Phase-matched third-harmonic generation via doubly resonant optical surface modes in 1D photonic crystals

Abstract

Efficient nonlinear conversion requires that interacting optical waves maintain a consistent phase relationship when traveling in a medium despite its dispersion. Birefringent phase-matching, which is often used to compensate for the dispersion, is not applicable to optically isotropic nonlinear materials. Here, we present a one-dimensional photonic crystal structure that allows the propagation of optical surface waves, both at the fundamental and third-harmonic frequencies, as an efficient medium for phase-matched third-harmonic generation. A unique advantage of this structure is that the effective refractive indices for the surface waves are similar to the refractive index of air at both frequencies. This allows phase-matching between the first and third harmonics, and a visible collinear beam of the third harmonic is produced at the prism-coupled output. Moreover, these optical surface waves propagate over long distances even if a lossy nonlinear nanofilm is deposited onto the photonic crystal surface. We provide experimental results for third-harmonic generation at a wavelength of 410 nm for a bare dielectric Ta2O5/SiO2 multilayer structure and for the same structure coated with a 15-nm GaAs film. A one-dimensional photonic-crystal structure acts as an efficient medium for the nonlinear process of third-harmonic generation. Valery Konopsky and co-workers from the Institute of Spectroscopy in Moscow and the Moscow Institute of Physics and Technology fabricated a onedimensional photonic crystal that consists of alternating layers of high-refractive-index Ta2O5 (index of 2.05) and low-index SiO2 (index of 1.46). Importantly, the structure provides very similar refractive indices for the fundamental and third-harmonic beams, allowing them to be phase matched as they propagate as surface waves along the structure. The team reports generation of 11 microwatt of third-harmonic light that has a wavelength of 411.6 nanometres and shows that optical surface waves propagate for long distances even when a lossy nonlinear nanofilm is deposited onto the surface of the photonic crystal.