All-solid-state VUV frequency comb at 160 nm using multi-harmonic generation in a non-linear femtosecond enhancement cavity

TL;DR

This paper demonstrates a solid-state vacuum ultraviolet frequency comb at 160 nm using multi-harmonic generation in a femtosecond enhancement cavity, achieving high power and spectral stability for advanced spectroscopy.

Contribution

It introduces a novel solid-state multi-harmonic generation method for VUV frequency combs, expanding the spectral range and power levels compared to gaseous media approaches.

Findings

Achieved 24 W average power in the 3rd, 5th, and 7th harmonics.

Generated a stable VUV frequency comb at 160 nm.

Demonstrated high-precision beat measurement with a CW laser.

Abstract

We report on the realization of a solid-state-based vacuum ultraviolet frequency comb, using multi-harmonic generation in an external enhancement cavity. Optical conversions in such arrangements were so-far reported only using gaseous media. We present a theory that allows selecting the most suited solid generation medium for specific target harmonics by adapting the bandgap of the material. Consequently, we experimentally use a thin AlN film grown on a sapphire substrate to realize a compact frequency comb multi-harmonic source in the DUV/VUV spectral range. Extending our earlier VUV source [Opt. Exp. 26, 21900 (2018)] with the enhancement cavity, a sub-Watt level Ti:sapphire femtosecond frequency comb is enhanced to 24 W stored average power, its 3rd, 5th and 7th harmonics are generated, and the target harmonic power at 160 nm increased by two orders of magnitude. The emerging non-linear effects in the solid medium together with suitable intra-cavity dispersion management support optimal enhancement and stable locking. To demonstrate the spectroscopic ability of the realized frequency comb, we report on the beat measurement between the 3rd harmonic beam and a 266 nm CW laser reaching about 1 MHz accuracy.