Ramifications of optical pumping on the interpretation of time-resolved photoemission experiments on graphene

Abstract

In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES) experiments the presence of the pump pulse adds a new level of complexity to the photoemission process in comparison to conventional ARPES. This is evidenced by pump-induced vacuum space-charge effects and surface photovoltages, as well as multiple pump excitations due to internal reflections in the sample-substrate system. These processes can severely affect a correct interpretation of the data by masking the out-of-equilibrium electron dynamics intrinsic to the sample. In this study, we show that such effects indeed influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In particular, we find a time-and laser fluence-dependent spectral shift and broadening of the acquired spectra, and unambiguously show the presence of a double pump excitation. The dynamics of these effects is slower than the electron dynamics in the graphene sample, thereby permitting us to deconvolve the signals in the time domain. Our results demonstrate that complex pump-related processes should always be considered in the experimental setup and data analysis.

Nonlocal plasmonic response of doped and optically pumped graphene, MoS2 , and black phosphorus

“…These hot electrons (and holes) then thermalize via carrier-carrier interactions to reach a Fermi-Dirac distribution in the conduction and valence bands within ∼10's fs, reaching attainable electronic temperatures of thousands of degrees [9,[17][18][19][20], and eventually relaxing to the atomic lattice temperature, with a minor overall temperature increase due to the high heat capacity of the lattice compared with electrons. Timeand angle-resolved photoemission spectroscopy (TR-ARPES) has corroborated this picture by monitoring the formation of Fermi-Dirac distributions of electrons and holes after ∼10's fs following optical pumping [21][22][23][24][25]. The presence of the so-called thermoplasmons when the electronic temperature is sufficiently high has been recently corroborated in highquality graphene through ultrafast near-field spatial imaging [9].…”

Ultrafast Band Structure Control of a Two-Dimensional Heterostructure

“…Such spectral shifts can be induced by space charge or surface photovoltage (SPV) effects. 26 The space charge effect is caused by interactions among the photoelectrons in vacuum, which sets up a charge cloud that propagates away from the sample. The acceleration of photoemitted electrons is consequently changed as they propagate to the detector, leading to shifts in the measured kinetic energy distributions.…”

Manipulating long-lived topological surface photovoltage in bulk-insulating topological insulators Bi2Se3 and Bi2Te3

“…Despite previous predictions 6 and observations of the SPV effect 7-10 in topological insulators, it is still not clear whether this is an intrinsic property of topological materials and under which conditions it can be realized. Moreover, previous reports rely on narrow temporal ranges and an indirect determination of the time constant, which has led to the misinterpretation of the SPV time profile [11][12][13] . Full characterization of the effect over a larger temporal range is needed in order to directly determine the time constant of the effect.…”