Electromagnetic metamaterials (MMs) and metasurfaces (MSs) are artificial media and surfaces with subwavelength separations of meta-atoms designed for anomalous manipulations of light properties. Owing to large scattering cross-sections of metallic/dielectric meta-atoms, it is possible to not only localize strong electromagnetic fields in deep subwavelength volume but also decompose and analyze incident light signal with ultracompact setup using MMs and MSs. Hence, by probing resonant spectral responses from extremely boosted interactions between analyte layer and optical MMs or MSs, sensing the variation of refractive index has been a popular and practical application in the field of photonics. Moreover, decomposing and analyzing incident light signal can be easily achieved with anisotropic MSs, which can scatter light to different directions according to its polarization or wavelength. In this paper, we present recent advances and potential applications of optical MMs and MSs for refractive index sensing and sensing light properties, which can be easily integrated with various electronic devices. The characteristics and performances of devices are summarized and compared qualitatively with suggestions of design guidelines.

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Color generation based on strategically designed plasmonic nanostructures is a promising approach for display applications with unprecedented high-resolution. However, it is disadvantageous in that the optical response is fixed once the structure is determined. Therefore, obtaining high modulation depth with reversible optical properties while maintaining its fixed nanostructure is a great challenge in nanophotonics. In this work, dynamic color tuning and switching using tungsten trioxide (WO 3 ), a representative electrochromic material, are demonstrated with reflection-type and transmission-type optical devices. Thin WO 3 films incorporated in simple stacked configurations undergo dynamic color change by the adjustment of their dielectric constant through the electrochromic principle. A large resonance wavelength shift up to 107 nm under an electrochemical bias of 3.2 V could be achieved by the reflection-type device. For the transmission-type device, on/off switchable color pixels with improved purity are demonstrated of which transmittance is modulated by up to 4.04:1.

Metalens is one of the most prominent applications among metasurfaces since it gives possibilities to replace the conventional lenses for compactness and multi-functionalities. Recently, many studies have been demonstrated to overcome the aberrations of the metalenses for high performance practical applications. Previous studies have used the methods that control the dispersion of meta-atoms for correcting chromatic aberrations and use doublet platform for correcting monochromatic aberrations. Despite these studies and the large demands for simultaneous correction of the aberrations in high numerical aperture metalens, the simultaneous correction has not been demonstrated yet. In this paper, we report the doublet metalens design with high numerical aperture which corrects longitudinal chromatic aberration and four monochromatic aberrations including spherical aberration, coma, astigmatism, and field curvature simultaneously for the three primary visible colors. Based on the novel doublet platform, the multi-wavelength targeted correction lens and geometric phase lens with color filtering functionality are utilized. Our doublet metalens has numerical apertures of 0.33, 0.38, and 0.47 for 445 nm, 532 nm, and 660 nm, respectively. The back focal length of our doublet metalens remains nearly 360 µm for target wavelengths and incident angles up to 30 degrees.

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of resonant scattering phenomenon that makes asymmetric line shape originating from destructive interference between broadband scattering within a continuum state (bright superradiant mode) and an excitation of discrete state (dark subradiant mode) similar to coupled oscillator system. It was firstly observed in scattering phenomenon of electrons from helium atom by Ugo Fano. [2] Since then, Fano-type resonances have been studied in many physical systems including not only classical atomic systems but also nanophotonic systems such as the field of plasmonics, photonic crystals, and metamaterials due to its unique narrow and asymmetric line shapes nature. [3,4] Recently, Fano-type spectral response in plasmonic nanostructures and metamaterials have attracted considerable attention due to its superior capability to manipulate various characteristics of the Fano resonance in a broad frequency range, which is applicable for practical applications including chemical or biosensing, nonlinear optics, slow light device, and spectroscopy. [5,6] Numerous metallic or dielectric metamateirals have been designed to demonstrate Fano resonances using a common method of breaking the structural symmetry of nanostructures, such as split-ring resonators, [7] asymmetric double bars, [8][9][10] nanoparticle clusters, [11,12] dolmen structures, [13] and hybridized structures. [14][15][16][17] Despite the superior capability of the Fano resonant metamaterials, a trade-off between the quality factor (Q factor) and resonance intensity is unfortunately inevitable. Numerous high Q metamaterials have been proposed so far. [18][19][20] However, as the Q factor increases with an extremely narrow line width, the resonance intensity decreases simultaneously, causing it difficult to detect or distinguish sharp and minute resonances that limit application possibilities. It is an important issue of the high Q metamaterials and is being investigated in a way that defines the Figure of merit (FoM) as the product of the Q factor and intensity. [21] This approach, however, does not solve the tradeoff problem, but suggest only an appropriate region where the two values are moderately high by defining the FoM. The way to resolve the trade-off relation still remains a problem. In addition to the abovementioned problem related to the resonant nature of the Fano resonance, studies for controlling the resonance characteristics are also an important issue since it provides large tunability and flexibility for a variety of practical applications, such as optical sensor, elector-optic modulator, and ultrasensitive Excitation and manipulation of Fano resonances in plasmonic nanostructure have attracted considerable attention due to its capability of degrees of freedom in artificial design especially for spectral positions and quality factors (Q factors). To utilize the high Q factor of Fano resonances in practical applications, their sharp peaks or dips should be well detected, which means a high intensity of resonance line shape. Thus far, the realization of F...

We demonstrated strong full-space modulation of visible intensities based on Fano resonances in all-dielectric VO2 metasurface gratings.

Dynamic control of light based on gate‐tunable metasurfaces has revolutionized traditional optoelectronic devices due to its unprecedented compactness and versatile functionalities. However, these devices are typically based on metal‐insulator‐metal geometries that enable field‐effect modulation of only reflected light. Transmittance modulation techniques based on dielectric metasurfaces, despite their large modulation depth, have a disadvantage of low modulation speed due to high resistance of dielectric materials. Here, a high‐efficiency transmittance modulator that enables high switching speed, as well as large modulation depth, is demonstrated using indium‐tin‐oxide‐based metasurfaces. To realize these devices, the hybrid plasmonic waveguide mode is used which allows electromagnetic energy storage within the nanoscale permittivity‐tunable region between metal and high‐refractive dielectric layers. Experimental measurements reveal a change in the transmittance (≈33%) by applying 6 V gate bias, and a fast modulation speed (≈826 kHz of 3 dB cut‐off frequency). This work provides a promising avenue for developing ultracompact optical components such as dynamic holograms, lenses with active focal lengths, or spatial light modulators.