Angle-insensitive and narrow band grating filter with a gradient-index layer

Ultra-narrowband absorption filter based on a multilayer waveguide structure

We propose a six-layer waveguide structure embedded in a single-layer grating based on guided-mode resonance (GMR), which can realize ultra-narrowband filtering with a tunable resonance wavelength. The filtering characteristics were analyzed and calculated by rigorous coupled-wave analysis (RCWA) and COMSOL Multiphysics. The narrowband resonance wavelength and absorption are tuned by changing the geometry and physical parameters of the structure such as the grating period and width, layer thickness, and materials. We designed and calculated the full width at half maximum (FWHM) and resonance absorption spectra in detail under different polarization states of electromagnetic waves. We obtained an absorption FWHM of 8.51e-5 nm for the transverse electric (TE) mode and 0.023 nm for the transverse magnetic (TM) mode, with the absorption coefficients having a value over 99.2%. The GMR filtering structure shows a good sensitivity and figure of merit (FOM) for refractive index sensing. For instance, a very high FOM of 17782.6/RIU for TM incidence is observed. These structures can have possible applications in optical information devices and sensors.

Artificial Structural Colors and Applications

Structural colors are colors generated by the interaction between incident light and nanostructures. Structural colors have been studied for decades due to their promising advantages of long-term stability and environmentally friendly properties compared with conventional pigments and dyes. Previous studies have demonstrated many artificial structural colors inspired by naturally generated colors from plants and animals. Moreover, many strategies consisting of different principles have been reported to achieve dynamically tunable structural colors. Furthermore, the artificial structural colors can have multiple functions besides decoration, such as absorbing solar energy, anti-counterfeiting, and information encryption. In the present work, we reviewed the typical artificial structural colors generated by multilayer films, photonic crystals, and metasurfaces according to the type of structures, and discussed the approaches to achieve dynamically tunable structural colors.

A high speed electrically switching reflective structural color display with large color gamut

Structural colors, which originate from the interactions between light and nanometer-scale structured materials, have the advantages of durability and environmentally friendly display compared with pigments and dyes. A large color gamut, high-speed, electrically-switching reflective structural color display is critical to dynamically tunable reflective structural color devices. Here, we report a theoretical design of an electrically switching reflective structural color display device with a large color gamut (∼157% sRGB, standard red green blue) and high speed (>10 MHz). Benefiting from the electric-switchable Epsilon-Near-Zero material and 1D dielectric grating with guided-mode resonance, the reflective display device can be electrically turned on or turned off by switching between a narrow band reflector and a transparent film. This design provides a promising solution towards reflective color displays, optical switches, spatial light modulators and so on.

Improving the sensitivity of guided-mode resonance sensors under oblique incidence condition

We present an investigation on the use of oblique incidence condition to enhance the sensitivity of guided-mode resonance (GMR) sensors. By adjusting the incident angle, the enhancement of GMR sensitivity in non-subwavelength regime can be obtained. The measured results show that the bulk sensitivity of the GMR sensors with period of 809 nm climbs to 177% or 292% as the incident angle increases from 15° to 25° or 35°, respectively. The same trend is also obtained for the grating period of 994 nm. Simulations based on the rigorous coupled wave analysis (RCWA) method were performed, and we also built a new slab waveguide model to describe the relationship between bulk sensitivity and the incident angle. The present investigation demonstrates a new method for enhancing the bulk sensitivity of GMR sensor. Moreover, simple fabrication techniques can be utilized since a large grating period was used.

Ultra-broadband perfect absorber utilizing refractory materials in metal-insulator composite multilayer stacks

We present an ultra-broadband perfect absorber composed of metal-insulator composite multilayer (MICM) stacks by placing the insulator-metal-insulator (IMI) grating on the metal-insulator-metal (MIM) film stacks. The absorber shows over 90% absorption spanning between 570 nm and 3539 nm, with an average absorption of 97% under normal incidence. The ultra-broadband perfect absorption characteristics are achieved by the synergy of guided mode resonances (GMRs), localized surface plasmons (LSPs), propagating surface plasmons (PSPs), and cavity modes. The polarization insensitivity is demonstrated by analyzing the absorption performance over arbitrary polarization angles. The ultra-broadband absorption remains more than 80% over a wide incident angle up to 50°, for both transverse electric (TE) and transverse magnetic (TM) modes. The ultra-broadband perfect absorber has tremendous potential for various applications, such as solar thermal energy harvesting, thermoelectrics, and imaging.

Full-color reflector using vertically stacked liquid crystal guided-mode resonators

In this work, we proposed a full-color reflector using three stacked red (R), green (G), and blue (B) reflection gratings which are combined with the tunable 90° twisted nematic liquid crystals (TNLCs). The color reflector based on guided-mode resonance (GMR) gratings reflects strongly at the resonance wavelength. The optical reflectivity of GMR gratings can then be controlled by using 90° TNLCs to change the polarization of incident light. The optical characteristics and the chromaticity of the designed reflectors were evaluated by simulation. An individual RGB chip with/without LC was demonstrated experimentally. The fabricated GMR reflector for red exhibits a high TE/TM polarization ratio of >10:1 and 80% optical reflectivity at resonant wavelength, while the GMR reflector for blue only allows 60% optical reflectivity and a degraded polarization ratio of 3:1 mainly due to high optical absorption of silicon. Nevertheless, the silicon-based GMR reflector enables a wide reflection bandwidth, so a full-color reflector can be realized by vertically stacking RGB tunable reflectors. The proposed full-color reflector therefore exhibits a wide-gamut color space with low driving voltage of <3  V, showing its promise for use in energy-saving reflective information systems.

Wide-angle and polarization independent perfect absorber based on one-dimensional fabrication-tolerant stacked array

We propose a wide-angle, polarization independent and fabrication-tolerant perfect absorber, which is based on a one-dimensional stacked array consisted of vertically cascaded two pairs of metal-dielectric bilayers. The results show that the absorption peaks are over 99% at the wavelength of 5.25 μm for different polarization angles, and remain very high within wide ranges of incident and azimuthal angles. We attribute those excellent performances to the excitation of the magnetic resonance (MR) and the guided mode resonance (GMR) for the TM and TE polarization, respectively, and are further expounded by the inductor-capacitor (LC) circuit model and the eigen equation of the GMR, respectively. More importantly, this one-dimensional absorber is very robust to the spacing distance between the neighboring stacks and the metallic strip thickness, which releases degrees of freedom in design and makes the absorber extremely flexible and simple in fabrication, thus it can be a good candidate for many fascinating applications.