Time-Resolved extreme UV micro-ARPES Electron Microscopy (TR-XUV-µ-ARPES)
tr-ARPES is one of the most direct techniques for us to study excited state dynamics in electronic structures. To fully utilize the capability of tr-ARPES technique, one is required to use a probe with large enough photon energy to access electronic state at different parts of the Brillouin zone where interesting physics are happening, for example, at the edge of the Brillouin zone in the direct gap of 2D semiconductors; one also need an instrument that allows collection of photoemitted electron at different angles and different kinetic energies; and to do time resolve measurement will further require one to use ultrafast pump and probe pulses.
Here in the Dani unit, we have tailored our tr-APRES setup to address all the above issues. On the optics side, we use a Yb-fiber laser to generate high energy probe pulses of 21eV at high repetition of 2Mhz or above with our home-built HHG setup, with this photon energy we can image large area in momentum space and cover the entire Brillouin zone of all materials; high repetition rate would ensure that we can image with minimum power avoiding space charge issues while maintaining a decent signal to noise ratio. For time resolved pump probe experiments, we divide a faction of the power from the laser to drive a tunable OPA that covers the UV to NIR wavelength, this provides flexibility for us to study excitation of different electronic states in various materials. To image the electrons, we use a state-of-the-art time-of-flight momentum microscope (Metis 1000, Specs GmbH) to collect simultaneously photoemitted electrons emitted at different emission angles and kinetic energies. In the momentum microscope, we can isolate and select photoelectrons emitted from micron-sized small sample area using an selected area aperture, this allows us to do tr-µ-ARPES on small samples. We have spent many years designing and testing; the whole system has finally come online in 2019. Since then, we have focused in studying the nature of excitons in different 2D Van der Waals materials, we have made direct observation of dark exciton and studied their formation dynamics , we have made the first observation of the hallmark feature of excitons in photoemission experiment, the anomalous dispersion feature in the photoemission spectrum  and we have revealed the unexpected tight confinement of interlayer exciton in a 2D heterostructure .