Time of flight measurements of the nanoscale heat transfer dynamic in terahertz quantum cascade lasers

In this work we report a unique experimental approach for high spatial resolution, time resolved measurements of the transverse and longitudinal heat transfer velocities in the active region and in the substrate of GaAs/AlGaAs THz QC devices. The experimental method is based on micro-probe band-to-band photoluminescence (PL) experiments carried out on QCLs operated in pulsed mode with pulse widths in the range PW = 0.3-2 mus, and a constant frequency PF = 40 KHz. A CCD gate of 30 ns was opened at regular time steps of 30 ns, both before, during and after the voltage pulse applied to the QCL. All the presented results will allow to predict the technological solutions to extend THz QCLs operation at the temperatures of actual interest for a progressively wider range of applications. To address this topic we will discuss: i) the effect induced by thinner active regions of the heating and cooling dynamics; ii) the role of the thermal boundary resistance and iii) the advantages of the double metal optical waveguide.