Development of a tunable high repetition rate XUV source for time-resolved photoemission studies of ultrafast dynamics at surfaces

The characterization of surfaces using photoelectron spectroscopy or photoemission electron microscopy provides sensitive probes of surface structure and electronic properties. Conventional extreme ultraviolet (XUV) light sources used for photoemission do not have ultrafast time resolution, which inhibits applying these techniques to the study of surface dynamics on their natural time scale. The high harmonics (HHG) of intense femtosecond laser pulses are capable of providing ultrashort XUV pulses for photoemission. However, for pulse-based photoemission measurements it is necessary to limit the density of electrons emitted by each pulse to prevent detrimental spacecharge effects. Therefore, to maintain reasonable data acquisition rates, the pulses must occur at a high repetition rate. Since the HHG process requires high peak fundamental laser powers, repetition rates have typically been limited to well below 1 MHz. In our lab, we can perform time-resolved XUV photoemission experiments at an 87 MHz repetition rate using a cavity-enhanced HHG source. Harmonics are generated at 87 MHz by resonantly enhancing a Yb:fiber laser capable of 1 μJ pulses in a passive optical cavity to pulse energies > 100 μJ. Average photon fluxes of up to 7x10¹¹ photons/s in a single isolated harmonic are delivered to a surface science end station. This delivered flux and repetition rate are comparable to a synchrotron light source, but with pulse durations nearly 1000 times shorter. In this paper, we discuss critical details of the source and its performance.