Extreme ultraviolet (XUV) frequency comb is a powerful tool in precision measurement. It also brings many new opportunities to the field of strong field physics since high harmonic generation related phenomena can be studied with high repetition rate. We demonstrate the generation of an XUV frequency comb with the aid of intra-cavity high harmonic generation process. The setup is driven by a high power infrared frequency comb, and an average power of 4.5 kW is reached in the femtosecond enhancement cavity. With Xe gas as the working media, harmonics up to the 19th order are observed. Power measurement indicates that as much as 115.9 μW (1.3 mW) are generated at ∼94 nm (∼148 nm). The shortest wavelength we can reach is ∼55 nm. The coherence of the generated light is tested with an optical-heterodyne-based measurement of the third harmonic. The resulted line width is ∼3 Hz. In addition, with this system, we also observe a strong suppression of below threshold harmonics from O2 compared to that from Xe. These results suggest that the current system is ready for precision spectroscopic measurements with few-electron atomic and molecular systems in XUV region as well as the study of strong field physics with an unprecedented 100MHz repetition rate.
Synopsis
We propose a circular-elliptical laser field, which consists of a circularly polarized field with its copla-nar counter-rotating elliptical third harmonic pulse, to generate polarized high-order harmonics and attosecond pulses (APs) by He atom. We show that the polarization of APs depends evidently on the ellipticity ξ of harmonic laser field. Most importantly, nearly circular APs can be obtained with a proper ξ. Our results contribute to the generation of high elliptically polarized APs with s-state atom.
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