From few-cycle THz pulses to terahertz quantum-cascade lasers

We present unique time-domain measurements of terahertz quantum cascade lasers. By coupling broadband terahertz pulses into the laser´s waveguide the losses and gain in the frequency range 0.5 - 3.5 THz are determined. Furthermore internal processes and the laser dynamics can be tracked directly in the time- domain. The performance of THz-QCLs fabricated as double metal resonators is studied as a function of the doping concentration. The double-metal waveguide concept allows the realization of micro-cavity THz lasers with extreme small mode volumes resulting in very low threshold currents.