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Frequency-domain electron spin resonance spectroscopy using continuously frequency-tunable terahertz photomixers

Frequency-domain electron spin resonance (FDESR) spectroscopy in the terahertz (THz) region using continuously tunable photomixers was demonstrated. Spectral resolution was greatly improved with the use of a pair of fiber stretchers. In this setup, the amplitude of the THz electric field was determined at each frequency by externally sweeping the optical path difference, resulting in a spectral resolution of about 1 MHz. With this technique, we observed narrow ESR spectra with a 20-MHz linewidth, enabling high-resolution FDESR spectroscopy in a broad frequency range.