Conversion efficiency for second-harmonic generation in photonic crystals

The potential of nonlinear optical phenomena in adding functionality to nanostructures like nanoparticles or photonic crystal waveguides are presented. The role of field localization and system resonances is discussed in the light of few examples in which harmonic generation is being drastically enhanced by these effects. In particular the results on single nanoparticle harmonic response are discussed as the basic example of nonlinear interaction of intense radiation with a single nanoparticle. Here the driving resonance mechanism of nonlinear interaction is proven to be due to surface plasmon oscillations that give rise to strong harmonic enhancement when proper resonance conditions are established. This response in further enriched when assemblies of nanoparticles are considered next, illustrating the role of the surrounding medium, particle shape and orientation and temperature. Switching from metals to semiconductors leads to exploitation of photonic resonances as opposed to plasmonic ones of previous examples. In photonic planar structures the nonlinear response gains also from the notion of quasi-phase matching introduced by the periodicity of the system and from exploiting leaky (or quasi-guided) optical modes that considerably expand the possibilities for an optimised output. Examples based on second and third harmonic generation from such systems are discussed in details. Finally, potential new routes for exploiting higher harmonic efficiencies or very broad-band resonance conditions are briefly mentioned