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 6 W. By using the multiplate continuum method and chirped mirrors, the resulting flux of HHG photons at 33-70 eV can be increased 50-fold (up to 1011 photons/s) by using post-compressed 30 fs pulses compared with the photon flux generated by the fundamental 190 fs pulses. Moreover, pulses from the 6 W 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 184 fs temporal resolution that is mainly limited by the pump pulse. The energy resolution is 176 meV.