Crystal response in the presence of femtosecond-laserinduced electron dynamics
Final Report Abstract
Intense femtosecond-laser pulses drive solid-state systems out of thermal equilibrium. In more detail, most of the laser energy is deposited in the electronic system, which results in a highly excited electronic system whereas the ions remain nearly unaffected. The intense light induces a non-equilibrium electronic distribution during and shortly after the pulse. The ultrafast electron dynamics then causes the electron system to thermalize in tens of femtoseconds. In this work we analyzed theoretically these ultrafast electron dynamics and the nature of the laser-induced electronic non-equilibrium distribution. Moreover, we investigated the effects of ultrafast electron dynamics arising from the highly excited electronic system on structural dynamics after a femtosecond-laser excitation. In order to simulate the different stages of an ultrafast light-matter interaction, we combined TDDFT simulations, which where used to simulate the system during and shortly after the excitations with Te-dependent DFT. Using the Octopus code we simulated electron dynamics in graphene, hBN/graphene and silicon under a temporal varying vectorpotential. Surprisingly, we found that although TDDFT is based on a unitary-time evolution, which does not include thermalization processes, a pseudothermalization process is present. Because of this finding, a direct transition from TDDFT to Te-dependent DFT seems possible without considering a further step including Boltzmann equations. In order to allow a smooth transition between both approaches, we switched to describing the electronic system in the microcanonical ensemble (N,V,E) instead of using the traditionally used description in the canonical ensemble (N,V,Te). Therefore, we extended CHIVES, our Te-dependent DFT code, to a microcanonical mode in which for the first time the electronic entropy is the system describing quantity. By studying graphene, graphite, and silicon we could identify that the first steps of the laser-induced phenomena remain conceptually the same independent of the ensemble choice, namely, the atoms follow the same microscopic pathways. For longer times, the choice of the ensemble could have an influence on the structural response. In addition, switching to a microcanonical description of the electrons allows to quantify a nonthermal state in which a temperature can not be well defined, which become most relevant on the attosecond timescale.
Publications
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Ultrafast Laser Nanostructuring. Springer Series in Optical Sciences. Springer International Publishing.
Stoian, Razvan & Bonse, Jörn
