What are hot electrons?

When a small metal particle is irradiated by light a portion of the absorbed energy is used to transfer electrons from the valence to the conduction band (interband transitions), the other part is absorbed by conduction electrons (intraband transitions). The specific heat of these conduction electrons is weak, thus these electrons can easily be raised to elevated temperatures. The details of how the electrons are initially distributed over the conduction band levels are different for each case (inter- and intraband mechanism). However, the excited electrons (so called non-thermal electrons) rapidly equilibrate via electron-electron scattering on a few 100 fs time scale to create a hot electron distribution (and the electrons are called hot electrons). The hot electron distribution then relaxes via electron-phonon scattering on a few picosecond time scale. During this time, the hot electrons, which can be given a temperature, are not in thermal equilibrium with the lattice. Their Fermi-Dirac distribution is therefore modified, part of the one-electron levels below the Fermi level being emptied whereas part of the levels above the Fermi level become occupied. This leads to a modification of the dielectric constant $varepsilon_m$ of a particle. This modification results in variation of the Surface Plasmon Resonance of the particle, which governs its optical properties. It is also the origin of the hot-electron contribution to the optical nonlinearity of particles.

The conclusion is that hot electrons are the electrons, induced by laser beams, that are not in thermal equilibrium with the lattice. During their relatively short existence they contribute to modification of the properties of plasmon-polaritons through the alteration of the dielectric function of a metal particle.