The role of a discontinuous free-electron density in harmonic generation from metal surfaces

We discuss a dynamical model of harmonic generation that arises from surfaces that demarcate regions of discontinuous free electron densities. These circumstances can arise from a simple metal mirror, to more complex structures such as layered structures composed of different metals or a metal and a conducting oxide. Using a modified hydrodynamic model we examine the simple case of a metal mirror, assuming the surface is characterized by an electron cloud that spills out into vacuum and shields the internal portions of the remaining medium. We also assess the relative importance of additional nonlinear sources that arise when a free electron discontinuity is present, and show that under the right circumstances both second and third harmonic generation can be very sensitive to the nature, density and thickness of the free electron cloud. Our findings suggest the possibility to control surface harmonic generation through surface charge engineering.