Customizing Radiative Decay Dynamics of Two-Dimensional Excitons via Position- and Polarization-Dependent Vacuum-Field Interference.

Embodying bosonic and interactive characteristics in two-dimensional space, excitons in transition metal dichalcogenides (TMDCs) have garnered considerable attention. The utilization of the strong-correlation effects, long-range transport, and valley-dependent properties requires customizing exciton decay dynamics. Vacuum-field manipulation allows radiative decay engineering without disturbing intrinsic material properties. However, conventional flat mirrors cannot customize the radiative decay landscape in TMDC's plane or support vacuum-field interference with desired spectrum and polarization properties. Here, we present a meta-mirror platform resolving the issues with more optical degrees of freedom. For neutral excitons of the monolayer MoSe2, the optical layout formed by meta-mirrors manipulated the radiative decay rate in space by 2 orders of magnitude and revealed the statistical correlation between emission intensity and spectral line width. Moreover, the anisotropic meta-mirror demonstrated polarization-dependent radiative decay control. Our platform would be promising to tailor two-dimensional distributions of lifetime, density, diffusion, and polarization of TMDC excitons in advanced opto-excitonic applications.

Spontaneous generation and active manipulation of real-space optical vortices.

Optical vortices are beams of light that carry orbital angular momentum1, which represents an extra degree of freedom that can be generated and manipulated for photonic applications2-8. Unlike vortices in other physical entities, the generation of optical vortices requires structural singularities9-12, but this affects their quasiparticle nature and hampers the possibility of altering their dynamics or making them interacting13-17. Here we report a platform that allows the spontaneous generation and active manipulation of an optical vortex-antivortex pair using an external field. An aluminium/silicon dioxide/nickel/silicon dioxide multilayer structure realizes a gradient-thickness optical cavity, where the magneto-optic effects of the nickel layer affect the transition between a trivial and a non-trivial topological phase. Rather than a structural singularity, the vortex-antivortex pairs present in the light reflected by our device are generated through mathematical singularities in the generalized parameter space of the top and bottom silicon dioxide layers, which can be mapped onto real space and exhibit polarization-dependent and topology-dependent dynamics driven by external magnetic fields. We expect that the field-induced engineering of optical vortices that we report will facilitate the study of topological photonic interactions and inspire further efforts to bestow quasiparticle-like properties to various topological photonic textures such as toroidal vortices, polarization and vortex knots, and optical skyrmions.

Ultrawide meta-film replication process for the mass production of a flexible microwave absorbing meta-surface.

The manufacturing process for an ultrawide flexible microwave absorbing meta-surface was developed and optimized experimentally. The developed replication process consists of four main steps to demonstrate double-square loop array meta-structures: (1) mechanical machining of a master mold, (2) soft mold replication and patterned film imprinting, (3) conductive ink blade filling, (4) lamination of a base flexible film to meta sheet. Based on experimental optimization of the individual steps, the manufacturing process for a large-area flexible meta-film was established successfully. The feasibility of a developed process has been demonstrated with a 200 mm × 500 mm fabricated meta-film with a focus on microwave absorbing uniformity in the X-band region.

Universal Metasurfaces for Complete Linear Control of Coherent Light Transmission.

Recent advances in metasurfaces and optical nanostructures have enabled complex control of incident light with optically thin devices. However, it has thus far been unclear whether it is possible to achieve complete linear control of coherent light transmission, that is, independent control of polarization, amplitude, and phase for both input polarization states, with just a single, thin nanostructure array. Here, it is proved possible, and a universal metasurface is proposed, a bilayer array of high-index elliptic cylinders that possesses a complete degree of optical freedom with fully designable chirality and anisotropy. The completeness of achievable light control is mathematically shown with corresponding Jones matrices, new types of 3D holographic schemes that were formerly impossible are experimentally demonstrated, and a systematic way of realizing any input-state-sensitive vector linear optical device is presented. The results unlock previously inaccessible degrees of freedom in light transmission control.

Photolithographic realization of target nanostructures in 3D space by inverse design of phase modulation.

The mass production of precise three-dimensional (3D) nanopatterns has long been the ultimate goal of fabrication technology. While interference lithography and proximity-field nanopatterning (PnP) may provide partial solutions, their setup complexity and limited range of realizable structures, respectively, remain the main problems. Here, we tackle these challenges by applying an inverse design to the PnP process. Our inverse design platform based on the adjoint method can efficiently find optimal phase masks for diverse target lattices and motifs. We fabricate a 2D rectangular array of nanochannels, which has not been reported for conventional PnP with normally incident light, as a proof of concept. With further demonstration of material conversion, our work provides versatile platforms for nanomaterial fabrication.

Ultrawideband electromagnetic metamaterial absorber utilizing coherent absorptions and surface plasmon polaritons based on double layer carbon metapatterns.

An ultrawideband electromagnetic metamaterial absorber is proposed that consists of double-layer metapatterns optimally designed by the genetic algorithm and printed using carbon paste. By setting the sheet resistance of the intermediate carbon metapattern to a half of that of the top one, it is possible to find an optimal intermediate metapattern that reflects and absorbs the EM wave simultaneously. By adding an absorption resonance via a constructive interference at the top metapattern induced by the reflection from the intermediate one, an ultrawideband absorption can be achieved without increasing the number of layers. Moreover, it is found that the metapatterns support the surface plasmon polaritons which can supply an additional absorption resonance as well as boost the absorption in a broad bandwidth. Based on the simulation, the [Formula: see text] absorption bandwidth is confirmed from 6.3 to 30.1 GHz of which the fractional bandwidth is 130.77[Formula: see text] for the normal incidence. The accuracy is verified via measurements well matched with the simulations. The proposed metamaterial absorber could not only break though the conventional concept that the number of layers should be increased to extend the bandwidth but also provide a powerful solution to realize a low-profile, lightweight, and low cost electromagnetic absorber.

Fabrication and characterization of resistive double square loop arrays for ultra-wide bandwidth microwave absorption.

Microwave absorbers using conductive ink are generally fabricated by printing an array pattern on a substrate to generate electromagnetic fields. However, screen printing processes are difficult to vary the sheet resistance values for different regions of the pattern on the same layer, because the printing process deposits materials at the same height over the entire surface of substrate. In this study, a promising manufacturing process was suggested for engraved resistive double square loop arrays with ultra-wide bandwidth microwave. The developed manufacturing process consists of a micro-end-milling, inking, and planing processes. A 144-number of double square loop array was precisely machined on a polymethyl methacrylate workpiece with the micro-end-milling process. After engraving array structures, the machined surface was completely covered with the developed conductive carbon ink with a sheet resistance of 15 Ω/sq. It was cured at room temperature. Excluding the ink that filled the machined double square loop array, overflowed ink was removed with the planing process to achieve full filled and isolated resistive array patterns. The fabricated microwave absorber showed a small radar cross-section with reflectance less than - 10 dB in the frequency band range of 8.0-14.6 GHz.

Directional radiation for optimal radiative cooling.

The omnidirectional radiation scheme has been widely applied to thermal emitters for radiative cooling. We quantitatively illustrate that significant net radiative absorption at high zenith angles limits the performance of such isotropic emitters, and demonstrate that simply cutting off components corresponding to high angles can substantially improve the cooling performance of commonly used isotropic emitter designs. We also present an expression for the ideal directional spectral emissivity at conditions below ambient temperature. As our approach can be applied to coolers with arbitrary surfaces, our results may serve as a basic guideline for designing practical systems with various surfaces, such as rooftops or façades of modern buildings with complicated geometries.

Extreme anti-reflection enhanced magneto-optic Kerr effect microscopy.

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems.

Laser Synthesis of MOF-Derived Ni@Carbon for High-Performance Pseudocapacitors.

Although nanosizing of multiphase pseudocapacitive nanomaterials could dramatically improve their electrochemical properties, a proper way to simultaneously control both the size and the phase of the pseudocapacitive materials is still elusive. Herein, we employed a commercial CO2 laser engraver to do the transformation of a metal-organic framework (MOF-74(Ni)) into size-controlled Ni nanoparticles (4-12 nm) in porous carbon. The produced Ni@carbon hybrid showed the best specific capacitance of 925 F/g with excellent cycling stability when the particle size is 5.5 nm. We found that the highly redox-active α-Ni(OH)2 is more predominantly formed than the less redox-active ß-Ni(OH)2 as the particle size becomes smaller. Our results substantiate that various MOFs could be created into high-performance pseudocapacitive materials with the controlled size and phase. It is believed that the laser-based synthesis could also serve as a powerful tool for the discovery of new MOF-derived materials in the field of energy storage and catalysis.

Spectrally sharp metasurfaces for wide-angle high extinction of green lasers.

In optical nanostructures used as artificial resonance-based color filters, there is unfortunate universal trade-off between spectral sharpness and angular tolerance as well as maximum extinction. We rigorously derive the maximum performance bounds of wavelength-rejection filters realized by single-layer plasmonic metasurfaces with a dominant resonance and weak near-field coupling, and propose a multi-layer approach to overcome these single-layer limits and trade-offs. We also present a realistic example that has a narrow full-width-at-half-maximum bandwidth of 24 nm with 10 dB extinction at 532 nm with good angular tolerance up to 60°. The performance of the proposed metasurface is close to the general theoretical bound.

Ideal spectral emissivity for radiative cooling of earthbound objects.

We investigate the fundamental limit of radiative cooling of objects on the Earth's surfaces under general conditions including nonradiative heat transfer. We deduce the lowest steady-state temperature attainable and highest net radiative cooling power density available as a function of temperature. We present the exact spectral emissivity that can reach such limiting values, and show that the previously used 8-13 µm atmospheric window is highly inappropriate in low-temperature cases. The critical need for materials with simultaneously optimized optical and thermal properties is also identified. These results provide a reference against which radiative coolers can be benchmarked.

High-Contrast Optical Modulation from Strain-Induced Nanogaps at 3D Heterogeneous Interfaces.

The realization of high-contrast modulation in optically transparent media is of great significance for emerging mechano-responsive smart windows. However, no study has provided fundamental strategies for maximizing light scattering during mechanical deformations. Here, a new type of 3D nanocomposite film consisting of an ultrathin (≈60 nm) Al2O3 nanoshell inserted between the elastomers in a periodic 3D nanonetwork is proposed. Regardless of the stretching direction, numerous light-scattering nanogaps (corresponding to the porosity of up to ≈37.4 vol%) form at the interfaces of Al2O3 and the elastomers under stretching. This results in the gradual modulation of transmission from ≈90% to 16% at visible wavelengths and does not degrade with repeated stretching/releasing over more than 10 000 cycles. The underlying physics is precisely predicted by finite element analysis of the unit cells. As a proof of concept, a mobile-app-enabled smart window device for Internet of Things applications is realized using the proposed 3D nanocomposite with successful expansion to the 3 × 3 in. scale.

Effects of dorsiflexor functional electrical stimulation compared to an ankle/foot orthosis on stroke-related genu recurvatum gait.

[Purpose] We evaluated the effects of functional electrical stimulation (FES) and an ankle/foot orthosis (AFO) in hemiplegic patients exhibiting excessive plantar flexion during the stance phase, associated with genu recurvatum. [Participants and Methods] In total, 12 stroke patients were recruited. We measured changes in knee and ankle joint angles, gait speed, and step and stride length during the gait cycle during barefoot walking, walking while wearing an AFO, and walking after FES application; we used a three dimensional gait analysis system. [Results] In terms of kinematic variables, FES walking was associated with significant increases in peak ankle dorsiflexion during swing, dorsiflexion angle at initial contact, peak ankle dorsiflexion during stance, knee angle at initial contact, and peak knee flexion in the loading response compared to AFO and barefoot walking. AFO walking was associated with a significant difference in peak ankle dorsiflexion during swing compared to barefoot walking. [Conclusion] FES afforded kinematic advantages to the ankle and knee joints compared to AFO in hemiplegic patients with a genu recurvatum gait.

Near-atomically flat, chemically homogeneous, electrically conductive optical metasurface.

Metasurfaces, or two-dimensional arrays of subwavelength-scale structures, can exhibit extraordinary optical properties. However, typical metasurfaces have a bumpy surface morphology that may restrict their practical applications. Here, we propose and demonstrate an optical metasurface that is composed of a thin metallic film, with hidden dielectric structures underneath, and a metal back mirror layer. Exploiting the large difference between the Thomas-Fermi screening length for longitudinal electric fields and the skin depth for transverse electromagnetic fields, the near-atomically flat top surface of the proposed structure can appear homogeneous chemically and electrically but highly inhomogenous optically. The size and shape of the hidden dielectric structures as well as the thickness of the top metallic layer can be tailored to acquire desired optical properties. We performed both theoretical and experimental studies of the proposed metasurface, finding a good agreement between them. This work provides a new platform for ultra-flat optical devices, such as a wavelength selective electrode, diffusive back reflector, meta-lens, and plasmonically enhanced optical biosensors.

Bright and vivid plasmonic color filters having dual resonance modes with proper orthogonality.

The mode orthogonality fundamentally influences the scattering spectra of multi-resonance systems, such as plasmonic color filters. We show that planar arrays of silver nanostructures with dual localized surface plasmon resonances and the right mode orthogonality can function as transmissive RGB color filters with peak transmittances higher than 70%, and color gamut areas larger than 90% of the sRGB space. These are the brightest and most saturated of all designs proposed thus far. We present the Pareto frontier from designs with more than 80% peak transmittance, to designs that achieve a color gamut larger than 120% of the sRGB space.

Ultralarge Area Sub-10 nm Plasmonic Nanogap Array by Block Copolymer Self-Assembly for Reliable High-Sensitivity SERS.

Effective surface enhancement of Raman scattering (SERS) requires strong near-field enhancement as well as effective light collection of plasmonic structures. To this end, plasmonic nanoparticle (NP) arrays with narrow gaps or sharp tips have been suggested as desirable structures. We present a highly dense and uniform Au nanoscale gap array enabled by the customized design of NP shape and arrangement employing block copolymer self-assembly. Block copolymer self-assembly in thin films offers uniform hexagonally packed nanopost template arrays over the entire surface of a 2 in. wafer. Conventional evaporative metal deposition over the nanotemplate surface allows precise geometric control and positional arrangement of metal NPs, constituting tunable, strong plasmonic near-field enhancement particularly at the "hot spots" near interparticular nanoscale gaps. Underlying field distribution has been investigated by a finite-difference time-domain simulation. In the detection of thiophenol, our Au nanogap array shows a remarkable enhancement of Raman intensity greater than ∼104, a standard deviation as small as 12.3% compared to that of the planar Au thin film. In addition, adenine biomolecules can be detected with a detection limit as low as 100 nM. Our approach proposes highly sensitive and reliable SERS on the basis of a scalable, low-cost bottom-up strategy.

Finite-difference time-domain analysis of increased penetration depth in optical coherence tomography by wavefront shaping.

Multiple scattering in turbid media inhibits optimal light focusing and thus limits the penetration depth in optical coherence tomography (OCT). However, the effects of multiple scattering in a turbid medium can be systematically controlled by shaping the incident wavefront. The authors utilize the reciprocity of Maxwell's equations and finite-difference time-domain numerical analysis to investigate the ultimate performance bounds of wavefront shaping-OCT under ideal and realistic configurations and compare them with the conventional method. The results reveal that the optimized impinging wavefront significantly enhances the penetration depth of OCT.

Direct Chemical Synthesis of Plasmonic Black Colloidal Gold Superparticles with Broadband Absorption Properties.

Self-assembly of plasmonic metal nanoparticles can provide an opportunity of creating colloidal superparticles with fascinating optical properties arising from interparticle plasmonic coupling, but typically requires multiple steps involving solvent and/or ligand exchange. We developed a direct, one-step chemical synthesis of plasmonic black colloidal Au superparticles with broadband absorption in visible and near-infrared regions. During the synthesis, the Au superparticles were formed through self-assembly of in-situ-formed Au nanoparticles driven by solvophobic interactions between nanoparticles and solvent. These superparticles could be solution-processed to fabricate a thin film, which exhibited near-perfect absorption over a broad range from 400 nm to 2.5 µm as well as the excellent antireflective property. Thanks to their broadband absorption property, the Au superparticles showed good performances for near-infrared surface-enhanced Raman spectroscopy and light-to-heat conversion.

Transferrable Plasmonic Au Thin Film Containing Sub-20 nm Nanohole Array Constructed via High-Resolution Polymer Self-Assembly and Nanotransfer Printing.

The fabrication and characterization of nanoscale hole arrays (NHA) have been extensively performed for a variety of unique characteristics including extraordinary optical transmission phenomenon observed for plasmonic NHAs. Although the size miniaturization and hole densification are strongly required for enhancement of high-frequency optical responses, from a practical point-of-view, it is still not straightforward to manufacture NHA using conventional lithography techniques. Herein, a facile, cost-effective, and transferrable fabrication route for high-resolution and high-density NHA with sub-50 nm periodicity is demonstrated. Solvent-assisted nanotransfer printing with ultrahigh-resolution combined with block copolymer self-assembly is used to fabricate well-defined Si nanomesh master template with 4-fold symmetry. An Au NHA film on quartz substrate is then obtained by thermal-evaporation on the Si master and subsequent transfer of the sample, resulting in NHA structure having a hole with a diameter of 18 nm and a density over 400 holes/µm2. A resonance peak at the wavelength of 650 nm, which is not present in the transmittance spectrum of a flat Au film, is observed for the Au NHA film. Finite-difference time-domain (FDTD) simulation results propose that the unexpected peak appears because of plasmonic surface guiding mode. The position of the resonance peak shows the sensitivity toward the change of the refractive index of surrounding medium, suggesting it as a promising label-free sensor application. In addition, other types of Au nanostructure arrays such as geometry-controlled NHA and nanoparticle arrays (NPAs) shows the outstanding versatility of our approach.