For the first time, energy dispersive cyclotron resonance (CR) spectra were measured during a single magnetic field pulse by using a rapid-scanning, fiber-coupled terahertz (THz) time-domain spectroscopy (TDS) system. The experimental setup, actually compact and portable, is depicted in the figure below. Broadband ultrashort THz pulses were generated and detected by using a customized photoconductive emitter-detector setup pumped by a femto-second fiber-laser. The emitter and the detector, based on low-temperature GaAs, were kept far enough from the magnet coil. Then, free-space propagation of the THz signal was used inside the cryostat that includes a high resistivity silicon beam splitter and an off-axis parabolic mirror for focussing the beam on a p-type germanium sample that sits on a mirror.
In the upper plot of the right figure, magnetic fields at which spectra were measured are shown by dots as a function of time. The lower plot shows THz transmission spectra measured at 77K and obtained at the frequency of 150 Hz, actually the scanning rate of THz pulses by the delay line (Δt) in figure 1. Absorption is decoded in colour (the low frequency part is greyed out because of poor S/N). Strong spectral light and heavy holes CR absorption peaks are displayed on the down sweep of the field at frequencies that follow linearly the magnetic field (2π νc = eB/m*). The peaks provide the light and heavy holes cyclotron effective masses m* of 0.04m0 and 0.3 m0 expected for a crystal with magnetic field parallel to the100 axis. In addition, a weaker line related to quantum effects is observed. Further developments now in progress are aimed at increasing the scanning speed, the intensity and the frequency range of THz pulses. This work paves the way to routine terahertz time-domain-spectroscopy at low temperatures and magnetic fields supplied by 60 Tesla coils. This development results from a team work between the Fraunhofer Institute for Physical Measurements Techniques, Kaiserslautern, Germany (Daniel Molter and Rene Beigang), the Max Plank Institute of Quantum Optics, Garching, Germany (Fritz Keilmann), and the LNCMI-Toulouse (Sylvie George, Michel Goiran and Jean Léotin).