Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened ...

Nanophotonic Structural Colors

Abstract Structural coloration using metasurfaces has been steadily researched to overcome the limitations of conventional color printing using pigments by improving the resolution, lowering the toxicity, and increasing the durability. Many metasurfaces have been demonstrated for dynamic structural coloration to convert images at the visible spectrum. However, the previous works cannot reach near-zero scattering when colors are turned-off, preventing it from being cryptographic applications. Herein, we propose a completely on/off switchable structural coloration with polarization-sensitive metasurfaces, enabling full-colored images to be displayed and hidden through the control of the polarization of incident light. It is confirmed that the nanostructure exhibits the polarization-dependent magnetic field distributions, and near-zero scattering is realized when the polarization of incident light is perpendicular to the long axis of the nanofins. Also, the metasurfaces are made up of triple-nanofin structures whose lengths affect locations of resonance peaks, resulting in full-color spectrum coverages. With such advantages, a QR code image, a two-color object image, and an overlapped dual-portrait image are obtained with the metasurfaces. Such demonstrations will provide potential applications in the fields of high-security information encryption, security tag, multichannel imaging, and dynamic displays.

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Near-zero reflection of all-dielectric structural coloration enabling polarization-sensitive optical encryption with enhanced switchability

A multispectral image camera captures image data within specific wavelength ranges in narrow wavelength bands across the electromagnetic spectrum. Images from a multispectral camera can extract a additional information that the human eye or a normal camera fails to capture and thus may have important applications in precision agriculture, forestry, medicine, and object identification. Conventional multispectral cameras are made up of multiple image sensors each fitted with a narrow passband wavelength filter and optics, which makes them heavy, bulky, power hungry, and very expensive. The multiple optics also create an image co-registration problem. Here, we demonstrate a single sensor based three band multispectral camera using a narrow spectral band red–green–blue color mosaic in a Bayer pattern integrated on a monochrome CMOS sensor. The narrow band color mosaic is made of a hybrid combination of plasmonic color filters and a heterostructured dielectric multilayer. The demonstrated camera technology has reduced cost, weight, size, and power by almost n times (where n is the number of bands) compared to a conventional multispectral camera.

/pdf/a-single-sensor-based-multispectral-imaging-camera-using-a-2ctnr7et7l.pdf

A single sensor based multispectral imaging camera using a narrow spectral band color mosaic integrated on the monochrome CMOS image sensor

This paper reports an experimental demonstration of moir\'e metalens which shows wide focal length tunability from negative to positive by mutual angle rotation at the wavelength of 900 nm. The moir\'e metalens was developed using high index contrast transmitarray meta-atoms made of amorphous silicon octagonal pillars, which is designed to have polarization insensitivity and full 2$\pi$ phase coverage. The fabricated moir\'e metalens showed focal length tunability at the ranges between $\pm$1.73 -- $\pm$5 mm, which corresponds to the optical power ranges between $\pm$578 -- $\pm$200 m$^{-1}$ at the mutual rotation between $\pm$90 degrees.

Demonstration of focal length tuning by rotational varifocal moir\'e metalens in an ir-A wavelength.

Dielectric metasurfaces: From wavefront shaping to quantum platforms

Conventional color image sensors employing absorptive color filters exhibit low overall light transmission, resulting in limited signal levels per sensor pixel. This issue is becoming critical beca...

High-Sensitivity Color Imaging Using Pixel-Scale Color Splitters Based on Dielectric Metasurfaces

Polarization imaging is key for various applications ranging from biology to machine vision because it can capture valuable optical information about imaged environments, which is usually absent in intensity and spectral content. Conventional polarization cameras rely on a traditional single-eye imaging system with rotating polarizers, cascaded optics, or micropolarizer-patterned image sensors. These cameras, however, have two common issues. The first is low sensitivity resulting from the limited light utilization efficiency of absorptive polarizers or cascaded optics. The other is the difficulty in device miniaturization due to the fact that these devices require at least an optical-path length equivalent to the lens’s focal length. Here, we propose a polarization imaging system based on compound-eye metasurface optics and show how it enables the creation of a high-sensitivity, ultra-thin polarization camera. Our imaging system is composed of a typical image sensor and single metasurface layer for forming a vast number of images while sorting the polarization bases. Since this system is based on a filter-free, computational imaging scheme while dramatically reducing the optical-path length required for imaging, it overcomes both efficiency and size limitations of conventional polarization cameras. As a proof of concept, we demonstrated that our system improves the amount of detected light by a factor of ∼2, while reducing device thickness to ∼1/10 that of the most prevalent polarization cameras. Such a sensitive, compact, and passive device could pave the way toward the widespread adoption of polarization imaging in applications in which available light is limited and strict size constraints exist.

Compound-eye metasurface optics enabling a high-sensitivity, ultra-thin polarization camera.

Metasurfaces can manipulate optical wavefronts by locally shifting the phase of incident light with metallic or dielectric optical nanoresonators that are generally arranged on a lattice with subwavelength spacing. However, such conventional metasurfaces inevitably generate a spatially discrete multi-level phase profile due to the spacing of their building blocks. This directly leads to an efficiency reduction and thus limits their capability. Here, we propose and demonstrate highly efficient transmissive metasurfaces with the ability to form a continuous phase profile. The proposed strategy relies on the fact that high-index dielectric nanobeams with gradually modulated widths can be interpreted to be a virtually impedance-matched material with spatial variations of its refractive index. By highly utilizing such features, one-dimensionally continuous, arbitrary phase profiles can be created in a simple manner with the width profile design. Since spatial transmittance variations can be minimized due to the impedance matching feature, this approach provides a nearly ideal phase profile for spatial light modulation with phase-only filtering operations. We demonstrate that this approach has the capability to improve the performance in various metasurface-based optical components, including polarization-dependent, large-angle beam deflectors and versatile multi-beam splitters. Considering that designing optical phases even in deep-subwavelength regimes is critical for free-space optics, the proposed approach will enable new classes of optical components with complex wavefront engineering.

Impedance-matched dielectric metasurfaces for non-discrete wavefront engineering

We demonstrate reservoir computing using a fiber delay line and membrane semiconductor optical amplifier on Si. Thanks to its small active volume and low fiber-coupling loss, the reservoir consumes only 43 mW for nonlinear activation.

Reservoir Computing with Low-Power-Consumption All-Optical Nonlinear Activation Using Membrane SOA on Si

We demonstrate continuous gradient metasurfaces that can form a non-discrete one-dimensional phase pattern on a surface. The capability of our approach for wavefront shaping is demonstrated in efficient and versatile optical multi-beam splitters.

Mitsumasa Nakajima

Papers

High-Sensitivity Color Imaging Using Pixel-Scale Color Splitters Based on Dielectric Metasurfaces

Compound-eye metasurface optics enabling a high-sensitivity, ultra-thin polarization camera.

Impedance-matched dielectric metasurfaces for non-discrete wavefront engineering

Reservoir Computing with Low-Power-Consumption All-Optical Nonlinear Activation Using Membrane SOA on Si

Continuous Gradient Dielectric Metasurfaces for Non-discrete Spatial Light Manipulation