice matched to GaSb. Two different laser structures were developed. The first one exhibited a well/barrier periodicity that was too short, which led to a laser emission near 2.65 μm originating from the waveguide rather than from the wells. With an improved well/barrier periodicity, the second structure exhibited laser emission up to 243 K at 2.93 μm in the pulsed regime (200 ns, 5 kHz). In this case, the laser photons were effectively produced by the hole-well active region. A minimum threshold of about 12.8 kA/cm2 at 80 K combined with a T0 around 70 K have been measured from this second structure.

Significant attention has recently been given to photoluminescence (PL) spectra and lifetime measurements on InAs/InAsSb superlattices, as high quality optical material with long carrier lifetimes are required for infrared detectors. The standard sample structure for PL measurements includes energy barriers to block photo-generated carriers from being lost through non-radiative recombination at interfaces between the superlattice and the surface or between the superlattice and the buffer/substrate. However, defect, surface, and/or interface states in AlSb, a commonly used barrier material, are known to contribute carriers to InAs quantum wells. Due to the similarity of the AlSb interface with the InAs/InAsSb superlattice, the effects of the barriers on the electrical and optical properties of the superlattice were investigated. Structures with AlSb barriers at the top and bottom of the superlattice, with no AlSb barriers, and with an AlSb barrier only at the top of the superlattice structure were studied. Hall Effect measurements revealed little change in the sheet carrier concentration at 10 K due to the barriers, but significant increases in low temperature mobility and a two-dimensional-like mobility temperature dependence were observed when barriers were present. Further high magnetic field measurements are necessary, however, to understand the transport properties of these samples due to the likelihood that multiple carriers are present. The photoluminescence (PL) spectra were almost identical regardless of the barriers, except for a 15% increase in intensity with the AlSb barrier between the buffer layer and the superlattice. The surface AlSb barrier had little effect on the intensity. The barriers are therefore recommended for PL measurements to increase the signal intensity; however, they complicate the analysis of single-field Hall Effect measurements.

We present experimental comparison of Type-I diode lasers emitting

The analysis of I-V and I-L curves in mid-IR quantum cascade lasers operating at room temperature is performed. When the ohmic component of the device resistance in the I-V curve is subtracted, the current I can be approximated by the exponential function, I = I s exp(eV j /e) where I s is the saturation current, Vj is the n-n junction voltage, and e is an energy parameter related to the tunneling mechanism which enables filling of upper states and emptying of lower states of the laser transition. Values of eare found to be in 0.68-1.45 eV range, and when divided by the number of stages in the cascade, the tunneling parameter of each stage is determined. The threshold related “kink” of differential I-V curves is shown. The effect of voltage saturation above the laser threshold is observed. Thus, the possibility of determination of the threshold using electrical measurements in quantum cascade lasers has been demonstrated.

We report on measurements of the temperature dependence of the gain and internal waveguide loss of a 3.4 μm, optically pumped InAs/InGaSb, type II, W laser. A high-resolution Fourier transform infrared spectrometer was used to measure the laser mode spectra below threshold. To obtain an accurate determination of the gain, a full-curve fit to the spectral output of the Fabry–Perot cavity was utilized. Our results indicate very low waveguide loss (a≈3 cm−1) at 78 K, with no apparent increase up to at least 120 K. Additional measurements of the gain properties of the device reveal a rapidly decreasing differential gain (dG/dP) and a rapidly increasing transparency pump power with increasing temperature. Moreover, measurements of the peak gain at constant pumping show a rapid decline with increasing temperature. Theoretical superlattice-empiricalpseudopotential-model-based calculations suggest that the substantial differences between the conduction and valence subband in-plane curvatures contribute to the rap...

We present a rationalized biquadratic interpolation algorithm for calculating quaternary semiconductor parameters and apply it to determine the band gaps of AlGaInAs, AlGaInP, AlInAsSb, GaInPAs, and GaInAsSb over complete compositional ranges. Extension to include bicubic corrections and quinary alloys are discussed. The band gaps of lattice-matched AlGaInAsSb and AlGaInPAs are reported.

We consider the digital alloy growth of mixed As/Sb layers by molecular beam epitaxy. Using the (InAs)n(GaSb)3n superlattice as an example, we determine an acceptable upper limit to the modulation period before significant deviation from the In0.25Ga0.75As0.25Sb0.75 bandgap occurs. We then consider the digital alloy growth of In0.2Ga0.8AsySb1� y layers on GaSb, and find that the digital alloying technique provides protection against compositional drift in the growth direction due to reduced sensitivity to variations in arsenic-flux and substrate temperature. r 2002 Elsevier Science B.V. All rights reserved.

We have grown single-quantum wells of strained Ga1−xInxSb (x

An optically pumped edge-emitting semiconductor laser emitting near 4.1 μm was designed with weak transverse mode confinement resulting in an exceptionally large transverse optical mode size. Consequently, a laser device is reported that exhibits a fast-axis divergence angle of ~4.2° full-width at half-maximum (FWHM). More notably, a large reduction of the lateral axis divergence is also observed as a result of the increased lateral coherency. The reduced confinement factor and differential gain serves to suppress lateral filamentation. Despite the higher threshold and lower power, a lateral divergence angle of ~3.2° FWHM and increased lateral brightness is achieved.

In this article, we report on a systematic study of mid-IR, W-Integrated Absorber (W-IA), lasers that employ strained InAs/InxGa1−xSb/InAs active layers, in which the indium content of the hole bearing InxGa1−xSb has been varied from xIn=0 to xIn=0.45. The output characteristics of the lasers improve as the In percentage is increased; the threshold temperature sensitivity (T0) values are observed to increase from ≈35 to ≈50 K. Further, the differential quantum efficiencies as a function of temperature are significantly improved in the devices with xIn⩾0.25. For samples with nominally eight monolayers (8 ML) InAs/7 ML InxGa1−xSb/8 ML InAs, the lasing wavelength at 84 K is observed to shift from 3.33 μm for xIn=0 out to a maximum of 4.62 μm for xIn=0.35. This large shift is well predicted by an empirical psuedopotential model; the model also predicts that the position of the hole wave function is sensitively dependent on strain level and that for xIn

We describe a method where the standard fabrication of broad area quantum cascade lasers is modified to provide a controlled amount of direct contact of device sidewalls with metal. We demonstrate that this provides sufficient levels of distributed losses to suppress the high order transverse modes in favor of the fundamental or near-fundamental transverse mode operation. We observe that the quantum cascade laser power and slope efficiency are degraded by a small amount, resulting in a large increase in brightness to accompany the power scaling.

The focused ion beam milling tool was used to convert a GaSb-based broad area gain-guided quantum well laser device with a standard Fabry-Perot cavity into one with an unstable resonator cavity. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Perot cavity device, where the coherency of the beam is severely disrupted by filamentation, the unstable resonator cavity device exhibits an ∼2× diffraction limited beam. The relatively small penalty in slope efficiency demonstrates that a much higher brightness can be reached in this class of broad area devices.

We have developed an empirical model of AlAsySb1−y composition during molecular beam epitaxy as a function of growth temperature, growth rate, As2-flux, and Sb2-flux, using response surface methodology. In order to facilitate data collection from a large number of experiments, we have applied desorption mass spectroscopy as an in situ measure of alloy composition. A quadratic model is found to fit the data well, identifying trends and several interactions between growth parameters.

We have investigated the effect of a brief As 2 -soak at the GaSb surface on the nature of the InAs-on-GaSb interface. We find that As 2 efficiently removes Sb from GaSb and that a controlled As 2 -soak may be a necessary step in forming a “GaAs-like” interface without structural or optical degradation. We observe that the thickness of the “GaAs-like” interfacial layer and the band-edge transition wavelength in InAs/GaSb superlattices both increase with increasing As 2 -soak duration.

We describe the photoluminescence spectroscopy (PL) and Fourier transform infrared absorbance spectroscopy characterization of a large set of InAs/GaSb type-II strained layer superlattice (SLS) samples. The samples are designed to probe the effect of GaSb layer thickness on the optical properties of the SLS, while the InAs-layer thickness is held fixed. As the GaSb layer thickness is increased, we observe a spectral blue shift of the PL peaks that is accompanied by an increase in intensity, narrower linewidths, and a large reduction in the temperature sensitivity of the luminescence. These effects occur despite a significant reduction in the electron-hole wave function overlap as the GaSb layer thickness is increased. In addition, we compare the results of empirical pseudopotential model (EPM) calculations to the observed blueshift of the primary band gap. The EPM calculations are found to be in very good agreement with the observed data.

Stimulated emission at 1.994 μm was demonstrated from an optically pumped, double quantum well, semiconductor laser that was digitally grown by modulated-molecular beam epitaxy. This “digital growth” consists of short period superlattices of the ternary GaInAs/GaInSb and GaAsSb/GaSb/AlGaSb/GaSb alloys grown by molecular beam epitaxy with the intent of approximating the band gaps of quaternary GaInAsSb and AlGaAsSb alloys in the active region and barriers of the laser, respectively. For a 50 μs pulse and a 200 Hz repetition rate, the threshold current density was 104 W/cm2 at 82 K. The characteristic temperature (T0) was 104 K, the maximum operating temperature was 320 K and the peak output power was 1.895 W/facet at 82 K with pumping power of 7.83 W.

We describe how cross-sectional scanning tunneling microscopy (STM) may be used to image the interfacial bonding across the nearly lattice-matched, non-common-atom GaSb/InAs heterojunction with atomic-scale precision. The method, which takes advantage of the length difference between interfacial and bulk bonds, appears equally applicable to AlSb/InAs and suggests how one might recover the complete structure of either heterojunction from atomic-resolution STM data.

We present a study of dual-wavelength optically pumped midinfrared semiconductor lasers, in which the effects of several design parameters are analyzed. We observe that when the short wavelength waveguide mode overlaps the long wavelength gain region, photon re-absorption occurs, favoring long wavelength emission. We demonstrate high power output from a laser emitting at two wavelength bands.

We use the technique of in situ desorption mass spectrometry (DMS) to probe the surface chemistry, and directly observe two phenomena that are responsible for anion intermixing at the InAs-on-GaSb interface: (1) the presence of excess but stable Sb on the GaSb surface that partially incorporates into the InAs layer, and (2) the removal of Sb from within the GaSb layer when the surface is over-exposed to an incident As-flux. We propose and demonstrate that anion intermixing in InAs/GaSb superlattices is minimized when the As-soak time is precisely adjusted to remove the excess surface Sb population while leaving the underlying GaSb layer intact.

We demonstrate optically pumped semiconductor lasers that are capable of simultaneously emitting at two different midinfrared wavelengths. The wavelengths can be independently chosen and designed into the heterostructure. The epitaxial III-V antimonide structure employs two sets of type-II quantum wells in a waveguide that is partitioned by a thin electrical barrier that is transparent to the pump radiation. Two-color devices emitting at wavelengths as far apart as ~4.0 and ~5.4 mum are reported.

The initial stages of molecular beam epitaxy of GaSb on highly mismatched GaAs(001) substrates were investigated. Transmission electron microscopy was used to analyze the defect structure in GaSb islands and at their interfaces with GaAs(001) at different stages of growth. Based on experimental observations, we propose that the semiperiodic net of 90° misfit dislocations at the GaSb/GaAs(001) interface nucleate homogeneously at the leading edges of advancing {111} planes. After nucleation, they glide inwards along the interface plane to reach their equilibrium position. Threading dislocations in GaSb layers were directly correlated with the misfit dislocation net. We demonstrate that there are no threading defects in GaSb islands when their interfaces consist solely of 90° misfit dislocations, and that threading dislocations in the GaSb epilayer are all associated with minority 60° misfit dislocations nucleated in growing islands. The number of threading dislocations per unit area of the GaSb film is found to be independent of GaSb coverage, indicating that island coalescence does not substantially increase the number of 60° dislocations.

Quantitative measurements of Sb surface segregation in pseudomorphic GaAs 0.8 Sb 0.2 layers grown on GaAs (001) using molecular beam epitaxy (MBE) were performed in situ using the line-of-sight mass spectrometry (MS) technique. It was observed that substantial surface accumulation of Sb occurs under normal growth conditions, giving rise to compositional broadening of the GaAsSb on GaAs interface. This compositional grading is found to strongly depend on the growth temperature as well as the presence of Sb at the GaAs film surface prior to the initiation of GaAsSb layer growth. We report the effect to substrate temperature and shutter sequence on the evolution of Sb surface accumulation near the interface and demonstrate that a compositionally abrupt interface can be formed by populating the GaAs surface with an amount of Sb equivalent to that expected on the GaAsSb surface during growth at steady state. Additionally, compositional abruptness at the GaAs on GaAsSb interface can be achieved by a “chemical flashoff” of the surface Sb population during exposure to an incident As 2 flux.

The reduction of threading dislocation density due to their mutual interactions in GaSb thin films grown on (001) GaAs substrates by molecular beam epitaxy has been investigated. The effectiveness of several buffer layer schemes including GaSb/AlSb strained layer superlattice and In 0.11 Ga 0.89 Sb/GaAs buffers for threading dislocation suppression was evaluated. High-resolution transmission electron microscopy shows that the GaSb/GaAs interface consists of a highly periodic network of 90° pure edge misfit dislocations, with an average spacing close to that for a fully relaxed system. This results in relatively low threading densities in the GaSb epilayer, despite their large lattice constant mismatch (8.2%). The threading dislocation density as a function of GaSb film thickness was determined by plan-view transmission electron microscopy and was found to decrease with film thickness due to mutual interactions among dislocations. It was found that a strained layer superlattice of GaSb/AlSb, with each layer close to its critical thickness, is the most effective in threading density reduction.

The interaction of a Sb2 molecular beam with the film surface during molecular beam epitaxy (MBE) of GaAs1 − xSbx layers was monitored by a quadrupole mass spectrometer (QMS) positioned in line-of-sight of the substrate. It was observed that the QMS signal from the monomeric Sb-species (massq = 121 or 123 amute) is proportional to the total Sb-flux leaving the film surface within a wide range of temperatures and surface composition. This enabled the accurate determination of the antimony incorporation coefficient, σ(Sb), and consequently the Sb-mole fraction during growth. Due to its applicability in growth conditions where σ(Sb) is less than unity, this compositional monitoring technique expands the practical range of MBE growth parameters for III-As1 − xSbx alloy growth. Real-time in situ control of alloy composition was demonstrated by implementing a feedback control scheme based on the QMS Sb signal. GaAs1 − xSbx alloys in which the Sb-fraction varied in a controlled manner in the direction of growth were deposited.

While conventional semiconductor lasers employ electrical injection for carrier excitation, optically pumped semiconductor lasers (OPSLs) have demonstrated high output powers and high brightness in the mid-infrared. An important consideration for optically pumped lasers is efficient absorption of the pump beam, which can be achieved through increasing the number of periods in the active region, by placing the active region in a cavity with an optical thickness of twice the pump wavelength between distributed Bragg reflectors (Optical Pumping Injection Cavity), or by periodically inserting the active quantum wells into an InGaAsSb waveguide designed to absorb the pump radiation (Integrated Absorber). A tunable optical pumping technique is utilized by which threshold intensities are minimized and efficiencies are maximized. The near-IR idler output of a Nd:YAG-pumped optical parametric oscillator (10 Hz, ~4 ns) is the tunable optical pumping source in this work. Results are presented for an OPSL with a type-II W active region embedded in an integrated absorber to enhance the absorption of the optical pump beam. Emission wavelengths range from 4.64 μm at 78 K to 4.82 μm at 190 K for optical pump wavelengths ranging from 1930-1950 nm. The effect of wavelength tuning is demonstrated and compared to single wavelength pumping (1940 nm) at a higher duty cycle (20- 30%). Comparisons are also made to other OPSLs, including a discussion of the characteristic temperature and high temperature performance of these devices.

Power scaling in broad area quantum cascade lasers results in the operation of high order transverse modes with a far-field profile consisting of two lobes propagating at large angles relative to the optical axis. We report a method of suppressing the high order transverse modes that can extract the fundamental mode and provide emission along the optical axis. By generating a lateral constriction in the waveguide in the form of short trenches defined by the focused ion beam milling technique, we report broad area devices in which most of the power is contained in a near diffraction-limited beam that provides high brightness.

We describe high peak-power, broad area mid-infrared semiconductor lasers. The laser structures incorporated 14 type-II quantum wells imbedded in thick waveguide/absorber regions composed of InGaAsSb. The Fabry-Perot lasers were optically pumped with the output from a passively-Q-switched Ho:YAG laser capable of delivering 100 kW at 2.09 um. The emission from the optically pumped semiconductor laser (OPSL) was near 4.1 um. The emission was spectrally and temporally resolved and showed evidence for the creation of a hot-carrier population at the higher pump powers. A roll-over in the OPSL output power curve was observed at pump-powers greater than ∼20 kW. The maximum single-ended OPSL power was near 470W.

A new approach to tunable mid-infrared lasers, an optically pumped, type-II, InGaSb/InAs gain medium with a chirped distributed feedback grating, has been developed. The chirped grating is patterned using an interferometric lithography (IL) technique with spherical wave fronts and etched into the top cladding of the laser slab waveguide structure. Because the period of grating increases gradually laterally, wavelength tuning is implemented by shifting pump stripe to different positions on the device with different grating periods. Fabry-Perot modes from the cleaved facets are successfully suppressed by fabricating the grating 6° tilted with respect to facets and adjusting the pump stripe normal to the grating. Continuous tuning of 30 nm around 3.1 μm with 320 mW single facet output power at 80K and a 1.6 nm FWHM is reported. The present device is designed in the 3- to 4-μm range which matches a low loss atmospheric transmission window, and covers an important region of molecular vibration spectra, in particular, the hydrocarbon C-H stretch at ~ 3.3 μm, making it suitable for atmospheric pressure remote gas sensing of industrially important small molecules such as methane, hydrogen chloride and ammonia.

A new method has been developed to fabricate chirped grating for type-II optically pumped tunable DFB lasers with a reduced longitudinal variation. Over 80 nm of continuous tuning has been achieved around 3.1 μm.

Citric acid/hydrogen peroxide at volume ratios from 0.2:1 to 20:1 was found to provide selective etching between , , , and various III–V semiconductor materials for use in new and based heterostructure transistors and optoelectronic devices. By choosing different concentration volume ratios of citric acid to hydrogen peroxide , highly selective as well as uniform (nonselective) etching regions were found to exist in these material systems. Etchant selectivities greater than 50 were found for most combinations of the III–V semiconductor materials under investigation, with selectivities of over 100 measured for and material combinations, and with selectivities of over 3850 calculated for to 13,650 for . The highest overall etch rates were measured for and the lowest etch rates were found for . The etch rate for the materials systematically decreased from the highest etch rate for the smallest Sb mole fraction examined of to the lowest etch rate for . The dramatic change in etch rate with citric acid/hydrogen peroxide volume ratio previously observed for the material system was also observed for , but this effect was not seen in higher Sb mole fraction alloys of examined. and were found to have very low etch rates with this etchant system at all volume ratios, making both materials suitable as an etch stop layer for simplified processing in device fabrication. Finally, citric acid/hydrogen peroxide can be used to preferentially etch these materials through a photoresist mask, since it does not erode photoresist at any volume ratio.