Dielectric metasurfaces as passive radiative coolers, colorimetric refractive index sensors, color filters, and one-way perfect absorber/reflectors with transparent sidebands

Abstract

Metamaterials define the class of synthetic, man-made materials with exotic properties that cannot be observed with natural materials. Their sub-wavelength counterparts are called metasurfaces. In particular, dielectric metasurfaces are extensively studied due to the advantages they offer in comparison to metamaterials, which are mainly their reduced thickness and not suffering from ohmic losses that are present in metals. Here, we design and implement dielectric metasurfaces in four important application areas, namely 1. Passive radiative coolers for spacecraft, 2. Colorimetric refractive index sensors, and 3. Color filters based on monolayer graphene. 4. A metasurface with a resonant one-way absorption/reflection with transmissive sidebands functionality In the first work, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design an optical solar reflector, which constitutes the physical interface between the spacecraft and space. Our proposed metasurface is based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal-radiation to broad plasmonic resonances, and consequently an experimental emissivity of 0.968, in a very wide range from 2.5 µm to 25 µm. In the solar spectrum, low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. This design with high-throughput, robustness, low-cost and high-performance, therefore, shows great promise not only for space missions but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling. In the second work, we propose a highly-sensitive refractive index sensor, utilizing the excitation of guided-modes of a novel, 2-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guided-mode resonances with an ultra-narrow linewidth (fullwidth at half-maximum) of 0.58 nm. Sensitivity is numerically investigated by considering the deposition of dielectric layers on the structure. For a layer thickness of 30 nm, the maximum sensitivity reached as high as 110 nm/refractive index unit (RIU), resulting in a very high Figure of Merit of 190. The fabricated devices with 30 nm Aluminum Oxide and Zinc Oxide coatings achieved a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric detection with polarization-insensitivity is confirmed practically by a simple optical microscope. Samples with different coatings have been observed to have clearly distinct colors, while the color of each sample is nearly identical upon azimuthal rotation. Excellent agreement is obtained between the numerical and experimental results regarding the spectral position of the resonances and sensitivity. The proposed device is, therefore, highly promising in efficient, highlysensitive, almost lossless, and compact molecular diagnostics platform in the fields of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring. In the third work, we numerically propose a graphene perfect absorber that can be utilized as a color filter, utilizing the excitation of guided-modes of a dielectric slab waveguide by a novel sub-wavelength dielectric grating structure. When the guided-mode resonance is critically coupled to the graphene, we obtain perfect absorption with an ultra-narrow bandwidth (full-width at half-maximum) of 0.8 nm. The proposed design not only preserves the spectral position of the resonance, but also maintains > 98% absorption at all polarization angles. The spectral position of the resonance can be tuned as much as 400 nm in visible and near-infrared regimes by tailoring geometrical parameters. The proposed device has great potential in efficient, tunable, ultra-sensitive, compact and easyto-fabricate advanced photodetectors and color selective notch filters. In the fourth and final work, we numerically propose the one-way perfect absorption of near-infrared (NIR) radiation in a tunable spectral range with high transmission in the neighboring spectral ranges. This functionality is obtained by using a 2-dimensional, guided-mode resonance based grating-waveguide metasurface that acts as a frequency-selective reflector, a spacer dielectric, and an absorbing oxide layer. Within the bandwidth of the excited guided-mode resonance excited at 1.82µm with a full-width at half-maximum of 19 nm), we confirmed perfect absorption when light is incident from one of the two opposite directions, whereas in the other direction, perfect reflection is observed. The forward-to-backward absorption ratio reached as high as 60, while the thickness of the entire structure is in the order of the operating wavelength. In addition to the spectral tunability of the excited resonances and their bandwidths, our proposed device supports transparency windows with 65% transmission in the adjacent frequency bands. Our 2D grating is also verified to enable near-absolute insensitivity to the polarization state of incident light. Geometrical parameter modification also gives our design great tunability, as we also designed a device with 300 nm absorption/reflection linewidth.