tr-ARPES is one of the most direct techniques for studying excited-state dynamics within electronic structures. To fully leverage the capabilities of tr-ARPES, the probe must have a sufficiently high photon energy to access electronic states across different regions of the Brillouin zone, where intriguing physics, such as the direct gap at the Brillouin zone edge of 2D semiconductors, can be observed. Additionally, the instrument should be able to collect photoemitted electrons at various angles and kinetic energies. Conducting time-resolved measurements also requires the use of ultrafast pump and probe pulses.
Schematic of our time resolved ARPES setup
In the Dani unit, we have customized our tr-APRES setup to address all the challenges outlined above. On the optics side, we employ a Yb-fiber laser to generate high-energy probe pulses of 21 eV at a high repetition rate of 2 MHz or more using our in-house high-harmonic generation (HHG) system. This photon energy allows us to image a large area in momentum space, covering the entire Brillouin zone of various materials. The high repetition rate ensures that we can image with minimal power to avoid space charge issues while maintaining a favorable signal-to-noise ratio.
For time-resolved pump-probe experiments, we direct a portion of the laser power to drive a tunable optical parametric amplifier (OPA) that covers wavelengths from UV to NIR. This setup provides the flexibility needed to explore excitations of different electronic states across a variety of materials. To image the electrons, we utilize a state-of-the-art time-of-flight momentum microscope (Metis 1000, Specs GmbH), which simultaneously collects photoemitted electrons at different emission angles and kinetic energies. The momentum microscope allows us to isolate and select photoelectrons emitted from micron-sized sample areas using a selected area aperture, enabling tr-µ-ARPES on small samples.
After years of design and testing, the system finally went online in 2019. Since then, we have focused on studying exciton dynamics in various 2D Van der Waals materials. Our research has led to direct observations of dark excitons and insights into their formation dynamics [1]. We achieved the first observation of the characteristic anomalous dispersion of excitons in photoemission spectra [2] and uncovered the unexpected tight confinement of interlayer excitons in a 2D heterostructure [3].
[1] J. Madéo, M. K. L. Man, et al., Science 370, 1199–1204 (2020).
[2] M. K. L. Man, J. Madéo, et al., Science Advanced 7, eabg0192 (2021).
[3] O. Karni, M. K. L. Man, et al., Nature 603, 247-252 (2022).