Enhance high harmonic generation (HHG) efficiency via compact multi-plate continuum post-compression for time-resolved angle-resolved photoemission spectroscopy.

Abstract

Time-resolved angle-resolved photoemission spectroscopy (Tr-APRES) gives direct insight into electron dynamics by providing temporal-, energy-, and momentum-resolved information in one experiment. A major obstacle to using high harmonic generation (HHG) probe pulses for photoemission spectroscopy is the low conversion efficiency, that is, the low flux of probe photons. We use a Yb-KGW based duo-laser source with an oscillator to pump two separate amplifiers and generate two synchronized pulsed laser sources with average energies of 7.5 and 6W. By using the multiplate continuum method and chirped mirrors, the resulting flux of HHG photons at 33-70eV can be increased 50-fold (up to 1011 photons/s) by using post-compressed 30fs pulses compared with the photon flux generated by the fundamental 190fs pulses. Moreover, pulses from the 6W amplifier are used to pump an optical parametric amplifier that can vary the wavelengths for photoexcitation. The system performance is demonstrated by applying Tr-ARPES to single-crystal graphite. The front tilt broadening is significantly suppressed by the off-plane mounted conical grating, leading to a 184fs temporal resolution that is mainly limited by the pump pulse. The energy resolution is 176meV.