High Brightness Extreme Ultraviolet Frequency Combs via Intracavity High-Order Harmonic Generation
A coherent XUV and soft x-ray light source can be realized through high-order harmonic generation (HHG). HHG transfers the spatial and temporal coherence of laser light to the soft x-ray and extreme ultraviolet (XUV) and is now being used for many exciting applications. This is typically realized with low repetition rate (< 100 kHz) amplified femtosecond laser systems producing high-energy pulses (>100 μJ), with average powers up to tens of Watts. In an alternative method, the modes of a frequency comb and a high-finesse external cavity can be locked, leading to >100 MHz repetition rates and multi-kiloWatt average powers for the driving laser while still supporting peak intensities sufficient for HHG.
I will discuss the advances we have made to improve the XUV yield of intracavity HHG by six orders of magnitude from the first demonstrations of this technique in 2005. We have generated HHG pulse trains at 154 MHz repetition rate with average power of more than 200 microWatt/harmonic. This has enabled the first direct frequency comb spectroscopy in the XUV, allowing MHz resolution of atomic lines at 82 nm and 63 nm with intrinsic absolute frequency calibration, bringing XUV spectroscopy out of the “light bulb” age. In addition to frequency metrology, this tabletop high average brightness XUV light source has promising applications in strong field and molecular physics studies, and more traditional XUV applications currently served by synchrotron facilities.
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