Time-resolved terahertz spectroscopy and imaging at the FELBE free-electron laser facility

Free-electron lasers are the only high-power, continuously tunable radiation sources in the terahertz range. Here we highlight some outstanding experiments performed at the FELBE free-electron laser at the Forschungszentrum Dresden-Rossendorf and discuss the influence of the beam parameters on the feasibility of these studies. FELBE is driven by a superconducting accelerator operating at a repetition rate of 13 MHz. The laser can be tuned in an extremely wide spectral range from 1.2 THz to 75 THz. The pulse duration is 1 – 25 ps, the pulse energy 10 nJ – 2 µJ, depending on the wavelength. Correspondingly, the average power can be up to several Watts. Most experiments at FELBE are studies in the field of condensed matter physics, with a special focus on semiconductor quantum structures. Here we present two time-resolved terahertz experiments, a study of two-photon currents in GaAs/AlGaAs quantum-well infrared photodetectors (QWIP) [1] and a pump-probe study of the relaxation dynamics in self-organized InGaAs quantum dots [2]. In a two-photon QWIP operated at 7.1 THz a quadratic dependence of the photocurrent on the THz intensity is observed over 3 orders of magnitude. Quadratic photocurrent signals were observed already at pulse energies of only few pJ. Fringe resolved autocorrelation traces were measured. An analysis showed that their duration is determined by the duration of the FELBE pulses rather than by intrinsic time constants. For the quantum dots a change of the relaxation time by 3 orders of magnitude was observed when the photon energy of the FELBE pulses was varied only by a factor of two (1.5 ns for 3.4 THz, 2 ps for 7 THz, respectively). All photon energies were below the longitudinal optical phonon energy in GaAs, hence the fast relaxation via those phonons was suppressed. The relaxation dynamics in these experiments is governed by polarons which decay into acoustic phonons. A theoretical description taking into account all possible acoustic phonon decay channels provides an excellent agreement with the measured relaxation times.
THz imaging in reflection geometry was demonstrated in an experiment, where an oil painting was scanned through the attenuated focused FELBE beam. The result is compared with earlier THz images, where broadband radiation from photoconductive antennas was applied. The higher spatial resolution and the improved contrast of the image can be explained by the higher frequency (3.4 THz) of FELBE compared to the spectrum of the THz antenna (below 1 THz). The wavelength was chosen in order to maximize the reflection from lead white pigments [3]. In general the spectral range from 1 – 15 THz seems more appropriate for art studies, since many pigments have spectral fingerprints in this region, while their transmission and reflection spectra are basically flat in the range below 1 THz [3].

References:

[1] H. Schneider, H.C. Liu, S. Winnerl , C.Y. Song, M. Walther and M. Helm, Opt. Express, 17, 12279 (2009).
[2] E.A. Zibik, T. Grange, B.A. Carpenter, N.E. Porter, R. Ferreira, G. Bastard, D. Stehr, S. Winnerl, M. Helm, H.Y. Liu, M. S. Skolnick, and L. R. Wilson, Nature Mat., 8, 803 (2009).
[3] K. Fukunaga, Y. Ogawa, Sh. Hayashi, and I. Hosako, IEICE Electronics Express 4, 258 (2007).