Subwavelength resonant lattices fashioned as nano- and microstructured films represent a basis for a host of device concepts. Whereas the canonical physical properties are fully embodied in a one-dimensional periodic lattice, the final device constructs are often patterned in two-dimensionally-modulated films in which case we may refer to them as photonic crystal slabs, metamaterials, or metasurfaces. These surfaces can support lateral modes and localized field signatures with propagative and evanescent diffraction channels critically controlling the response. The governing principle of guided-mode, or lattice, resonance enables diverse spectral expressions such that a single-layer component can behave as a sensor, reflector, filter, or polarizer. This structural sparsity contrasts strongly with the venerable field of multi-layer thin-film optics that is basis for most optical components on the market today. The lattice resonance effect can be exploited in all major spectral regions with appropriate low-loss materials and fabrication resources. In this paper, we highlight resonant device technology and present our work on design, fabrication, and characterization of optical elements operating in the near-IR, mid-IR, and long-wave IR spectral regions. Examples of fabricated and tested devices include biological sensors, high-contrast-ratio polarizers, narrow-band notch filters, and wideband high reflectors.
Periodic optical lattices consisting of isolated-particle arrays in vacuum are treated with rigorous electromagnetics. These structures possess a wealth of interesting properties including perfect reflection across small or large spectral bandwidths depending on the choice of materials and design parameters. Pertinent spectral expressions have been observed theoretically and experimentally via one-dimensional (1D) and two-dimensional (2D) structures commonly known as resonant gratings, metamaterials, and metasurfaces. The physical cause of perfect reflection and related properties is guided-mode resonance mediated by lateral Bloch modes excited by evanescent diffraction orders in the subwavelength regime. Here, we review recent results on differentiation of local Mie resonance and guided-mode lattice resonance in causing resonant reflection by periodic particle assemblies. We treat a classic 2D periodic array consisting of dielectric spheres. To disable Mie resonance, we apply antireflection (AR) coatings to the spheres. Reflectance maps for coated and uncoated spheres demonstrate that perfect reflection persists in both cases. We find that the Mie scattering efficiency of an AR-coated sphere is greatly diminished. Additionally, in a 1D cylindrical rod-type lattice, we investigate and compare local field profiles in periodic assemblies and in the constituent isolated particles. In general, the lattice and particle resonance wavelengths differ. When the lateral leaky-mode field profiles approach the isolated-particle Mie field profiles, the resonance locus tends towards the Mie resonance wavelength. This correspondence is referred to as Mie modal memory. These fundamentals may help distinguish Mie effects and leaky-mode effects in generating the observed spectra in this class of optical devices while elucidating the basic resonance properties across the entire spectral domain.
Periodic arrays of resonant dielectric nano- or microstructures provide perfect reflection across spectral bands whose extent is controllable by design. At resonance, the array yields this result even in a single subwavelength layer fashioned as a membrane or residing on a substrate. The resonance effect, known as guided-mode resonance, is basic to modulated films that are periodic in one dimension (1D) or in two dimensions (2D). It has been known for 40 years that these remarkable effects arise as incident light couples to leaky Bloch-type waveguide modes that propagate laterally while radiating energy. Perfect reflection by periodic lattices derives from the particle assembly and not from constituent particle resonance. We show that perfect reflection is independent of lattice particle shape in the sense that it arises for all particle shapes. The resonance wavelength of the Bloch-mode-mediated zero-order reflectance is primarily controlled by the period for a given lattice. This is because the period has direct, dominant impact on the homogenized effective-medium refractive index of the lattice that controls the effective mode index experienced by the mode generating the resonance. In recent years, the field of metamaterials has blossomed with a flood of attendant publications. A significant fraction of this output is focused on reflectors with claims that local Fabry-Perot or Mie resonance causes perfect reflection with the leaky Bloch-mode viewpoint ignored. In this paper, we advance key points showing the essentiality of lateral leaky Bloch modes while laying bare the shortcomings of the local mode explanations. The state of attendant technology with related applications is summarized. The take-home message is that it is the assembly of particles that delivers all the important effects including perfect reflection.
We review guided-mode resonant photonic lattices by addressing their functionalities and potential device applications. The 1D canonical model is rich in properties and conceptually transparent, with all the main conclusions being applicable to 2D metasurfaces and periodic photonic slabs. We explain the operative physical mechanisms grounded in lateral leaky Bloch modes. We summarize the band dynamics of the leaky stopband. With several examples, we demonstrate that Mie scattering is not causative in resonant reflection. Illustrated applications include a wideband reflector at infrared bands as well as resonant reflectors with triangular profiles. We quantify the improved efficiency of a silicon reflector operating in the visible region relative to loss reduction as realizable with sample hydrogenation. A resonant polarizer with record performance is presented.
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