Observation of heat pumping effect by radiative shuttling.

Heat shuttling phenomenon is characterized by the presence of a non-zero heat flow between two bodies without net thermal bias on average. It was initially predicted in the context of nonlinear heat conduction within atomic lattices coupled to two time-oscillating thermostats. Recent theoretical works revealed an analog of this effect for heat exchanges mediated by thermal photons between two solids having a temperature dependent emissivity. In this paper, we present the experimental proof of this effect using systems made with composite materials based on phase change materials. By periodically modulating the temperature of one of two solids we report that the system akin to heat pumping with a controllable heat flow direction. Additionally, we demonstrate the effectiveness of a simultaneous modulation of two temperatures to control both the strength and direction of heat shuttling by exploiting the phase delay between these temperatures. These results show that this effect is promising for an active thermal management of solid-state technology, to cool down solids, to insulate them from their background or to amplify heat exchanges.

Transient measurement of near-field thermal radiation between macroscopic objects.

The involvement of evanescent waves in the near-field regime could greatly enhance spontaneous thermal radiation, offering a unique opportunity to study nanoscale photon-phonon interaction. However, accurately characterizing this subtle phenomenon is very challenging. This paper proposes a transient all-optical method for rapidly characterizing near-field radiative heat transfer (NFRHT) between macroscopic objects, using the first law of thermodynamics. Significantly, a full measurement at a fixed gap distance is completed within tens of seconds. By simplifying the configuration, the transient all-optical method achieves high measurement accuracy and reliable reproducibility. The proposed method can effectively analyze the NFRHT in various material systems, including SiO2, SiC, and Si, which involve different phonon or plasmon polaritons. Experimental observations demonstrate significant super-Planckian radiation, which arises from the near-field coupling of bounded surface modes. Furthermore, the method achieves excellent agreement with theory, with a minimal discrepancy of less than 2.7% across a wide temperature range. This wireless method could accurately characterize the NFRHT for objects with different sizes or optical properties, enabling the exploration of both fundamental interests and practical applications.

Observation of multiple bulk bound states in the continuum modes in a photonic crystal cavity.

Obtaining bound states in the continuum (BICs) in photonic crystals gives rise to the realization of resonances with high quality factors for lasing and nonlinear applications. For BIC cavities in finite-size photonic crystals, the bulk resonance band turns into discrete modes with different mode profiles and radiation patterns. Here, photonic-crystal BIC cavities encircled by the photonic bandgap of lateral heterostructures are designed. The mirror-like photonic bandgap exhibits strong side leakage suppression to confine the mode profile in the designed cavity. Multiple bulk quantized modes are observed both in simulation and experiment. After exciting the BIC cavity at different positions, different resonance peaks are observed. The physical origin of the dependence between the resonance peak and the illuminating position is explained by analyzing the mode profile distribution and further verified by numerical simulations. Our findings have potential applications regarding the mode selectivity in BIC devices to manipulate the lasing mode in photonic-crystal surface-emitting lasers or the radiation pattern in nonlinear optics.

Defect-rich graphene-coated metamaterial device for pesticide sensing in rice.

Performing sensitive and selective detection in a mixture is challenging for terahertz (THz) sensors. In light of this, many methods have been developed to detect molecules in complex samples using THz technology. Here we demonstrate a defect-rich monolayer graphene-coated metamaterial operating in the THz regime for pesticide sensing in a mixture through strong local interactions between graphene and external molecules. The monolayer graphene induces a 50% change in the resonant peak excited by the metamaterial absorber that could be easily distinguished by THz imaging. We experimentally show that the Fermi level of the graphene can be tuned by the addition of molecules, which agrees well with our simulation results. Taking chlorpyrifos methyl in the lixivium of rice as a sample, we further show the molecular sensing potential of this device, regardless of whether the target is in a mixture or not.

A Semisolid Micromechanical Beam Steering System Based on Micrometa-Lens Arrays.

Optical beam steerers have been widely employed for information acquisitions. Numerous beam steering schemes have been developed, and each of them can satisfy practical requirements for certain scenarios. However, there is still a lack of a comprehensive approach that is able to balance all of the critical technical parameters for wide range of applications. Here, a semisolid micromechanical beam steering system based on micrometa-lens arrays (MMLAs) is demonstrated. It is operated by manipulating the probe beam over two sets of decentered MMLAs potentially driven by high-speed piezo-electric motors. Small f-numbers, well-corrected aberration, and easy lateral reproduction of micrometa-lenses optimize the overall technical parameters. As a proof-of-concept, we implement such a device exhibiting diffraction-limited resolution within a large field of view of 30° × 30°. A three-dimensional depth sensing is also performed to demonstrate its potential in light detection and ranging applications.

Microwave Metamaterial Absorbers with Controllable Luminescence Features.

Traditional microwave absorbers can hide target objects from the detection of radar waves without tackling optical waves. However, in actual scenarios, the objects might still be observed by a hyperspectral remote sensor or an imaging system from comparison with environmental optical features, in particular for variable backgrounds. For this aspect, a comprehensive stealthing technique able to deal with microwaves and optical features simultaneously adapting to the variable environment is highly desired. In this work, an ultrathin flexible metamaterial that can simultaneously realize wideband microwave absorption and controllable visible and near-infrared luminescence spectra is proposed. The designed artificial coat can absorb more than 80% of the incident energy in the X-band (8-12 GHz) within a large incidence angle range up to 54° at low polarization sensitivity, while its real-time visible and near-infrared luminescence spectrum can be electrically adjusted through an integrated emission system. The method proposed here can be extended to broader wave bands and find important applications in multifunctional stealthing technologies.

Flexible dynamic structural color based on an ultrathin asymmetric Fabry-Perot cavity with phase-change material for temperature perception.

Dynamic structural color has attracted considerable attentions due to its good tunable characteristics. Here, an ultrathin asymmetric Fabry-Perot (FP)-type structural color with phase-change material VO2 cavity is proposed. The color-switching performance can be realized by temperature regulation due to the reversible monoclinic-rutile phase transition of VO2. The various, vivid structural color can be generated by simply changing the thickness of VO2 and Ag layers. Moreover, the simple structural configuration enables a large-scale, low-cost preparation on both rigid and flexible substrates. Accordingly, a flexible dynamic structural color membrane is adhered on a cup with a curved surface to be used for temperature perception. The proposed dynamic structural color has potential applications in anti-counterfeiting, temperature perception, camouflage coatings among other flexible optoelectronic devices.

Research on Enhanced Detection of Benzoic Acid Additives in Liquid Food Based on Terahertz Metamaterial Devices.

It is very important for human health to supervise the use of food additives, because excessive use of food additives will cause harm to the human body, especially lead to organ failures and even cancers. Therefore, it is important to realize high-sensibility detection of benzoic acid, a widely used food additive. Based on the theory of electromagnetism, this research attempts to design a terahertz-enhanced metamaterial resonator, using a metamaterial resonator to achieve enhanced detection of benzoic acid additives by using terahertz technology. The absorption peak of the metamaterial resonator is designed to be 1.95 THz, and the effectiveness of the metamaterial resonator is verified. Firstly, the original THz spectra of benzoic acid aqueous solution samples based on metamaterial are collected. Secondly, smoothing, multivariate scattering correction (MSC), and smoothing combined with first derivative (SG + 1 D) methods are used to preprocess the spectra to study the better spectral pretreatment methods. Then, Uninformative Variable Elimination (UVE) and Competitive Adaptive Reweighted Sampling (CARS) are used to explore the optimal terahertz band selection method. Finally, Partial Least Squares (PLS) and Least square support vector machine (LS-SVM) models are established, respectively, to realize the enhanced detection of benzoic acid additives. The LS-SVM model combined with CARS has the best effect, with the correlation coefficient of prediction set (Rp) is 0.9953, the root mean square error of prediction set (RMSEP) is 7.3 × 10-6, and the limit of detection (LOD) is 2.3610 × 10-5 g/mL. The research results lay a foundation for THz spectral analysis of benzoic acid additives, so that THz technology-based detection of benzoic acid additives in food can reach requirements stipulated in the national standard. This research is of great significance for promoting the detection and analysis of trace additives in food, whose results can also serve as a reference to the detection of antibiotic residues, banned additives, and other trace substances.

Confinement and Protection of Skyrmions by Patterns of Modified Magnetic Properties.

Magnetic skyrmions are versatile topological excitations that can be used as nonvolatile information carriers. The confinement of skyrmions in channels is fundamental for any application based on the accumulation and transport of skyrmions. Here, we report a method that allows effective position control of skyrmions in designed channels by engineered energy barriers and wells, which is realized in a magnetic multilayer film by harnessing the boundaries of patterns with modified magnetic properties. We experimentally and computationally demonstrate that skyrmions can be attracted or repelled by the boundaries of areas with modified perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction. By fabricating square and stripe patterns with modified magnetic properties, we show the possibility of building reliable channels for confinement, accumulation, and transport of skyrmions, which effectively protect skyrmions from being destroyed at the device edges. Our results are useful for the design of spintronic applications using either static or dynamic skyrmions.

Ultrathin Conformal Magnetic Invisible Cloak for Irregular Objects.

Magnetic invisible cloaking has been previously demonstrated but only limited to objects with rotational geometries either in spherical or cylindrical shapes, for which the classic analytical bilayer scheme could be strictly applied to design the hiding coat. In this work, we show that a quasi-static cloaking effect could be achieved for irregular objects, e.g., metals with sharp edges, using a numerical optimization scheme. In the quasi-static limit, it is unambiguously proved that the disturbance of the irregular geometries could be well compensated by the inhomogeneous distribution of the soft ferromagnetic (FM) layer either in permeability values or in shapes under the framework of a bilayer cloak. An FM mesh coat with a constant thickness of 0.5 mm was successfully engineered to meet the specific requirements. Experimentally, good cloaking performance with a field disturbance of less than 0.5% has been achieved for a 2 × 2 × 5 cm3 brass bar in a wide frequency range from ∼10 to 250 kHz. A commercial metal scanner was also applied to verify the practical potential. The general strategy to hide almost arbitrary objects was discussed in the end. In principle, the numerical conformal coat engineered by the composite material proposed here could be broadly extended for objects with various geometries.

Broadband Electromagnetic Wave Tunneling with Transmuted Material Singularity.

Subwavelength channels filled with near-zero-index (NZI) media can realize extraordinary optical functionalities, for example, tunneling electromagnetic wave without reflections, but usually confined in a narrow wavelength band due to the material singularity (refractive index n≈0), which seriously limits the practical potentials. In this Letter, we show this limit can be fundamentally overcome by an alternative, named near-zero-index-featured (NZIF) structure, with the singularity transmuted via a controlled optical conformal mapping, enabling the device implementation with nonmagnetic normal dielectrics (i.e., relative permittivity >1). Their equivalence is strictly examined through a subwavelength tunneling waveguide. Classic wave tunneling features in a broad frequency range are revealed in various confined geometries. These properties are robust against the disturbance of several kinds of structural defects benefited from the infinite effective local wavelength. The broadband and lossless NZIF medium proposed here provides a promising way to pursue the fascinating light controlling functionalities as initially enabled by singular NZI materials.

High-performance silicon-graphene hybrid plasmonic waveguide photodetectors beyond 1.55 µm.

Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range. However, the reported fast graphene photodetectors mainly operate in the 1.55 µm wavelength band. In this work, we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon-graphene hybrid plasmonic waveguide, which enables efficient light absorption in graphene at 1.55 µm and beyond. When operating at 2 µm, the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz. When operating at 1.55 µm, the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz (setup-limited) and a high responsivity of ~0.4 A/W even with a low bias voltage of -0.3 V. This work paves the way for achieving high-responsivity and high-speed silicon-graphene waveguide photodetection in the near/mid-infrared ranges, which has applications in optical communications, nonlinear photonics, and on-chip sensing.

Spin-dependent switchable metasurfaces using phase change materials.

Metasurfaces have been widely investigated for various applications enabled by their strong light manipulation capabilities. Their monolithic designs offer the convenience to incorporate novel natural materials in order to realize advanced electromagnetic (EM) functionalities. Here, based on the usage of the phase change material vanadium dioxide (VO2), a switchable metasurface that could work at two different working states is proposed. With insulating VO2, we show that helicity-dependent metasurface could be rigorously designed by adopting two phase variables, i.e., initial phase and Pancharatnam-Berry (P-B) phase, which is verified by showing an asymmetric photonic spin Hall effect (APSHE). When VO2 goes into the metallic phase (e.g., by raising the operating temperature above ~341K), the loss factor of the unit cell will be enhanced, and in this case with the assistance of multi-mode resonances, the metasurface will turn into a perfect broadband circular-polarization-insensitive EM absorber. Based on these, switchable beam splitters and focus-lenses have been designed and discussed in the paper. The method proposed here may pave a new way to pursue active and multifunctional optical devices.

Broadband metamaterial absorber with an in-band metasurface function.

Metamaterials (MMs) have received wide attention due to their strong ability to control wave propagation. Here we propose a thin MM that can realize broadband absorption, as well as in-band phase-controlled reflection. It absorbs more than 90% energy of the incident circularly polarized (CP) light between 7.94-9.76 and 10.99-12.26 GHz and, meanwhile, works as a half-wave plate near 10.35 GHz. The schemes based on both natural and artificial magnetic materials have been discussed to produce the anisotropic phase. A reflection beam steering metasurface is proposed using the absorption-selective MM. The metasurface-integrated MM absorber may have important applications in aerospace technologies such as radome.

Far-field radiative thermal rectifier based on nanostructures with vanadium dioxide.

Radiative thermal rectifiers capable of realizing asymmetric heat flux transfer have attracted a lot of research interests recently, mainly focusing on the engineering of the emissivity spectra. In this Letter, we propose a far-field radiative thermal rectifier utilizing the phase change material vanadium dioxide (VO2). The thermal rectifier consists of a metamaterial infrared absorber and a two-layer thin-film structure acting as the active and the passive components, respectively. Numerical optimization has been carried out to control the emissivity spectra of both parts and maximize the overall rectification effect. A large thermal rectification factor of 3.5 is predicted at a temperature bias of ΔT=100 K.

Observing of the super-Planckian near-field thermal radiation between graphene sheets.

Thermal radiation can be substantially enhanced in the near-field scenario due to the tunneling of evanescent waves. Monolayer graphene could play a vital role in this process owing to its strong infrared plasmonic response, however, which still lacks an experimental verification due to the technical challenges. Here, we manage to make a direct measurement about plasmon-mediated thermal radiation between two macroscopic graphene sheets using a custom-made setup. Super-Planckian radiation with efficiency 4.5 times larger than the blackbody limit is observed at a 430-nm vacuum gap on insulating silicon hosting substrates. The positive role of graphene plasmons is further confirmed on conductive silicon substrates which have strong infrared loss and thermal emittance. Based on these, a thermophotovoltaic cell made of the graphene-silicon heterostructure is lastly discussed. The current work validates the classic thermodynamical theory in treating graphene and also paves a way to pursue the application of near-field thermal management.

Transparent transmission-selective radar-infrared bi-stealth structure.

We report a multifunctional metamaterial composite structure that not only provides the broadband radar and thermal infrared bi-stealth function but also possesses an in-band microwave transmission window and high optical transparency. It is composed of four metasurface layers made of indium tin oxide (ITO) films with different surface resistances, which are specifically designed to sequentially control the infrared emission, microwave absorption and transmission. The fabricated sample exhibits a low reflectivity less than 10% in 1.5-9 GHz and a transmission peak of 50% around 3.8 GHz up to the incident angle of 30 degrees. In the infrared atmosphere window, a low thermal emissivity of about 0.52 is achieved. Meanwhile, it keeps good optical transparency by the use of the ITO films. The optically transparent, low-infrared-emissivity, radar-reflectionless and frequency-selective-transmission properties will enable the promising application of communication-compatible multispectral stealth technology.

Giant power enhancement for quasi-omnidirectional light radiation via ε-near-zero materials.

We theoretically demonstrate a giant power enhancement effect for a line current source in a ε-near-zero (ENZ) two-dimensional (2D) shell with proper physical dimensions. Compared with the traditional high-ε dielectric approach, the ENZ scheme has the prominent advantage that the radiation performance is less sensitive to the outer radius of the shell, which is critically important for real applications where micro-nano fabrications are often involved. The enhancing performance is independent on the position of the source inside the ENZ shell and could be substantially strengthened by incorporating more sources, while the quasi-omnidirectional radiation pattern could be managed to have negligible variance, as evidenced by a particular example with an inner radius of the shell equal to 0.156λ0. Compared with the single source case, two identical sources with a phase difference less than 134° will raise the total radiation power more than 4 folds and the maxima will be about 30 when they are in phase. The field analysis shows that this quasi-isotropic radiation enhancement is mainly contributed by the amplification of the isotropic zeroth order mode radiation while the higher orders with anisotropic emission patterns are effectively suppressed by the specifically designed ENZ shell. In the end, a practicable device employing 4H-silicon carbide (4H-SiC) naturally available with ENZ properties in the mid-infrared regime is numerically proposed, which could provide more than 10 times of radiation enhancement through optimizing the permittivity of the inner dielectric cylinder. These results may find very important applications in the design of novel devices for mid-infrared photon sources or detectors.

Multilayered analog optical differentiating device: performance analysis on structural parameters.

Analogy optical devices (AODs) able to do mathematical computations have recently gained strong research interest for their potential applications as accelerating hardware in traditional electronic computers. The performance of these wavefront-processing devices is primarily decided by the accuracy of the angular spectral engineering. In this Letter, we show that the multilayer technique could be a promising method to flexibly design AODs according to the input wavefront conditions. As examples, various Si-SiO2-based multilayer films are designed that can precisely perform the second-order differentiation for the input wavefronts of different Fourier spectrum widths. The minimum number and thickness uncertainty of sublayers for the device performance are discussed. A technique by rescaling the Fourier spectrum intensity has been proposed in order to further improve the practical feasibility. These results are thought to be instrumental for the development of AODs.

Design of optical wavelength demultiplexer based on off-axis meta-lens.

We propose an off-axis meta-lens-based optical wavelength demultiplexer. The performance of the device initially proposed with four output channels (1527-1596 nm, channel spacing 23 nm) composed of an optical fiber array is analyzed by both scalar diffraction theory and ray tracing method. The results show that the fiber energy coupling efficiency of the demultiplexer could be larger than 89% and the channel bandwidth is about 9 nm. Influences of the two key parameters, focal length f and off-axis angle α, are also investigated. We find that the minimum spectral linewidth of the channel is inversely proportional to the sine of α and nearly independent of f for a meta-lens with large F-number (F>5), while the effective spectral range is negatively (positively) dependent on α (f). These results are significant in guiding us to build small and compact demultiplexing devices for optical telecommunication.