Unraveling novel surface phenomena: intricacies of the nonlinear optical properties of aluminum through bound electrons

By uncovering novel aspects of second harmonic generation, we show that there are unusual, remarkable consequences of resonant absorption, namely an unexpectedly critical role that bound electrons play for light-matter interactions across the optical spectrum, suggesting that a new basic approach is required to fully explain the physics of surface phenomena. By tackling an issue that is never under consideration given the generic hostile conditions to the propagation of light under resonant absorption, we demonstrate through simulations and experimental observations of second harmonic generation from aluminum nanolayers that bound electrons are responsible for a unique signature neither predicted nor observed previously: a hole in the second harmonic spectrum. A hydrodynamic-Maxwell theory developed in other contexts explains these and other findings in metals, semiconductors, and conductive oxides exceptionally well and becomes the basis for renewed studies of surface physics.