Phase-Coherent Frequency Combs in the Vacuum Ultraviolet via High-Harmonic Generation inside a Femtosecond Enhancement Cavity

We demonstrate the generation of phase-coherent frequency combs in the vacuum utraviolet spectral region. The output from a mode-locked laser is stabilized to a femtosecond enhancement cavity with a gas jet at the intracavity focus. The resulting high-peak power of the intracavity pulse enables efficient high-harmonic generation by utilizing the full repetition rate of the laser. Optical-heterodyne-based measurements reveal that the coherent frequency comb structure of the original laser is fully preserved in the high-harmonic generation process. These results open the door for precision frequency metrology at extreme ultraviolet wavelengths and permit the efficient generation of phase-coherent high-order harmonics using only a standard laser oscillator without active amplification of single pulses.

Figure 1

Schematic setup of intracavity high-harmonic generation. The incident pulse train is stabilized to a high finesse cavity, enhancing pulse energy nearly 3 orders of magnitude while maintaining a high repetition frequency. A gas target at the cavity focus enables phase-coherent HHG, resulting in a phase-stable frequency comb in the vacuum utraviolet (VUV) spectral region. The photo inset shows the actual spatial mode profile of the 3rd harmonic coupled out of the cavity.

Figure 2

Characterization and measurement of the fs enhancement cavity. (a) Transmission through fs enhancement cavity as the laser cavity length is scanned. The dominant peak results from optimal alignment of frequency comb modes to resonant modes of the fs enhancement cavity. (b) Cavity ring-down measurements for the entire pulse spectrum (top trace) and for a 5 nm region centered at 800 nm. (c) Incident and transmitted spectrum of the pulse. The pulse durations measured by frequency-resolved-optical gating are 48 fs (incident) and 60 fs (transmitted). (d) Visible plasma generation at the cavity focus. A pair of electrodes for ion collection is useful for signal optimization.

Figure 3

Detection of the 3rd, 5th, and 7th harmonic signals on a phosphor screen after diffraction from a grating.

Figure 4

(a) Setup for coherent heterodyne detection between 3rd harmonic generated in Xe gas and that generated by bound optical nonlinearities in BBO. (b) Measurement of the detected beat signal and repetition frequency. The linewidth shown in the inset is resolution bandwidth limited at 1 Hz.