Continuous-Wave Laser Diodes Based on a Novel InGaAsNSb Material System

These devices could replace bulky, optically pumped solid-state light emitters and cryogenically cooled semiconductor lasers in security applications.

Laser sources operating in the spectral region from 2 to 3.5 μm are in demand for ultra-sensitive laser spectroscopy, medical diagnostics, home security, industrial process monitoring, infrared countermeasures, and optical wireless communications. Currently, solid-state lasers and optical parametric oscillators and amplifiers are used as coherent light sources in this spectral region.

Solid-state and parametric sources are being optically pumped by near-infrared diode lasers. This intermediate energy transfer step from the near-infrared pumping diode to the mid-infrared emitting device reduces power-conversion system efficiency. GaSb-based technology of high-power, room-temperature-operated mid-IR type-I QW diode lasers operating above 3 μm has been developed. Room-temperature CW operation was demonstrated for 3 to 3.4 μm diode lasers.

In GaInAsSb/AlGaAsSb quantum wells (QWs), the band offsets at the heterointerfaces are unevenly distributed between conduction and valence bands leading to excessive electron and deficient hole confinements. Since the electrons are strongly localized in deep conduction band QWs, the thermal redistribution of holes between the shallow valence band QWs and the optical waveguide layers creates Coulomb barriers, which can improve the hole confinement to some extent. Even in this case, however, the bulk heavy-hole states of the waveguide material with very high density of states (DOS) remain energetically close to the lasing states in the uppermost hole subband and, therefore, unfavorably affect the population of the lasing states. This situation can be improved by using the compressively strained QW layers, which deepens the heavy-hole QWs and, therefore, enhances the population of the upper (lasing) hole subband and increases the structure optical gain.

Compressive strain in active QWs improves the laser differential gain by balancing the DOS in the joint lasing sub-bands. Compressive strain splits the first heavy-hole (HH) and first light-hole (LH) subbands and reduces the band-edge heavy-hole DOS, which otherwise is unfavorably increased by HH-LH subband mixing. This mechanism of gain improvement, however, works well only for compressive strain level up to 1%. Strain values beyond that range have little additional effect on the band-edge heavy-hole DOS, since at such a high strain the HH and LH sub-bands are already well separated in energy.

Due to the inherently low valence band offsets at the quaternary GaInAsSb/ AlGaAsSb interfaces, the QW compressive strain manifests itself not only through the band-edge HH DOS reduction but, mainly, by increasing the effective barrier height for the quantum-confined HH states. Compressive strain moves the position of the QW HH states upwards thus making the heavy-hole QW deeper. Improved HH confinement, in turn, reduces the thermally activated hole redistribution between the QW subbands and the adjoining bulk barrier states and, therefore, increases the occupation of the uppermost hole subband states participating in the lasing transition. This ultimately enhances the laser differential gain and reduces the threshold current density. Enhancement of the optical gain in Sb-based lasers through the strain-induced increase of the HH confinement remains efficient for high compressive strains in the 1% to 2% range, while preserving the benefit of the balanced joint DOS achieved at the lower strain level.

Effect of the QW strain on the hole confinement becomes even more important in InGaAsSb/AlGaAsSb QW lasers when moving to longer emission wavelengths. In this case, more indium is needed in the QW composition, which correspondingly requires higher QW arsenic concentration to avoid excessive strain build-up. Higher arsenic content lowers the valence band position in the QW material and deteriorates the hole confinement. As a result, in QWs with indium concentration above 50%, which brings the laser emission above 3 μm, the QW compressive strain becomes completely responsible for the hole confinement in structures with 25% aluminum in the waveguide. As a result, to ensure the adequate hole confinement in the long-wavelength lasers, highly strained (up to 2%) QWs should be complemented by the waveguides with higher aluminum contents of about 35%.

Strain-induced deepening of the HH QW reduces thermal redistribution of the holes between QW and waveguide and, for the same total carrier concentration, increases the occupation of the upper heavy-hole sub-band states participating in lasing transition. The implication of the improved hole confinement for optical gain is straightforward — structure with better hole confinement demonstrates higher differential gain and lower threshold concentration.

Due to large conduction band offsets in the In(Al)GaAsSb system, the effect of thermal electron redistribution into the higher electron subbands and electron continuum was negligible. The resulting difference in electron and hole spatial distributions creates band bending, which to some extent improves the hole confinement in shallow hole QW of structure.

Devices designed with barrier layers consisting of the quatemary alloy A1GaAsSb and emitting at 3.1 μm demonstrated a continuous output power of 80 mW at room temperature.

Devices designed with quinternary AlInGaAsSb barrier layers produced a continuous 130 mW of 3 μm optical power at room temperature. Room temperatures pulsed power outputs of 750 mW at 3.1 micrometers and over 1W at 3.0 micrometers were observed.

This work was done by Gregory Belenky of the State University of New York at Stony Brook for the Air Force Office of Scientific Research.

AFRL-0101