Influence of non-collisional laser heating on the electron dynamics in dielectric materials

Abstract

Abstract. The electron dynamics in dielectric materials induced by intense femtosecond laser pulses is theoretically addressed. The laser driven temporal evolution of the energy distribution of electrons in the conduction band is described by a kinetic Boltzmann equation. In addition to the collisional processes for energy transfer such as electron-phonon-photon and electron-electron interactions, a non-collisional process for photon absorption in the conduction band is included. It relies on direct transitions between sub-bands of the conduction band through multiphoton absorption. This mechanism is shown to significantly contribute to the laser heating of conduction electrons for large enough laser intensities. It also increases the time required for the electron distribution to reach the equilibrium state as described by the Fermi-Dirac statistics. Quantitative results are provided for quartz irradiated by a femtosecond laser pulse with a wavelength of 800 nm and for intensities in the range of tens of TW/cm 2 , lower than the ablation threshold. The change in the energy deposition induced by this non-collisional heating process is expected to have a significant influence on the laser processing of dielectric materials.

Evidence of noncollisional femtosecond laser energy deposition in dielectric materials

“…IV the observed photoelectron energy distributions are directly compared with the theoretical predictions, within the challenging accuracydemanding linear scale in the present context. We emphasize that only the maximum energy of ejected electrons has been considered in previous works [13,17,18]. The present model allows us to predict photoelectron spectra which are in a good agreement with experimental results for various laser intensities.…”

“…The electron dynamics in the bulk is described by a Boltzmann kinetic equation which domain of validity is fulfilled by using moderate laser intensities producing electron densities of the order of 10 18 -10 20 cm −3 [5,12,13]. This particle density is small enough to ensure only binary collisions and an average distance between particles larger than the De Broglie length allowing one to consider particles as evolving classically (Boltzmann equation is classical, whereas collision operators are evaluated through quantum calculations).…”

“…These processes eventually lead to the energy transfer to the lattice. This is a collisional picture theoretically described either by the Drude model, multiple rate equations [12], or the kinetic Boltzmann equation [4,5,13] which can provide the energy distribution of excited electrons related to the laser energy deposition. This distribution can be experimentally obtained through photoemission spectroscopy [14][15][16][17].…”

“…(i) Collisional models as solving the state-of-the-art quantum Boltzmann equation, including all possible electronic excitation and relaxation processes, provide the electron energy distribution [4,5,12]. But the interband process has never been included except in [13]. (ii) Noncollisional models based on a resolution of the time-dependent Schrödinger equation or optical Bloch equations [20][21][22][23]25].…”

Optical Bloch modeling of femtosecond-laser-induced electron dynamics in dielectrics

“…Nevertheless, the impact ionization process was not tackled, neither an in-depth treatment of electron collisions whereas their influence is crucial since they lead to decoherence effects and to the laser energy deposition into the material. On the other hand, kinetic-type descriptions have been developed, accounting for all main collisional processes [30,31]. But they generally do not account for the band structure beyond a single parabolic band and are currently computationally too expensive for their coupling to a Maxwell solver.…”