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

TL;DR

This paper demonstrates a frequency-stable, ultra-narrow-band free-electron laser at 240 GHz with a linewidth of 0.5 MHz, achieved by injection-locking with a solid-state source, advancing FEL applications in spectroscopy.

Contribution

It presents the first demonstration of injection-locked FEL at 240 GHz with sub-MHz linewidth, enabling new high-resolution spectroscopic techniques.

Findings

FEL emission concentrated into a single mode

Linewidth of 0.5 MHz matches Fourier transform limit

Potential for FEL-based pulsed electron paramagnetic resonance

Abstract

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.