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.

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We present a new approach to dielectric metasurface design that relies on a single resonator per unit cell and produces robust, high quality-factor Fano resonances. Our approach utilizes symmetry breaking of highly symmetric resonator geometries, such as cubes, to induce couplings between the otherwise orthogonal resonator modes. In particular, we design perturbations that couple "bright" dipole modes to "dark" dipole modes whose radiative decay is suppressed by local field effects in the array. Our approach is widely scalable from the near-infrared to radio frequencies. We first unravel the Fano resonance behavior through numerical simulations of a germanium resonator-based metasurface that achieves a quality-factor of ~1300 at ~10.8 µm. Then, we present two experimental demonstrations operating in the near-infrared (~1 µm): a silicon-based implementation that achieves a quality-factor of ~350; and a gallium arsenide-based structure that achieves a quality-factor of ~600 -the highest near-infrared quality-factor experimentally demonstrated to date with this kind of metasurfaces. Importantly, large electromagnetic field enhancements appear within the resonators at the Fano resonant frequencies. We envision that combining high quality-factor, high field enhancement resonances with nonlinear and active/gain materials such as gallium arsenide will lead to new classes of active optical devices.Metasurfaces are currently the subject of intensive research worldwide since they can be tailored to produce a wide range of optical behaviors. However, metasurfaces generally exhibit broad spectral resonances, and it is difficult to obtain narrow (i.e. high quality-factor, Q) spectral features. Attaining such high-Q features from metasurfaces would greatly expand their application space, particularly in the areas of sensing, spectral filtering, and optical modulation. Early metasurfaces were fabricated from metals and exhibited particularly broad resonances at infrared and optical frequencies as a result of Ohmic losses. Dielectric resonator-based metasurfaces were introduced to overcome these losses and have enabled, among others, wave-front manipulation and cloaking devices, perfect reflectors, and ultrathin lenses [1-10] but, although absorptive losses were reduced, the metasurface resonances remained broad due to strong coupling with the external field (i.e. large radiation losses).Recently, new strategies based on "electromagnetically induced transparency" or "Fano resonances" have been developed that show great promise for achieving high-Q resonances [11][12][13][14][15]. In this approach, the resonator system is designed to support both "bright" and "dark" resonances. The incident optical field readily couples to the bright resonance, but cannot couple directly to the dark resonance. Through proper design, a weak coupling between the two resonances can be introduced, allowing energy from the incident wave to be indirectly coupled to the dark resonance. The metasurface transmission and reflection spectra resulting from ...

Pattern formation on surfaces undergoing low-energy ion bombardment is a common phenomenon. Here, a recently developed in situ spectroscopic light scattering technique was used to monitor periodic ripple evolution on Si(OO1) during Ar+ sputtering. Analysis of the rippling kinetics indicates that under high flux sputtering at low temperatures the concentration of mobile species on the surface is saturated, and, surprisingly, is both temperature and ion flux independent. This is due to an effect of ion collision cascades on the concentration of mobile species. This new understanding of surface dynamics during sputtering allowed us to measure straightforwardly the activation energy for atomic migration on the surface to be L2 f 0.1 eV. The technique is generalizable to any material, including high temperature and insulating materials for which surface migration energies are notoriously difficult to measure. PRL, 1999 Low energy (500 -LOO0 eV) ion bombardment is a common technique used in many thin film applications such as forming shallow junctions, sputter etching and deposition, ion beam assisted growth, reactive ion etching, and plasma assisted chemical vapor deposition. Under certain conditions, ion sputtering is known to produce patterns on surfaces. Features such as ripples, bumps or cones are common [ 1,2,3]. Typical length scales of these features are of order 10-1000 n m In some cases, these features are nuisances, such as in sample thinning for transmission microscopy or depth profiling by secondary ion mass spectroscopy. However these nano-scale patterns also hold promise in applications as varied as optical devices, templates for liquid crystal orientation, and strain-free patterned substrates for heteroepitaxial growth of quantum dots or wires.Rippling has been observed in amorphous materials (Si02 In this study, a recently developed in situ light scattering spectroscopic technique [7] was used to monitor ripple evolution on Si(OO1) during Arc sputtering. The technique allowed the first systematic study of the temporal and spatial evolution of both ripple wavelength and ripple amplitude as functions of both temperature and ion beam flux.Theoretical models describe rippling as arising from competition between ion beam roughening/etching and surface diffusion or viscous flow mediated relaxation 181. Our results here help illuminate the range of validity of these models. Additionally, we describe the method by which the activation energy for surface migration can be found.A salient feature of these experiments was our use of a very high ion flux compared to other rippling experiments. At high fluxes, the annihilation process for mobile species becomes dominated by collision cascades. Sputter-induced rippling can, in principle, occur on the surface of any material. As such, it holds great promise as a method for measuring the activation energy for surface migration on high temperature or insulating materials, which are generally notoriously difficult to measure.Rippling experiments were perf...