# What are hot electrons?

What are they? How are they created? And what do they have to do with plasmons? I searched the web, but I would like more reliable and straightforward sources.

Hot electrons (a type of ‘hot carriers’) are electrons that have gained very high levels of kinetic energy after being accelerated by a strong electric field in areas of high field intensities within a semiconductor. Because of their high kinetic energy, they can get injected and trapped in areas where they shouldn’t be, forming a space charge that may cause the semiconductor to degrade or become unstable.

As for what they have to do with plasmons, here’s a quote from a relevant study (my second link below):

Hot electrons can be extracted from plasmonic particles and directed into a molecular electronic device, which represents a new mechanism of transfer from light to electronic transport.

Theory of hot electrons: general discussion (Behind paywall)

Exploiting Plasmon-Induced Hot Electrons in Molecular Electronic Devices

Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H2 on Au

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.