Far-from-Equilibrium Electron–Phonon Interactions in Optically Excited Graphene

Abstract

Comprehending far-from-equilibrium many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equilibrium functions. In this work, we use time-and angle-resolved photoelectron spectroscopy to study such far-from-equilibrium many-body interactions for the prototypical material graphene. In accordance with theoretical simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequilibrium conditions. These observations can be understood by ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequilibrium dynamics on manybody interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equilibrium states of matter.

Ultrafast dynamics of bright and dark excitons in monolayer WSe2 and heterobilayer WSe2/MoS2

“…We resonantly excite the bright A1s exciton of WSe 2 with 1.7 eV 40 fs light pulses (s-polarized). Photoemission from the occupied band structure and the excitons is induced with time-delayed 26.5 eV 20 fs light pulses (p-polarized) that are created in a table-top 500 kHz repetition rate high-harmonic generation beamline [25,70,71]. For each delay between the pump and the probe laser pulse, the ToF-MM (Surface Concept GmbH) collects three-dimensional data cubes that contain information on the two in-plane momenta k x and k y and the energy E of the detected photoelectrons [25,42].…”

Ultrafast Nonadiabatic Phonon Renormalization in Photoexcited Single-Layer MoS2

“…Electron–phonon coupling (EPC) plays a crucial role in the aforesaid ultrafast phenomena, ,− and thus, it is of utmost importance to master and comprehend microscopic channels governing phonon dynamics in extreme nonequilibrium conditions. Complementary to the time-resolved photoemission methods that provide an important access to the electron–hole thermalization process ,,− and electronic structure changes, , there are several ultrafast techniques, such as ultrafast electron diffraction scattering, − coherent phonon spectroscopy, − and time-resolved Raman spectroscopy, − that can precisely track the phonon relaxation channels following the photoexcitation and corresponding EPC strength. , For instance, ultrafast electron diffraction had uncovered highly anisotropic non-thermal phonon relaxation in black phosphorus and mapped momentum-resolved electron–phonon scattering channels and strengths in various transition-metal dichalcogenides (TMDs). ,,, Intriguingly, these methods are able to analyze photo-induced phonon frequency modifications and uncover the relevant microscopic processes, as it was done, for example, for zone-center strongly coupled E 2g optical mode in graphite with coherent phonon and time-resolved Raman spectroscopies, as well as for the amplitude CDW mode in TiSe 2 by means of ultrafast electron diffraction . In combination with other time-resolved spectroscopy approaches, the latter technique allowed us to pinpoint the phonon modes that play an active role in unconventional superconductivity of FeSe thin films on SrTiO 3 and to extract the correlation-induced EPC constants .…”

Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy

“…Identifying bilayer graphene with a particular twist angle is challenging, particularly substrate-agnostic, fast, large-area mapping. Angle-dependent characterization of twisted bilayer graphene has been demonstrated through optical absorption , and reflection, − photoemission, ,,− photoluminescence, , and Raman ,,, spectroscopies, which are often correlated with higher resolution electron microscopy , or scanning probe microscopy. − Many of these techniques rely on specific substrate properties, such as transparent or contrast-enhancing, , which may be incompatible with characterization during particular stages of manufacturing . Spectroscopic imaging ellipsometry has emerged as a tool to determine the optical constants of graphene and other two-dimensional materials and provide thickness information with single-atomic layer precision , with a lateral resolution down to ∼1 μm on a wide range of substrates, including as-grown directly on metal catalyst foils .…”