Nanophotonic control of thermal radiation for energy applications [Invited]

PISC-Net: A Comprehensive Neural Network Framework for Predicting Metasurface Infrared Emission Spectra

Multifunctional metasurfaces have exhibited extensive potential in various fields, owing to their unparalleled capacity for controlling electromagnetic wave characteristics. The precise resolution is achieved through numerical simulation in conventional metasurface design methodologies. Nevertheless, the simulations using these approaches are inherently computationally costly. This paper proposes the Physical Insight Self-Correcting Convolutional Network (PISC-Net), which enables rapid prediction of infrared radiation spectra of metasurfaces with remarkable generalization capacity. In contrast to preceding prediction networks, we have enhanced the cognitive ability of the network to recognize physical mechanisms by designing parameter-communication modules and integrating a priori knowledge grounded in the parameter association mechanism. Additionally, we proposed an effective strategy for constructing data sets that facilitate precise tuning of absorption bands in the entire spectral range (3-14 μm) and serves to reduce the costs associated with data set development. Transfer learning is employed to obtain precise predictions for large-period metasurfaces from limited data sets. This approach demonstrates that a network trained exclusively on simulation data could predict experimental outcomes accurately, as proved by the comparative analysis between simulation, experimental testing, and prediction results. The average mean square error is less than 4%.

Review on the Scientific and Technological Breakthroughs in Thermal Emission Engineering

The emission of thermal radiation is a physical process of fundamental and technological interest. From different approaches, thermal radiation can be regarded as one of the basic mechanisms of heat transfer, as a fundamental quantum phenomenon of photon production, or as the propagation of electromagnetic waves. However, unlike light emanating from conventional photonic sources, such as lasers or antennas, thermal radiation is characterized for being broadband, omnidirectional, and unpolarized. Due to these features, ultimately tied to its inherently incoherent nature, taming thermal radiation constitutes a challenging issue. Latest advances in the field of nanophotonics have led to a whole set of artificial platforms, ranging from spatially structured materials and, much more recently, to time-modulated media, offering promising avenues for enhancing the control and manipulation of electromagnetic waves, from far- to near-field regimes. Given the ongoing parallelism between the fields of nanophotonics and thermal emission, these recent developments have been harnessed to deal with radiative thermal processes, thereby forming the current basis of thermal emission engineering. In this review, we survey some of the main breakthroughs carried out in this burgeoning research field, from fundamental aspects to theoretical limits, the emergence of effects and phenomena, practical applications, challenges, and future prospects.

Localized thermal emission from topological interfaces

The control of thermal radiation by shaping its spatial and spectral emission characteristics plays a key role in many areas of science and engineering. Conventional approaches to tailoring thermal emission using metamaterials are hampered both by the limited spatial resolution of the required subwavelength material structures and by the materials' strong absorption in the infrared. In this work, we demonstrate an approach based on the concept of topology. By changing a single parameter of a multilayer coating, we were able to control the reflection topology of a surface, with the critical point of zero reflection being topologically protected. The boundaries between subcritical and supercritical spatial domains host topological interface states with near-unity thermal emissivity. These topological concepts enable unconventional manipulation of thermal light for applications in thermal management and thermal camouflage.

Dynamically manipulating long-wave infrared polarized thermal radiation by a vanadium dioxide metasurface

Dynamically manipulating the spectra and polarization properties of thermal radiation is the key to counter an infrared polarization imaging system (IPIS) under the different background environments. In this Letter, we propose a phase-change metasurface thermal emitter (PCMTE) composed of vanadium dioxide (VO2) dipole antenna arrays to dynamically manipulate polarized radiation spectra in the long-wave infrared (LWIR) region of 8-14 µm. During the thermally induced and reversible insulator-to-metal transition (IMT) in VO2, by simulating the LWIR images at different polarization angles for the PCMTE and background plates, the PCMTE can realize dynamically tunable LWIR camouflage; then, their degree of linear polarization (DoLP) can be calculated, which can demonstrate that the PCMTE can also achieve dynamically tunable LWIR polarization camouflage at the specific radiation angles and backgrounds. Our proposed PCMTE provides an effective scheme for adaptive IR polarization camouflage.

Machine Learning Aided Design and Optimization of Thermal Metamaterials

Artificial Intelligence (AI) has advanced material research that were previously intractable, for example, the machine learning (ML) has been able to predict some unprecedented thermal properties. In this review, we first elucidate the methodologies underpinning discriminative and generative models, as well as the paradigm of optimization approaches. Then, we present a series of case studies showcasing the application of machine learning in thermal metamaterial design. Finally, we give a brief discussion on the challenges and opportunities in this fast developing field. In particular, this review provides: (1) Optimization of thermal metamaterials using optimization algorithms to achieve specific target properties. (2) Integration of discriminative models with optimization algorithms to enhance computational efficiency. (3) Generative models for the structural design and optimization of thermal metamaterials.

Hybrid supervised and reinforcement learning for the design and optimization of nanophotonic structures

From higher computational efficiency to enabling the discovery of novel and complex structures, deep learning has emerged as a powerful framework for the design and optimization of nanophotonic circuits and components. However, both data-driven and exploration-based machine learning strategies have limitations in their effectiveness for nanophotonic inverse design. Supervised machine learning approaches require large quantities of training data to produce high-performance models and have difficulty generalizing beyond training data given the complexity of the design space. Unsupervised and reinforcement learning-based approaches on the other hand can have very lengthy training or optimization times associated with them. Here we demonstrate a hybrid supervised learning and reinforcement learning approach to the inverse design of nanophotonic structures and show this approach can reduce training data dependence, improve the generalizability of model predictions, and significantly shorten exploratory training times. The presented strategy thus addresses several contemporary deep learning-based challenges, while opening the door for new design methodologies that leverage multiple classes of machine learning algorithms to produce more effective and practical solutions for photonic design.

Experimental Investigation of the Thermal Emission Cross Section of Nanoresonators Using Hierarchical Poisson-Disk Distributions

Effective cross sections of nano-objects are fundamental properties that determine their ability to interact with light. However, measuring them for individual resonators directly and quantitatively remains challenging, particularly because of the very low signals involved. Here, we experimentally measure the thermal emission cross section of metal-insulator-metal nanoresonators using a stealthy hyperuniform distribution based on a hierarchical Poisson-disk algorithm. In such distributions, there are no long-range interactions between antennas, and we show that the light emitted by such metasurfaces behaves as the sum of cross sections of independent nanoantennas, enabling direct retrieval of the single resonator contribution. The emission cross section at resonance is found to be on the order of λ_{0}^{2}/3, a value that is nearly 3 times larger than the theoretical maximal absorption cross section of a single particle, but remains smaller than the maximal extinction cross section. This measurement technique can be generalized to any single resonator cross section, and we also apply it to a lossy dielectric layer.

Omnidirectional and near-unity nonreciprocal thermal radiation with trilayer cavities-enhanced approach

From the standpoint of thermal radiation, omnidirectional nonreciprocal thermal radiation (NTR) is strongly desired for thermal energy harvesting. Here, we propose theoretically lithographic free thermal emitter made in a dielectric-Weyl semimetal (WSM)-dielectric fashion and terminated by a metallic substrate. By engineering the structural parameters, a surprising result of spectrally selective as well as omnidirectional (along both polar and azimuthal angles) NTR is realized. It is shown that the magnitude and sign of the contrast between emission (e) and absorption (α) can be managed simultaneously. The suggested structure shows good nonreciprocity stability in a wide range of polar and azimuthal angles for transverse magnetic (TM) polarized incident wave. The ability to fine tune nonreciprocal radiative properties of our design suggests a relatively simple way to manifest the NTR with high performance, which could lead to the development of power scavenging and conversion devices.

Octopus-inspired deception and signaling systems from an exceptionally-stable acene variant

Multifunctional platforms that can dynamically modulate their color and appearance have attracted attention for applications as varied as displays, signaling, camouflage, anti-counterfeiting, sensing, biomedical imaging, energy conservation, and robotics. Within this context, the development of camouflage systems with tunable spectroscopic and fluorescent properties that span the ultraviolet, visible, and near-infrared spectral regions has remained exceedingly challenging because of frequently competing materials and device design requirements. Herein, we draw inspiration from the unique blue rings of the Hapalochlaena lunulata octopus for the development of deception and signaling systems that resolve these critical challenges. As the active material, our actuator-type systems incorporate a readily-prepared and easily-processable nonacene-like molecule with an ambient-atmosphere stability that exceeds the state-of-the-art for comparable acenes by orders of magnitude. Devices from this active material feature a powerful and unique combination of advantages, including straightforward benchtop fabrication, competitive baseline performance metrics, robustness during cycling with the capacity for autonomous self-repair, and multiple dynamic multispectral operating modes. When considered together, the described exciting discoveries point to new scientific and technological opportunities in the areas of functional organic materials, reconfigurable soft actuators, and adaptive photonic systems.

Designing Metasurfaces for Efficient Solar Energy Conversion

Metasurfaces have recently emerged as a promising technological platform, offering unprecedented control over light by structuring materials at the nanoscale using two-dimensional arrays of subwavelength nanoresonators. These metasurfaces possess exceptional optical properties, enabling a wide variety of applications in imaging, sensing, telecommunication, and energy-related fields. One significant advantage of metasurfaces lies in their ability to manipulate the optical spectrum by precisely engineering the geometry and material composition of the nanoresonators' array. Consequently, they hold tremendous potential for efficient solar light harvesting and conversion. In this Review, we delve into the current state-of-the-art in solar energy conversion devices based on metasurfaces. First, we provide an overview of the fundamental processes involved in solar energy conversion, alongside an introduction to the primary classes of metasurfaces, namely, plasmonic and dielectric metasurfaces. Subsequently, we explore the numerical tools used that guide the design of metasurfaces, focusing particularly on inverse design methods that facilitate an optimized optical response. To showcase the practical applications of metasurfaces, we present selected examples across various domains such as photovoltaics, photoelectrochemistry, photocatalysis, solar-thermal and photothermal routes, and radiative cooling. These examples highlight the ways in which metasurfaces can be leveraged to harness solar energy effectively. By tailoring the optical properties of metasurfaces, significant advancements can be expected in solar energy harvesting technologies, offering new practical solutions to support an emerging sustainable society.

General deep learning framework for emissivity engineering

Wavelength-selective thermal emitters (WS-TEs) have been frequently designed to achieve desired target emissivity spectra, as a typical emissivity engineering, for broad applications such as thermal camouflage, radiative cooling, and gas sensing, etc. However, previous designs require prior knowledge of materials or structures for different applications and the designed WS-TEs usually vary from applications to applications in terms of materials and structures, thus lacking of a general design framework for emissivity engineering across different applications. Moreover, previous designs fail to tackle the simultaneous design of both materials and structures, as they either fix materials to design structures or fix structures to select suitable materials. Herein, we employ the deep Q-learning network algorithm, a reinforcement learning method based on deep learning framework, to design multilayer WS-TEs. To demonstrate the general validity, three WS-TEs are designed for various applications, including thermal camouflage, radiative cooling and gas sensing, which are then fabricated and measured. The merits of the deep Q-learning algorithm include that it can (1) offer a general design framework for WS-TEs beyond one-dimensional multilayer structures; (2) autonomously select suitable materials from a self-built material library and (3) autonomously optimize structural parameters for the target emissivity spectra. The present framework is demonstrated to be feasible and efficient in designing WS-TEs across different applications, and the design parameters are highly scalable in materials, structures, dimensions, and the target functions, offering a general framework for emissivity engineering and paving the way for efficient design of nonlinear optimization problems beyond thermal metamaterials.

Efficient Strategy for Radiative Cooling Based on Ultra-Broad-Band Infrared Regulation of Flexible Bilayer Film

Flexible thermal radiation films with adjustable broad-band infrared radiation could maintain various heat-generating electronic devices working stably in corresponding operating temperatures, making them good candidates for radiative cooling (RC) material. However, the controllable radiation peaks of the metamaterial were narrow, and manipulation was a time-consuming and complex process. Herein, we design a simple TiN/Si bilayer film with controllable broad-band radiation peaks at a thermal radiation wavelength of 3.5-20 μm by impedance matching. Meanwhile, the different bilayer films applied to aluminum devices at different temperatures exhibit outstanding heat dissipation efficiency and maintain the corresponding equilibrium temperature to ensure that devices work stably for a long time. Moreover, the bilayer films deposited on the flexible PI substrates exhibit preferable thermostability and higher tensile strength than existing radiative cooling materials deposited on PDMS, PE, PMMA or TPX, etc. This work provides an effective strategy to realize efficient radiation cooling for flexible electronic devices and spacecraft appendages.

Indirect measurement of infrared absorption spectrum through thermal emission of meta-cavity array

Controlling thermal emission is essential for various infrared spectroscopy applications. Metasurfaces can be utilized to control multiple degrees of freedom of thermal emission, enabling the compact thermal emission materials and devices. Infrared spectroscopy such as FTIR (Fourier transform infrared spectroscopy), usually requires external infrared radiation source and complex spectroscopic devices for absorption spectrum measurement, which hinders the implementation of integrated compact and portable measurement equipment. Measuring absorption spectrum through the thermal emission of pixelated thermal emitter array can facilitate the integration and miniaturization of measurement setup, which is highly demanded for on-chip spectroscopy applications. Here, we experimentally demonstrate an integrated technology that allows for indirect measurement of the absorption spectrum through the thermal emission of meta-cavity array. This indirect measurement method opens a new avenue for compact infrared spectroscopy analysis.

New Horizons in Near-Zero Refractive Index Photonics and Hyperbolic Metamaterials

The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide efficient knobs to control light-matter interactions. This Perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular, light and thermal emission, nonlinear optics, sensing applications, and time-varying photonics.

Asymmetric Directional Control of Thermal Emission

Control over the directionality of thermal emission plays a fundamental role in efficient heat transport. Although nanophotonic technologies have demonstrated the capability for angular-selective thermal emission, achieving asymmetric directional thermal emission in reciprocal systems with energy directed to a single output angle remains challenging due to symmetric band dispersion. In this work, a general strategy for achieving asymmetric directional thermal emission in reciprocal systems is presented. With periodic perturbation and broken mirror symmetry, metasurfaces behave as resonant metagratings whose resonances can be diffracted to symmetric output angles with distinct efficiency, allowing for high emissivity toward a single direction. An asymmetric directional thermal emitter is experimentally demonstrated at mid-infrared wavelengths with high emissivity (ɛ = 0.61) at the observation angle of +30°, and low emissivity (ɛ < 0.3) at other angles. This work highlights the potential for manipulating the directionality of thermal emission, which holds promise for developing ultrathin customized thermal sources and impacts on various thermal-engineering applications.

A tunable infrared emitter based on phase-changing material GST for visible-infrared compatible camouflage with thermal management

Visible-infrared compatible camouflage is important to increase the counter-detection ability of a target due to the fast development of detection systems. However, most of the previously reported visible-infrared compatible camouflage structures are not suitable when the temperature of targets and type of background environment change. In this paper, we propose a tunable infrared emitter composed of ZnS/Ge/Ag/Ge2Sb2Te5/Ag films and numerically demonstrate visible-infrared compatible camouflage and radiation heat dissipation. Firstly, the proposed infrared emitter can produce different structural colors as the thickness of the ZnS film changes, which can be applied to visible camouflage. Secondly, the crystallization fraction of the Ge2Sb2Te5 (GST) layer could help to engineer the average emissivity of the proposed infrared emitter, achieving tunable mid-infrared (MIR) camouflage, radiation heat dissipation, and long-infrared (LIR) camouflage in wavelength ranges of 3-5 μm, 5-8 μm, and 8-14 μm, respectively. Finally, we numerically demonstrate the visible camouflage and infrared camouflage for different application scenarios by using the simulated visible and infrared images. This work has promising application potential in visible-infrared compatible camouflage technology.

Unsupervised machine learning to classify the confinement of waves in periodic superstructures

We propose a rigorous method to classify the dimensionality of wave confinement by utilizing unsupervised machine learning to enhance the accuracy of our recently presented scaling method [Phys. Rev. Lett.129, 176401 (2022)10.1103/PhysRevLett.129.176401]. We apply the standard k-means++ algorithm as well as our own model-based algorithm to 3D superlattices of resonant cavities embedded in a 3D inverse woodpile photonic band gap crystal with a range of design parameters. We compare their results against each other and against the direct usage of the scaling method without clustering. Since the clustering algorithms require the set of confinement dimensionalities present in the system as an input, we investigate cluster validity indices (CVIs) as a means to find these values. We conclude that the most accurate outcome is obtained by first applying direct scaling to find the correct set of confinement dimensionalities, and subsequently utilizing our model-based clustering algorithm to refine the results.

Visibly transparent multifunctional camouflage coating with efficient thermal management

Camouflage technology has attracted growing interest in many thermal applications. In particular, high-temperature infrared (IR) camouflage is crucial to the effective concealment of high-temperature objects but remains a challenging issue, as the thermal radiation of an object is proportional to the fourth power of temperature. Here, we proposed a coating to demonstrate high-temperature IR camouflage with efficient thermal management. This coating is a combination of hyperbolic metamaterial (HMM), gradient epsilon near zero (G-ENZ) material, and polymer. HMM makes the coating transparent in the visible range (300-700 nm) and highly reflective in the IR region, so it can serve as a thermal camouflage in the IR. G-ENZ and polymer support BE mode (at higher angles ∼50° to 90° in the 11-14 µm atmospheric window) and vibrational absorption band (in 5-8 µm non-atmospheric for all angles), respectively. So it is possible to achieve efficient thermal management through radiative cooling. We calculate the temperature of the object's surface, considering the emissivity characteristics of the coating for different heating temperatures. A combination of silica aerogel and coating can significantly reduce the surface temperature from 2000 K to 750 K. The proposed coating can also be used in the visible transparent radiative cooling due to high transmission in the visible, high reflection in the near-IR (NIR), and highly directional emissivity in the atmospheric window at higher angles, and can therefore potentially be used as a smart window in buildings and vehicles. Finally, we discuss one more potential future application of such a multifunctional coating in water condensation and purification.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

Incandescent temporal metamaterials

Regarded as a promising alternative to spatially shaping matter, time-varying media can be seized to control and manipulate wave phenomena, including thermal radiation. Here, based upon the framework of macroscopic quantum electrodynamics, we elaborate a comprehensive quantum theoretical formulation that lies the basis for investigating thermal emission effects in time-modulated media. Our theory unveils unique physical features brought about by time-varying media: nontrivial correlations between fluctuating electromagnetic currents at different frequencies and positions, thermal radiation overcoming the black-body spectrum, and quantum vacuum amplification effects at finite temperature. We illustrate how these features lead to striking phenomena and innovative thermal emitters, specifically, showing that the time-modulation releases strong field fluctuations confined within epsilon-near-zero (ENZ) bodies, and that, in turn, it enables a narrowband (partially coherent) emission spanning the whole range of wavevectors, from near to far-field regimes.

Photonic structures in radiative cooling

Radiative cooling is a passive cooling technology without any energy consumption, compared to conventional cooling technologies that require power sources and dump waste heat into the surroundings. For decades, many radiative cooling studies have been introduced but its applications are mostly restricted to nighttime use only. Recently, the emergence of photonic technologies to achieves daytime radiative cooling overcome the performance limitations. For example, broadband and selective emissions in mid-IR and high reflectance in the solar spectral range have already been demonstrated. This review article discusses the fundamentals of thermodynamic heat transfer that motivates radiative cooling. Several photonic structures such as multilayer, periodical, random; derived from nature, and associated design procedures were thoroughly discussed. Photonic integration with new functionality significantly enhances the efficiency of radiative cooling technologies such as colored, transparent, and switchable radiative cooling applications has been developed. The commercial applications such as reducing cooling loads in vehicles, increasing the power generation of solar cells, generating electricity, saving water, and personal thermal regulation are also summarized. Lastly, perspectives on radiative cooling and emerging issues with potential solution strategies are discussed.

Deep learning-based inverse design of microstructured materials for optical optimization and thermal radiation control

Microstructures with engineered properties are critical to thermal management in aerospace and space applications. Due to the overwhelming number of microstructure design variables, traditional approaches to material optimization can have time-consuming processes and limited use cases. Here, we combine a surrogate optical neural network with an inverse neural network and dynamic post-processing to form an aggregated neural network inverse design process. Our surrogate network emulates finite-difference time-domain simulations (FDTD) by developing a relationship between the microstructure's geometry, wavelength, discrete material properties, and the output optical properties. The surrogate optical solver works in tandem with an inverse neural network to predict a microstructure's design properties that will match an input optical spectrum. As opposed to conventional approaches that are constrained by material selection, our network can identify new material properties that best optimize the input spectrum and match the output to an existing material. The output is evaluated using critical design constraints, simulated in FDTD, and used to retrain the surrogate-forming a self-learning loop. The presented framework is applicable to the inverse design of various optical microstructures, and the deep learning-derived approach will allow complex and user-constrained optimization for thermal radiation control in future aerospace and space systems.

Broadband hyperbolic thermal metasurfaces based on the plasmonic phase-change material In3SbTe2

Thermal radiation modulation facilitated by phase change materials (PCMs) needs a large thermal radiation contrast in broadband as well as in a non-volatile phase transition, which are only partially satisfied by conventional PCMs. In contrast, the emerging plasmonic PCM In₃SbTe₂ (IST) that undergoes a non-volatile dielectric-to-metal phase transition during crystallization offers a fitting solution. Here, we have prepared IST-based hyperbolic thermal metasurfaces and demonstrated their capabilities to modulate thermal radiation. By laser-printing crystalline IST gratings with different fill factors on amorphous IST films, we have achieved multilevel, large-range, and polarization-dependent control of the emissivity modulation (0.07 for the crystalline phase and 0.73 for the amorphous phase) over a broad bandwidth (8-14 μm). With the convenient direct laser writing technique that supports large-scale surface patterning, we have also demonstrated promising applications of thermal anti-counterfeiting with hyperbolic thermal metasurfaces.

Fabrication and Characterization of Acicular Micro-Textured Copper Sheet Device for Low-Temperature Heat Radiation

An acicular microtextured sheet was developed as a heat radiation device from the high-temperature source to the cooling medium in the infrared (IR) spectrum. The copper surface was modified by acicular micro-texturing to place a semi-regular micro-/nano-cone structure onto it. FT-IR (Fourier transformation IR) spectroscopy was utilized to measure the transmittance diagram in near-IR to far-IR wavelengths. The wavelength (λ) of 6.7 μm, where the highest absorbance valley was detected in the diagram, was equivalent to the doubled size of the micro-cone average height, with H_ave = 3.3 μm; λ ~ 2 × H_ave. The electromagnetic waves in the far-IR wavelength were emitted by acicular micro-textured metallic sheets. The heat radiation transfer experiment was performed to describe this low-temperature heat radiation behavior. No temperature rise was detected on the black-colored polycarbonate (BC-PC) plate away from the bare copper sheet without textures, located on the high-temperature source. The temperature increased by 4 K on the BC-PC plate using the acicular textured copper sheet device. The emitter temperature also decreased significantly by 50 K or 50% of the heat source temperature.

Recent Advances in Thermoregulatory Clothing: Materials, Mechanisms, and Perspectives

Personal thermal management (PTM) is a promising approach for maintaining the thermal comfort zone of the human body while minimizing the energy consumption of indoor buildings. Recent studies have reported the development of numerous advanced textiles that enable PTM systems to regulate body temperature and are comfortable to wear. Herein, recent advancements in thermoregulatory clothing for PTM are discussed. These advances in thermoregulatory clothing have focused on enhancing the control of heat dissipation between the skin and the localized environment. We primarily summarize research on advanced clothing that controls the heat dissipation pathways of the human body, such as radiation- and conductance-controlled clothing. Furthermore, adaptive clothing such as dual-mode textiles, which can regulate the microclimate of the human body, as well as responsive textiles that address both thermal performance (warming and/or cooling) and wearability are discussed. Finally, we include a discussion on significant challenges and perspectives in this field, including large-scale production, smart textiles, bioinspired clothing, and AI-assisted clothing. This comprehensive review aims to further the development of sustainably manufactured advanced clothing with superior thermal performance and outstanding wearability for PTM in practical applications.

High-index-contrast photonic structures: a versatile platform for photon manipulation

In optics, the refractive index of a material and its spatial distribution determine the characteristics of light propagation. Therefore, exploring both low- and high-index materials/structures is an important consideration in this regard. Hollow cavities, which are defined as low-index bases, exhibit a variety of unusual or even unexplored optical characteristics and are used in numerous functionalities including diffraction gratings, localised optical antennas and low-loss resonators. In this report, we discuss the fabrication of hollow cavities of various sizes (0.2-5 μm in diameter) that are supported by conformal dielectric/metal shells, as well as their specific applications in the ultraviolet (photodetectors), visible (light-emitting diodes, solar cells and metalenses), near-infrared (thermophotovoltaics) and mid-infrared (radiative coolers) regions. Our findings demonstrate that hollow cavities tailored to specific spectra and applications can serve as versatile optical platforms to address the limitations of current optoelectronic devices. Furthermore, hollow cavity embedded structures are highly elastic and can minimise the thermal stress caused by high temperatures. As such, future applications will likely include high-temperature devices such as thermophotovoltaics and concentrator photovoltaics.

Scaling Theory of Wave Confinement in Classical and Quantum Periodic Systems

Functional defects in periodic media confine waves-acoustic, electromagnetic, electronic, spin, etc.-in various dimensions, depending on the structure of the defect. While defects are usually modeled by a superlattice with a typical band-structure representation of energy levels, determining the confinement associated with a given band is highly nontrivial and no analytical method is known to date. Therefore, we propose a rigorous method to classify the dimensionality of wave confinement. Starting from the confinement energy and the mode volume, we use finite-size scaling to find that ratios of these quantities raised to certain powers yield the confinement dimensionality of each band. Our classification has negligible additional computational costs compared to a band structure calculation and is valid for any type of wave, both quantum and classical, and in any dimension. In the quantum regime, we illustrate our method on electronic confinement in 2D hexagonal boron nitride (BN) with a nitrogen vacancy, in agreement with previous results. In the classical case, we study a three-dimensional photonic band gap cavity superlattice, where we identify novel acceptorlike behavior. We briefly discuss the generalization to quasiperiodic lattices.

Metasurfaces as Energy Valves for Sustainable Energy Management

Control of light absorption and transmission by metal-insulator-metal (MIM) metasurfaces are promising for applications in optical windows. This study shows the realization of photo-thermal energy conversion for radiative cooling by MIM metasurfaces with thin metal substrate and Indium-Tin-Oxide (ITO). High transparency of ITO at visible wavelengths and high absorption at mid-infrared wavelengths were realized for future applications of efficient cooling or heating applicable for living and working spaces. The MIM (ITO/CaF₂/ITO) metasurface was patterned with low-resolution photo-lithography as a demonstration of further simplification and possible scalability of the patterning for practical window applications.

Metasurfaces Assisted Twisted α-MoO3 for Spinning Thermal Radiation

Spinning thermal radiation has demonstrated applications in engineering, such as radiation detection and biosensing. In this paper, we propose a new spin thermal radiation emitter composed of the twisted bilayer α-MoO3 metasurface; in our study, it provided more degrees of freedom to control circular dichroism by artificially modifying the filling factor of the metasurface. In addition, circular dichroism was significantly enhanced by introducing a new degree of freedom (filling factor), with a value that could reach 0.9. Strong-spin thermal radiation resulted from the polarization conversion of circularly polarized waves using the α-MoO3 metasurface and selective transmission of linearly polarized waves by the substrate. This allowed for extra flexible control of spinning thermal radiation and significantly enhanced circular dichroism, which promises applications in biosensing and radiation detection. As a result of their unique properties, hyperbolic materials have applications not only in spin thermal radiation, but also in areas such as near-field thermal radiation. In this study, hyperbolic materials were combined with metasurfaces to offer a new idea regarding modulating near-field radiative heat transfer.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Optical emissivity dataset of multi-material heterogeneous designs generated with automated figure extraction

Optical device design is typically an iterative optimization process based on a good initial guess from prior reports. Optical properties databases are useful in this process but difficult to compile because their parsing requires finding relevant papers and manually converting graphical emissivity curves to data tables. Here, we present two contributions: one is a dataset of thermal emissivity records with design-related parameters, and the other is a software tool for automated colored curve data extraction from scientific plots. We manually collected 64 papers with 176 figures reporting thermal emissivity and automatically retrieved 153 colored curve data records. The automated figure analysis software pipeline uses Faster R-CNN for axes and legend object detection, EasyOCR for axes numbering recognition, and k-means clustering for colored curve retrieval. Additionally, we manually extracted geometry, materials, and method information from the text to add necessary metadata to each emissivity curve. Finally, we analyzed the dataset to determine the dominant classes of emissivity curves and determine the underlying design parameters leading to a type of emissivity profile.

Spinning thermal radiation from twisted two different anisotropic materials

Thermal radiation has applications in numerous fields, such as radiation cooling, thermal imaging, and thermal camouflage. Micro/nanostructures such as chiral metamaterials with polarization-dependent or symmetry-breaking properties can selectively emit circularly (spin) polarized polarization waves. In this paper, we propose and demonstrate the spinning thermal radiation from two twisted different anisotropic materials. Taking industrial polymer and biaxial hyperbolic material α-MoO₃ as an example, it is found that broadband spinning thermal radiation can be obtained from 13 µm to 18 µm. The spin thermal radiation of the proposed twisted structure originates from the combined effect of polarization conversion of circularly polarized wave and selective absorption of linearly polarized wave by the top and bottom layers of anisotropic materials, respectively. Besides, the narrowband spinning thermal radiation with 0.9 circular dichroism is achieved at wavelength of 12.39 µm and 18.93 µm for finite thickness α-MoO₃ due to the epsilon-near-zero mode, and the magnetic field distribution can confirm the phenomenon. This work achieves broadband and narrowband spin thermal radiation and significantly enhances circular dichroism, which may have applications in biological sensing and thermal detection.

Electrically Driven Hyperbolic Nanophotonic Resonators as High Speed, Spectrally Selective Thermal Radiators

We introduce and experimentally demonstrate electrically driven, spectrally selective thermal emitters based on globally aligned carbon nanotube metamaterials. The self-assembled metamaterial supports a high degree of nanotube ordering, enabling nanoscale ribbons patterned in the metamaterial to function both as Joule-heated incandescent filaments and as infrared hyperbolic resonators imparting spectral selectivity to the thermal radiation. Devices batch-fabricated on a single chip emit polarized thermal radiation with peak wavelengths dictated by their hyperbolic resonances, and their nanoscale heated dimensions yield modulation rates as high as 1 MHz. As a proof of concept, we show that two sets of thermal emitters on the same chip, operating with different peak wavelengths and modulation rates, can be used to sense carbon dioxide with one detector. We anticipate that the combination of batch fabrication, modulation bandwidth, and spectral tuning with chip-based nanotube thermal emitters will enable new modalities in multiplexed infrared sources.

A Symmetrical Terahertz Triple-Band Metamaterial Absorber Using a Four-Capacitance Loaded Complementary Circular Split Ring Resonator and an Ultra-Thin ZnSe Substrate

In this research work, a symmetrical four-capacitance loaded complementary circular split ring resonator is proposed, which uses an ultra-thin Zinc Selenide (ZnSe) substrate to realize a low-profile triple-band metamaterial (MTM) perfect absorber for application in the terahertz (THz) frequency range. The electromagnetic properties of the proposed structure were calculated and investigated using the Finite Integration Technique (FIT). The proposed structure exhibited three highly absorptive (nearly perfect) peaks at the resonance frequencies of 15.68 THz, 37.48 THz, and 39.55 THz. Furthermore, the absorber was found to be insensitive to the polarization and incident wave angles, due to its symmetrical design. The effects of the conductor type, substrate thickness, unit cell dimension, resonator gap, and substrate type on the reflection and absorption spectra were investigated. To validate the numerical results, the proposed design was analyzed using High-Frequency Simulation Software (HFSS) and Advanced Design System (ADS). The surface current, electric field, and magnetic field distributions at the three-resonance frequency were analyzed. It was concluded that the overall performance of the proposed MTM structure was superior compared to those reported in the literature. The proposed design could be a good candidate for application in stealth technology, imaging, and thermal energy harvesting.

Potential for identification of wild night-flying moths by remote infrared microscopy

There are hundreds of thousands of moth species with crucial ecological roles that are often obscured by their nocturnal lifestyles. The pigmentation and appearance of moths are dominated by cryptic diffuse shades of brown. In this study, 82 specimens representing 26 moth species were analysed using infrared polarimetric hyperspectral imaging in the range of 0.95–2.5 µm. Contrary to previous studies, we demonstrate that since infrared light does not resolve the surface roughness, wings appear glossy and specular at longer wavelengths. Such properties provide unique reflectance spectra between species. The reflectance of the majority of our species could be explained by comprehensive models, and a complete parametrization of the spectral, polarimetric and angular optical properties was reduced to just 11 parameters with physical units. These parameters are complementary and, compared with the within-species variation, were significantly distinct between species. Counterintuitively to the aperture-limited resolution criterion, we could deduce microscopic features along the surface from their infrared properties. These features were confirmed by electron microscopy. Finally, we show how our findings could greatly enhance opportunities for remote identification of free-flying moth species, and we hypothesize that such flat specular wing targets could be expected to be sensed over considerable distances.

Engineering Plasmonic Environments for 2D Materials and 2D-Based Photodetectors

Two-dimensional layered materials are considered ideal platforms to study novel small-scale optoelectronic devices due to their unique electronic structures and fantastic physical properties. However, it is urgent to further improve the light–matter interaction in these materials because their light absorption efficiency is limited by the atomically thin thickness. One of the promising approaches is to engineer the plasmonic environment around 2D materials for modulating light–matter interaction in 2D materials. This method greatly benefits from the advances in the development of nanofabrication and out-plane van der Waals interaction of 2D materials. In this paper, we review a series of recent works on 2D materials integrated with plasmonic environments, including the plasmonic-enhanced photoluminescence quantum yield, strong coupling between plasmons and excitons, nonlinear optics in plasmonic nanocavities, manipulation of chiral optical signals in hybrid nanostructures, and the improvement of the performance of optoelectronic devices based on composite systems.

Microemitter-Based IR Spectroscopy and Imaging with Multilayer Graphene Thermal Emission

IR analyses such as Fourier transform infrared spectroscopy (FTIR) are widely used in many fields; however, the performance of FTIR is limited by the slow speed (∼10 Hz), large footprint (∼ millimeter), and glass bulb structure of IR light sources. Herein, we present IR spectroscopy and imaging based on multilayer-graphene microemitters, which have distinct features: a planar structure, bright intensity, a small footprint (sub-μm²), and high modulation speed of >50 kHz. We developed an IR analysis system based on the multilayer-graphene microemitter and performed IR absorption spectroscopy. We show two-dimensional IR chemical imaging that visualizes the distribution of the chemical information. In addition, we present high-spatial-resolution IR imaging with a spatial resolution of ∼1 μm, far higher than the diffraction limit. The graphene-based IR spectroscopy and imaging can open new routes for IR applications in chemistry, material science, medicine, biology, electronics, and physics.

A point-like thermal light source as a probe for sensing light-matter interaction

Historically, thermal radiation is related to 3D cavities. In practice, however, it is known that almost any hot surface radiates according to Planck’s law. This approximate universality roots in the smooth electromagnetic mode structure of free space, into which the radiation is emitted. Here, we study the effect for a strongly patterned mode structure and use quasi-transparent point-like thermal light emitters as a probe. As such, we choose current-driven graphene nanojunctions for which the emission into free space obeys Planck’s law. Placed in front of a mirror, however, this process is highly sensitive to a node/antinode pattern of light modes. By varying the distance, we can sample the latter with atomic precision, and observe a deep imprint on the observed spectrum. The experiment allows an unprecedented view on thermal radiation in a spatially/spectrally patterned electromagnetic environment.

Multiband infrared emissions limited in the grazing angle from metal-dielectric-metal metamaterials

Thermal radiation management remains a challenge because of the incoherent and isotropic nature of electromagnetic waves. In this study, a multiband and angular-selective infrared emitter, consisting of a simple one-dimensional (1D) metal-dielectric-metal metamaterial, is demonstrated. Although this structure has been well known as spectrally selective emitters, we analytically reveal that when the dielectric layer thickness is much smaller than the wavelength of interest (< 1/10), directive emission at nearly equal to the grazing angles (> 80°) can be obtained at multiple resonant wavelengths. As the absorption peaks can be entirely characterized by geometrical parameters, this angular selective technology offers flexible control of thermal radiation and can be adjusted to specific applications.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Low-Power Laser Sailing for Fast-Transit Space Flight

Space exploration is of paramount importance to advancing fundamental science and the global economy. However, today's space missions are limited by existing propulsion technologies. Here, we examine the use of laser-driven light sailing for agile Earth orbital maneuvering and for fast-transit exploration of the solar system and interstellar medium. We show that laser propulsion becomes practical at laser powers ≥100 kW and laser array sizes ∼1 m, which are feasible in the near term. Our analysis indicates that lightweight (1-100 g) wafer-scale (∼10 cm) spacecraft may be propelled by lasers to orbits that are beyond the reach of current systems. We discuss material requirements and photonic designs and introduce new figures of merit. We show that lightsails made of silicon nitride and boron nitride are particularly well suited for the discussed applications. Our architecture may pave the way to ubiquitous Earth orbital networks and fast-transit low-cost missions across the solar system.

Self-adaptive control of infrared emissivity in a solution-processed plasmonic structure

Active control of optical properties, particularly in the infrared (IR) regime, is critical for the regulation of thermal emission. However, most photonic structures and devices are based on a sophisticated design, making the dynamic control of their IR properties challenging. Here, we demonstrate self-adaptive control of IR absorptivity/emissivity in a simple stacked structure that consists of an oxide plasmonic nanocrystal layer and a phase change material (VO₂) layer, both fabricated via a solution process. The resonance wavelength and emission intensity for this structure depend on the phase of the VO₂. This has potential applications for thermal emission structures (e.g., self-adaptive radiative cooling and IR camouflage). The proposed structure is a candidate low-cost and scalable active photonic platform.

Non-Planck thermal emission from two-level media

Thermal emission is a universal phenomenon of stochastic electromagnetic emission from absorbing bodies at elevated temperatures. A defining feature of this emission is the monotonic and rapid growth of its intensity with the object's temperature for most known materials. This growth originates from the Bose-Einstein statistics of the thermal photonic field. The fact that the material's ability to emit light may change with temperature, however, is often overlooked. Here, we carry out a theoretical study of thermal emission from structures incorporating two-level media. We investigate this effect in a range of geometries including thin films and compact nanoparticles and establish the general dependencies in the evolution of thermal emission from such systems. Thermal emission turns out to be essentially non-Planckian and exhibits a universal asymptotic behavior in the limit of high temperatures. These results might have important implications for the design of thermal energy harvesting and thermal vision systems.

Theory of exciton thermal radiation in semiconducting single-walled carbon nanotubes

Spectral control of thermal radiation is an essential strategy for highly efficient and functional utilization of thermal radiation energy. Among the various proposed methods, quantum confinement in low-dimensional materials is promising because of its inherent ability to emit narrowband thermal radiation. Here, we theoretically investigate thermal radiation from one-dimensional (1D) semiconductors characterized by the strong quantum correlation effect due to the Coulomb interaction. We derive a simple and useful formula for the emissivity, which is then used to calculate the thermal radiation spectrum of semiconducting single-walled carbon nanotubes as a representative of 1D semiconductors. The calculations show that the exciton state, which is an electron-hole pair mutually bound by the Coulomb interaction, causes enhancement of the radiation spectrum peak and significant narrowing of its linewidth in the near-infrared wavelength range. The theory developed here will be a firm foundation for exciton thermal radiation in 1D semiconductors, which is expected to lead to new energy harvesting technologies.

Thermal Management by Engineering the Alignment of Nanocellulose

One of the grand current research challenges is to improve the energy efficiency of residential and commercial buildings, which cumulatively consume more than 40% of the energy generated globally. In addition to improving the comfort of the inhabitants and mitigating the growing energy consumption problem, new building materials and technologies could provide a safe strategy for geoengineering to mitigate global climate change. Herein, recent progress in developing such advanced materials from nanocellulose, which is often derived from wood or even dirty feedstocks like waste, is reviewed. By using chemical and bacteria-enabled processing, nanocellulose can be used to fabricate broadband photonic reflectors, thermally super-insulating aerogels, solar gain regulators, and low-emissivity coatings, with potential applications in windows, roofs, walls, and other components of buildings envelopes. These material developments draw inspiration from advanced energy management found in nature, such as the nanoporous photonic structures that evolved in cuticles of beetles. Fabrication of such materials takes advantage of mesoscale liquid crystalline self-assembly, which allows for pre-designed control of cellulose nanoparticle orientations at the mesoscale. With the potential fully realized, such materials could one day transform the current energy-lossy buildings into energy plants on Earth and possibly even enable extraterrestrial habitats.

Ultranarrow and Wavelength-Scalable Thermal Emitters Driven by High-Order Antiferromagnetic Resonances in Dielectric Nanogratings

Engineering wavelength-selective thermal emission is a promising technology associated with several advanced applications, including thermal imaging, gas sensing, far/near-field thermophotovoltaics, and so on. However, the majority of reported approaches suffer from low Q-factor emission due to intrinsic loss of metallic components or rely on thick structures like multilayers to ensure unitary emissivity, making it challenging to design compatible high-Q narrowband emitters. In this work, we propose a mechanism to tailor thermal emission by taking advantage of optically induced high-order antiferromagnetic (AFM) resonances in a simple subwavelength 2D Si nanobar. Such AFM modes, stemmed from hybrid magnetic dipoles and high-order Fabry-Perot modes, exhibit both pronounced resonant responses and superior light confinement ability. We first reveal its essential roles in ultranarrowband emission control with a sharp (Q ∼ 400) and near-perfect emissivity available. Especially, the measured angle-resolved emission spectra further indicate that the AFM-induced emission peak, being nearly immune to changes of nanogratings' periods and incident angle, is able to be flexibly engineered in a wide waveband by merely tuning the width-to-height ratio of nanobars. Our work provides a promising strategy to design extremely high-Q thermal emitters possessing robust narrowband performance, large spectral tunability and desirable compatibility with advanced planar nanofabrication techniques, which will be more favorable in practice compared with metallic counterparts. Besides, we anticipate that, the revealed mechanism of high-order AFM modes can also stimulate advanced applications in diverse research communities including but not limited to multipolar physics, nonlinear nano-optics, energy harvesting, etc.

Extreme Nonreciprocal Near-Field Thermal Radiation via Floquet Photonics

By using Floquet driving protocols and interlacing them with a judicious reservoir emission engineering, we achieve extreme nonreciprocal thermal radiation. We show that the latter is rooted in an interplay between a direct radiation process occurring due to temperature bias between two thermal baths and the modulation process that is responsible for pumped radiation heat. Our theoretical results are confirmed via time-domain simulations with photonic and rf circuits.

Spatially Shaping Waves to Penetrate Deep inside a Forbidden Gap

It is well known that waves with frequencies within the forbidden gap inside a crystal are transported only over a limited distance-the Bragg length-before being reflected by Bragg interference. Here, we demonstrate how to send waves much deeper into crystals in an exemplary study of light in two-dimensional silicon photonic crystals. By spatially shaping the wave fronts, the internal energy density-probed via the laterally scattered intensity-is enhanced at a tunable distance away from the front surface. The intensity is up to 100× enhanced compared to random wave fronts, and extends as far as 8× the Bragg length, which agrees with an extended mesoscopic model. We thus report a novel control knob for mesoscopic wave transport that pertains to any kind of waves.

Ultralight and Mechanically Robust Fibrous Sponges Tailored by Semi-Interpenetrating Polymer Networks for Warmth Retention

People living in very cold climates urgently desire warmth retention equipment to remain healthy. However, creating materials that exhibit both effective warm retention and robust mechanical properties to maintain stable structures is extremely challenging. Herein, we report a facile and time-saving strategy for preparing ultralight, mechanically robust, and high-performance warmth retention materials via direct electrospinning and thermal crosslinking. Fluffy fibrous assemblies with stereoscopic fiber networks are fabricated with a humidity-induced electrospinning technique, followed by heating to create semi-interpenetrating polymer networks (semi-IPNs) within fibers to acquire fibrous sponges (FSs). The semi-IPN-based FSs (semi-IPN FSs) present integrated properties of high tensile stress (∼1 MPa), good fatigue resistance (∼0% plastic deformation after 1000 cyclic tensile or compressive tests), and nondestructive resilience in liquid nitrogen (-196 °C). Furthermore, the semi-IPN FSs exhibit a low volume density of ∼2.2 mg cm^-3, effective heat preservation ability (low thermal conductivity ∼25.8 mW m^-1 K^-1), and desired waterproofness and breathability. The successful synthesis of semi-IPN FSs provides a novel attempt to develop high-performance materials with robust mechanical properties for numerous applications.

Beyond the Visible: Bioinspired Infrared Adaptive Materials

Infrared (IR) adaptation phenomena are ubiquitous in nature and biological systems. Taking inspiration from natural creatures, researchers have devoted extensive efforts for developing advanced IR adaptive materials and exploring their applications in areas of smart camouflage, thermal energy management, biomedical science, and many other IR-related technological fields. Herein, an up-to-date review is provided on the recent advancements of bioinspired IR adaptive materials and their promising applications. First an overview of IR adaptation in nature and advanced artificial IR technologies is presented. Recent endeavors are then introduced toward developing bioinspired adaptive materials for IR camouflage and IR radiative cooling. According to the Stefan-Boltzmann law, IR camouflage can be realized by either emissivity engineering or thermal cloaks. IR radiative cooling can maximize the thermal radiation of an object through an IR atmospheric transparency window, and thus holds great potential for use in energy-efficient green buildings and smart personal thermal management systems. Recent advances in bioinspired adaptive materials for emerging near-IR (NIR) applications are also discussed, including NIR-triggered biological technologies, NIR light-fueled soft robotics, and NIR light-driven supramolecular nanosystems. This review concludes with a perspective on the challenges and opportunities for the future development of bioinspired IR adaptive materials.