Donald M. Gianardi

High power and high brightness from an optically pumped InAs/InGaSb type-II midinfrared laser with low confinement

We report on optically pumped semiconductor lasers emitting near 3.8 μm that exhibit high power and low output divergence. The lasers incorporate multiple InAs/InGaSb/InAs type-II wells imbedded in an InGaAsSb waveguide that is designed to absorb the pump emission. When operated at 85 K, 0.25 mm×2.5 mm broad area devices produce >5 W of peak power under long pulse conditions. Moreover, these extremely bright devices exhibit a fast axis divergence of only ∼15° full width at half maximum (FWHM), coupled with a slow axis divergence of ∼6° FWHM. The first is due to the reduced optical confinement in the transverse direction, while the latter is attributed to the suppression of filament formation, which is another beneficial consequence of the low optical confinement.

Spectral blueshift and improved luminescent properties with increasing GaSb layer thickness in InAs–GaSb type-II superlattices

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.

2 μm GaInAsSb/AlGaAsSb midinfrared laser grown digitally on GaSb by modulated-molecular beam epitaxy

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.

Absorbance spectroscopy and identification of valence subband transitions in type-II InAs/GaSb superlattices

We describe the molecular-beam epitaxy growth, as well as both the structural and optical characterization of a set of InAs/GaSb type-II strained-layer superlattice samples, in which the GaSb layer thickness is systematically increased. Absorbance spectroscopy measurements show well-defined features associated with transitions from the various valence subbands to the lowest conduction subband, and also a significant blueshift of the band edge when the GaSb layers thickness is increased. Empirical pseudopotential method calculations are shown to successfully predict the blueshift and help identify the higher-energy transitions.

High performance optically pumped antimonide lasers operating in the 2.4–9.3μm wavelength range

We provide an update on the further development of optically pumped semiconductor lasers based on the InAs∕InGaSb∕InAs type-II quantum wells. We show increased power generation, as well as the inherent flexibility to produce devices that can emit at any wavelength in the ∼2.4μm to ∼9.3μm range with consistently high photon-to-photon conversion rates.

High-temperature performance in ∼4 μm type-II quantum well lasers with increased strain

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<0.25, the holes are no longer confined in the W active re...

Optically pumped integrated absorber 3.4 μm laser with InAs-to-InGaAsSb type-II transition

We report optically pumped lasing at λ∼3.4 μm from an integrated absorber structure in which the electrons confined in the InAs quantum wells recombine with holes in adjacent InGaAsSb layers to provide the gain. This type-II laser exhibits an estimated photon-to-photon conversion rate of ∼24% at 85 K. The self-consistent empirical pseudopotential method calculations suggest that Coulomb attraction can lead to a strong enhancement in carrier overlap, and the resulting small shift in transition energy is consistent with that observed.

Linewidth analysis of the photoluminescence from InAs/GaSb/InAs/AlSb type-II superlattices

We present photoluminescent (PL) linewidth measurements on InAs:GaSb, type-II superlattices as a function of temperature and power. The observed PL linewidth for the samples, studied at 80 K, was 40–60 meV, which is significantly larger than a thermally broadened line of width 2 kT. The larger linewidth is well explained by a combination of homogeneous and inhomogeneous broadening. The data suggest that the inhomogeneous broadening is dominated by interface roughness and that the roughness amplitude at the InAs–GaSb interface is on the order of 1 ML. A significant fraction of the broadening can be accounted for by the presence of interfacial regions which show a 1 ML decrease in the GaSb layer thickness. To account for homogeneous broadening, a Lorentzian function of width δ (full width at half maximum) is employed to smooth and broaden the synthetic spectrum which is calculated from a simple model of the spontaneous emission rate. A δ=10 meV was found to give the best fit of the synthetic spectra to the ...

Optically pumped type-II antimonide mid-IR lasers with integrated absorber layers

We report on optically pumped mid-IR semiconductor lasers that are based on type-II wells. A systematic study of the effect of increasing the In-content in the InxGa1-xSb hole-well suggests that improved hole confinement results in improved power conversion efficiency at elevated temperatures that is also accompanied by a reduction in threshold power and a reduction in T0, the characteristics for threshold.

High-peak power from optically-pumped mid-IR semiconductor lasers

We present work on the high-peak power pumping of optically pumped mid-IR semiconductor lasers. The lasers incorporated 14 type-II InAs/InGaSb/InAs quantum wells (QW). Thick quaternary absorber layers (InGaAsSb) surrounded the QWs, which allowed a large fraction of pump light to be absorbed. The devices were optically pumped with the output of a passively Q-switched Ho:YAG laser at λ = 2.09 μm. The Ho:YAG maximum output power was ~90 kW; this allows the optically pumped semiconductor lasers (OPSLs) to be pumped at several thousand times above threshold. Emission from the QW was observed near 4.1 μm. As the pump power was increased, the QW spectra were observed to broaden and eventually saturate. Under low power pumping conditions, the OPSL pulse tracked the Ho:YAG pulse, which has a 16-ns full-width half-maximum. As the pump power was increased, the OPSL pulse duration increased and the pulse eventually split into two peaks. This may be due to a large increase in the free-carrier absorbance rates in the bulk-like quaternary absorber. Emission from the quaternary was observed near 2.2 μm and its intensity, with respect to the QW intensity, increased significantly as the pump power was increased. This indicates that at the higher pump powers, large fractions of photogenerated carriers are reservoired in the thick absorber layers. The maximum OPSL single-ended peak power was 490 W. This is the highest reported peak power from a mid-IR semiconductor laser. A rate equation model describing the time evolution of the carriers in the QW, the stored carrier population in the reservoir, and the photon population gives good agreement with the data.