ULTRADEX · Ultrafast energy transfer and dissipation in electronically excited materials: calculations from first principles

Ultrafast energy transfer and dissipation in electronically excited materials: calculations from first principles

Work carried out to achieve the project's objectives: The objectives of this project were: to extend the scope of electronic structure computational methods, allowing us to quantitatively simulate (without phenomenological parameters) the coupled electronic and atomic dynamics in highly photoexcited materials on a picosecond and sub-picosecond timescale, and to use these new methods to calculate the transfer of energy from electronic states to the lattice vibrations in a photoexcited semiconductor and to follow the redistribution of vibrational energy through to its ultimate thermalisation in the lattice. To this end, we have developed the methodology to calculate the initial optically excited electronic distribution in a material hit with a very short, polarized laser pulse, and used it to calculate the generation mechanism for short lived forces (several femtoseconds) driving phonon motion in bismuth. We have developed a first-principles based computational approach to predict how the rate of generation of phonons in a material due to electron-phonon coupling varies with time as the excited electronic distribution decays, and calculated how the generated phonon populations evolve with time as they couple anharmonically to other phonon modes in the system. We have used this to explain features present in recent time resolved diffuse x-ray scattering experiments on germanium. The main results: We have developed the capability to calculate atomic forces in the presence of an arbitrary photo-excited electronic distribution. The approach we have developed allows us to calculate the initial polarization- and energy-dependent optically excited distribution using first principles calculations. We used this method to investigate bismuth under ultrafast excitation, where very recent optical and x-ray measurements were available. Our calculated driving force was in excellent agreement with that implied by experiment. This work was published in a paper titled "First-Principles Calculation of Femtosecond Symmetry-Breaking Atomic Forces in Photoexcited Bismuth" in Physical Review Letters 114, 055502 (2015). In collaboration with Dr. Ivana Savic (Tyndall National Institute, Ireland), we have developed an approach to solve the phonon-phonon scattering rate equations and calculate the time evolution of phonon populations through anharmonic coupling to other phonons, using anharmonic interactions calculated from first principles using finite displacements of atoms in a supercell. This allows us to follow how the energy, that is initially imparted to a small number of phonons that are directly generated in the ultrafast optical excitation, is redistributed to other phonon modes in the system. In collaboration with Dr. Felipe Murphy-Armando (Tyndall National Institute, Ireland), we have added the capability to include phonon generation due to electron-phonon coupling as a function of time in this approach. As optically excited electrons lose energy, we have found that the rate at which particular phonons are generated can change dramatically on picosecond timescales. We used first principles calculations of the electron-phonon couplings to find the time-dependent rate of generation of phonons and include its contribution in our calculations of the rate of change of phonon populations due to phonon-phonon coupling. We have applied this to the case of photoexcited germanium, and used this method to explain interesting features seen in the diffuse x-ray scattering experiments of our collaborators in the group of Prof. David Reis (SLAC, Stanford, USA). The paper describing this work is in preparation. Conclusions, their potential impact and use and any socio-economic impact of the project. Please mention any target groups such as policy makers or civil society for whom the research could be relevant: The final results of this project will allow better simulation of photoexcited materials and their behaviour on picosecond timescales. This work will be highly relevant to the field of ultrafast science and will aid experimentalist in understanding phenomena observed in pump-probe and time-resolved x-ray scattering measurements. For theoreticians this work represents an important initial step towards developing a complete approach to calculate the time evolution of the optically excited electronic distribution and its eventual thermalisation.

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