Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for probing electronic structure as a function of momentum and energy in solids. However, it fails to provide direct information on electronic reaction and scattering process to excitations in nonequilibrium states.

Time- and angle-resolved photoemission spectroscopy (trARPES) enables the Fermi surface of a material to be monitored as it responds to photoexcitation. It allows one to examine ultrafast phenomena such as gap collapse and reformation in the most direct way possible: by actually monitoring the behaviour of the electron states after optical excitation. In detail, a femtosecond pump pulse excites spectral weight transfer by creating electron-hole pair or resonantly drives special collective excitations, and a subsequent UV/XUV pulse probes the transient electronic structure as conventional ARPES does after a certain time delay. TrARPES is capable of studying ultrafast electron changes by photoexcitation, recovery of excited states by electron-electron and electron-phonon interactions, and light-induced phase transition.

Tr-ARPES system at CFEL

The newly developed trARPES system at CFEL is characterized by its high resolution (< 50 meV), tunable pump source (NIR and MIR), tunable probe source (UV+ XUV, 6 ~ 40 eV) and high probe fluence (10­9 ~ 1010 ph/s). This system is capable of vibrationally stimulating low energy excitations and investigating their relations to electronic structures, which require high energy resolution and high photon energy (more momentum space).

Schematic overview of our tr-ARPES experimental system. MIR pump is achieved by an optical parameter amplifier setup. XUV probe is obtained via HHG generation in argon gas, ranging from 10 to 40 eV. The probe beam is selected by a grating setup, and focused by a toroidal mirror. Electrons are detected by a hemispherical analyzer, which enables the detection of energy and momentum simultaneously.

XUV beamline

The XUV beamline consists of 4 vacuum chambers holding a gasjet, a grating, a slit and 3 toroidal mirrors. NIR beam is focused to a gasjet to generate HHG in Ar gas, the desired XUV photoenergy (10 ~ 40 eV) is selected by rotating the grating and closing the slit right after.

The MPSD XUV beamline at CFEL.
ARPES end station. We use a two-stage load lock system to transfer samples from air to the UHV chamber. The second stage has an electron-beam heating system to treat samples up to 1500 K, and few evaporators for film/sample growth.
6-axis motorized cryostat, with the lowest temperature at the sample position is 10 K. The rotation of the sample allows full Fermi surface measurements.
Gold-coated toroidal mirror mounted on a 6-axis stage. XUV beam at grazing incidence is focused onto the sample by the toroid mirror.
Picture of the gas nozzle. 800 nm beam is focused into argon gas, which is jetted out of this nozzle, producing XUV photons from 10 to 40 eV via high harmonic generation.

Tr-ARPES system at Rutherford Appleton Laboratory

Our efforts in this field have been taking place on the new Artemis beamline at the Rutherford Appleton Laboratory, where we are investigating the response of the quasi-two dimensional material TaS2 as it undergoes a photoinduced phase transition from a Mott insulator to a metal. Using the apparatus at Artemis, we aim to explore the charge density wave gap regions of the Fermi surface as the Mott gap collapses on the femtosecond timescale. We can also utilise varying pump photon energies to understand the response to excitation at and below the gap energy.

The Material Science Station

The excitation laser and probe XUV photons are combined and focussed down onto the sample at the centre of this chamber. The sample sits at the end of a cryostat cold finger, which can be cooled to 14K. The chamber contains low energy electron diffraction (LEED) apparatus for sample orientation and characterisation and a hemispherical electron analyser for the trARPES measurements.

Experimental hutch

We use XUV photons created through high harmonic generation to produce the photoelectrons needed for this experiment. These photons are produced in the UHV chambers seen in this photo. They are then focussed into the Material Science Station at the end of the beamline, which contains the sample and measurement apparatus.

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