Second-Harmonic Generation at Simple Metal Surfaces

The frequency dependence of second-harmonic generation at simple metal surfaces is calculated by use of the time-dependent density-functional approach. The perpendicular surface contribution to the second-harmonic current is found to be significantly larger than had been assumed in previous work. For $2\omega$ close to the threshold for electron emission, the self-consistently screened nonlinear electronic response becomes resonantly enhanced in analogy to the well-known local field enhancement seen in linear response near $0.8{\omega }_p$, where ${\omega }_p$ is the bulk plasma frequency.