We explore the application of a single-step nanoimprinting technique using water-soluble Polyvinyl Alcohol (PVA) to fabricate tunable metasurfaces. These metasurfaces display multiplexed structural color and meta holography. The structured PVA achieved below 100 nm, accompanied by aspect ratios approaching 10. Under increasing relative humidity conditions, the PVA metaatom can expand by approximately 35.5%, allowing precise control of wavefronts. Here, we demonstrate the optical security metasurfaces for multiplexed encryption, capable of revealing, concealing, or eliminating information based on changes in relative humidity, both irreversibly and reversibly.
Here, we propose liquid-crystal (LC) integrated metasurfaces to create intensity-tunable metaholography in the visible spectrum. The metasurface is composed of gap-shifted split ring resonators, which exhibit different optical responses depending on the input polarization states of light. The gap-shifted split ring resonators enable the perfect absorption of right-circularly polarized light (RCP), while they reflect left-circularly polarized with varied retardation phases. the integration of LC enables electrically tunable metaholograms, where the intensity of the holographic images can be adjusted by controlling the applied voltages on LC. The circular dichroism in reflection reaches (experiments) 0.91 and (simulations) 0.99.
Here, we demonstrated vivid structural coloration and polarization-sensitive color metasurfaces using bandgap-engineered a-Si:H, whose extinction coefficient is near zero at the entire visible spectrum. The scattering response of high-index nanostructures is numerically analyzed with multipole expansion, and we vitrify that the low-optical losses of bandgap-engineered a-Si:H significantly improve color coverage of metasurface, achieving comparable coverage with the Adobe RGB gamut. Also, we demonstrated the application of optical encryption with polarization-sensitive structural coloration, achieving near-zero reflection when optical information is encrypted. We believe that structural coloration with low-loss a-Si:H will be widely used with its advantageous benefits compared to chemical pigments.
We have developed two deep neural networks (inverse network / forward network) for obtaining metasurface operating at visible bandwidth. Unlike other studies, the neural networks involve not only the geometry of the metasurface but also incorporate refractive index information for metasurface designers. With the inverse network, inverse designs of metasurfaces displaying specific spectra have been conducted. Also, using the forward network, we have demonstrated a dual-mode metasurface with a reflective image / transmissive hologram, and an achromatic metalens. The networks provide vast design choices and fast calculation speed for engineers.
Here, we introduce low-loss hydrogenated amorphous silicon (a-Si:H), whose bandgap is optimized to suppress their extinction coefficient at visible frequencies. The bandgap of a-Si:H has been manipulated by adjusting the deposition parameters of plasma-enhanced chemical vapor deposition. Low-loss a-Si:H have been applied for beam-steering metasurfaces at the entire visible spectrums. Beam steering metasurface has achieved measured efficiencies of 42%, 65%, and 75% at the wavelengths of 450, 532, and 635 nm, respectively.
Here, we developed two types of tunable PVA metaphotonic platforms, which are 1) PVA metasurface that simultaneously displays a far-field holographic image and reflective structural coloration under coherent and incoherent light, respectively, and 2) humidity-responsive structural coloration that enables to generate RGB color in a single pixel. Three types of PVA metasurfaces have been designed: 1) revisable holography/structural coloration with pure PVA structures, 2) relative-humidity sensitive optical decryption with PVA-hydrogenated amorphous silicon structures, and 3) revisable holography/structural coloration with Pt-coated PVA structures. Considering that the aforementioned PVA structures have been fabricated with low-coat single-step nanoimprinting methods, they will be widely applied for various tunable optical components by significantly reducing their production costs.
Here, we developed humidity-responsive full color nano-pixels with 700nm resolution using 3D nanoimprinted polyvinyl alcohol (PVA) and disordered silver nanoparticles (NPs). The nano-pixels are designed as Fabry-Pérot structure which is composed of an aluminum mirror substrate, humidity-responsive PVA spacer, and disordered silver nanoparticles as top metal layer. The proposed structure has three main merits as follows. It has 1) high resolution thanks to 3D nanoimprinted PVA technique, and 2) millisecond-response time (441ms), 3) vivid color generation thanks to disordered silver NPs which enhance the penetration speed of water molecules into the PVA layer and have highly absorbing dielectric optical property.
The article comments on a recently proposed innovative process that uses direct laser writing to achieve vivid, fine-tunable color at centimeter scale by leveraging the fabrication speed and the spatial resolution of pixelated F-P cavity structures.
Advancements in micro/nanofabrication have enabled the realization of practical micro/nanoscale photonic devices such as absorbers, solar cells, metalenses, and metaholograms. Although the performance of these photonic devices has been improved by enhancing the design flexibility of structural materials through advanced fabrication methods, achieving large-area and high-throughput fabrication of tiny structural materials remains a challenge. In this aspect, various technologies have been investigated for realizing the mass production of practical devices consisting of micro/nanostructural materials. This review describes the recent advancements in soft lithography, colloidal self-assembly, and block copolymer self-assembly, which are promising methods suitable for commercialization of photonic applications. In addition, we introduce low-cost and large-scale techniques realizing micro/nano devices with specific examples such as display technology and sensors. The inferences presented in this review are expected to function as a guide for promising methods of accelerating the mass production of various sub-wavelength-scale photonic devices.
Dielectric metasurfaces working at visible frequencies have been steadily investigated to realize practical flat optical components. However, recently investigated dielectrics, TiO2 and GaN suffer high fabrication costs since a precursor of TiO2 is expensive, and GaN requires two-step etching process. Here, this work suggests optical-loss-suppressed hydrogenated amorphous silicon (a-Si:H) for functional metasurfaces. Optical losses in the visible frequencies are manipulated by adjusting deposition conditions of plasma-enhanced chemical vapor deposition. Optical properties of a-Si:H are optimized for geometric metasurfaces, and it exhibits a high refractive index over 3.0 with low extinction coefficient (<0.1). Using them, highly efficient beam-steering metasurfaces, encapsulated metalenses, and bright structural coloration has been demonstrated. Considering that our manipulation efficiency approaches 42%, 65%, and 75% at the wavelength of 450, 532, 635 nm, it will be dominant materials for a functional photonic platform with low-fabrication costs.
Hydrogels such as PVA have yet to be fully exploited in metasurfaces due to their fairly low refractive index and fabrication feasibility. Here, we demonstrate one-step nanoimprinted PVA metasurfaces with resolutions reaching sub-100 nm, and aspect ratios approaching 10. We then demonstrate three distinct relative humidity dependent dual-mode optical security applications using the PVA metasurfaces. Through the swelling of the PVA when exposed to high humidity conditions and careful design of meta-atoms with different dimensions orientated at specific angles, multiplexed color prints and holograms can be selectively uncovered or destroyed.
Tunable optical devices powered by metasurfaces provide a new path for functional planar optics. Lenses with tunable focal lengths could play a key role in various fields with applications in imaging, displays, and augmented and virtual reality devices. Here, we present a method for computationally designing an RGB-achromatic bifocal metalens and experimentally realize it through a scalable one-step nanoimprinting technique using a TiO2 nanoparticle embedded resin combined with an electrically tunable liquid crystal cell.
Tunable metasurfaces have been steadily investigated to miniaturize optical devices by altering active module that is equipped with mechanical actuators. However, almost all of recent tunable metasurfaces focused on switching functionality between two distinct states or more. Here, we proposed spin-selective metasurfaces, which perfectly absorb certain circularly polarized light, while reflecting counter circularly polarized light with desired phases. The metasurfaces consist of split ring resonators on metal-dielectric metal structures to enlarge their plasmonic responses, approaching near-zero reflection of LCP, and 14% reflection of RCP at the wavelength of 635 nm. With the metasurface, we implanted it on electrically tunable liquid crystals for intensity tuning of encoded holograms. 23-steps of hologram intensities are experimentally demonstrated with liquid crystal integrated split ring resonators. Considering that previous spin-selective metasurfaces are designed with a complex fabrication process, our liquid crystal integrated split ring resonators will be a dominant option for altering active and bulky optical components.
Metasurfaces have attracted great attention due to their ability to manipulate the phase, amplitude, and polarization of light in a compact form. Tunable metasurfaces have been investigated recently through the integration with mechanically moving components and electrically tunable elements. Two interesting applications, in particular, are to vary the focal point of metalenses and to switch between holographic images. We present the recent progress on tunable metasurfaces focused on metalenses and metaholograms, including the basic working principles, advantages, and disadvantages of each working mechanism. We classify the tunable stimuli based on the light source and electrical bias, as well as others such as thermal and mechanical modulation. We conclude by summarizing the recent progress of metalenses and metaholograms, and providing our perspectives for the further development of tunable metasurfaces.
We report an approach assisted by deep learning to design spectrally-sensitive multi-band absorbers that work in the visible range. We propose a five-layered metal-insulator-metal grating structure composed of aluminum and silicon dioxide, and design its structural parameters by using an artificial neural network (ANN). For a spectrally-sensitive design, spectral information of resonant wavelengths is additionally provided as input as well as the reflection spectrum. The ANN facilitates highly robust design of grating structure that has an average mean squared error of 0.023. The optical properties of the designed structures are validated using electromagnetic simulations and experiments. Analysis of design results for gradually-changing target wavelengths of input show that the trained ANN can learn physical knowledge from data. We also propose a method to reduce the size of the ANN by exploiting observations of the trained ANN for practical applications.
Passive radiative cooling has attracted great attention due to its capability to dissipate heat without energy consumption [1,2]. Here, we demonstrate a one dimensional photonic structure for high-performance daytime radiative cooling [3]. Structural parameters of the proposed photonic structure are optimized for both wavelength range of the solar and atmospheric transparency window simultaneously. The types of materials and thicknesses of up-to 10 layers of multilayer are optimized by genetic algorithms. We develop an objective function in the solar region to achieve high-performance daytime radiative cooling with a focus on minimizing solar absorption power. Among the four material candidates of SiO2, Si3N4, MgF2, and HfO2, proper materials are recommended and the best thickness are optimized for desired optical functionalities for daytime radiative cooling. The designed structures minimize the solar power absorbed while strongly emit thermal radiation at the atmospheric transparency window at 8-13 um wavelength region.
Mie scatterer resonantly scatters when wavelength of incident light is similar to the size of the scatterer. The scattering of Mie resonator can be analyzed using multipole decomposition; silicon nanostructure has multipole scattering modes in visible regime. When the Mie scatterers are arrayed, the scattering response can be greatly amplified. To properly design array of Mie scatterer, i.e. metasurface, the hybridization of radiation mode of scatterer and lattice effect, i.e. guided-mode resonance (GMR), must be understood. Herein, we would like to provide the scattering mechanisms behind the hybridization between individual scattering mode and lattice effect, and use them to realize gradient structural coloration by silicon-based metasurface. We believe that a solid understanding of the coupling between individual Mie resonators and the lattice resonances can be a strong basis for designing planar spectral filters.
This research reveals that the optical properties of hydrogenated a-Si can be modulated by varying chamber atmosphere of PE-CVD. When substrates temperature and chamber pressure were adjusted, the refractive index and the extinction coefficient was modulated. The highest refractive index at 450 nm is 4.3, and the lowest one is 2.6, which achieve 1.7 modulable indexes in the visible region. Also, this research provides lower than 0.1 extinction coefficient at the same wavelength. The lowest extinction coefficient can be applied to metasurfaces designs, and we propose 50%, and 85% conversion efficiency at 450 nm and 635 nm respectively. An experimental demonstration of these metasurfaces will be conducted.
Chiral metamaterials consisting of periodic asymmetric unit cells have a different complex refractive index depending on spin-states of impending light. These phenomena can be applied to chiral sensing applications, so enhanced chiral responses have been attracted to develop advanced spectroscopic devices. However, chiral devices, which made of metallic chiral metasurfaces, have weak circular dichroism due to ohmic loss of metal. Here, we proposed simulation results of chiral metasurfaces consist of helically located gold metallic nanodisc, called oligomers. The oligomers are located with C4 chirality, resulting in non-conversion efficiency for reducing noise when they are used for spectroscopy. Also, the oligomers have ultra-sharp circular dichroism that has been rarely reported in the near-infrared region. These results may have wide applications, including spectroscopy, thermal detectors, and biochemical distributors.
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