Yeong Hwan Ko

Properties of wideband resonant reflectors under fully conical light incidence

Applying numerical modeling coupled with experiments, we investigate the properties of wideband resonant reflectors under fully conical light incidence. We show that the wave vectors pertinent to resonant first-order diffraction under fully conical mounting vary less with incident angle than those associated with reflectors in classical mounting. Therefore, as the evanescent diffracted waves drive the leaky modes responsible for the resonance effects, fully-conical mounting imbues reflectors with larger angular tolerance than their classical counterparts. We quantify the angular-spectral performance of representative resonant wideband reflectors in conic and classic mounts by numerical calculations with improved spectra found for fully conic incidence. Moreover, these predictions are verified experimentally for wideband reflectors fashioned in crystalline and amorphous silicon in distinct spectral regions spanning the 1200-1600-nm and 1600-2400-nm spectral bands. These results will be useful in various applications demanding wideband reflectors that are efficient and materially sparse.

Flat-top narrowband filters enabled by guided-mode resonance in two-level waveguides

Resonant nanogratings and periodic metasurfaces express diverse spectral and polarization properties on broadside illumination by incident light. Cooperative resonance interactions may yield shaped spectra for particular applications, in contrast to a multilayer dielectric mirror. Here, we provide guided-mode resonance filters with flat-top spectra suitable for wavelength division multiplexing systems. Applying a single one-dimensional grating layer sandwiched by two waveguides, we theoretically achieve high-efficiency flat-top spectra in the near-infrared region. This result is obtained by inducing simultaneous nearly degenerate resonant modes. The resonance separation under this condition controls the width of the flat-top spectrum. This means we can implement spectral widths ranging from a sub-nanometer to several nanometers applying fundamentally the same device architecture.

Wideband dielectric metamaterial reflectors: Mie scattering or leaky Bloch mode resonance?

Metamaterials are important, as they possess properties not found in simple materials. Photonic device technology applying metamaterials supports many new and useful applications. Here, we address the fundamental physics of wideband metamaterial reflectors. We show that these devices operate because of resonant leaky Bloch modes propagating in the periodic lattice. Moreover, in contrast to published literature, we demonstrate that Mie scattering in individual array particles is not a causal effect. In particular, by connecting the constituent particles by a matched sublayer and thereby destroying the Mie cavity, we find that the resonance bandwidth actually expands even though localized Mie resonances have been extinguished. There is no abrupt change in the reflection characteristics on addition of a sublayer to any metamaterial array consisting of discrete particles. Thus, the physics of the discrete and connected arrays is the same. The resonant Bloch mode picture is supported by numerous additional examples and analyses presented herein.

Flat-top bandpass filters enabled by cascaded resonant gratings

Narrow bandpass filters are applied in laser systems, imaging, telecommunications, and astronomy. Traditionally implemented with thin-film stacks, there is recent interest in alternative means incorporating photonic resonance effects. Here, we demonstrate a new approach to bandpass filters that engages the guided-mode resonance effect working with a cavity-based Fabry-Perot resonance to flatten and steepen the pass band. Both of these resonance mechanisms are native to simple resonant bandpass filters placed in a cascade. Numerical examples provide quantitative spectral properties including pass-band shape and sideband levels. Thus, we compare the spectra of single-layer 1D and 2D resonant gratings with the dual-cascade design incorporating identical gratings. Two- and three-cavity designs are measured against a classic multi-cavity thin-film filter with 151 layers. Whereas these initial results show comparable and improved results achieved with sparse structures, the challenge remains of developing a suitable fabrication technology to capitalize on this promise.

Modal Processes in Two-Dimensional Resonant Reflectors and Their Correlation With Spectra of One-Dimensional Equivalents

We explain modal processes in 2-D guided-mode resonance reflectors with subwavelength periods in terms of the mode structure of quasi-equivalent 1-D grating-based reflectors. The 1-D gratings are designed via a second-order effective-medium theory. The principal features in the reflection spectra of the 2-D devices show good quantitative agreement with the corresponding 1-D grating spectra for small modulation strength. Thereby, clear connections are established with the TE and TM modal states of a reflector. For reflectors made with silicon that has a high index of refraction, there is qualitative agreement between the 2-D spectra and the concomitant 1-D modal signatures. Two-dimensional reflectors with periodic rods and holes are treated. In both cases, it is found that the spectra are dominated by contributions from a single polarization state in the 1-D grating equivalent. The results and methods provided herein enable improved understanding of the physical properties of 2-D resonant reflectors and the related 2-D modulated devices, including photonic crystal slabs. Hence, this methodology facilitates the design of 2-D reflectors in general as is straightforwardly applied to device architectures, materials, and spectral regions beyond those treated here.

Guided-mode resonance nanophotonics: fundamentals and applications

We review principles and applications of nanophotonic devices based on electromagnetic resonance effects in thin periodic films. We discuss the fundamental resonance dynamics that are based on lateral Bloch modes excited by evanescent diffraction orders in these subwavelength devices. Theoretical and experimental results for selected example devices are furnished. Ultra-sparse nanogrids with duty cycle less than 10% are shown to provide substantially wide reflection bands and operate as effective polarizers. Narrow-passband resonant filters with extensive low sidebands are presented with focus on the zero-contrast grating architecture. This study is extended to long-wave operation in the THz region. Examples of fabricated guided-mode resonance devices with outstanding performance are given. This includes an unpolarized wideband reflector using serial single-layer reflectors, an ultra-sparse silicon nanowire grid as wideband reflector and polarizer, resonant bandpass filter with wide low sidebands, and a spatial/spectral filter permitting compact nonfocusing spatial filtering. The guided-mode resonance concept applies in all spectral regions, from the visible band to the microwave domain, with available low-loss materials.

Guided-mode resonance nanophotonics in materially sparse architectures

The guided-mode resonance (GMR) concept refers to lateral quasi-guided waveguide modes induced in periodic layers. Whereas these effects have been known for a long time, new attributes and innovations continue to appear. Here, we review some recent progress in this field with emphasis on sparse, or minimal, device embodiments. We discuss properties of wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material to eliminate local reflections and phase changes. This critical interface therefore possesses zero refractive-index contrast; hence we call them “zero-contrast gratings.” Applying this architecture, we present single-layer, wideband reflectors that are robust under experimentally realistic parametric variations. We introduce a new class of reflectors and polarizers fashioned with dielectric nanowire grids that are mostly empty space. Computed results predict high reflection and attendant polarization extinction for these sparse lattices. Experimental verification with Si nanowire grids yields ~200-nm-wide band of high reflection for one polarization state and free transmission of the orthogonal state. Finally, we present bandpass filters using all-dielectric resonant gratings. We design, fabricate, and test nanostructured single layer filters exhibiting high efficiency and sub-nanometer-wide passbands surrounded by 100-nm-wide stopbands.

Divergence-tolerant resonant bandpass filters.

Bandpass filters based on subwavelength dielectric gratings are grounded in physical principles that are totally distinct from their thin-film counterparts. Ease in fabrication, design scalability, material sparsity, and on-chip integration compatibility makes them a promising alternative especially for long-wavelength applications. Here we demonstrate the interesting attribute of resonant bandpass filters of high angular stability for fully conical light incidence. Fashioning an experimental bandpass filter with a subwavelength silicon grating on a quartz substrate, we show that fully conical incidence provides an angular full width at half-maximum linewidth of ∼9.5° compared to a linewidth of ∼0.1° for classical incidence. Slow angular variation of the central wavelength with full conical incidence arises via a corresponding slow angular variation of the resonant second diffraction orders driving the pertinent leaky modes. Moreover, full conical incidence maintains a profile with a single passband as opposed to the formation of two passbands characteristic of resonant subwavelength gratings under classical incidence. Our experimental results demonstrate excellent stability in angle, spectral profile, linewidth, and efficiency.

Fiber-facet-integrated guided-mode resonance filters and sensors: experimental realization

Guided-mode resonant (GMR) thin films integrated on fiber tips are known to realize compact filters and sensors. However, limited progress in experimental realization has been reported to date. Here we provide a considerable advance in this technology, as we experimentally demonstrate efficient fiber-facet mounted device prototypes. To retain a large aperture for convenient coupling, we design and fabricate silicon nitride-based resonators on the tip of a multimode fiber. We account for light propagation along the multimode fiber with exact numerical methods. This establishes the correct amplitude and phase distribution of the beam incident on the tip-mounted GMR element, thus enabling us to properly predict the resonance response. To fabricate the integrated GMR structures on the tips of fibers, we employ standard microfabrication processes, including holographic interference lithography and reactive-ion etching. The experimental results agree with simulation with an example device achieving high efficiency of ∼77% in transmission. To investigate fiber sensor operation, an etched silicon nitride fiber tip filter is surrounded with solutions of various refractive indices, yielding an approximate sensitivity of 200 nm/RIU.

Experimental demonstration of wideband multimodule serial reflectors

We demonstrate unpolarized wideband reflectors fashioned with orthogonal serial resonant reflectors. Unpolarized incident light generates internal TM- and TE-polarized reflections that are made to cooperate to extend the bandwidth of the composite spectral reflectance. The experimental results presented show ~42% band extension by a two-grating module. In addition, good angular tolerance is found because the orthogonal arrangement simultaneously supports classical and fully conic mountings at oblique angles. The resulting spectra form contiguous zero-order reflectance across wide spectral/angular regions. Furthermore, using a multimodule device with serial reflectors fabricated with silicon-on-quartz wafers with different device layer thicknesses, extreme band extension is achieved providing ~56% fractional bandwidth with reflectance exceeding 98%. These results imply potential for developing lossless unpolarized mirrors operating in diverse spectral regions of practical interest.