We recently introduced laminate metamaterials composed of a dielectric ABC layer sequence made by atomic-layer deposition. The ABC sequence breaks inversion symmetry, allowing for second-harmonic generation. Here, we discuss 3D polymeric woodpile photonic crystals conformally coated with such ABC laminate metamaterials (unpublished). In our experiments on such meta-crystals with 24 layers and 600 nm rod spacing at around 800-900 nm fundamental wavelength, we find up to 1000-fold enhancement of the second-harmonic conversion efficiency as compared to the same ABC laminate on a planar glass substrate (for 45 degrees angle of incidence with respect to the substrate and p-polarization).
To clarify the underlying mechanism, we have performed extensive numerical calculations based on solving the full-wave problem for the fundamental wave, computing the second-harmonic 3D source-term distribution assuming tensor elements for the ABC laminate as found previously, and numerically computing the resulting emitted second-harmonic wave. This analysis indicates that the enhancement is consistent with guided-mode resonant excitations at the fundamental wavelength inside of the 3D meta-crystal slab, leading to a standing-wave behavior providing beneficial local-field enhancements.
Recently introduced generalized Snell’s law provides wide possibilities for wavefront manipulations using metasurfaces. In contrast to conventional blazed gratings, their metasurface-based counterparts are planar (no grooves) and do not require complicated fabrication techniques. However, all previously demonstrated metasurfaces for anomalous reflection have revealed their deficiency due to parasitic energy coupling into non-desired diffraction modes. This negative effect becomes especially pronounced for metasurfaces designed to have a large separation angle between the incident and reflected beams. The reason is the used inaccurate approach for metasurface synthesis. It approximates that the surface is uniform on the sub-wavelength scale and the phase of the local reflection coefficient follows the linear profile dictated by the generalized reflection law. While the former assumption could be made, the latter one, as we show in the presentation, is incorrect. Moreover, the conventional synthesis approach does not take into account requirements on the amplitude of the local reflection coefficient. In the presentation, we will demonstrate an original alternative design scheme for metasurface gratings based on the generalized surface impedance model. It appears that perfect coupling of an incident plane wave into a single reflected plane wave requires energy channeling along the metasurface plane. To verify our design approach, we fabricate and measure an optical metasurface that reflects a normally incident beam at a very steep angle of 80 deg. The comparison of the two approaches shows that our new scheme provides double increase in the efficiency and complete absence of parasitic reflections.