Chalcogenide material Ge2Sb2Te5 (GST) has bistable phases, the so‐called amorphous and crystalline phases that exhibit large refractive index contrast. It can be reversibly switched within a nanosecond time scale through applying thermal bias, especially optical or electrical pulse signals. Recently, GST has been exploited as an ingredient of all‐optical dynamic metasurfaces, thanks to its ultrafast and efficient switching functionality. However, most of these devices provide only two‐level switching functionality and this limitation hinders their application to diverse all‐optical systems. In this paper, the method to expand switching functionality of GST metasurfaces to three level through engineering thermo‐optically creatable hybrid state that is co‐existing state of amorphous and crystalline GST‐based meta‐atoms is proposed. Furthermore, the novel hologram technique is introduced for providing the visual information that is only recognizable in the hybrid state GST metasurface. Thanks to thermo‐optical complexity to make the hybrid state, the metasurface allows the realization of highly secured visual cryptography architecture without the complex optical setup. The phase‐change metasurface based on multi‐physical design has significant potential for applications such as all‐optical image encryption, security, and anti‐counterfeiting.

Broadband‐operating active devices within a small‐footprint are highly on demand in various nanophotonic fields such as fiber‐optic communication systems and chip‐based integrated optical circuits. As pioneering approaches, diverse platforms of active metasurfaces (AMs) have been proposed due to their superior tunable functionality and ultra‐compact size. However, most of previous researches provide only limited operating bandwidth because they generally rely on resonant light–matter interaction between active material and plasmonic antenna. In this study, an active wavefront switching metasurface that can operate over 500 nm bandwidth at near‐infrared spectral bands is experimentally realized by utilizing nonresonant U‐shaped Ge2Sb2Te5 nanoantennas. Two different sizes of the U‐shaped antenna are designed to exhibit large transmittance contrast and their optical phases are determined by imposing the orientation angle variation. As an example of the functionality, anomalous refraction angle switching and dispersionless active hologram are demonstrated. The devices provide high signal‐to‐noise ratio (>7 dB) for overall operation bandwidth. It is believed that the proposed AMs can be an innovative platform for real device application thanks to their not only broadband and low‐noise operation but also fast speed, low power consumption switching within a small‐footprint.

Polarization is one of the fundamental properties of light, which is inherited from the vectorial nature of electromagnetic waves. Measuring states of polarizations (SOPs) is widely used for remote sensing applications. A number of ultracompact polarimeters that analyze SOPs at a single detection without splitting of incident beams have been demonstrated recently. However, the development of a broadband polarimeter remains challenging due to the resonant feature of their building blocks. Here, an ultracompact polarimeter that retrieves the SOP of incident light with broad operation bandwidth at near-infrared wavelengths is proposed. Surface plasmon polaritons (SPPs) excited by X-shaped aperture arrays are utilized as intensity probes. Full-Stokes parameters are retrieved by detecting the SPP intensities. The proposed polarimeter is demonstrated experimentally and can measure SOPs at wavelengths from 750 nm to 1050 nm. This work can help to develop more compact polarization-sensitive optical systems, such as polarimetry, ellipsometry, and optical routers.

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...

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