Terahertz time domain spectroscopy (THz TDS) is a powerful spectroscopic technique that allows the time-resolved measurement of light-matter interaction with broadband and powerful THz pulses. Contrary to other spectroscopies, amplitude and phase information are directly retrieved in a single scan, making THz TDS a powerful tool for studying absorption and gain dynamics in semiconductor heterostructures. The time information allows further to retrieve depth information as is used in time-of-flight tomographic imaging.


THz TDS allows us to study and characterize intersubband dynamics of quantum cascade lasers with optical transitions in the THz frequency range (THz QCLs). These lasers are compact sources of coherent THz radiation and consist of a semiconductor gain medium confined in a plasmonic waveguide. The main motivation for our experiments is to get information about internal parameters, such as gain and loss dynamics, all with the aim of improving the performance of THz QCLs.
(D. Bachmann, J. Darmo)

By scaling the incident THz field amplitudes to several tens of kV/cm, the pulses can be used to actively manipulate and control the intersubband excitations, giving rise to non-equilibrium states of matter. From the dynamics following the ultrafast excitation pulse, considerable insight into the microscopic properties of the sample can be gained. We use various techniques to generate these intense, broadband THz pulses, such as four-wave mixing in plasma filaments, optical rectification in nonlinear crystals and large area photoconductive switches. In conjunction with THz metamaterials, the available field strength can reach values in the MV/cm regime, paving the road to THz spectroscopy under extreme conditions.
(D. Dietze, J. Darmo)