Enhancement cavities for few-cycle pulses.

We address the challenge of increasing the bandwidth of high-finesse femtosecond enhancement cavities and demonstrate a broad spectrum spanning 1800 cm-1 (195 nm) at -10 dB around a central wavelength of 1050 nm in an EC with an average finesse exceeding 300. This will benefit a host of spectroscopic applications, including transient absorption spectroscopy, direct frequency comb spectroscopy, and Raman spectroscopy. The pulse circulating in the EC is composed of only 5.4 optical cycles, at a kilowatt-level average power. Together with a suitable gating technique, this paves the way to the efficient generation of multi-megahertz-repetition-rate isolated extreme ultraviolet attosecond pulses via intracavity high-order harmonic generation.