Strong guided mode resonant local field enhanced visible harmonic generation in an azo-polymer resonant waveguide grating

Guided mode resonance (GMR) enhanced second- and third-harmonic generation (SHG and THG) is demonstrated in an azo-polymer resonant waveguide grating (RWG), comprised of a poled azo-polymer layer on top of a textured SU8 substrate with a thin intervening layer of TiO₂. Strong SHG and THG outputs are observed by matching either incoming fundamental- or out-going harmonic-wavelength to the GMR wavelengths of the azo-polymer RWG. Without the azo-polymer coating, pure TiO₂ RWGs, do not generate any detectable SHG using a fundamental beam peak intensity of 2 MW/cm². Without the textured TiO₂ layer, a planar poled azo-polymer layer results in 3650 times less SHG than the full nonlinear RWG structure under identical excitation conditions. Rigorous coupled-wave analysis calculations confirm that this enhancement of the nonlinear conversion is due to strong local electric fields that are generated at the interfaces of the TiO₂ and azo-polymer layers when the RWG is excited at resonant wavelengths associated with both SHG and THG conversion processes.