研究目的
Investigating the efficient generation of high harmonics at 10 MHz repetition rate using an external femtosecond enhancement cavity seeded by a post-compressed FCPA laser, focusing on the benefits of shorter pulses and low finesse cavities for improving HHG efficiency and stability.
研究成果
The study demonstrated efficient HHG at 10 MHz using a post-compressed FCPA laser and a low finesse enhancement cavity, achieving stable operation with over 0.1 mW outcoupled power at 149 nm. Shorter pulses and lower finesse cavities were found to alleviate plasma nonlinearity effects and improve HHG efficiency and stability, making the system suitable for time-resolved photoemission spectroscopy.
研究不足
The study was limited by the sensitivity of the cavity-locking to disturbances such as nonlinearity of the plasma, mechanical vibrations, and fluctuations in the externally compressed pulses. The use of higher transmission input couplers improved stability but reduced the power enhancement ratio.
1：Experimental Design and Method Selection
The study utilized an external femtosecond enhancement cavity (fsEC) seeded by a post-compressed 10 MHz fiber chirped pulse amplifier (FCPA) laser to generate high harmonics. The methodology included spectral broadening through self-phase modulation (SPM) inside a photonic crystal fiber (PCF) followed by chirp removal using grating pairs.
2：Sample Selection and Data Sources
The experiment used noble gases (Xe, Kr, Ar) as the HHG medium, introduced into the cavity via a fused-silica capillary nozzle. The high harmonics were separated from the fundamental beam using a Brewster-angled MgO output coupler.
3：List of Experimental Equipment and Materials
Yb-based FCPA laser, LMA-35 photonic crystal fiber (Thorlabs), transmission gratings (1000 lines/mm), quarter-wave plate (QWP), half-wave plate (HWP), polarizing beam splitter (PBS), Brewster-angled MgO output coupler, VUV toroidal grating, phototube (Hamamatsu R1187).
4：Experimental Procedures and Operational Workflow
The laser output was guided to the fsEC either directly or via an external pulse compression stage. The compressed pulses were characterized by frequency-resolved optical gating (FROG). The cavity was locked using Pound-Drever-Hall (PDH) locking electronics, and the HHG process was driven by introducing noble gases at the focus of the cavity.
5：Data Analysis Methods
The outcoupled harmonic power was measured using a phototube, and the harmonic spectra were analyzed without correcting for the wavelength dependency of the diffraction grating efficiency, luminescent yield, or gas reabsorptions.
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