Abstract
Femtosecond laser excitation of solid-state systems creates non-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering we gain a detailed understanding of the complex non-equilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wavevector resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond timescale. We determine the evolving non-equilibrium phonon occupations which we find to deviate markedly from thermal occupations.
Highlights
Rapid CommunicationsFemtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and to lattice vibrations of the solid
The ability to measure dynamical processes in real time with femtosecond time resolution has in recent years enabled observations of unexpected, nonequilibrium dynamical phenomena [1,2,3,4]
PHYSICAL REVIEW B 101, 100302(R) (2020) (b) electron diffraction (UED) [7,18,29] to track the momentumresolved phonon occupation dynamics throughout the Brillouin zone (BZ), which we find to be distinctly different from a thermalized occupation for the first 4.9 ps after laser excitation
Summary
Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering, we gain a detailed understanding of the complex nonequilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wave-vector-resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy, a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond time scale. We determine the evolving nonequilibrium phonon occupations, which we find to deviate markedly from thermal occupations
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