Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide

Abstract

Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.

Correlated electronic decay in expanding clusters triggered by intense XUV pulses from a Free-Electron-Laser

“…Quasi-free energetic electrons excited within nanoscale samples and bulk solids thermalize through multielectron as well as lattice collisions47. Another deexcitation mechanism is related to an autoionization of excited atomic states.…”

Transient Changes of Optical Properties in Semiconductors in Response to Femtosecond Laser Pulses

“…Such estimation has been performed for GaAs in [32], however, not with the code XTANT. The current version of XTANT cannot treat properly the relaxation of X-ray excited GaAs.…”

“…This is due to the specific band properties of GaAs which, in particular, imply that the effective masses of electrons and holes differ by a factor of ∼10. As a consequence, there is no fast global thermalization of electrons and holes in both valence and conduction bands [32]. Instead, electrons and holes thermalize separately within the conduction and the valence band, respectively, at the timescale of a few hundred fs.…”

“…Including the temperature dependence of the band gap into a two-temperature model (combined with an extended Drude-model), and fitting the model to the experimental data, allowed us to extract the electron-lattice thermalization time in FEL-excited GaAs [32]. Since the excitation level here is low, we expected the Drude model to be applicable.…”

Direct measurements of multi-photon induced nonlinear lattice dynamics in semiconductors via time-resolved x-ray scattering

“…For instance, earlier work using TRXS on GaAs has demonstrated the feasibility of measuring the crystal lattice response upon heating at nanosecond regimes22 as well as dynamics of coherent phonon excitation and its propagation near the melting threshold23. More recently, an alternate effort has been made to deduce electron-lattice coupling mechanisms in GaAs by modeling data from x-ray pumped, transient optical reflectivity measurements at FEL facilities24. However, the levels of pumped fluences presented in these studies were very high (tens of mJ/cm 2 ) and do not provide a full picture of how varying levels of excitation powers affect the evolution of crystal lattice responses.…”