Microwave frequency modulation in continuous-wave far-infrared ESR utilizing a quasioptical reflection bridge

We report the development of the frequency-modulation (FM) method for measuring electron spin resonance (ESR) absorption in the 210-420 GHz frequency range. We demonstrate that using a high-frequency ESR spectrometer without resonating microwave components enables us to overcome technical difficulties associated with the FM method due to nonlinear microwave-elements, without sacrificing spectrometer performance. FM was achieved by modulating the reference oscillator of a 13 GHz Phase Locked Dielectric Resonator Oscillator, and amplifying and frequency-multiplying the resulting millimeter-wave radiation up to 210, 315 and 420 GHz. ESR spectra were obtained in reflection mode by a lock-in detection at the fundamental modulation frequency, and also at the second and third harmonic. Sensitivity of the setup was verified by conduction electron spin resonance measurement in KC$_{60}$.