# Cavity-enhanced high harmonic generation for XUV time-resolved ARPES

## TL;DR

This paper introduces a cavity-enhanced high harmonic generation source for time-resolved ARPES, enabling high photon flux, energy resolution, and temporal resolution for studying electronic structures in solids.

## Contribution

It demonstrates a practical, high-flux XUV source using a femtosecond enhancement cavity for improved TR-ARPES measurements.

## Key findings

- Achieved over 10^11 photons/sec flux at 8-40 eV
- Operates at 60 MHz repetition rate
- Temporal resolution of 190 fs and energy resolution of 22 meV

## Abstract

With its direct correspondence to electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a ubiquitous tool for the study of solids. When extended to the temporal domain, time-resolved (TR)-ARPES offers the potential to move beyond equilibrium properties, exploring both the unoccupied electronic structure as well as its dynamical response under ultrafast perturbation. Historically, ultrafast extreme ultraviolet (XUV) sources employing high-order harmonic generation (HHG) have required compromises that make it challenging to achieve a high energy resolution - which is highly desirable for many TR-ARPES studies - while producing high photon energies and a high photon flux. We address this challenge by performing HHG inside a femtosecond enhancement cavity (fsEC), realizing a practical source for TR-ARPES that achieves a flux of over 10$^{11}$ photons/s delivered to the sample, operates over a range of 8-40 eV with a repetition rate of 60 MHz. This source enables TR-ARPES studies with a temporal and energy resolution of 190 fs and 22 meV, respectively. To characterize the system, we perform ARPES measurements of polycrystalline Au and MoTe$_2$, as well as TR-ARPES studies on graphite.