Frequency Comb Spectroscopy from IR to XUV
Technical Report,01 May 2015,30 Apr 2019
Regents of the University of Colorado Boulder United States
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Key research highlights arising from the AFOSR funding support in 2017 include the following 1 Achieving phase-matching condition for high repetition rate XUV frequency comb generation. We recently submitted a paper titled Phase-matched extreme-ultraviolet frequency-comb generation to Nature Photonics and it is accepted for publication now. Laser-driven high-order harmonic generation HHG provides tabletop sources of broadband extreme-ultraviolet XUV light with excellent spatial and temporal coherence. These sources are typically operated at low repetition rates, less than 100 kHz, where phase-matched frequency conversion into the XUV is readily achieved. However, there are many applications that demand improved counting statistics or frequency-comb precision afforded by operation at high repetition rates, 10 MHz. Unfortunately, at such high repetition rates, phase matching is prevented by the accumulated steady-state plasma in the generation volume, setting stringent limitations on the XUV average power. We have now solved this problem and reached, for the first time, phase matched XUV generation at high repetition rate. Specifically, we use gas mixtures at high temperatures as the generation medium to increase the translational velocity of the gas, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at a repetition rate of 77 MHz, enabling a record generated power of 2 mW in a singleharmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy of few-electron atoms and ions for precision tests of fundamental physical laws and constants.
- Atomic and Molecular Physics and Spectroscopy