Angle-resolved photoemission spectroscopy (ARPES) provides priceless information about the quantum nature of electrons inside solids. Extending this method into the ultrafast time domain, time- and angle-resolved photoemission spectroscopy (trARPES) and two photon photoemission (2PPE) spectroscopy employ a combination of two femtosecond (10-15 s) laser pulses to additionally access dynamical properties. The femtosecond time resolution is achieved by the variation of the time delay between pump and probe pulse: The pump pulse creates a non-equilibrium distribution of electrons in the sample and the resulting relaxation dynamics in the intermediate state, defined by interactions in the solid, are monitored by the time-delayed probe pulse.
In trARPES, high energy (XUV) probe pulses are used to gain access to a wide area of the Brillouin zone. In 2PPE spectroscopy, both pump and probe laser pulses have energies below the sample work function (Φ), providing a background-free mapping of the unoccupied band structure. In both cases, first a photon of the pump pulse is absorbed by an electron below the Fermi level EF of the sample into an excited state. The photoemission occurs when the same electron absorbs a photon of the probe pulse, which excites the electron above the vacuum level Evac.
Analyzing the excited state’s time and energy scales provides important information on carrier relaxation pathways and coupling to other degrees of freedom. In addition, trARPES and 2PPE spectroscopy provide a sensitive tool to study optical control of electronic states, such as photoinduced phase transitions and hidden phases not accessible in equilibrium. We apply this method to a variety of materials, including novel 2-dimensional semiconductors and semimetals, interfaces, molecular systems and strongly correlated materials.