High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system

Abstract

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively; the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm 2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm 2 .

Angle-resolved photoemission spectroscopy and its application to topological materials

“…The rapid development of commercially available, high-repetition-rate (≥10 kHz), amplified femto second lasers has led to the increased use of time-resolved ARPES. In the past 10 years, various time-resolved ARPES systems 55,57,[125][126][127][176][177][178][179][180] with different energy reso lution (tens to hundreds of meV), temporal resolution (approximately hundreds of attoseconds 58,59 to hundreds of femto seconds) and pump frequencies (approximately tens of meV to several eV) have been developed, enabling the electron dynamics in correlated materials to be probed. Moreover, by carefully choosing the pump frequency, it is possible to observe new quantum states induced by light, such as Floquet-Bloch states 64,181,182 (discussed below).…”

“…Moreover, the emergence of light sources with microscale or even nanoscale spot sizes has given rise to the possibility of performing spatially resolved ARPES measurements [48][49][50][51] , which will have a central role in probing the electronic structure of microscale and nanoscale materials as well as materials with phase separation or multiple domains [52][53][54] . Finally, the implementation of time-resolved ARPES with ultrafast lasers or X-ray sources 20,[55][56][57][58][59] makes it possible to study ultrafast electronic dynamics in the time domain [60][61][62][63][64][65][66][67][68] and enables the unoccupied states above the Fermi level (E F ) [69][70][71] to be probed. The advances in ARPES have made it an effective and ideal tool for the identification of the unique bulk and surface electronic structures of topological materials.…”

Work function seen with sub-meV precision through laser photoemission

“…We set up a fibre-laser-based source of hν = 5.988 eV 29 and docked it to an ARPES spectrometer equipped with a helium lamp ( Fig. 1b; also see Supplementary Note 1 and Supplementary Fig.…”

“…This underlies the precise extraction of the work function via ARPES through the relationship ϕ s = hν − (E f − E s ). Simple though it may seem, the precise detection via ARPES was aided by the use of a fibre-laser-based light source 29 whose beam can be aligned and focused to take control over the trajectory of the slow photoelectrons in ARPES setups 30 ; see the subsequent section, Laser-ARPES setup for slow photoelectrons. The slow end of the photoelectron distribution seen in fibre-laser ARPES retained the sharpness expected in Einstein's theory 31 and allowed us to monitor the work function with sub-meV precision.…”

Ultrafast extreme ultraviolet photoemission without space charge

“…Currently, to study dynamics in the low-fluence regime, workers in the field turn away from the clarity and fidelity of XUV ARPES and instead pursue other methods that have the required sensitivity but are limited in scope and harder to interpret. For example, low-fluence experiments are typical for the more sensitive techniques of optical spectroscopy, 64 two-photon photoemission, 65 and laser-based ARPES using 6 eV probe light 66 but have been extremely difficult using space-charge limited HHG systems.…”