Development of high-performance InAs quantum cascade lasers

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In order to develop low-threshold current density, high-power and high-temperature quantum-cascade lasers (QCLs), we fabricated InAs-based QCLs by molecular beam epitaxy and investigated laser characteristics, especially their relationship with the band structure of an active region. The summary of the research results is as follows:1、Low-threshold current density operationWe demonstrated low-threshold current density (0.4 kA/cm^2) operation at liquid-nitrogen temperature by employing InAs/Al_<0.2>Ga_<0.8>Sb superlattice active layers. The emission wavelength was 10 μm, which was in close agreement with the designed wavelength. This is among the lowest threshold current density of QCLs operating in the mid-infrared spectrum region.2、Room-temperature operationWe demonstrated the room-temperature operation of InAs-based QCLs emitting at 8.9 μm. In order to decrease thermal carrier leakage from the active layer, a diagonal intersubband transition was used. The observed threshold current density at 300 K was 12.0 kA/cm^2 and maximum operation temperature was 305 K.3.High-power operationIn order to increase the dynamic range of the operation current, a scheme to increase the doping concentration in the injection layers was employed. The observed room-temperature threshold current density was 4.0 kA/cm^2, which is the lowest threshold current density in InAs-based QCLs. We demonstrated the output optical peak power above 1 W at liquid-nitrogen temperature and 125 mW at room-temperature.4.High-temperature operationStructure dependence on optical gain coefficient and its influence on the maximum operation temperature were investigated. We found that the active layer which exhibits the maximum optical gain coefficient presents the highest operation temperature. The observed maximum operation temperature was +100 ℃, which is the highest operation temperature in InAs-based QCLs.