Periodic guided-mode resonance structures which provide perfect reflection across sizeable spectral bandwidths have been known for decades and are now often referred to as metasurfaces and metamaterials. Although the underlying physics for these devices is explained by evanescent-wave excitation of leaky Bloch modes, a growing body of literature contends that local particle resonance is causative in perfect reflection. Here, we address differentiation of Mie resonance and guided-mode resonance in mediating resonant reflection by periodic particle assemblies. We treat a classic 2D periodic array consisting of silicon spheres. To disable Mie resonance, we apply an optimal antireflection (AR) coating to the spheres. Reflectance maps for coated and uncoated spheres demonstrate that perfect reflection persists in both cases. It is shown that the Mie scattering efficiency of an AR-coated sphere is greatly diminished. The reflectance properties of AR-coated spherical arrays have not appeared in the literature previously. From this viewpoint, these results illustrate high-efficiency resonance reflection in Mie-resonance-quenched particle arrays and may help dispel misconceptions of the basic operational physics.

We demonstrate band flip in one-dimensional dielectric photonic lattices presenting numerical and experimental results. In periodic optical lattices supporting leaky Bloch modes, there exists a second stop band where one band edge experiences radiation loss resulting in guided-mode resonance (GMR), while the other band edge becomes a nonleaky bound state in the continuum (BIC). To illustrate the band flip, band structures for two different lattices are provided by calculating zero-order reflectance with respect to wavelength and incident angle. We then provide three photonic lattices, each with a different fill factor, consisting of photoresist gratings on Si3N4 sublayers with glass substrates. The designs are fabricated using laser interferometric lithography. The lattice parameters are characterized and verified with an atomic force microscope. The band transition under fill-factor variation is accomplished experimentally. The measured data are compared to simulation results and show good agreement.

Resonant periodic nanostructures provide perfect reflection across small or large spectral bandwidths depending on the choice of materials and design parameters. This effect has been known for decades, observed theoretically and experimentally via onedimensional and two-dimensional structures commonly known as resonant gratings, metamaterials, and metasurfaces. The physical cause of this extraordinary phenomenon is guided-mode resonance mediated by lateral Bloch modes excited by evanescent diffraction orders in the subwavelength regime. In recent years, hundreds of papers have declared Fabry-Perot or Mie resonance to be basis of the perfect reflection possessed by periodic metasurfaces. Treating a simple one-dimensional cylindrical-rod lattice, here we show clearly and unambiguously that Mie resonance does not cause perfect reflection. In fact, the spectral placement of the Bloch-mode-mediated zero-order reflectance is primarily controlled by the lattice period by way of its direct effect on the homogenized effective-medium refractive index of the lattice. In general, perfect reflection appears away from Mie resonance. However, when the lateral leaky-mode field profiles approach the isolated-particle Mie field profiles, the resonance locus tends towards the Mie resonance wavelength. The fact that the lattice fields "remember" the isolated particle fields is referred here as "Mie modal memory." On erasure of the Mie memory by an index-matched sublayer, we show that perfect reflection survives with the resonance locus approaching the homogenized effective-medium waveguide locus. The results presented here will aid in clarifying the physical basis of general resonant photonic lattices.

We treat operational principles and resulting properties of periodic photonic lattices. These lattices can be fashioned in the lab as 2D or 1D patterned films on substrates or modeled as membrane particle arrays for simplest geometry. Even though the basic periodic element, namely the diffraction grating, has been known for 100+ years, new solutions and applications based on periodic spatial modulations continue to appear. In recent literature, corresponding assemblies and devices are called metamaterials or metasurfaces. Current lithographic technology enables fabrication of spatial modulations on subwavelength scales in one, two, or three dimensions. The resulting diffractive optical elements or metasurfaces may support waveguide modes when designed with proper refractive indices and dimensions. Waveguide modes that are guided or quasi-guided in waveguide gratings experience stopbands and passbands as the light frequency is varied. Associated with these leaky bands, there appear resonant bright channels and non-resonant dark channels in the spectra. The bright state corresponds to a high reflectivity guided-mode resonance (GMR) whereas the dark channel represents a bound state in the continuum (BIC). We review past results placing these physical observations in historical context. The BIC states correspond to symmetry-blocked resonance channels. In earlier work, the BIC was often referenced as the non-leaky band edge. We show theoretical dispersion charts and experimental sampling of those revealing GMR and BIC channels. Along similar lines, recent results show experimental lattice band dynamics of simple resonance systems. Finally, the connection of the classic Rytov effective medium formalism to the GMR/BIC states is presented. We show that all key resonancelattice properties are embodied in Rytov-equivalent homogeneous waveguides. This interesting fact overwhelmingly proves the waveguide character of the lattice systems.

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