S. A. Anson

Auger recombination in narrow-gap semiconductor superlattices incorporating antimony

A comparison is performed between measured and calculated Auger recombination rates for four different narrow-gap superlattices based on the InAs/GaSb/AlSb material system. The structures are designed for optical or electrical injection for mid-infrared laser applications, with wavelengths ranging from 3.4 to 4.1 μm. The electronic band structures are computed employing an accurate 14-band restricted basis set (superlattice K⋅p) methodology that utilizes experimental information about the low-energy electronic structure of the bulk constituents. The superlattice band structures and their associated matrix elements are directly employed to compute Auger recombination rates. Varying amounts of Auger recombination suppression are displayed by the various superlattices as compared to bulk mid-infrared systems. The greatest disagreement between theory and experiment is shown for the structure predicted to have the most Auger suppression, suggesting the suppression is sensitive either to theoretical or growth u...

Theoretical performance of mid-infrared broken-gap multilayer superlattice lasers

We present calculations of the differential gain and threshold current densities for a 3.7 μm multiple quantum well structure consisting of a “well” composed of several periods of an InAs/InGaSb superlattice alternating with a quinternary alloy “barrier.” We find serious limitations to the optical properties of active regions composed of these multiple quantum wells, and propose a four-layer superlattice structure which corrects these problems.

Carrier recombination dynamics in a (GaInSb/InAs)/AlGaSb superlattice multiple quantum well

We have used the 830 nm, subpicosecond output of a mode‐locked Ti:sapphire laser, together with subpicosecond 3.55 μm pulses from a synchronously pumped optical parametric oscillator, to perform room‐temperature, time‐resolved, differential transmission measurements on a multiple quantum well structure with AlGaSb barriers and GaInSb/InAs superlattice wells. From these measurements, we have determined a Shockley–Read–Hall rate of 2.4×108 s−1 and an Auger coefficient of 7×10−27 cm6/s. In addition, we estimate the carrier capture efficiency into the wells to be ∼52% and have demonstrated that carrier cooling, cross‐well transport, and capture are complete within ∼10 ps after excitation.

III-V interband 5.2 μm laser operating at 185 K

We report the operation of a III-V interband laser at a wavelength beyond 5 μm and temperatures above 90 K. The active region consists of a strain compensated broken gap four layer superlattice of InAs/Ga0.6In0.4Sb/InAs/Al0.3Ga0.42In0.28As0.5Sb0.5 grown by molecular beam epitaxy. The maximum operating temperature under 2.01 μm pulsed optical excitation was 185 K at a wavelength of 5.2 μm. The peak pump intensity at the 80 K threshold was 62 kW/cm2, and the characteristic temperature (T0) of the threshold intensity was 37 K. This T0 is comparable to the best observed values for 3–4.5 μm lasers based on the InAs/GaInSb material system.

Differential gain, differential index, and linewidth enhancement factor for a 4 μm superlattice laser active layer

We describe temporally and spectrally resolved measurements of the material differential gain, differential refractive index, and linewidth enhancement factor for a multilayer superlattice intended for use in midwave-infrared semiconductor lasers. We find good agreement between measured quantities and theoretical predictions based on a superlattice K⋅p formalism. The superlattice was designed for suppression of Auger recombination and intersubband absorption, and we find that the strategies employed in this process result in other characteristics that are desirable in a semiconductor laser gain medium. Specifically, for carrier densities and wavelengths appropriate to threshold in an optimized cavity configuration, this structure has a differential gain of approximately 1.5×10−15 cm2, a value comparable to that reported for near-infrared strained quantum wells. The peak gain and peak differential gain are nearly spectrally coincident, leading to a small value for the differential index. The large differen...

Generation of 3-4- mu m femtosecond pulses from a synchronously pumped, critically phase-matched KTiOPO4 optical parametric oscillator.

The operation of a Ti:sapphire-pumped, femtosecond optical parametric oscillator (OPO) based on the nonlinear material KTiOPO4 is described. By empirically optimizing the parametric interaction geometry, we have extended the maximum idler wavelength beyond that reported for similar systems. The OPO typically produces 175-fs idler and signal pulses tunable in the ranges of 2.9–3.96 and 1.05–1.16 μm, respectively.

Optimization of active regions in midinfrared lasers

The ideal performance of bulk, quantum well, and superlattice active regions for III–V interband midinfrared lasers are compared according to the maximum net gain per unit current density. Based on this figure of merit, which is appropriate for high-power as well as near-threshold operation, InAsSb quantum well active regions should have an order of magnitude lower threshold current than bulk InAs at room temperature. Optimized four-layer superlattices based on the InAs/GaInSb material system, however, should have two to ten times lower threshold currents than the quantum well active regions. Optimal thicknesses for these active regions were evaluated assuming a separate confinement region design. For the four-layer superlattices the optimal thickness is substantially thinner than has been commonly grown: 3 periods rather than 40 periods.

Experimental and theoretical density-dependent absorption spectra in (GaInSb/InAs)/AlGaSb superlattice multiple quantum wells

Carrier density-dependent absorption ~or gain! spectra, along with carrier lifetimes, are essential material properties for diode laser performance. The accuracy of calculated absorption spectra depend on both the accuracy of the computed band structure and the model used for the density dependence of the absorption. If the results are to be used to optimize device performance, it is crucial that such computed quantities be verified through comparison with measured results. Here, we directly measure the densitydependent absorption spectra using time-resolved differential transmission with a broadly tunable, midinfrared, optical parametric oscillator 8 and compare the results with a bandfilling model for the absorption based on a Knp calculation of the band structure. 9 We find that the measured and calculated spectra are in good agreement over a wide range of carrier densities including those expected during diode laser operation, and over a wide spectral range that includes contributions from higher conduction subbands. These results indicate that the calculated band structure, together with the band filling model, are sufficiently accurate to provide the basis for optimizing these structures for diode laser active regions. The structure under investigation was grown by molecular beam epitaxy on an undoped GaSb substrate and contains ten 225 A quantum wells separated by 400 A Al 0.2Ga0.8Sb barriers. Each well @shown in Fig. 1~a!# is composed of a short segment of a type II ~misaligned! GaInSb/InAs superlattice consisting of five 33 A Ga 0.75In0.25Sb layers and four

Comparison of mid-infrared laser diode active regions

The development of mid-infrared interband diode lasers has been hindered by factors such as Auger recombination and intervalence band absorption, which become increasingly important at longer wavelengths. A number of structures have been proposed in which the effects of these processes are reduced. The maximum gain per unit volumetric current density can be used as a figure of merit for comparing different active region materials. Using this figure of merit, we compare a series of structures with band gaps near 0.3 eV (i.e., wavelengths near 4 microns). The figure of merit is obtained from gain spectra calculated using superlattice K(DOT)p theory and a combination of calculated and measured recombination rates. We show that devices based on active regions incorporating type-I InAsSb/AlInAsSb or InAsSb/InAsP quantum wells should have room temperature threshold currents 7 - 13 times smaller than those of devices based on bulk InAs. However, devices using type-II superlattice active regions should have room temperature threshold currents that are a factor of 3 - 4 times smaller than those of the type-I quantum wells. The figure of merit can also be used to determine the optimal thickness of the active region as a function of waveguide loss and optical mode width.

Transient grating measurement of in-plane ambipolar diffusion in a 4-micron-band-gap, four-layer superlattlce

Summary form only given. We describe time-resolved transient grating measurements of in-plane ambipolar transport in a narrowband-gap superlattice that has a period comprised of four layers: InAs/GaInSb/InAs/AlGaInAsSb. The structure was designed as an active region for mid-infrared semiconductor lasers. Two 140-fs, 840-nm pulses from a mode-locked Ti:sapphire laser were interfered in the superlattice to create the transient grating, which was subsequently interrogated in a time-resolved manner by diffraction of a weak, mid-infrared, 160-fs probe pulse. The diffraction efficiency was measured at 300 K as a function of time-delay, grating period, and carrier density.