Optical third-harmonic generation at interfaces

Optical third-harmonic generation (THG) is generally a weak process but is dipole allowed, therefore it occurs in all materials, including dielectric materials with inversion symmetry. We report that when using focused high-intensity ultrashort laser pulses, this normally weak THG process becomes highly operative at a simple air-dielectric interface and is much stronger than the bulk of most dielectric materials. We characterized this nonlinear optical response at interfaces as a phenomenological surface-enhanced THG in transmission and/or in reflection. This surface THG is further cascaded in transmission or reflection from layered composite dielectric materials of a high-low index of refraction, resulting in a marked increase in photon conversion efficiency than that of a single interface. Although the present THG efficiency is lower than that of a typical phase-matching harmonic crystal, it is important to note that the surface-enhanced optical THG is a fundamental physical process occurring at all interfaces and is relatively free from the constraint of a phase-matching condition and wavelength restriction. Using optical THG at an interface, it becomes possible to generate wavelengths at which harmonic crystals are unavailable. These findings may lead to a new development of surface-enhanced studies and prompt a reexamination of the processes of high-harmonic generation at interfaces using a focused beam.