High-power narrow spectrum GaSb-based DBR lasers emitting near 2.1 µm.

Stable high-power narrow-linewidth operation of the 2.05-2.1 µm GaSb-based diode lasers was achieved by utilizing the sixth-order surface-etched distributed Bragg reflector (DBR) mirrors. The DBR multimode devices with 100 µm wide ridge waveguides generated ∼850mW in the continuous wave (CW) regime at 20°C. The device CW output power was limited by thermal rollover. The laser emission spectrum was defined by Bragg reflector reflectivity at all operating currents in a wide temperature range. The devices operated at DBR line with detuning from gain peak exceeding 10 meV.

Engineering Dirac Materials: Metamorphic InAs1-xSbx/InAs1-ySby Superlattices with Ultralow Bandgap.

Quasiparticles with Dirac-type dispersion can be observed in nearly gapless bulk semiconductors alloys in which the bandgap is controlled through the material composition. We demonstrate that the Dirac dispersion can be realized in short-period InAs1-xSbx/InAs1-ySby metamorphic superlattices with the bandgap tuned to zero by adjusting the superlattice period and layer strain. The new material has anisotropic carrier dispersion: the carrier energy associated with the in-plane motion is proportional to the wave vector and characterized by the Fermi velocity vF, and the dispersion corresponding to the motion in the growth direction is quadratic. Experimental estimate of the Fermi velocity gives vF = 6.7 × 105 m/s. Remarkably, the Fermi velocity in this system can be controlled by varying the overlap between electron and hole states in the superlattice. Extreme design flexibility makes the short-period metamorphic InAs1-xSbx/InAs1-ySby superlattice a new prospective platform for studying the effects of charge-carrier chirality and topologically nontrivial states in structures with the inverted bandgaps.

External cavity type-I quantum well cascade diode lasers with a tuning range of 440 nm near 3 µm.

A three-stage cascade GaSb-based diode laser heterostructure with an enhanced optical gain spectral bandwidth was designed and fabricated. The gain broadening was achieved by varying the thickness of the type-I quantum wells in different stages of the cascade active region from 10 to 14 nm. The structures were processed into bent ridge gain chips with virtually eliminated feedback from the anti-reflection-coated angled facet. The external cavity devices based on a novel gain chip design demonstrated a record wide tuning range from 2.79 to 3.23 µm in a Littrow cavity configuration at 20°C.

Laterally coupled distributed feedback type-I quantum well cascade diode lasers emitting near 3.22 µm.

The laterally coupled distributed feedback (LC-DFB) GaSb-based type-I quantum well cascade diode lasers using the second- and the sixth-order gratings to stabilize the output spectrum near 3.22 µm were designed and fabricated. The laser heterostructure contained three cascades. The devices were manufactured using a single dry etching step defining the ∼5-µm-wide ridge with ∼5-µm-wide gratings sections adjacent to the ridge sides. The grating coupling coefficients were estimated to be about 1 cm-1. The stability of the single-frequency operation was ensured by alignment of the DFB mode to the relatively wide gain peak. The 2-mm-long second-order LC-DFB lasers generated above 10 mW of continuous-wave (CW) output power at 20°C in epi-side-up configuration and demonstrated power conversion efficiency above 2%. The sixth-order LC-DFB lasers showed lower efficiency but still generated several milliwatts of CW output power. The devices demonstrated a CW current tuning range of about 3.5 nm at the temperature of 20°C.

Materials design parameters for infrared device applications based on III-V semiconductors.

The collaborative development of infrared detector materials by the Army Research Laboratory and Stony Brook University has led to new fundamental understandings of materials, as well as new levels of control and flexibility in III-V semiconductor crystal growth by molecular beam epitaxy. Early work on mid-wave strained layer superlattice (SLS) cameras led to a subsequent focus on minority carrier lifetime studies, which resulted in the proposal of the Ga-free SLS on GaSb substrates. The later demonstration of virtual substrate technology allowed the lattice constant to become a design parameter and enabled growth of undistorted bulk InAsSb. When grown in that manner, InAsSb has a bandgap bowing parameter large enough to cover absorption wavelengths across the entire long-wavelength band (8-12 µm). Even longer wavelengths are achieved with a general Ga-free SLS approach, with a virtual substrate having a lattice constant significantly larger than that of GaSb and with InAsSb in both bi-layers in the period. Since these layers can also be made very thin, the general Ga-free SLS does not suffer from the relatively low optical absorption and poor hole transport, which is characteristic of the special Ga-free SLS on GaSb for long-wavelength designs. Finally, the general Ga-free InAsSb SLS provides a method to induce and control sustained atomic ordering, which is yet another new design parameter.