Ultrafast electron-lattice coupling in cobalt films probed by nanoscale extreme ultraviolet transient gratings

Extreme ultraviolet transient gratings (EUV TG) are a recently developed tool, originally conceived for probing thermoelastic dynamics and more recently extended to magnetic dynamics through the use of core-resonant EUV probing. Resonant EUV probes also enable probing the ultrafast electronic response. Here we report on the observation of a prominent electronic response observed from a cobalt film in transmission geometry, when the photon energy is tuned to the M2,3 edge of the sample (59.6 eV). The observed dynamics is consistent with a population grating of electronic excitations, which represents the initial step of the EUV TG excitation process and, in a subpicosecond timescale, decays through electron-lattice coupling. On longer timescales, the resulting lattice excitation grating drives the thermoelastic response, regularly observed in EUV TG experiments and featured by acoustic oscillations that modulate the time decay of the lattice temperature grating. The ultrafast electronic relaxation dynamics is no longer observed when the photon energy is largely off resonance, indicating the need for a resonant EUV probe to detect it. While a photon energy scan in a narrow range around the edge does not show sizable changes in the electron-lattice relaxation dynamics, the showcased capability of imposing a nanoscale spatial modulation of the excitation pulse enables the possibility to use a variable EUV TG periodicity to study electronic relaxation dynamics at the nanoscale.