Broadband Reflection Research Articles

Enhanced broadband reflection properties of PSCLCs films supported by electrospun nanofiber networks loaded with Cs0.33WO3 nanoparticles

ABSTRACT The introduction of nanoparticles into cholesteric liquid crystals has long provided inspiration for innovative device materials. A novel material design strategy was demonstrated to achieve a broadband reflection of polymer-stabilised cholesteric liquid crystals (PSCLCs) based on Cs0.33WO3 nanoparticles and the polymer nanonetworks in the PSCLCs. Through the adjustment of nanoparticle and chiral dopant (C6M) concentrations and meticulous control of polymerisation conditions, an appropriate pitch gradient distribution was achieved in the polymer-stabilised cholesteric liquid crystals (PSCLCs). This was attained by striking a delicate balance between the polymerisation speed and chiral dopant diffusion rate, resulting in optimal PSCLCs optical properties. Modification of Cs0.33WO3 nanoparticles by oleic acid was successfully accomplished as confirmed by IR spectroscopy. SEM and EDS analyses further demonstrated the distribution of nanoparticles in the spun fibre film. Moreover, POM observations showed that liquid crystals with nanofiber networks maintained the planar texture and temperature stability. The prepared broadband reflective film opens an avenue towards developing the reflection bandwidth in the PSCLCs, essential for intelligent windows and infrared shielding devices.

Near-Infrared Reflective Polymer Films Based on UV-327-Doped Zinc Oxide Nanoparticles.

We prepared cholesteric liquid crystal (CLC) films with broadband reflective properties by admixing organic dye UV-327 into inorganic zinc oxide nanoparticles (ZnO NPs), utilizing the principle of pitch distribution from a large to a small gradient along the film thickness direction, leading to broadband reflection. ZnO NPs are poorly dispersed and easy to gather, but they do not decompose easily. The addition of UV-327 makes up for the above shortcomings. UV-327 is an organic compound with good compatibility and dispersion with liquid crystal systems. Therefore, we used the method of mixing two UV-absorbing dyes (UV-327 and ZnO NPs) to obtain CLC films. UV-absorbing dyes (UV-327 and ZnO NPs) made the liquid crystal films form a UV intensity gradient in the direction of thickness, prompting the polymerizable monomers to polymerize faster on the stronger side of the light, leading to the relative diffusion of chiral molecules and polymerizable monomers, forming the concentration gradient of chiral molecules in the direction of thickness. The pitch has a gradient distribution as the chiral concentration varies. Then, anchored by the polymer network, the pitch gradient distribution no longer changes. Broadened reflective bandwidth can reach up to 881 nm. Furthermore, the film covers the near-infrared wavelength band well, which can be applied to future smart windows or laser shielding for medical and military applications. It is also believed that this achievement will optimize the preparation technology of broadband reflective CLC films in the future.

A miniaturized spot-focusing microwave sensor for broad-band reflectivity measurements

This study presents a novel spot-focusing microwave sensor designed for on-site monitoring of absorbing materials and coatings, a critical process in assessing the performance of equipment in service. The sensor addresses the challenge of integrating miniaturization, wide bandwidth, and focused radiation-requirements that are typically at odds in microwave sensor design. The developed approach utilizing a multi-layer lens nesting structure to achieve narrow-beam radiation over an ultra-wideband (UWB) frequency range. Each layer of the lens system is engineered to concentrate beams within specific frequency bands, enabling consistent performance across a broad spectrum. In addition, we introduce a composite feeding structure characterized by exponential gradient slots that enhance UWB performance while minimizing cross-polarization effects. The sensor demonstrates exceptional capabilities, achieving over 8 dB of radiation gain and maintaining a voltage standing wave ratio (VSWR) under 2.5 within a 2–22 GHz frequency band. Its compact size (95 mm in diameter and 75 mm in height) does not compromise its ability to emit a Gaussian beam with a narrow radiation pattern of less than ±30°, simulating plane-wave characteristics essential for near-field on-site measuring. Accompanying the sensor is a novel on-site reflectivity measuring method that leverages time-domain gating technology to mitigate interference signal impact. Measurement results validate the sensor and method’s precision, with an average error margin of 0.78 dB. This breakthrough in sensor design and methodology marks a significant advancement in the on-site evaluation of absorbing materials and coatings.

Evaluation of spatial downscaling for satellite retrieval of evapotranspiration from the nonparametric approach in an arid area

Evapotranspiration (ET) data is critical for monitoring the limited hydrological resources in arid areas. At present, the spatial resolution of ET derived by the thermal infrared images from satellite data is coarse (always more than 500 m). The low-resolution data cannot meet the needs of the high-precision mapping of ET. Therefore, it is important to undertake a downscaling procedure to improve the spatial resolution of ET. This paper selects the oasis region and surrounding deserts of Zhangye City of China as study area and compares with two downscaling procedures that include input and output downscaling procedure. The ET is estimated with the Remote Sensing-Non Parametric (RS-NP) model. The two downscaling procedures use Enhanced Spatial and Temporal Adaptive Reflectance Fusion Model (ESTARFM) to improve the spatial resolution of ET from 1-km to 100 m. Compared with the result derived from the output downscaling procedure, the high-resolution ET derived from the input downscaling procedure has a slightly better performance with R2 of 0.92 and RMSE of 41.8 W/m2, respectively. Neglectingthe downscaling of land surface temperature and broadband reflectance (BBR) result in larger errors of ET estimation with bias values of 73.0 W/m2 and 23.7 W/m2, respectively. Those values account for almost 1/3 and 1/10 of ET values, respectively. In non-vegetated areas, those proportions increase further around 90.6 % and 21.3 %, respectively. Furthermore, the influence of the neglect of the downscaling of the near-infrared reflectance on the ET estimation dominates that of BBR with a proportion of 42 %. The input downscaling procedure is suitable for generating the high-precision map of ET, especially in vegetated areas and during northern hemisphere summer months. Our study is helpful to obtain ET maps in fine spatial resolutions and contribute to the comprehensive understanding of the importance of the downscaling of each kind of input for reliable ET estimations.

Omnidirectional broadband phase modulation by total internal reflection.

Phase modulation plays a crucial role in shaping optical fields and physical optics. However, traditional phase modulation techniques are highly dependent on angles and wavelengths, limiting their applicability in smart optical systems. Here, we propose a first-principle theory for achieving constant phase modulation independent of incident angle and wavelength. By utilizing a hyperbolic metamaterial and engineering-specific optical parameters, different reflective phase jumps are achieved and tailored for both transverse electric (TE) and transverse magnetic (TM) waves. The aimed reflection phase difference between TE and TM waves can be thus achieved omnidirectionally and achromatically. As an example, we propose a perfect omnidirectional broadband reflection quarter wave plate. This work provides fundamental insights into manipulating optical phases through optical parameter engineering.

Quasi-bound states in the continuum in asymmetric hetero-bilayer metasurfaces

Bound states in the continuum (BICs) are completely confined states of open optical structures existing above the light line. However, BICs must be turned into quasi-BICs having a finite Q-factor to be accessible. To achieve quasi-BICs with a high Q-factor at normal incidence, diverse optical structures have been explored by inducing asymmetry by etching a tiny part of the structure. However, it is difficult to introduce a small asymmetry by the etching process for a high Q-factor. Here, we propose novel asymmetric hetero-bilayer metasurface using a low-refractive-index top grating and high-refractive-index bottom grating. The thin upper grating is intended to introduce perturbation accurately, and the bottom grating functions as a broadband reflector. We identified the effect of the geometric perturbation and the top grating material on the Q-factor, resonance wavelength, and background reflectance. As the geometric asymmetry increases, both the Q-factor and the background reflectance decreased, and the resonance wavelength changed accordingly. The Q-factor followed the inverse quadratic of the geometric perturbation and the inverse cube of the relative permittivity. The effect of geometric perturbation decreased up to 10 times with a lower refractive index. Taking advantage of a high-Q hetero-bilayer metasurface, a gas sensor was numerically investigated, and it distinguished a change in the refractive index of 5 × 10−7. Furthermore, the asymmetric hetero-bilayer metasurface showed a quasi-BIC with a high Q-factor on a rigid substrate.

Characterization of porosity in periodic 3D nanostructures using spectroscopic scatterometry

Periodic nanostructures have important applications in nanophotonics and nanostructured materials as they provide various properties that are advantageous compared to conventional solid materials. However, there is a lack of metrology techniques that are suitable for large-scale manufacturing, as the traditional tools used in nanotechnology have limited throughput and depth resolution. In this work, we use spectroscopic scatterometry as a fast and low-cost alternative to characterize the porosity of three-dimensional (3D) periodic nanostructures. In this technique, the broadband reflectance of the structure is measured and fitted with physical models to predict the structure porosity. The process is demonstrated using 3D periodic nanostructures fabricated using colloidal phase lithography at various exposure dosages. The measured reflectance data are compared with an optical model based on finite-difference time-domain and transfer-matrix methods, which show qualitative agreement with the structure porosity. We found that this technique has the potential to further develop into an effective method to effectively predict the porosity of 3D nanostructures and can lead to real-time process control in roll-to-roll nanomanufacturing.

A Bioinspired Bilevel Metamaterial for Multispectral Manipulation toward Visible, Multi-Wavelength Detection Lasers and Mid-Infrared Selective Radiation.

Manipulation of the electromagnetic signature in multiple wavebands is necessary and effective in civil and industrial applications. However, the integration of multispectral requirements, particularly for the bands with comparable wavelengths, challenges the design and fabrication of current compatible metamaterials. Here, a bioinspired bilevel metamaterial is proposed for multispectral manipulation involving visible, multi-wavelength detection lasers and mid-infrared (MIR), along with radiative cooling. The metamaterial, consisting of dual-deck Pt disks and a SiO2 intermediate layer, is inspired by the broadband reflection splitting effect found in butterfly scales and achieves ultralow specular reflectance (average of 0.013) over the entire 0.8-1.6µm with large scattering angles. Meanwhile, tunable visible reflection and selective dual absorption peaks in MIR can be simultaneously realized, providing structural color, effective radiative thermal dissipation at 5-8µm and 10.6µm laser absorption. The metamaterial is fabricated by a low-cost colloidal lithography method combined with two patterning processes. Multispectral manipulation performances are experimentally demonstrated and a significant apparent temperature drop (maximum of 15.7°C) compared to the reference is observed under a thermal imager. This work achieves optical response in multiple wavebands and provides a valuable way to effectively design multifunctional metamaterials inspired by nature.

Preparation of polymer‐stabilized cholesteric liquid crystal films capable of broadband reflection using a three‐layer system of self‐diffusion

AbstractThrough a simple and effective method for preparing broadband reflective liquid crystal films, the advantages of the three‐layer system are expanded based on the two‐layer system. The cholesteric liquid crystals form a prepolymerized layer after semi‐curing. In the design of the top prepolymerization layer, there is less content of the chiral compound R5011, which results in the formation of cholesteric liquid crystal molecules with a larger pitch. Mainly by controlling the content of polymerizable monomers and UV‐absorbing dyes in cholesteric liquid crystals (CLCs), a polymer network with density gradient distribution suitable for substance diffusion is formed, and the oleophobic substrate on one side of the low‐density polymer network is more likely to be peeled off. In the design of the lowest prepolymerization layer, the content of R5011 is higher, which results in the formation of cholesteric liquid crystals with a larger pitch. One side substrate of two cholesteric liquid crystal cassettes is peeled off and merged into a new liquid crystal cassette whose left and right sides are stacked with septum pads (20 μm), and the nematic liquid crystals are poured into the middle layer to form a three‐layer integrated film. The polymer network of the polymer‐stabilized cholesteric liquid crystals (PSCLCs) itself in the bilayer was used as a diffusion channel. Chiral compounds and UV‐absorbing dyes were diffused from the cholesteric liquid crystal layer to the intermediate nematic liquid crystal layer by diffusion gradient method, resulting in liquid crystal films with arbitrary wavelength bands and ultrawide broadband reflection.

Preparation and application of broadband reflective polymer-stabilised cholesteric liquid with nanoscale fibres

ABSTRACT The bandwidth broadening of polymer-stabilised cholesteric liquid crystal (PSLC) has the potential to enhance its optical performance. In this study, a novel approach was presented for preparing broadband reflective films by using nanofibers loaded with zinc oxide nanoparticles as network supports in chiral nematic liquid crystals. The introduction of nanofiber played a pivotal role in replacing some monomers, resulting in cost-efficiency. Experiments were conducted to investigate the influence of different compositions and conditions on the broadband reflection performance. Specially, the effect of the concentration of nanoparticles loaded in nanofibers, direct-current (DC) voltage, monomer content, ultraviolet (UV) light intensity, polymerisation temperature, and UV irradiation time were examined. The microstructure of nanofiber and optical texture of PSLC were characterised using SEM and POM, respectively. With the above optimisations, the broadening from 246 nm to 412 nm was achieved and the films embodied brilliant UV shielding and infrared (IR) heat control effectiveness. This study provides a potential method for the preparation of smart liquid crystal devices in the future.

Controlling sound waves in gradient spoof-fluid-spoof waveguides

In this work, we theoretically and experimentally demonstrate that effective trapping, guiding, and manipulation of sound waves can be realized in spoof-fluid-spoof acoustic waveguides with gradient index modulation. Empowered by the abundant mode evolution physics between propagation waves and spoof acoustic surface waves in the gradient waveguide structure, various functional sound propagation phenomena, including broadband transmission, broadband reflection, Fabry–Pérot resonances, and Fano resonances, are unveiled. The underlying principle stems from the interplay of various mechanisms composed of gradient mode conversion, high-order mode resonances, and symmetry-protected bound states in the continuum. These effects can be effectively modulated through the manipulation of the fluid gap and doped defects within the waveguide structure. Our findings can offer possibilities for manipulating sound waves in a versatile manner and holding significant potential for various acoustic applications such as sensing, filtering, insulation, and wavefront engineering.

Electrically controllable broadband reflective linear cross-polarization conversion based on liquid crystals.

This paper presents an electrically controllable reflective broadband linear polarization (LP) converter based on liquid crystals (LCs) for cross-polarization conversion (CPC) in the terahertz frequency range. The proposed structure achieves a high polarization conversion ratio (PCR) exceeding 0.9 within the frequency range of 236.8 - 269.6 GHz. A vital feature of this design is the dynamic control of polarization conversion by re-orienting the nematic liquid crystal molecules through voltage bias switching between 'on' and 'off' states, allowing for precise manipulation of cross-polarized and co-polarized reflected waves. Experimental results validate the simulation outcomes, demonstrating excellent agreement. In contrast to conventional reflective polarization converters with fixed frequency responses, the proposed electrically controllable polarization conversion offers significant potential in imaging and optical communications.

Improved Transformer-Based Target Matching of Terahertz Broadband Reflective Metamaterials With Monolayer Graphene

On-demand metamaterial designs aided by artificial intelligence have hitherto received tremendous attention recently. However, the traditional deep neural networks (DNNs) still show the limited generalization ability in the inverse design of tunable graphene-based terahertz (THz) metamaterial. In this article, we propose two kinds of DNNs based on the self-attention mechanism to implement the inverse design of tunable broadband reflectors working in the THz band. Moreover, the proposed networks have been improved, so that they could adapt to different types of input vector or matrix in terms of different kinds of on-demand design requirements. Besides, adaptive batch normalization (BN) layers are introduced in our improved networks to accelerate the converging speed with low computational consumption. It could be shown in experiments that the proposed networks exhibit higher accuracy and faster convergence speed than the traditional neural networks, such as multilayer perceptron (MLP) and convolutional neural network (CNN). Finally, this work may provide a key guide for developing THz metamaterials with 2-D materials employing DNNs based on self-attention mechanism.

A Review of Developments in Polymer Stabilized Liquid Crystals.

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Super‐White Boron Nitride Aerogel‐Enabled High‐Energy Laser Irradiation Protection

AbstractAerogels are ultralight solid materials with three‐dimensional interconnected porous structures that offer considerable advantages for various protective applications. Lasers have led to various technological revolutions in aerospace, medical science, and industrial manufacturing. However, shielding against high‐energy laser irradiation using aerogels has rarely been reported. A super‐white (reflectivity >98%) boron nitride (BN) aerogel constructed using BN nanoribbons is developed in this study for high‐energy laser protection. Large numbers of BN nanoribbons serve as randomized and disordered optical nanobarriers for intense backlight scattering, which contribute to high broadband reflectivity. Combined with the low thermal conductivity of the aerogel structure and the excellent high‐temperature stability of BN, the resulting BN aerogel exhibits a high protective threshold of 2.1 × 104 W cm−2. Moreover, the BN aerogel possesses a soft porous network and a low thermal expansion coefficient, which is promising for tolerating the thermal stress and shock of local high‐temperature fields caused by laser irradiation. The successful laser shielding by the super‐white BN aerogel may be helpful for designing and fabricating advanced aerogel‐based lightweight anti‐laser materials.

Metal-free design of a multilayered metamaterial with chirped Bragg grating for enhanced radiative cooling.

A wideband, all-dielectric metamaterial structure for enhancing radiative cooling is investigated. The structure is optimized to reflect most of the solar irradiance window (between 0.3 µm-3 µm), which is one of the biggest challenges in highly efficient radiative cooling coatings. The design is based on the principles of Bragg gratings, which constitutes a simple synthesis procedure to make a broadband reflector of reduced dimensions, without metallic layers, while keeping a flat enough response in the entire bandwidth. Numerical results show that reflection of solar irradiation can be easily tailored and maximized using this method, as well as the net cooling power of the device, about ∼79 W/m2 at daytime (about double at night-time) and a temperature reduction of 23 K (assuming no heat exchange) and 7 K assuming a heat exchange coefficient of 10 W/m2/K, for a device and ambient temperatures of 300 K and 303 K, respectively. This occurs even in detriment of absorption in the atmospheric window (8 µm-13 µm). Results also show the importance of efficiently reflecting solar irradiance for such technologies and its relevance in synthesis and design without using metallic components.

Bioinspired “aerogel grating” with metasurfaces for durable daytime radiative cooling for year-round energy savings

Passive radiative cooling strategy shows a great potential in mitigating global warming and reducing energy consumption. However, existing radiative coolers generally show poor solar-reflection and be easily dampened by environmental aging, making the cooling ineffective. Mimicking the biological design for thermoregulation has given promise for realizing high performance and long time cooling. Here, inspired by structural whiteness in butterflies, we developed a sustainable cellulose nanocrystal aerogel grating (CAG) with designable metasurfaces for efficient radiative cooling, which can be achieved by using copper meshes as well as manipulating the 2D confined freeze casting process. The simulation indicates that the regular tubes and irregular gully on the aerogel surface act as gratings to achieve diffraction and broadband reflection of sunlight. Such CAG exhibits ultrahigh solar reflectance (97.4 %), high infrared emittance (94 %), and anti environmental aging function (anti-ultraviolet light/ contamination). It achieves a sub-ambient temperature drop of 10.5 °C under direct sunlight and still approximately 9.4 °C even after 6 months of exposure to hot weather. Building simulations demonstrate that CAG can save 47 % of the worldwide energy consumption for cooling each year. This work provides insight into the design of bioinspired thermal-regulating materials to save energy consumption.

Design of a 3-bit encoded THz ultra-wideband vortex beam generator based on a metasurface

With the rapid development of high-speed THz communications, there has been a growing interest in exploring the potential of the orbital angular momentum (OAM) of electromagnetic waves. In this study, we propose a cross-polarized reflective broadband metasurface operating in the THz band to harness the benefits of OAM in the optical field. We quantitatively analyze the reflection amplitude and phase characteristics of the metasurface elements, which supports the design of a reflective broadband element surface array with an ordered arrangement of 3-bit elements. By combining the 3-bit metasurface elements in an array, linearly polarized waves can effectively be converted into vortex beams in the operating frequency range of 0.6THz-1.3THz. The simulation results demonstrate that the designed metasurface element structure can achieve not only efficient cross-polarization reflection amplitude but also effective phase control by adjusting the size parameters. Our proposed metasurface is able to convert linearly polarized waves into vortex beams with an efficiency exceeding 85% and can achieve high-purity OAM beam acquisition. Furthermore, the metasurface structure is simple to implement and can be easily integrated with photoelectric circuits, making it ideal for use in ultrahigh-speed THz communications.

Large omnidirectional mid-infrared photonic bandgap in a one-dimensional ternary photonic crystal consisting of isotropic dielectric, elliptical metamaterial and plasma

Photonic bandgaps (PBGs) in traditional one-dimensional (1-D) binary photonic crystals (PhCs) consisting of two kinds of isotropic dielectrics strongly shift towards shorter wavelengths as incident angle increases. Such blueshift property of PBGs intensively limits the widths of omnidirectional photonic bandgaps (OPBGs). Very recently, researchers achieved a special kind of PBGs called angle-insensitive PBGs in novel 1-D binary PhCs consisting of isotropic dielectric and elliptical metamaterial (EMM). The emergence of such angle-insensitive PBGs provides us an opportunity to achieve large OPBGs. Herein, we periodically introduce plasma layers into a 1-D binary PhC consisting of isotropic dielectric and EMM with an angle-insensitive PBG to achieve a large OPBG at mid-infrared wavelengths. The EMM is mimicked by an all-dielectric subwavelength multilayer. The broaden effect of the OPBG originates from the plasmonic property of plasma and the angle-insensitive property of the PBG. The width of the OPBG reaches 4.19 μm. Our work provides a feasible route to achieving large OPBGs in 1-D PhCs and would promote the development of OPBG-based devices, such as omnidirectional broadband reflectors and omnidirectional filters.

Investigation on Absorption and Photocurrent in Silicon Absorber with Varied Pyramid Texture Angles by Ray Tracer

In this work, ray tracing is used to investigate the effects of pyramid texture angle towards light absorption and photocurrent in 250 μm-thick crystalline silicon (c-Si) absorber. Upright pyramids with texture angles of 10-50o are investigated. Planar c-Si absorber is used as a reference. When the pyramid angle increases, the broadband reflection reduces due to enhanced light scattering which leads to improved light absorption. At angle of 50o, the weighted average reflection (WAR) reduces to 14.7% and broadband light absorption increases. The optical path length enhancement increases to 12 at wavelength of 1100 nm. The reflection and photogenerated current density (Jg) exhibit an inverse relationship with increasing zenith angle. With increasing zenith angle, the reflection from the c-Si absorber increases and this results in lower light absorption and Jg. In the passivated emitter rear cell (PERC) solar cell, the planar solar cell exhibits short-circuit current density (Jsc) of 26 mA/cm2 with conversion efficiency of 13.6%. When both the pyramids and the silicon nitride (SiNx) anti-reflective coating (ARC) are incorporated on the solar cell, the Jsc increases to 39 mA/cm2 and conversion efficiency increases to 20.5%. This is attributed to the enhanced light-trapping and light-coupling effects in the device.