Polarization tunable all-dielectric color filters based on cross-shaped Si nanoantennas

Mie Resonances Enabled Subtractive Structural Colors with Low-Index-Contrast Silicon Metasurfaces

All-dielectric structural colors are attracting increasing attention due to their great potential for various applications in display devices, imaging security certification, optical data storage, and so on. However, it remains a great challenge to achieve vivid structural colors with low-aspect-ratio silicon nanostructures directly on a silicon substrate, which is highly desirable for future integrated optoelectronic devices. The main obstacle comes from the difficulty in achieving strong Mie resonances by Si nanostructures on low-index-contrast substrates. Here, we demonstrate a generic principle to create vivid bright field structural colors by using silicon nanopillars directly on top of the silicon substrate. Complementary colors across the full visible spectrum are achieved as a result of the enhanced absorption due to Mie resonances. It is shown that the color saturation increases with the increasing of the nanopillar height. Remarkably, blue and black colors are generated by trapezoid nanopillar arrays as a result of the absorption at long wavelengths or all visible wavelengths. Our strategy provides a powerful scheme for accelerating the integrated optoelectronic applications in nanoscale color printing, imaging, and displays.

Full-color reflective filter in a large area exploiting a sandwiched metasurface

Metasurface-based color filters show great potential in imaging devices and color printing. However, it is still a great challenge to meet the high demand for large-area flexible displays with structural color filters. Here, a reflective color filter is developed with a sandwiched metasurface, where the photoresist grating, complementary silver grating and silicon nitride grating are sequentially stacked on the substrate. Analytical results show that bandpass reflective spectra can be achieved due to the combined influence of guided mode resonance and cavity resonance, and full-spectrum colors including three primary colors can be generated by merely varying the period of the metasurface. With only photolithography and deposition technology involved, large-area samples incorporating pixelated metasurfaces are easily fabricated. Metasurfaces with three periods of 540 nm, 400 nm and 320 nm are experimentally obtained having peak reflective efficiency of ∼ 60%, demonstrating red, green and blue colors as theoretical results. A stripe sample with the structural period varying from 250 nm to 550 nm is fabricated in an area of 10 mm × 30 mm, displaying full-color reflections as simulated. Finally, with metasurfaces of three structural periods, the pixelated Soochow University logo is fabricated in a larger area of ∼ 30 mm × 30 mm. Therefore, the proposed structure shows high compatible to roll-to-roll nano-imprinting for large-area flexible displays, with the photoresist film can be easily substituted by UV film in addition.

All-dielectric multi-resonant bullseye antennas

Integrated devices that generate multiple optical resonances in the same volume can enhance on-chip nonlinear frequency generation, nonlinear spectroscopy, and quantum sensing. Here, we demonstrate circular Bragg antennas that exhibit multiple spatially overlapping, polarization-selective optical resonances. Using templated atomic layer deposition of TiO₂, these devices can be fabricated on arbitrary substrates, making them compatible with a wide range of nonlinear materials and sensing targets, and couple efficiently to underlying films. In this work, we detail the design, simulation, and fabrication of all-dielectric multi-resonant bullseye antennas and characterize their performance using polarized broadband reflection spectroscopy.

Resonant subwavelength control of the phase of spin waves reflected from a Gires-Tournois interferometer

Subwavelength resonant elements are essential building blocks of metamaterials and metasurfaces, which have revolutionized photonics. Despite similarities between different wave phenomena, other types of interactions can make subwavelength coupling significantly distinct; its investigation in their context is therefore of interest both from the physics and applications perspective. In this work, we demonstrate a fully magnonic Gires–Tournois interferometer based on a subwavelength resonator made of a narrow ferromagnetic stripe lying above the edge of a ferromagnetic film. The bilayer formed by the stripe and the film underneath supports two propagative spin-wave modes, one strongly coupled with spin waves propagating in the rest of the film and another almost completely reflected at the ends of the bilayer. When the Fabry–Perot resonance conditions for this mode are satisfied, the weak coupling between both modes is sufficient to achieve high sensitivity of the phase of waves reflected from the resonator to the stripe width and, more interestingly, also to the stripe-film separation. Such spin-wave phase manipulation capabilities are a prerequisite for the design of spin-wave metasurfaces and may stimulate development of magnonic logic devices and sensors detecting magnetic nanoparticles.

Structural-color nanoprinting with hidden watermarks

Nanostructured metasurfaces can manipulate the spectrum and polarization of incident light at the nanoscale, which suggests a new integration of color nanoprints and polarizing-related components. Herein, we design and experimentally demonstrate a structural-color nanoprint carrying hidden watermarks, enabled with the polarization-assisted spectrum manipulation of light. Specifically, under unpolarized white light, the watermarks are concealed and a structural-color nanoprinting-image occupies the metasurface plane. Meanwhile, once linearly polarized white light is incident on the same metasurface, the hidden information can be decoded, and the same nanoprinting-image covered with watermarks appears. The proposed metasurface represents a paradigm for displaying color nanoprinting-images with or without watermarks, showing a flexible switch between the two operating modes and providing an easily camouflaged scheme for anticounterfeiting, encryption, information multiplexing, high-density optical storage, etc.

Bright Field Structural Colors in Silicon-on-Insulator Nanostructures

Structural coloration with artificially nanostructured materials is emerging as a prospective alternative to traditional pigments for the high resolution, sustainable recycling, and long-time durability. However, achieving bright field structural colors with dielectric nanostructures remains a great challenge due to the weak scattering in an asymmetric environment. Here, we demonstrate all-dielectric bright field structural colors with diffraction-limited resolution on the silicon-on-insulator platform. The backscattering is strongly enhanced from the constructive interference between Mie resonances of individual Si antennas and Fabry-Perot resonances supported by the SiO₂ layer. The fabricated colors with varying hues and saturations show strong insensitivity with respect to the interparticle spacing and, remarkably, the viewing angle under resonant conditions. Compared with creating a quasi-homogeneous environment, our strategy is solid and complementary metal-oxide semiconductor integrable, paving the way for practical applications of structural colors in nanoscale color printing, microdisplays, and imaging.

Dynamically tunable transmissive color filters using ultra-thin phase change materials

Structural color filters (i.e. plasmonics and nano-cavities) provide vivid and robust color filtering in applications such as CMOS image sensors but lack simplicity in fabrication and dynamic tuning. Here we report a dynamically tunable, transmissive color filter by incorporating an ultra-thin phase change layer inside a thin-film optical resonator. The transmitted color spectrum can be designed over the entire visible range and shifted by around 50 nm after phase transition. Angle dependence shows little color variation within a ±30° viewing angle. Crucially, only film deposition is required to fabricate our phase change color filter, showing great potential for large-scale and inexpensive production. The dynamically tunable color filter, described in this paper, could be a promising component in display, CMOS sensor, and solar cell technology.

Polarization tunable color filters based on all-dielectric metasurfaces on a flexible substrate

Structural color filters based on all-dielectric materials are considered to be promising alternatives to metal nanostructures due to significant advantages, such as high-quality resonance effects and low losses of Ohmic effects. We demonstrate a polarization tunable color filter based on all-dielectric metasurfaces, which is based on the arrays of asymmetric monocrystalline silicon nanoblocks on the flexible substrate. By adjusting the physical dimensions of nanoblocks, the filter can exhibit a variety of bright transmission colors. Furthermore, the designed dielectric metasurfaces are sensitive to the linear polarization direction of the incident light, thus a wide range of color images can be created by changing the polarization angles. All of the color filter including the dielectric silicon nanoblocks, the overcladding, and the flexible substrate can be delaminated from the handler substrates and the optical property is reconfigurable, which will find applications in the functional color display, polarization detection and imaging, and secured optical tag.

Light guiding, bending, and splitting via local modification of interfaces of a photonic waveguide

A general approach to the surface control of the localization, guiding, and redirecting of volume-mode light in photonic waveguides via tailoring their interfaces (surfaces) is proposed. The approach is demonstrated for dielectric rod-type photonic crystal slabs, whose regular and defect parts are distinguished by whether the nanocylinders are covered by metal caps. Thus, the rod-array part of the structure is not changed, while the local modifications are only applied to the interfaces. The basic functionalities, i.e., localized volume wave guiding, bending, and splitting are achievable. Selective dual-mode operation is possible due to the co-existence of a defect mode and a chainlike mode in one structure.

Structural color filters based on an all-dielectric metasurface exploiting silicon-rich silicon nitride nanodisks

An all-dielectric metasurface is deemed to serve a potential platform to demonstrate spectral filters. Silicon-rich silicon nitride (SRN), which contains a relatively large portion of silicon, can exhibit higher refractive indices, when compared to silicon nitride. Meanwhile, the extinction coefficient of SRN is smaller than that of hydrogenated amorphous silicon, leading to reduced absorption loss in the shorter wavelength. SRN is therefore recommended as a scattering element from the perspective of realizing all-dielectric metasurfaces. In this work, we propose and embody a suite of highly efficient structural color filters, capitalizing on a dielectric metasurface that consists of a two-dimensional array of SRN nanodisks that are embedded in a polymeric layer. The SRN nanodisks may support the electric dipole (ED) and magnetic dipole (MD) resonances via Mie scattering, thereby leading to appropriate spectral filtering characteristics. The ED and MD are identified from field profile observation with the assistance of finite-difference time-domain simulations. The manufactured color filters are observed to produce various colors in both transmission and reflection modes throughout the visible band, giving rise to a high transmission of around 90% in the off-resonance region and a reflection ranging up to 60%. A variety of colors can be realized by tuning the resonance by adjusting the structural parameters such as the period, diameter, and height of the SRN nanodisks. The spectral position of resonances might be flexibly tuned by tailoring the polymer surrounding the SRN nanodisks. It is anticipated that the proposed coloring devices will be actively used for color displays, imaging devices, and photorealistic color printing.

Toward Electrically Tunable, Lithography-Free, Ultra-Thin Color Filters Covering the Whole Visible Spectrum

The possibility of real-time tuning of optical devices has attracted a lot of interest over the last decade. At the same time, coming up with simple lithography-free structures has always been a challenge in the design of large-area compatible devices. In this work, we present the concept and the sample design of an electrically tunable, lithography-free, ultra-thin transmission-mode color filter, the spectrum of which continuously covers the whole visible region. A simple Metal-Insulator-Metal (MIM) cavity configuration is used. It is shown that using the electro-optic dielectric material of 4-dimethyl-amino-N-methyl-4-stilbazoliumtosylate (DAST) as the dielectric layer in this configuration enables efficient electrical tuning of the color filter. The total thickness of the structure is 120 nm, so it is ultra-thin. The output color gets tuned from violet to red by sweeping the applied voltage from −12 to +12 Volts (V). We present an in-detail optimization procedure along with a simple calculation method for the resonance wavelength of the MIM cavity that is based on circuit theory. Such power-efficient structures have a large variety of potential applications ranging from optical communication and switching to displays and color-tunable windows.

Plasmonic- and dielectric-based structural coloring: from fundamentals to practical applications

Structural coloring is production of color by surfaces that have microstructure fine enough to interfere with visible light; this phenomenon provides a novel paradigm for color printing. Plasmonic color is an emergent property of the interaction between light and metallic surfaces. This phenomenon can surpass the diffraction limit and achieve near unlimited lifetime. We categorize plasmonic color filters according to their designs (hole, rod, metal-insulator-metal, grating), and also describe structures supported by Mie resonance. We discuss the principles, and the merits and demerits of each color filter. We also discuss a new concept of color filters with tunability and reconfigurability, which enable printing of structural color to yield dynamic coloring at will. Approaches for dynamic coloring are classified as liquid crystal, chemical transition and mechanical deformation. At the end of review, we highlight a scale-up of fabrication methods, including nanoimprinting, self-assembly and laser-induced process that may enable real-world application of structural coloring.

Liquid-crystal tunable color filters based on aluminum metasurfaces

Designing color pixels using plasmonic nanostructures and metasurfaces has become a luring area of research in recent years. Here, we experimentally demonstrated the voltage tunability of a dynamic plasmonic color filter by using an aluminum grating integrated with the nematic liquid crystal (LC). Along with a typical substrate coated with rubbed polyimide film, the aluminum grating itself serves as a molecular alignment layer to form a twisted LC cell. This hybrid structure allows electrically controlled transmission color by applying the voltage. A significant spectral tunability of such a device has been demonstrated by applying the small voltage from 0 to 4 V_rms.