Appl. Phys. Lett. 110, 094106 (2017)

Terahertz magneto-spectroscopy has been performed with light intensities far beyond that of conventional light sources by making use of the FLARE free-electron laser in The Netherlands and strong 33 T Bitter magnets. Mykhaylo Ozerov and co-workers from Radboud University tested the performance of the spectrometer by performing electron spin resonance (ESR) measurements of polycrystalline 2,2-diphenyl-1-picrylhydrazyl (DPPH), a well-known paramagnetic material. Terahertz pulses from FLARE with an average power of 100 mW were focused to a nearly diffraction limited Gaussian spot, resulting in THz electric and magnetic fields with strengths of 7–70 kV cm−1 and 2–20 mT, respectively. The photon energy of the THz pulse from FLARE was tunable from 0.3 to 3 THz in frequency. The ESR spectra of the DPPH sample were captured with sufficiently high resolution to reveal a 3 GHz fine structure. The spectral resolution Δν/ν is better than 0.1%, which is an order of magnitude higher than typical values for a RF-linac-based free-electron laser.