Excited carriers in noble metal nanostructures have recently attracted considerable attention for photocatalysis, optical detection, and photonic circuitry. However, most nanoarchitectures fail to yield highly energetic carriers, which are usually necessary for these applications. The main reason for this bottleneck is the inefficiency of intraband carrier absorption in metals, dictated by momentum conservation. Due to the small momentum of free-space photons, direct electronic transitions in the conduction band are forbidden under the dipole approximation.
In a recent paper in Nature Communications, the Harutyunyan lab showed that this selection rule can be lifted by using platforms with extreme electromagnetic confinement. By squeezing light into nanometer-scale gaps, they effectively create “large momentum photons” that can be absorbed by metal electrons, efficiently promoting intraband transitions. Using this approach, Lemasters et al. demonstrate that highly energetic carriers can be excited in gold that are not thermally equilibrated, i.e., they cannot be described by a single temperature or Fermi-Dirac distribution. These non-Fermi electronic distributions can be promising for applications in photovoltaics, hot carrier-induced catalysis, and other photonic platforms. For more details see: https://www.nature.com/articles/s41467-024-48928-4