Direct visualization of phase-matched efficient second harmonic and broadband sum frequency generation in hybrid plasmonic nanostructures

Second harmonic generation and sum frequency generation (SHG and SFG) provide effective means to realize coherent light at desired frequencies when lasing is not easily achievable. They have found applications from sensing to quantum optics and are of particular interest for integrated photonics at communication wavelengths. Decreasing the footprints of nonlinear components while maintaining their high up-conversion efficiency remains a challenge in the miniaturization of integrated photonics. Here we explore lithographically defined AlGaInP nano(micro)structures/Al2O3/Ag as a versatile platform to achieve efficient SHG/SFG in both waveguide and resonant cavity configurations in both narrow- and broadband infrared (IR) wavelength regimes (1300-1600 nm). The effective excitation of highly confined hybrid plasmonic modes at fundamental wavelengths allows efficient SHG/SFG to be achieved in a waveguide of a cross-section of 113 nm x 250 nm, with a mode area on the deep subwavelength scale (lambda(2)/135) at fundamental wavelengths. Remarkably, we demonstrate direct visualization of SHG/SFG phase-matching evolution in the waveguides. This together with mode analysis highlights the origin of the improved SHG/SFG efficiency. We also demonstrate strongly enhanced SFG with a broadband IR source by exploiting multiple coherent SFG processes on 1 mu m diameter AlGaInP disks/Al2O3/Ag with a conversion efficiency of 14.8% MW(-1)which is five times the SHG value using the narrowband IR source. In both configurations, the hybrid plasmonic structures exhibit >1000 enhancement in the nonlinear conversion efficiency compared to their photonic counterparts. Our results manifest the potential of developing such nanoscale hybrid plasmonic devices for state-of-the-art on-chip nonlinear optics applications. Nonlinear optics: plasmonic nanostructures Plasmonic waveguides and microdisk cavities can provide an efficient means of nonlinear wavelength conversion of light in a chip-compatible platform. Zhe Li and coworkers from Ireland, China and Switzerland fabricated nano- and micro-scale waveguides and cavities from the compound semiconductor AlGaInP, which were then transferred to plasmonic substrates composed of SiO2/Si coated with thin layers of metal oxide (Al2O3) and silver (Ag). When pumped with near-infrared light in the 1300 to 1600 nm region, efficient second harmonic generation (SHG) and sum frequency generation (SFG) were observed with a conversion efficiency of up to 14.8% MW-1. Control experiments with the same structures on plain glass substrates indicate that the plasmonic versions offer >1000 times enhancement in the nonlinear conversion efficiency. The results suggest a promising future for integrated optics employing plasmonics.