Michael L. Tilton

Pseudopotential methods for superlattices: Applications to mid-infrared semiconductor lasers

Many mid-infrared semiconductor laser sources are now being developed with superlattice active regions. Calculations of gain, index of refraction, and intervalence subband absorption for these laser materials require accurate subband energies, wave functions, and radiative matrix elements. We have recently begun using a solution method based on the empirical pseudopotential method (EPM). This method shows particular strength in analyzing structures with short periods or thin layers, for which the standard method, based on k⋅p perturbation theory and the envelope function approximation, may be problematical. We will describe the EPM applied to bulk solids and then demonstrate our direct generalization of the method for applications to superlattice structures. Calculations for recently developed mid-infrared semiconductor lasers using type-II superlattice active regions will be used to illustrate the method.

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.

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.

Modeling broad-area semiconductor optical amplifiers

The authors report on the wave optics modeling of broad-area semiconductor amplifiers used in a double-pass reflective configuration. The results show excellent agreement with recent reports of 2.5 and 12 W of nearly diffraction-limited output from such systems. Several design issues that must be considered in a practical laser communication system based on this class of amplifier are discussed. >

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...

Modeling multiple-longitudinal-mode dynamics in semiconductor lasers

We present a time-dependent coupled-wave equation analysis for simulating both single- and multilongitudinal-mode dynamics in semiconductor lasers, while incorporating frequency-dependent gain in a particularly simple way. In our first application, we analyze the response of Fabry-Perot semiconductor lasers to feedback levels ranging from -60 to -30 dB. This allows us to anchor, continuously, from single-mode phenomena at weak feedback levels to multimode behavior at stronger feedback levels. To our knowledge, this is the first simulation of multilongitudinal-mode dynamics that are induced in a laser at feedback levels exceeding -30 dB, Feedback-induced phenomena provide a significant test of both the single-mode and multimode capabilities of our coupled-wave model. In our second application, we analyze a modern, large-scale device for which multiple longitudinal modes, as well as compound cavity modes, are a significant possibility. Specifically, we consider facet-coating requirements for a new class of monolithic master-oscillator power-amplifier (MOPA) devices.

Performance comparison of optically pumped type-II midinfrared lasers

We report a comparative study on the performance of three optically pumped, type-II quantum well lasers with differing quantum well (QW) confinement. One of the active regions emphasized hole confinement, another emphasized electron confinement, while the third incorporated both electron and hole confinements. In all cases the wells were inserted in a thick InxGa1−xAsySb1−y waveguide/absorber region. The lasing wavelengths at 84K were 2.26, 3.44, and 2.37μm, respectively. The maximum peak output powers and differential quantum efficiencies η at 84K were similar for the hole well and W lasers (≈13W,η≈0.55), but significantly reduced in the electron-well-only laser (2.3W,η=0.14). Waveguide loss measurements via the traditional quantum efficiency versus cavity length method and by a Hakki-Paoli method revealed that all three lasers had low waveguide loss that either increased slowly or not at all with increasing temperature. However, the laser’s internal efficiency, ηi, showed a linear decline with increasin...

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 ...

Gain and loss in an optically pumped mid-infrared laser

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...

Calculation of prolate functions for optical analysis

The linear and circular prolate functions are basis sets for the bare cavity eigenmodes of stable and unstable laser resonators having rectangular or circular symmetry. Early calculations of these functions used power series expansions. A matrix technique which was first used by Sanderson and Streifer is used here to demonstrate rapid and accurate numerical calculations of prolate functions.