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The thermal dynamic behaviors of terahertz quantum cascade lasers are investigated using a spectrally matching terahertz quantum-well photodetector. The THz output power of a lasing device under a pulse excitation is found to decrease with time, which is attributed to the increase of active-region temperature. The measurements show that the duration of output lasing pulse ranges from a few microseconds up to 77 μs, depending on device bias and heat-sink temperature. Atheoretical analysis based on a two-dimensional heat diffusion model is presented. The model calculates lasing quenching time based on an "average active-region temperature" criterion as well as a "reserve of gain" criterion. The best fit for quenching time is found when the vertical thermal conductivity in the quantum well region is two orders of magnitude lower than that of substrate. The measured lasing quenching time is in very good agreement with theoretical modeling calculated using both criteria. [ABSTRACT FROM AUTHOR]

This paper investigates the heat conduction behavior of a terahertz (THz) quantum-cascade laser (QCL) active region by measuring its temperature using in-situ microprobe band-to-band photoluminescence (PL) technique. The heat resistance of different regions in QCL structure is derived from the temperature measurement. Experimental results show that thinning the substrate from 300 μm thick to 140 μm lowers thermal resistance of the device by 21%, which helps achieving continuous-wave operation. A thermodynamic differential equation was numerically solved and the temperature profiles and thermal behavior of various regions within actively biased QCL devices under various conditions were obtained. The simulation confirms the measured results that the substrate accounts for 59% of the total device thermal resistance. [ABSTRACT FROM AUTHOR]

Generation of terahertz (THz), and sub-THz, coherent waves has been demonstrated at room temperature in GaAs-AIGaAs asymmetric quantum wells by mixing two CO2 laser beams. Two overlapping regimes were studied: the double resonance regime where three states are involved and the optical rectification which relies on two states only. The measured conversion efficiency of ~ 10-7 W-1 is in reasonable agreement with theoretical predictions. In addition, this technique of THz generation provides a model system to study the emission profile from an ensemble of radiating dipoles. For instance, backward emission and strong diffraction effects have been observed. [ABSTRACT FROM AUTHOR]