  By uncovering novel aspects of second harmonic generation in aluminum we show
that there are unusual and 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 different basic
approach is required to fully explain the physics of surfaces. We tackle an
issue that is never under consideration given the generic hostile conditions to
the propagation of light under resonant absorption. Unlike most noble metals,
aluminum displays Lorentz-like behavior and interband transitions centered near
810 nm, thus splitting the plasmonic range in an atypical manner and setting
its linear and nonlinear optical properties apart. Studies of aluminum
nanostructures having complex topologies abound, as do reported inconsistencies
in the linear spectral response of surface plasmons and harmonic generation.
Our experimental observations of second harmonic generation from aluminum
nanolayers show 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 explains these findings exceptionally
well and becomes the basis for renewed studies of surface physics.
