Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide-mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes from UV to microwave frequencies in many different configurations. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto-optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry: refractive index and fluorescence biosensors, solar cells and photodetectors, signal processing, polarizers and wave plates, spectrometers, active tunable filters, mirrors for lasers and optical security features. The aim of this review is to discuss the latest developments in the field including numerical modeling, manufacturing, the physics, and applications of RWGs. Scientists and engineers interested in using RWGs for their application will also find links to the standard tools and references in modeling and fabrication according to their needs.

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Recent Advances in Resonant Waveguide Gratings

We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (approximately 4 cm(2)) that is scalable. We show efficiency improvements of approximately 70% that result not only from greater absorption, but also from electrical enhancements. The methodology is generally applicable to organic solar cells and the experimental findings reported in our manuscript corroborate theoretical expectations.

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Photonic Crystal Geometry for Organic Solar Cells

Self-assembled hydrogel materials constructed from cyclodextrin polymer (P-CD)/adamantane-modified poly acrylic acid (PAA-Ad) were designed and prepared via host-guest interactions. It was observed that the prepared supramolecular hydrogels had an interconnected three-dimensional porous network. In addition, the obtained hydrogels showed a recovery performance and it was confirmed that the host-guest interactions between β-cyclodextrin and adamantane were the main driving force for the formation of the hydrogels. The mechanical properties of the hydrogels could be adjusted by varying the concentrations of PAA-Ad. In particular, the prepared supramolecular hydrogels exhibited superior performances in water purification. The results demonstrated that the hydrogels possessed different mechanisms in the adsorption of the four typical poisonous organic dye molecules used, including bisphenol A (BPA), 4-aminoazobenzene (N-Azo), methylene blue (MB), and rhodamine B (RhB). The hydrogels mainly adsorbed N-Azo by host-guest interaction and adsorbed BPA by host-guest interaction and hydrogen bond synergy. They also adsorbed MB and RhB by hydrogen bonding and electrostatic interaction.

Facile preparation of self-assembled hydrogels constructed from poly-cyclodextrin and poly-adamantane as highly selective adsorbents for wastewater treatment.

Metasurfaces made up of subwavelength arrays of Mie scatterers can be engineered to control the optical properties of incident light. The hybridization of the fundamental Mie resonances with lattice resonances greatly enhances the scattering cross-section of individual Mie scatterers. Through careful design of the locations of these hybridized modes using two differently engineered hydrogenated amorphous silicon nanorods, we numerically calculate and experimentally fabricate two examples of full color printing; one with spectral colors comparable to the Adobe RGB gamut, and another with gradients of color. We identify and characterize the mechanisms behind each and provide a framework that can be used to design any all-dielectric metasurfaces of subwavelength Mie scatterers for spectral modulation.

Spectral Modulation through the Hybridization of Mie-Scatterers and Quasi-Guided Mode Resonances: Realizing Full and Gradients of Structural Color

Diffractive photonic devices manipulate light via local and nonlocal optical modes. Local devices, such as metasurfaces, can shape a wave front at multiple selected wavelengths, but inevitably modify light across the spectrum; nonlocal devices, such as grating filters, offer great frequency selectivity but limited spatial control. Here, we introduce a rational design paradigm using quasibound states in the continuum to realize multifunctional nonlocal devices: metasurfaces that produce narrow band spatially tailored wave fronts at multiple selected wavelengths and yet are otherwise transparent.

Multifunctional Nonlocal Metasurfaces.

Resonant waveguide gratings (RWGs) are thin-film structures, where coupled modes interfere with the diffracted incoming wave and produce strong angular and spectral filtering The combination of two finite-length and impedance matched RWGs allows the creation of a passive beam steering element, which is compatible with up-scalable fabrication processes Here, we propose a design method to create large patterns of such elements able to filter, steer, and focus the light from one point source to another The method is based on ellipsoidal mirrors to choose a system of confocal prolate spheroids where the two focal points are the source point and observation point, respectively It allows finding the proper orientation and position of each RWG element of the pattern, such that the phase is constructively preserved at the observation point The design techniques presented here could be implemented in a variety of systems, where large-scale patterns are needed, such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays

Light refocusing with up-scalable resonant waveguide gratings in confocal prolate spheroid arrangements

Transmission Microsphere-Assisted Dark-Field Microscopy

Resonant waveguide gratings (RWGs) are subwavelength structures of great interest for biosensors, optical filters and optical security applications. We demonstrate and characterize a beam steering device, where the in-coupling and out-coupling processes make use of different RWGs that share the same ultrathin dielectric waveguide. This device enables selective color-filtering and redirection of a white light source (such as a white LED). Furthermore, this structure is compatible with up-scalable fabrication processes such as roll-to-roll replication, and is relevant for high-volume production. Because of its color selectivity and its use in low coherence illumination conditions, such a beam steering device could be implemented in a variety of optical applications such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays.

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Color-Selective and Versatile Light Steering with up-Scalable Subwavelength Planar Optics

The invention relates to an optical combiner (1) comprising an array (2) of pairs of resonating waveguide gratings (23,25). The waveguiding layer (20) of the array (2), comprising the resonating waveguide gratings (23,25), is configured to guide at most ten guided light modes in the visible wavelength range. At least a portion of the resonating waveguide grating array (2) is configured to incouple a portion of incident light (10) on said array (2) and to coupled out a fraction of that incident light. The outcoupled fraction (10') has a predetermined wavelength λ in said visible and near-infrared wavelength range and has a predetermined spectral width Δλ. The optical combiner (1) is preferably configured to be used in a near- eye display apparatus (100). The invention is also achieved by a near-eye display apparatus (100) that is adapted to combine projected images provided by a light emitter (110) with light provided by a scene observed by an observer looking through the optical combiner (1).

Giorgio Quaranta

Papers

Recent Advances in Resonant Waveguide Gratings

Light refocusing with up-scalable resonant waveguide gratings in confocal prolate spheroid arrangements

Transmission Microsphere-Assisted Dark-Field Microscopy

Color-Selective and Versatile Light Steering with up-Scalable Subwavelength Planar Optics

Optical combiner and applications thereof