W. W. Bewley

Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption

The interband cascade laser differs from any other class of semiconductor laser, conventional or cascaded, in that most of the carriers producing population inversion are generated internally, at semimetallic interfaces within each stage of the active region. Here we present simulations demonstrating that all previous interband cascade laser performance has suffered from a significant imbalance of electron and hole densities in the active wells. We further confirm experimentally that correcting this imbalance with relatively heavy n-type doping in the electron injectors substantially reduces the threshold current and power densities relative to all earlier devices. At room temperature, the redesigned devices require nearly two orders of magnitude less input power to operate in continuous-wave mode than the quantum cascade laser. The interband cascade laser is consequently the most attractive option for gas sensing and other spectroscopic applications requiring low output power and minimum heat dissipation at wavelengths extending from 3 μm to beyond 6 μm.

Continuous-wave operation of λ=3.25 μm broadened-waveguide W quantum-well diode lasers up to T=195 K

Mid-infrared (λ=3.25 μm) broadened-waveguide diode lasers with active regions consisting of 5 type-II “W” quantum wells operated in continuous-wave (cw) mode up to 195 K. At 78 K, the threshold current density was 63 A/cm2, and up to 140 mW of cw output power was generated. A second structure with ten quantum wells operated up to 310 K in pulsed mode.

Near-room-temperature mid-infrared interband cascade laser

A 25-stage interband cascade laser with a W active region and a third hole quantum well for the suppression of leakage current has exhibited lasing in pulsed mode up to 286 K. A peak output power of 160 mW/facet and a slope efficiency of 197 mW/A per facet (1.1 photons per injected electron) were measured at 196 K. Above 200 K, the characteristic temperature was higher (T0=53 K) and the threshold current densities lower than for a previously reported W interband cascade laser without the third hole quantum well.

Interband cascade laser emitting at λ=3.75μm in continuous wave above room temperature

We report a five-stage interband cascade laser that operates at λ=3.75μm in cw mode up to a maximum temperature of 319K. With gold electroplating, epitaxial-side-up mounting, and one facet coated for high reflectivity, a 3mm×9.2μm ridge emits over 10mW of cw power at 300K.

Auger coefficients in type-II InAs/Ga1−xInxSb quantum wells

Two different approaches, a photoconductive response technique and a correlation of lasing thresholds with theoretical threshold carrier concentrations have been used to determine Auger lifetimes in InAs/GaInSb quantum wells. For energy gaps corresponding to 3.1–4.8 μm, the room-temperature Auger coefficients for seven different samples are found to be nearly an order-of-magnitude lower than typical type-I results for the same wavelength. The data imply that at this temperature, the Auger rate is relatively insensitive to details of the band structure.

Interband cascade lasers

Recent advances in midwave infrared interband cascade lasers (ICLs) include novel distributed feedback configurations, vertical-cavity surface-emitting lasers, light-emitting devices, frequency combs, dual-comb spectroscopy, and ICLs incorporated into photonic integrated circuits on silicon and III-V platforms.

W-structured type-II superlattice long-wave infrared photodiodes with high quantum efficiency

Results are presented for an enhanced type-II W-structured superlattice (WSL) photodiode with an 11.3μm cutoff and 34% external quantum efficiency (at 8.6μm) operating at 80K. The new WSL design employs quaternary Al0.4Ga0.49In0.11Sb barrier layers to improve collection efficiency by increasing minority-carrier mobility. By fitting the quantum efficiencies of a series of p-i-n WSL photodiodes with background-doped i-region thicknesses varying from 1to4μm, the authors determine that the minority-carrier electron diffusion length is 3.5μm. The structures were grown on semitransparent n-GaSb substrates that contributed a 35%–55% gain in quantum efficiency from multiple internal reflections.

High-temperature continuous-wave 3–6.1 μm “W” lasers with diamond-pressure-bond heat sinking

Optically pumped type-II W lasers emitting in the mid-infrared exhibited continuous-wave (cw) operating temperatures of 290 K at λ=3.0 μm and 210 K at λ=6.1 μm. Maximum cw output powers for 78 K were 260 mW at λ=3.1 μm and nearly 50 mW at λ=5.4 μm. These high maximum temperatures were achieved through the use of a diamond-pressure-bonding technique for heat sinking the semiconductor lasers. The thermal bond, which is accomplished through pressure alone, permits topside optical pumping through the diamond at wavelengths that would be absorbed by the substrate.

Mid-infrared interband cascade lasers operating at ambient temperatures

We discuss the state-of-the-art performance of interband cascade lasers emitting in the 3?5??m spectral band. Broad-area devices with five active stages display pulsed threshold current densities as low as 400?A?cm?2 at room temperature. Auger decay rates are extracted from the analysis of threshold current densities and differential slope efficiencies of nearly 30 lasers, and found to be significantly lower than was anticipated based on prior information. New designs also produce ICLs with room-temperature internal losses as low as ?6?cm?1. The combination of these advances with improvements to the processing of narrow ridges has led to the fabrication of a 4.4-?m-wide ridge emitting at 3.7??m that lased to 335?K in continuous mode. This is the highest continuous-wave (cw) operating temperature for any semiconductor laser in the 3.0?4.6??m spectral range. A 10-?m-wide ridge with high-reflection and anti-reflection facet coatings produced up to 59?mW of cw power at 298?K, and displayed a maximum wall-plug efficiency of 3.4%.

Interband Cascade Lasers With Low Threshold Powers and High Output Powers

The midwave infrared interband cascade laser (ICL) can operate at threshold power densities 30 times lower than those of the quantum cascade laser. This is ultimately attributable to the much longer interband carrier lifetime, rather than to specifics of the cavity dimensions and mirror reflectivities. The ICL is therefore an attractive candidate for insertion into the portable, battery-powered chemical sensors now being developed for this spectral region. We review the characteristics of ICLs operating at wavelengths from 2.9 to 5.5 μm, and show that their Auger coefficients vary by less than a factor of 3 throughout this range. Consequently, the ICL performance degrades only modestly with increasing wavelength. We report that an epitaxial-side-down-mounted ICL ridge of width 30 μm and λ = 3.7 μm emits more than 300 mW of continuous wave (CW) output power at room temperature with M2 ≤ 3.1. A distributed-feedback ICL with a fourth-order grating etched into its corrugated sidewalls produces 55 mW of CW power in a single spectral mode at T = 25 °C.