Resonant Pumping of Collective Excitations
High-intensity long-wavelength pump perturbations can be used to resonantly excite collective modes of quantum materials, effectively controlling their properties dynamically.
With a particular focus o lattice vibrations (i.e. phonons), we note that the energy of optical phonons falls commonly in the 10-to-100 meV range (2.5-25 THz, >10 μm). Resonant light-phonon coupling has been shown to drive new phases of matter, such as light-induced room temperature superconductivity [1,2], metal-to-insulator transitions  and transient magnetism . Simply put, these out-of-equilibrium states are stimulated by the transient modulation of the lattice constant along specific directions, which selectively modifies the electron hopping between different atomic sites, i.e. the kinetic term to the system's Hamiltonian. As a consequence of the intrinsic strong electron interactions in quantum materials, dynamical changes of the ratio between electrons' kinetic energy and the on-site electron interaction may lead to novel out-of-equilibrium phases as disparate as superconductivity and Mott insulators .
Light-induced lattice vibrations are expected to manifest in dynamical changes of the electronic band structure  and will be tracked via TR-ARPES. Although it is not uncommon to observe coherent phonon dynamics upon non-resonant visible/near-infrared light excitations, the non-thermal electronic bath excited by high-energy photons screens and masks pure lattice vibration effects. On the other hand, how the electronic band structure evolves upon resonant-phonon excitation has not been investigated with momentum resolution so far. The UPE, by visualizing the electronic band structure with momentum resolution, aims to track how the resonant pumping of collective excitations may be used to control the properties of quantum materials.
 Fausti et al. Science 331, 189 (2011)
 Mitrano et al. Nature 530, 461 (2016)
 Rini et al. Nature 449, 72 (2007)
 Nova et al. Nat. Phys. 13, 132 (2016)
 Kennes et al. Nat. Phys 13, 479 (2017)
 Gerber et al. Science 357, 71 (2017)