The FEL at ELBE has several advantages for time-resolved and nonlinear spectroscopy. The wavelength range of FELBE (5-250 µm) corresponds to the energy (5-250 meV) of many characteristic excitations in solids, semiconductors, and semiconductor quantum structures, i.e. phonons, plasmons, polarons, polaritons, magnons, impurity binding energies, and intersubband transitions in quantum wells, wires and dots. Thus FELBE can be employed for time-resolved studies of their relaxation dynamics. For this purpose, the FELBE beam is ideally suited since quasi-cw operation at 13 MHz repetition rate enables efficient experiments yielding high S/N ratio at short measurement time. In addition, the high peak power can also induce nonlinear processes and permits active modification of the energy spectrum and the dynamics.

A powerful spectroscopic method is the pump-probe technique, where one short pulse is used for excitation, and a second, time-delayed pulse for probing the changes induced by the first one. In a general situation, the two pulses can have different wavelengths (colors). In particular, one of the pulses can be derived from a modelocked laser synchronized to the FEL.

Research topics include

• Carrier dynamics in III-V semiconductor quantum wells, nanowires, quantum dots (e.g. inter- and intra-subband/miniband relaxation), as well as 2D semiconductor materials like graphene and monolayer transition metal dichalcogenides

• Infrared and THz optical properties of semiconductor nanostructures in the far field and in the near field

• Terahertz devices: THz detector, THz plasma FET, THz emitter, THz quantum cascade laser, etc.

• Resonant coherent effects: High-intensity laser pulses closely resonant with the intersubband energy or with intra-excitonic transitions can induce coherent phenomena such as dressed states and Autler-Townes splittings, which are spectral signatures of Rabi oscillations

• Coherent control: Ultrashort IR pulses with controlled phases can actively influence the electron dynamics on short time scales

• Nonlinear optics in the THz frequency range