Brighter than a Thousand Suns: Luminescence from Noble Metals

Metals usually appear shiny and reflect light, but even seemingly smooth mirrors can luminesce under photoexcitation, i.e. absorb and reemit light. Although this phenomenon has been known for almost 50 years, and plays a significant role in the interaction of light waves with plasmon modes of metallic nanostructures, given the inherent momentum mismatch between electrons and photons the microscopic origin of luminescence remains a controversial topic. In particular, the comparison of one-photon and multi-photon processes has remained challenging. Under UV excitation, interband transitions may occur, which could indeed induce luminescence by electron-hole recombination. But how is non-linear luminescence generated under excitation below the dominant interband transition?