High-performance InAs-based interband cascade lasers

Currently, there are only two types of mid-infrared lasers that are capable of continuous-wave (CW) operation above room temperature: quantum cascade (QC) lasers and interband cascade (IC) lasers. Both of them share the cascade feature for carrier recycling. The most successful QC lasers, based on the inter-subband transition and the well-established InGaAs/InAlAs/InP material system, are able to deliver several watts of optical power. In contrast, IC lasers, based on the interband transition and the unique InAs/GaSb/AlSb type-II broken-bandgap material system, have the threshold power density more than an order of magnitude lower than that of QC lasers (e.g., 0.3 kW/cm2 vs. 11 kW/cm2). As a result, IC lasers become a better solution for low-power applications in the mid-infrared region. \nGaSb-based IC lasers have achieved the best performance around 3.7 μm with a threshold current density as low as 100 A/cm2 at 300 K. However, their waveguide cladding layers, consisting of thick InAs/AlSb superlattice, have a low thermal conductivity and are challenging to grow by molecular beam epitaxy. These problems become more severe at longer lasing wavelengths due to the requirement of thicker cladding layers. InAs-based IC lasers, utilizing highly doped InAs as the optical cladding layer, have been developed to address these issues. The goal of this dissertation is to use modeling and experiments to explore several aspects of InAs-based IC lasers, including far-field patterns, high-temperature operation, long-wavelength operation, wide-tunability, and single frequency mode operation. \nThe beam quality is critical for the laser application. The higher-order spatial modes naturally appear when the laser ridge is wider than the lasing wavelength in the medium. For InAs-based IC lasers with a thin top cladding layer, the top contact configuration can have a major influence on the spatial modes, which are observed in the measurement of far-field patterns. The physical origin is identified by waveguide modeling based on an effective index method. \nRadical design innovations, including “shortened injector” and “carrier rebalancing,” have significantly improved the performance of both GaSb-based and InAs-based IC lasers. Furthermore, a hybrid waveguide, consisting of an inner cladding layer with InAs/AlSb superlattice and an outer cladding layer with highly doped InAs, has significantly increased the modal gain of InAs-based IC lasers. As a result, CW operations above room temperature have been achieved at wavelengths of 4.6~4.8 μm. The threshold current density, 247 A/cm2 at 300 K in pulsed mode, is the lowest ever reported among the mid-infrared semiconductor lasers at similar wavelengths. The pulsed operating temperature is as high as 377 K. \nLong-wavelength operations are vigorously explored. With the hybrid waveguide mentioned above, the lasing temperature reaches 324 K at a wavelength of 6.4 μm. Further design improvement and optimization are presented. In addition, the lasing wavelength is extended to 11.2 μm at 130 K. Several things are found to hinder the progress. The waveguide loss is dramatically increased, mainly because the lasing wavelength approaches the plasmon wavelength of the heavily-doped InAs. The negative differential resistance is observed and may be related to the unexpected high threshold. \nA wide tuning range is highly desirable for many applications such as spectroscopy and biochemical analysis. A repeatable, large electrical tunable range of 180 cm-1 (or 900 nm in wavelength near λ~7 μm) is achieved by a novel active region consisting of three InAs quantum wells. This challenges the conventional idea that the carrier density pinning at the threshold level would not allow a significant tuning by Stark effect. The gain analysis, based on the calculation of the field dependent wavefunction overlap, well explains the physical mechanism. This strategy is very useful for the design of tunable lasers. \nFor sensitive detection of important gas molecules such as carbonyl sulfide/COS (4.5 μm), single-mode distributed feedback IC lasers are highly desirable for tunable laser absorption spectroscopy. A grating is patterned using interference lithography to etch through the thin top cladding into the top spacer layer of the IC laser structure. Single-mode emission with a side mode suppression ratio of 30 dB is obtained in continuous wave operation at temperatures up to 180 K near 4.5 μm. A total tuning range of 16 nm is achieved for a single device, with a temperature-tuning rate of 0.4 nm/K and a current-tuning rate of 0.016 nm/mA. The impact of the grating on device performance is evaluated and discussed in comparison with Fabry–Perot lasers.