Characterization of a high harmonics beam line for the application of time-resolved spectroscopy

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The mission of this project is to characterize high harmonic pulses to apply to time-resolved photoelectron spectroscopy. The research issues we have accomplished are 1) the efficient pulse compression to 5 fs using metal-coated hollow fiber and 2) the development of a new characterization technique of high harmonic pulses. These issues were raised 1) to generate high harmonic pulses efficiently from 5-fs laser pulses and 2) to apply high harmonic pulses to time-resolved spectroscopy more conveniently.Few-cycle laser pulses are used to generate attosecond high harmonic pulses. To generate intense attosecond pulses, the pulse energy of few-cycle laser pulses should be boosted. Therefore, we used an aluminum coated hollow fiber in the fiber compression to improve the transmittance efficiency, improved from 40% to 70% under the same experimental condition. Finally, we could obtain the 5-fs, 660-μJ pulses after compressed by chirped mirrors. These will be very useful to generate attosecond pulses.We propose a new characterization technique of high harmonic pulses based on the spectral shearing interferometry. In this method, optical pulses are copied on the photoelectron wave packet by the photoelectric effect. Using neon or argon atoms as a target, a pair of spectral sheared photoelectron wave packets is generated because neon and argon have two ionization energies. From the spectral sheared photoelectron wave packets, the spectral dispersion can be extracted. This method is quite simple and has high sensitivity. We applied this method to the 19th harmonic of Ti:sapphire laser and could generate 3.7-fs pulses, although the pulse duration of the laser pulse was 36 fs. The nonadiabatic rising of the laser electric field can explain the relatively short pulse duration.