Solid state spectroscopy using an infrared free-electron laser

A free electron laser covering a wavelength range of nearly two spectral decades (3-300 µm or 1-100 THz) has been in operation at our research center for a few years. I will present a few examples of the research done with it, taking advantage if its high peak and average power and picosecond time structure.
I will present measurements of carrier relaxation in semiconductor quantum dots, where the relaxation time varies by three orders of magnitude (2 ps to 1.5 ns), if the energy level spacing is changed by only a factor of two (30 meV to 15 meV). This can be explained in the quantum dot polaron picture.
The high average power allows one to perform near-field microscopy experiments. We can image ferroelectric domains on BaTiO3 surfaces purely due to their optical contrast resulting from a slight anisotropy of the dielectric function. Sweeping the FEL wavelength across a phonon resonance results in a reversal of optical contrast, which demonstrates the resonant nature of the interaction. The spatial resolution is 150 nm, about 1/100 of the wavelength.
Finally I will discuss a nonlinear mixing experiment, where THz sidebands of 800 nm radiation are generated in a semiconductor quantum well system, and briefly describe first cyclotron resonance experiments using the FEL in the pulsed magnetic field.