Sub-MHz Linewidth at 240 GHz from an Injection-Locked Free-Electron Laser

Radiation from an ultra-stable 240 GHz solid-state source has been injected, through an isolator, into the cavity of the University of California Santa Barbara (UCSB) MM-wave free-electron laser (FEL). High-power FEL emission, normally distributed among many of the cavity's longitudinal modes, is concentrated into the single mode to which the solid state source has been tuned. The linewidth of the FEL emission is 0.5 MHz, consistent with the Fourier transform limit for the 2 microsecond pulses. This demonstration of frequency-stable, ultra-narrow-band FEL emission is a critical milestone on the road to FEL-based pulsed electron paramagnetic resonance spectroscopy.