Terahertz free-electron lasers and their applications for the spectroscopy of solids

In free-electron lasers (FEL) optical gain and stimulated emission are provided by relativistic electrons, wiggling through a periodic magnetic-field arrangement. Since the emitted wavelength only depends on the energy of the electrons and the magnetic field strength, an in principle arbitrary lasing wavelength may be achieved. Thus FELs are mostly used in spectral ranges, where conventional lasers with reasonably high power hardly exist. This is true, among others, for the long-wavelength infrared and terahertz (THz) range and represents the motivation to build such bulky THz sources.

Here I will discuss the operation principle of an FEL and then describe the THz FEL at FZD in Dresden. This FEL, called FELBE, was first operated in 2004 and covers now a wavelength range of nearly two spectral decades (3-300 µm or 1-100 THz). The key feature which distinguishes FELBE from other FEL user facilities is the possibility of delivering a continuous train of (micro)pulses, made possible by the superconducting accelerator cavities. The FEL thus provides picosecond optical pulses at a repetition rate of 13 MHz.

In order to pinpoint interesting scientific experiments using the FEL, one has to focus on its unique features, which are high peak power (up to 1 MW), high average power (tens of Watts), and short pulses (1-20 ps, depending on the wavelength). These features lend themselves to nonlinear optical experiments, near-field microscopy, and time resolved pump-probe studies, respectively. I will present experiments belonging to each of these three classes, namely

(1) pumping the intra-excitonic 1s-2p transition in semiconductor quantum wells, which leads to THz sideband generation and the observation of the AC Stark effect;
(2) near-field micro-spectroscopy of ferroelectrics, which allows imaging of ferroelectric domains in BaTiO3 purely due to their optical contrast resulting from a slight anisotropy of the dielectric function;
(3) carrier relaxation in semiconductor quantum dots. Here 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).