Time- and angle-resolved photoemission (TR-ARPES)
ARPES relies on the photoelectric effects, i.e. the emission of an electron via the absorption of ultraviolet light. By detecting the angle of emission and kinetic energy of photoelectrons, we offer direct insights into electronic properties and collective excitations of complex quantum materials.
The extension of the momentum space (and the related band dispersion) explored via ARPES is directly proportional to the photon energy of the ultraviolet light (see figure on the right). The TR-ARPES endstation at ALLS uses low-energy (6 eV) and high-energy pulses (>15 eV) in an effort to map the momentum space of quantum materials.
ARPES is extended into the time domain (TR-ARPES) via pump-probe stroboscopic technique: an ultrashort intense pulse (the pump) is used to excite the system under investigation, while a delayed ultraviolet pulse (the probe) photoemites electrons. By changing the relative delay between pump and probe, a movie of the femtosecond electron dynamics inside the material is recorded.
The figure on the left displays an example of TR-ARPES measurement: the superconducting gap of a high-temperature cuprate superconductor (Bi2212 UD82) fills up upon the pump excitation as a consequence of the enhancement of the density of phase fluctuations.
In order to explore the transient evolution of spectral gaps and, in general, low-energy spectral features, narrow bandwidth probe pulses are needed. The UPE at ALLS generates narrow bandwidth (sub 30 meV) high-energy pulses.