Abstract:Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently, allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using Gallium Arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10 4 relative to 2 unpatterned GaAs. At the magnetic dipole resonance we measure an absolute nonlinear conversion efficiency of ~2 × 10 −5 with ~3.4 GW/cm 2 pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

All-dielectric metasurfaces, two-dimensional arrays of subwavelength low loss dielectric inclusions, can be used not only to control the amplitude and phase of optical beams, but also to generate new wavelengths through enhanced nonlinear optical processes that are free from some of the constraints dictated by the use of bulk materials. Recently, high quality factor (Q) resonances in these metasurfaces have been revealed and utilized for applications such as sensing and lasing. The origin of these resonances stems from the interference of two nanoresonator modes with vastly different Q. Here we show that nonlinear optical processes can be further enhanced by utilizing these high-Q resonances in broken symmetry all-dielectric metasurfaces. We study second harmonic generation from broken symmetry metasurfaces made from III–V semiconductors and observe nontrivial spectral shaping of second-harmonic and multifold efficiency enhancement induced by high field localization and enhancement inside the nanoresonators.

(Received s The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates *m is typically greater than 109/cm2. Such high dislocation densities degrade both the electronic so and photonic properties of the material. The density of dislocations can be decreased b~~@ orders of magnitude using cantilever epitaxy (CE), which employs prepattemed sapphire substrates to provide reduced-dmension mesa regions for nucleation and etched trenches ($)42 between them for suspended lateral growth of Gall or AIGaN. The substrate k prepattemed d~w ith narrow lines and etched to a depth that permits coalescence of laterally growing III-N~@ nucleated on the mesa stiaces before vertical growth fills the etched trench. Low a dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.The great potential of wide-band-gap Group III nitrides (III-N) has been limited in many applications by the very high density of treading dislocations (TDs) that form when the III-N materials are grown on latticemismatched substrates [1]. Growth of GaN on a planar substrate of sapphire, SiC, orSi(111) produces TD areal densities on~e order of 108to 1010/cm2.Although such high TD densltles do not appear to seriously degrade light-emitting diode (LED) performance due to the vertical character of the TDs and the short minority carrier difision lengths found in III-nitrides, they cause unacceptably short lifetimes for laser diodes (LDs) and excessive leakage current under reverse bias for p-n junction devices such as field-effect transistors (FETs) and high-electron-mobility transistors (HEMTs). To solve these problems, a GaN substrate with <1OG TDs/cm2 will be required.Several approaches have achieved considerable success in reducing TD densities to the 106/cm2range in selected regions of a wafer, but these techniques are very time-consuming to implement. These include epitaxial lateral overgrowth (ELO or LEO) [2,3], pendeoepitaxy (PE) [4], and lateral overgrowth from trenches (LOFT) [5]. While each technique produces selective areas on a wafer that possess the low TD densities (

Resonant semiconductor metasurfaces are an emerging versatile platform for nonlinear photonics. In this work, we investigate second-harmonic generation from metasurfaces consisting of two-dimensional square arrays of gallium arsenide nanocylinders as a function of the polarization of the fundamental wave. To this end, we perform nonlinear second harmonic microscopy, where the pump wavelength is tuned to the resonances of the metasurfaces. Furthermore, imaging the generated nonlinear signal in Fourier space allows us to analyze the spatial properties of the generated second harmonic. Our experiments reveal that the second harmonic is predominantly emitted into the first diffraction orders of the periodic arrangements, and that its intensity varies with the polarization angle of the fundamental wave. While this can be expected from the structure of the GaAs nonlinear tensor, the characteristics of this variation itself are found to depend on the pump wavelength. Interestingly, we show that the metasurface can reverse the polarization dependence of the second harmonic with respect to an unstructured GaAs wafer. These general observations are confirmed by numerical simulations using a simplified model for the metasurface. Our results provide valuable input for the development of metasurface-based classical and quantum light sources based on parametric processes.