Development of an Extreme Ultraviolet Frequency Comb for Precision Spectroscopy

We describe an extreme-ultraviolet frequency comb produced via high repetition rate, intracavity high harmonic generation. The experimental apparatus consists of a high power frequency comb of 80 W average power, which excites a robust femtosecond enhancement cavity. On resonance, the enhancement cavity contains \u3e8 kW of average power and is capable of driving the high harmonic process at a repetition rate of 154 MHz producing a frequency comb. More than 200 microwatts of XUV power is generated in each harmonic. While it is difficult to build a continuous wave laser below 100 nm, this source functions as millions of intrinsically calibrated narrow-linewidth sources in the XUV. With this unique radiation source, we were able to conduct high resolution, direct frequency comb spectroscopy on neutral argon atoms at a wavelength of 82 nm. This measurement demonstrates the efficacy of this technique for precision measurement