We have performed investigations of resonance effects inside a gallium nitride one-dimensional photonic crystal slab in order to enhance the second-harmonic generated from an beam incident on the surface of the slab. Convenient conditions on the incident beam propagation direction and polarization are first identified by experimental or theoretical linear optical studies. Giant enhancements in the second-harmonic conversion have been obtained by comparison with the unpatterned GaN layer. The combined role of the resonant coupling of the fundamental field and of the second-harmonic field has been observed by rotating the polarization of the fundamental beam.

We present a concept of graded photonic crystals used to enhance the control of light propagation. Gradual modifications of the lattice periodicity make it possible to bend the light at the micrometer scale. This effect is tailored by parametric studies of the isofrequency curves. As a demonstration, we propose a two-dimensional graded photonic crystal that could provide frequency-selective tunable bending. (c) 2005 Optical Society of America.

There is a major interest in fabricating III-nitride photonic structures for efficient nonlinear optical processes such as second-harmonic generation because they offer fairly large non linear coefficients and high optical damage thresholds. However, the use of III-nitrides has not been possible practically due to the high dispersion and the small birefringence for phase-matching. Recently, we have shown experimentally the enhancement of second-harmonic generation in epitaxial GaN-based photonic crystals [1]. By using calculated and experimental equifrequency surfaces, it is possible to identify the geometrical configurations that will allow quasi-phase matching to be satisfied and observed experimentally in the available wavelength tuning range of the laser. The second-harmonic field generated has been measured in reflection from the surface of photonic crystals etched into a GaN layer. A very large second-harmonic enhancement is observed when simultaneously the incident beam at the fundamental frequency ω excites a resonant Bloch mode and the second-harmonic field generated is coupled into a resonant Bloch mode at 2ω.

In order to determine the angular geometry that satisfies quasi-phase matching conditions for enhanced second-harmonic generation (SHG), the equi-frequency surfaces of the resonant photonic modes (that lie above the light line) of a one-dimensional GaN photonic crystal have been experimentally and theoretically studied as a function of frequency, angle of incidence, and azimuthal direction. Enhancement of the SHG has been observed when the angular configuration satisfies the quasi-phase matching conditions, i.e., when both the fundamental and second-harmonic fields coincide with resonant modes of the photonic crystal. The SHG enhancement achieved to the double resonance was 5000 times with respect to the unpatterned GaN layer. A smaller, but still substantially enhanced SHG level was also observed when the fundamental field is coupled into a resonant mode, while the second-harmonic field is not.