Optical EngineeringHeat transport in terahertz quantum cascade lasers
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We review the basic microscopic phenomena controlling the extraction of energy from the active regions of terahertz quantum cascade lasers. Using time-resolved microprobe photoluminescence, we measure the in-plane (v//) and cross-plane (v) heat transfer velocities and find a strong anisotropy, with v// slightly reduced by 30% and v reduced by a factor of 6.5 with respect to the corresponding bulk values. Combining this information with measurements of the cross-plane thermal conductivity enables us to estimate a phonon mean free path value L ~ 70 nm at 80 K, reduced by about one order of magnitude with respect to the bulk value in GaAs, but much larger than the average spacing between the interfaces in the active region. The latter finding is consistent with a heat transport model considering both thermal boundary resistance and phonon dispersion modification.