Studies on Dual Helmholtz Resonators and Asymmetric Waveguides for Ventilated Soundproofing.

Achieving the simultaneity of ventilation and soundproofing is a significant challenge in applied acoustics. Ventilated soundproofing relies on the interplay between local resonance and nonlocal coupling of acoustic waves within a sub-wavelength structure. However, previously studied structures possess limited types of fundamental resonators and lack modifications from the basic arrangement. These constraints often force the specified position of each attenuation peak and low absorption performance. Here, we suggest the in-duct-type sound barrier with dual Helmholtz resonators, which are positioned around the symmetry-breaking waveguides. The numerical simulations for curated dimensions and scattered fields show the aperiodic migrations and effective amplifications of the two absorptive domains. Collaborating with the subsequent reflective domains, the designed structure holds two effective attenuation bands under the first Fabry-Pérot resonance frequency. This study would serve as a valuable example for understanding the local and non-local behaviors of sub-wavelength resonating structures. Additionally, it could be applied in selective noise absorption and reflection more flexibly.

Implementation of particle swarm optimization for complete inverse design of multilayered optical filters.

Particle swarm optimization is implemented for the complete inverse design of multilayered optical filters. To achieve this, a model is designed to optimize the thickness and material of each layer, as well as the total number of layers, simultaneously. The performance of the model is evaluated by repeating the optimization process, enabling clarification of the effects of model parameters on the final output. The designed model is also demonstrated for the optimization of various optical filters, including bandstop filters, bandpass filters, and anti-reflection coatings. The results confirm that particle swarm optimization is capable of designing arbitrary optical filters that cannot be designed using conventional design theories.

Thermally-curable nanocomposite printing for the scalable manufacturing of dielectric metasurfaces.

Metasurfaces consisting of artificially designed meta-atoms have been popularized recently due to their advantages of amplitude and phase of light control. However, the electron beam lithography method for metasurface fabrication has high cost and low throughput, which results in a limitation for the fabrication of metasurfaces. In this study, nanocomposite printing technology is used to fabricate high-efficiency metasurfaces with low cost. To demonstrate the efficiency of the proposed fabrication method, a metahologram is designed and fabricated using a nanocomposite. The metahologram exhibits conversion efficiencies of 48% and 35% at wavelengths of 532 and 635 nm, respectively. The nanocomposite is composed of polymers with nanoparticles, so durability tests are also performed to evaluate the effects of temperature and humidity on the metasurfaces. The test verifies that at temperatures below the glass transition temperature of the base resin, the nanostructures do not collapse, so the efficiency of the metasurfaces remains almost the same. The surrounding humidity does not affect the nanostructures at all. Hence, the durability of the nanocomposite metasurfaces can be further enhanced by replacing the base resin, and this nanocomposite printing method will facilitate practical metasurface use at low cost.

Printable Nanocomposite Metalens for High-Contrast Near-Infrared Imaging.

Printable metalenses composed of a silicon nanocomposite are developed to overcome the manufacturing limitations of conventional metalenses. The nanocomposite is synthesized by dispersing silicon nanoparticles in a thermally printable resin, which not only achieves a high refractive index for high-efficiency metalenses but also printing compatibility for inexpensive manufacturing of metalenses. The synthesized nanocomposite exhibits high refractive index >2.2 in the near-infrared regime, and only 10% uniform volume shrinkage after thermal annealing, so the nanocomposite is appropriate for elaborate nanofabrication compared to commercial high-index printable materials. A 4 mm-diameter metalens operating at the wavelength of 940 nm is fabricated using the nanocomposite and one-step printing without any secondary operations. The fabricated metalens verifies a high focusing efficiency of 47%, which can be further increased by optimizing the composition of the nanocomposite. The printing mold is reusable, so the large-scale metalenses can be printed rapidly and repeatedly. A compact near-infrared camera combined with the nanocomposite metalens is also demonstrated, and an image of the veins underneath human skin is captured to confirm the applicability of the nanocomposite metalens for biomedical imaging.

Revealing Structural Disorder in Hydrogenated Amorphous Silicon for a Low-Loss Photonic Platform at Visible Frequencies.

The high refractive index of hydrogenated amorphous silicon (a-Si:H) at optical frequencies is an essential property for the efficient modulation of the phase and amplitude of light. However, substantial optical loss represented by its high extinction coefficient prevents it from being utilized widely. Here, the bonding configurations of a-Si:H are investigated, in order to manipulate the extinction coefficient and produce a material that is competitive with conventional transparent materials, such as titanium dioxide and gallium nitride. This is achieved by controlling the hydrogenation and silicon disorder by adjusting the chemical deposition conditions. The extinction coefficient of the low-loss a-Si:H reaches a minimum of 0.082 at the wavelength of 450 nm, which is lower than that of crystalline silicon (0.13). Beam-steering metasurfaces are demonstrated to validate the low-loss optical properties, reaching measured efficiencies of 42%, 62%, and 75% at the wavelengths of 450, 532, and 635 nm, respectively. Considering its compatibility with mature complementary metal-oxide-semiconductor processes, the low-loss a-Si:H will provide a platform for efficient photonic operating in the full visible regime.

Recent Progress on Ultrathin Metalenses for Flat Optics.

As technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Single-step manufacturing of hierarchical dielectric metalens in the visible.

Metalenses have shown a number of promising functionalities that are comparable with conventional refractive lenses. However, current metalenses are still far from commercialization due to the formidable fabrication costs. Here, we demonstrate a low-cost dielectric metalens that works in the visible spectrum. The material of the metalens consists of a matrix-inclusion composite in which a hierarchy satisfies two requirements for the single-step fabrication; a high refractive index and a pattern-transfer capability. We use a UV-curable resin as a matrix to enable direct pattern replication by the composite, and titanium dioxide nanoparticles as inclusions to increase the refractive index of the composite. Therefore, such a dielectric metalens can be fabricated with a single step of UV nanoimprint lithography. An experimental demonstration of the nanoparticle composite-based metalens validates the feasibility of our approach and capability for future applications. Our method allows rapid replication of metalenses repeatedly and thereby provides an advance toward the use of metalenses on a commercial scale.

Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces.

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated amorphous silicon (a-Si:H) is deposited on the fused silica substrate, using plasma-enhanced chemical vapor deposition (PECVD). Typical amorphous silicon deposited by evaporation causes severe optical loss, impinging the operation at visible frequencies. Hydrogen atoms inside the amorphous silicon thin film can reduce the structural defects, improving optical loss. Nanostructures of a few hundreds of nanometers are required for the operation of metasurfaces in the visible frequencies. Conventional photolithography or direct laser writing is not feasible when fabricating such small structures, due to the diffraction limit. Hence, electron beam lithography (EBL) is utilized to define a chromium (Cr) mask on the thin film. During this process, the exposed resist is developed at a cold temperature to slow down the chemical reaction and make the pattern edges sharper. Finally, a-Si:H is etched along the mask, using inductively coupled plasma-reactive ion etching (ICP-RIE). The demonstrated method is not feasible for large-scale fabrication due to the low throughput of EBL, but it can be improved upon by combining it with nanoimprint lithography. The fabricated device is characterized by a customized optical setup consisting of a laser, polarizer, lens, power meter, and charge-coupled device (CCD). By changing the laser wavelength and polarization, the diffraction properties are measured. The measured diffracted beam powers are always equal, regardless of the incident polarization, as well as wavelength.

Facile Nanocasting of Dielectric Metasurfaces with Sub-100 nm Resolution.

This work presents a facile nanocasting technique to fabricate dielectric metasurfaces at low cost and high throughput. A flexible polymer mold is replicated from a master mold, and then the polymer mold is used to shape particle-embedded UV-curable polymer resin. The polymer mold is compatible with flexible and curved substrates. A hard-polydimethylsiloxane improves mechanical stability of the polymer mold providing sub-100 nm patterning resolution. The patterned resin itself can work as a metasurface without secondary operations because dielectric particles sufficiently increase the refractive index of the resin. The absence of the secondary operations allows our method to have higher productivity and cost competitiveness than those of typical nanoimprint lithography. Experimental demonstration verifies the feasibility of our method, and the replicated metasurface exhibits a conversion efficiency of 46% in the visible, which is comparable to metasurfaces based on low-loss dielectrics. Given that conventional dielectric metasurfaces have been fabricated by electron beam lithography at formidable cost due to low throughput, our method will be a promising nanofabrication platform and thereby facilitate commercialization of dielectric metasurfaces.

Geometric metasurface enabling polarization independent beam splitting.

A polarization independent holographic beam splitter that generates equal-intensity beams based on geometric metasurface is demonstrated. Although conventional geometric metasurfaces have the advantages of working over a broad frequency range and having intuitive design principles, geometric metasurfaces have the limitation that they only work for circular polarization. In this work, Fourier holography is used to overcome this limitation. A perfect overlap resulting from the origin-symmetry of the encoded image enables polarization independent operation of geometric metasurfaces. The designed metasurface beam splitter is experimentally demonstrated by using hydrogenated amorphous silicon, and the device performs consistent beam splitting regardless of incident polarizations as well as wavelengths. Our device can be applied to generate equal-intensity beams for entangled photon light sources in quantum optics, and the design approach provides a way to develop ultra-thin broadband polarization independent components for modern optics.

"Crypto-Display" in Dual-Mode Metasurfaces by Simultaneous Control of Phase and Spectral Responses.

Although conventional metasurfaces have demonstrated many promising functionalities in light control by tailoring either phase or spectral responses of subwavelength structures, simultaneous control of both responses has not been explored yet. Here, we propose a concept of dual-mode metasurfaces that enables simultaneous control of phase and spectral responses for two kinds of operation modes of transmission and reflection, respectively. In the transmission mode, the dual-mode metasurface acts as conventional metasurfaces by tailoring phase distribution of incident light. In the reflection mode, a reflected colored image is produced under white light illumination. We also experimentally demonstrate a crypto-display as one application of the dual-mode metasurface. The crypto-display looks a normal reflective display under white light illumination but generates a hologram that reveals the encrypted phase information under single-wavelength coherent light illumination. Because two operation modes do not affect each other, the crypto-display can have applications in security techniques.

Recent Advances in Tunable and Reconfigurable Metamaterials.

Metamaterials are composed of nanostructures, called artificial atoms, which can give metamaterials extraordinary properties that cannot be found in natural materials. The nanostructures themselves and their arrangements determine the metamaterials' properties. However, a conventional metamaterial has fixed properties in general, which limit their use. Thus, real-world applications of metamaterials require the development of tunability. This paper reviews studies that realized tunable and reconfigurable metamaterials that are categorized by the mechanisms that cause the change: inducing temperature changes, illuminating light, inducing mechanical deformation, and applying electromagnetic fields. We then provide the advantages and disadvantages of each mechanism and explain the results or effects of tuning. We also introduce studies that overcome the disadvantages or strengthen the advantages of each classified tunable metamaterial.

Complete amplitude and phase control of light using broadband holographic metasurfaces.

Reconstruction of light profiles with amplitude and phase information, called holography, is an attractive optical technology with various significant applications such as three-dimensional imaging and optical data storage. Subwavelength spatial control of both amplitude and phase of light is an essential requirement for an ideal hologram. However, traditional holographic devices suffer from their restricted capabilities of incomplete modulation in both amplitude and phase of visible light; this results in sacrifice of optical information and undesirable occurrences of critical noises in holographic images. Herein, we have proposed a novel metasurface that is capable of completely controlling both the amplitude and phase profiles of visible light independently with subwavelength spatial resolution. The full, continuous, and broadband control of both amplitude and phase was achieved using X-shaped meta-atoms based on the expanded concept of the Pancharatnam-Berry phase. The first experimental demonstrations of the complete complex-amplitude holograms with subwavelength definition at visible wavelengths were achieved, and excellent performances with a remarkable signal-to-noise ratio as compared to those of traditional phase-only holograms were obtained. Extraordinary control capability with versatile advantages of our metasurface paves a way to an ideal holography, which is expected to be a significant advancement in the field of optical holography and metasurfaces.

Polarization-sensitive tunable absorber in visible and near-infrared regimes.

A broadband tunable absorber is designed and fabricated. The tunable absorber is comprised of a dielectric-metal-dielectric multilayer and plasmonic grating. A large size of tunable absorber device is fabricated by nano-imprinting method. The experimental results show that over 90% absorption can be achieved within visible and near-infrared regimes. Moreover, the high absorption can be controlled by changing the polarization of incident light. This polarization-sensitive tunable absorber can have practical applications such as high-efficiency polarization detectors and transmissive polarizer.

Electrically tunable metasurface perfect absorber for infrared frequencies.

We theoretically investigate a metasurface perfect absorber based on indium-tin-oxide as active material. Our design scheme relies on conventional metal-oxide-semiconductor model and the Drude model. Inducing a voltage into the device causes a blue-shift of 50 nm in the reflectance spectrum in the infrared region. The total thickness of the device is only 3.5% of the working wavelength λ = 2.56 µm, and the rate of reflectance change reaches 5.16 at λ = 2.56 µm. Because the material that we use has advantages of easy fabrication and fast response, our design approach can be used for numerous applications on active plasmonic sensors and filters.

Fabrication of three-dimensional suspended, interlayered and hierarchical nanostructures by accuracy-improved electron beam lithography overlay.

Nanofabrication techniques are essential for exploring nanoscience and many closely related research fields such as materials, electronics, optics and photonics. Recently, three-dimensional (3D) nanofabrication techniques have been actively investigated through many different ways, however, it is still challenging to make elaborate and complex 3D nanostructures that many researchers want to realize for further interesting physics studies and device applications. Electron beam lithography, one of the two-dimensional (2D) nanofabrication techniques, is also feasible to realize elaborate 3D nanostructures by stacking each 2D nanostructures. However, alignment errors among the individual 2D nanostructures have been difficult to control due to some practical issues. In this work, we introduce a straightforward approach to drastically increase the overlay accuracy of sub-20 nm based on carefully designed alignmarks and calibrators. Three different types of 3D nanostructures whose designs are motivated from metamaterials and plasmonic structures have been demonstrated to verify the feasibility of the method, and the desired result has been achieved. We believe our work can provide a useful approach for building more advanced and complex 3D nanostructures.

Demonstration of a Hyperlens-integrated Microscope and Super-resolution Imaging.

The use of super-resolution imaging to overcome the diffraction limit of conventional microscopy has attracted the interest of researchers in biology and nanotechnology. Although near-field scanning microscopy and superlenses have improved the resolution in the near-field region, far-field imaging in real-time remains a significant challenge. Recently, the hyperlens, which magnifies and converts evanescent waves into propagating waves, has emerged as a novel approach to far-field imaging. Here, we report the fabrication of a spherical hyperlens composed of alternating silver (Ag) and titanium oxide (TiO2) thin layers. Unlike a conventional cylindrical hyperlens, the spherical hyperlens allows for two-dimensional magnification. Thus, incorporation into conventional microscopy is straightforward. A new optical system integrated with the hyperlens is proposed, allowing for a sub-wavelength image to be obtained in the far-field region in real time. In this study, the fabrication and imaging setup methods are explained in detail. This work also describes the accessibility and possibility of the hyperlens, as well as practical applications of real-time imaging in living cells, which can lead to a revolution in biology and nanotechnology.