Coherent probing of quantum cascade laser emission by terahertz time-domain spectroscopy

In this contribution we present unique time-domain spectroscopy measurements of THz quantum cascade lasers. By coupling broad-band THz pulses into the active zone of the laser the internal processes and dynamics are accessed. Currently these devices are measured by emission spectroscopy which can only access their parameters in terms of output power, wavelength and line width of the emission. By applying the coherent measurement scheme the laser's active zone can be directly inspected. This gives the urgently needed information about the real gain bandwidth and the internal dynamics of this type of laser devices.