This study investigates the enhancement of near-infrared (NIR) absorption in MXenes using a multiscale modeling approach that integrates density functional theory calculations and finite-difference time domain (FDTD) simulations for obtaining full-wave electromagnetic solutions. We performed a comprehensive investigation of the MXene material architectures, leading to the design of patterned structures with a significantly enhanced absorptivity across the 1-3 mu m range, particularly for O2-terminated MXenes. The findings reveal that pyramidal and stacked cubic patterns significantly improve electric field confinement by creating localized resonant modes and side modes along the edges and base of the structures. The detailed understanding of these geometric resonances provides a pathway for designing MXene-based devices optimized for applications such as thermal emitters, infrared sensors, and wavelength-selective absorbers.

Passive radiative cooling, an innovative approach for cooling buildings and devices, has attracted considerable attention in recent years. One significant challenge in radiative cooling is the need for surfaces with selective spectral emissivity that aligns with the atmospheric transmission of earth. The spectral emissivity of the surface in the 8-13 mu m serves as a crucial factor in enhancing the net cooling capacity of the surface. In this study, we achieved a spectral surface emissivity that is aligned with the atmospheric transmission window using layered dielectrics augmented with a metamaterial interface. By harnessing the strong coupling within the gap of a bowtie antenna resulting from the interaction of light with metallic surfaces, we achieve a broadband absorption around a resonance frequency within the atmospheric window spectrum. Additionally, we successfully attained broadband reflection in the visible region with a reflectivity of more than approximately 97 % and in the nearinfrared spectra through the design and optimization of alternating layers with high and low refractive indices (SiO2-TiO2) deposited on a thin silver layer. Our results indicate that merging the metamaterial surface with dielectric layers eliminates the need for thick layers in conventional radiative cooling structures. This configuration significantly improves cooling performance, and it results in a more compact and thinner stack.

Surface temperature management of refractory metals, such as Tungsten, is crucial for their use in high-temperature applications like aerospace, defense, and laser instruments. In this study, distributed Bragg reflectors (DBR) were utilized to reduce the temperature caused by intense laser illumination. To address this problem, we developed a coupled optical and thermal model and investigated multiple factors, including material composition, morphology, architecture, and source-related parameters, such as wavelength. Our results indicate that our DBR coating designs can significantly decrease the temperature of the Tungsten.

Refractory metals, which include niobium, tantalum, molybdenum, and tungsten, are critical components in applications in extreme environments due to their attractive thermomechanical properties. However, their low reflectivity below 1500 nm has prompted researchers to focus on increasing their reflection at shorter wavelengths. In this study, we applied an adjoint-based optimization technique to improve the spectral reflectivity of refractory metals in the broadband spectrum (300–3000 nm). An optimized periodic multilayer consisting of SiO2/TiO2 is selected as a starting point for the process. Then, the adjoint-based method is implemented to enhance the reflection of the surfaces. This approach involves an iterative procedure that guarantees improvement in every iteration. In every iteration, both the direct and adjoint solutions of Maxwell’s equations are computed to predict the scattering characteristics of a particular microstructure on a surface and measure its effectiveness. The results of our study indicate that the final designs not only increase reflectivity to over 90% but also have thermomechanical benefits that make them suitable for use in harsh environments. We also explored the effect of initial geometry on the results. Overall, our study shows that the adjoint-based optimization technique is an effective method for creating high-performing broadband reflectors with refractory metal substrates coated with dielectric multilayers.

Sensors fabricated by using a silicon-on-insulator (SOI) platform provide promising solutions to issues such as size, power consumption, wavelength-specific nature of end reflectors and difficulty to detect ternary mixture. To address these limitations, we proposed and investigated a broadband-thermally tunable vanadium dioxide (VO2)-based linear optical cavity sensor model using a finite element method. The proposed structure consists of a silicon wire waveguide on a silicon-on-insulator (SOI) platform terminated with phase-change vanadium oxide (VO2) on each side to provide light confinement. A smooth transmission modulation range of 0.8 (VO2 in the insulator state) and 0.03 (VO2 in the conductive phase state) in the 125 to 230 THz spectral region was obtained due to the of Fabry–Pérot (FP) effect. For the 3.84 μm cavity length, the presented sensor resulted in a sensitivity of 20.2 THz/RIU or 179.56 nm/RIU, which is approximately two orders of magnitude higher than its counterparts in the literature. The sensitivity of the 2D model showed direct relation with the length of the optical cavity. Moreover, the change in the resonating mode line width Δν of approximately 6.94 THz/RIU or 59.96 nm/RIU was also observed when the sensor was subjected to the change of the imaginary part k of complex refractive index (RI). This property of the sensor equips it for the sensing of aternary mixture without using any chemical surface modification. The proposed sensor haspotential applications in the areas of chemical industries, environmental monitoring and biomedical sensing.

Distributed Bragg reflectors (DBRs) can experience thermomechanical issues under an intense incident beam. A remedy for this issue is to pattern the DBR structure to facilitate thermal expansion in the structure. However, finite-size patterns reduce the design's electromagnetic performance, including the reflectivity amplitude and the operational bandwidth. Here, we introduce a silica aerogel cavity medium between the DBR layer and the substrate to increase the reflection by stimulating the cavity modes. By altering the cavity layer's thickness and the pattern width, the interference of reflected lights can be controlled and enhance the reflectivity at desired wavelengths. The results show a significant enhancement in the reflection by introducing the cavity in the design. For a comprehensive investigation of the mechanisms, the interaction of the DBR layers with the substrate, the effect of patterns, and the propagation of electric fields, especially in the cavity layer, are examined.

Vanadium dioxide (VO2) has attracted extensive attention due to its reversible transition from the insulator to metal phase at a critical temperature of 68 degrees C. Below the critical temperature VO2 transmits the infrared radiation in the insulator phase, whereas above the critical temperature VO2 reflects the infrared portion of the incident radiation. However, smart surface interfaces for high-temperature emitter surfaces require the opposite functionality within the 1-3 mu m spectral range. Here, we demonstrate that a core-shell structure, composed of VO2@Si, which is deposited on a thin layer of Ag, achieves the inverted optical functionality within the 1-3 mu m spectral range, making it ideal as smart interfaces for radiative heat applications as high-temperature emitters. The proposed material architecture also increases the thermal stability of VO2 in addition to enhancing its optical properties in near-infrared region. The results were obtained using numerical simulations. Our results indicate that in its metallic state, the core-shell structure with metallic underlayer promotes efficient absorption in the near-infrared spectrum. On the other hand, in its insulating state dielectric resonances within the core-shell structure along with the metallic underlayer, resulting in increased reflection, offer inverse optical functionalities. Our findings present a significant step toward designing dynamic filters that can efficiently capture and respond to changing conditions in the near-infrared spectrum. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Distributed Bragg reflectors (DBRs) have been developed as an effective way for reflecting light in several applications. In this work, a cavity medium is introduced between the DBR and substrate to increase the reflection by stimulating the cavity modes. The DBR has been designed carefully based on the quarter-wavelength rule utilizing the transfer matrix method (TMM). The thickness, number of layers, and material composition have been optimized, and the surface reflectance of a DBR-coated substrate with and without a cavity layer is compared. Employing FDTD simulations, the optimal thickness of the cavity layer for the incident wavelength of the interest is obtained. The results show a significant enhancement in the reflection by introducing the cavity in the design.

Vanadium dioxide (VO2) exhibits reversible insulator-to-metal phase transition at 68 degrees C, making it promising for diverse applications. However, its low thermal stability in the metallic phase and insufficient emissivity in the near-mid infrared range limit practical use. To address these challenges, we propose a core-shell structure deposited on a metal layer. This configuration enhances VO2's optical properties in both phases, facilitating efficient absorption and reflection of near-infrared radiation.

Passive radiative cooling has garnered significant attention in recent years due to its potential in addressing the energy consumption of conventional cooling systems. Plasmonic and metamaterial structures have been found to be effective broadband absorbers due to their selective emissive spectra, thin thickness, design flexibility, and the ability to excite plasmonic or photonic resonances. This study explores the use of bowtie shape plasmonic metamaterials for the development of novel, structurally simple radiative cooling devices. We show that by designing and optimizing a periodic high index-low index alternating layers (SiO2-TiO2), broadband reflection in visible and near-infrared spectrums is achievable. While to achieve broadband absorption in the transparency window (8-13 um), metamaterial is utilized.

Engineering the spectral characteristics of surfaces is of great importance for different applications including but not limited to energy and electronics. Advancements in nano−/micro-fabrication techniques accelerated the progress made in the field by allowing the realization of structures that are comparable to wavelengths of operation, which can exhibit desired characteristics in a broad range of wavelengths. Therefore, understanding both fundamental mechanisms and design principles of such spectrally selective surfaces becomes very important. The aim of this chapter is to demonstrate some examples of such spectrally selective surfaces, as well as their design principles and physical mechanisms, for applications such as passive radiative cooling and highly reflecting/absorbing coatings for high-temperature applications. Having broadband features is attractive for thermal control, plasmonic particles, and surfaces which can bring additional functionality to these broadband spectrally selective filters. Potential uses of such surfaces are also discussed.

Ferroelectric/dielectric layered stacks are of special interest as gate oxides in the pursuit of designing low-power transistors, where the electrostatics of such stacks are thought to provide a means to allow for voltage amplification in the semiconductor channel. Strain and thickness dependence of the response of such a gate stack in relation to voltage amplification in a semiconductor channel becomes important to identify, which is what we study in this work using a thermodynamic approach. For a ferroelectric multidomain state as the stable phase in the stack, our findings show that a limited magnitude of voltage amplification appears to be feasible. Voltage amplification at the semiconductor surface is computed to hardly exceed 1.2 in thick bilayers (40 nm) for strains stabilizing the multidomain state and attains even less than this value for the thinner stacks.

Passive radiative cooling, an innovative approach for cooling buildings and devices, has attracted considerable attention in recent years. In particular, the spectral emissivity distribution of surfaces plays a crucial role for an object to radiate at wavelengths for which the atmosphere is transparent and solar irradiance is low. Here, we study the role of spectral emissivity distributions using different performance metrics: cooling power (CP) and equilibrium temperature (TEq). We investigated the roles of environmental factors, such as ambient temperature and level of thermal insulation from surroundings, on spectral emissivity distributions. Based on these emissivity distributions, we report the conditions at which the suitable profile for cooling power maximization and equilibrium temperature minimization changes. We discuss the realization of spectral emissivity distributions using various optical materials for cooling power maximization and equilibrium temperature minimization separately under different environmental conditions. The impacts of material selection on the realization of desired emissivity profiles and corresponding outcomes are analyzed. As progress in this emerging field gains traction, development of radiative cooling structures with suitable spectral emissivity profiles under different circumstances will become essential.

New high density storage media and spintronic devices come about with a progressing demand for the miniaturization of ferromagnetic structures. Vortex ordering of magnetic dipoles in such structures has been repeatedly observed as a stable state, offering the possibility of chirality in these states as a means to store information at high density. Electric pulses and magnetoelectric coupling are attractive options to control the chirality of such states in a deterministic manner. Here, we demonstrate the chirality reversal of vortex states in ferromagnetic nanodiscs via pulsed electric fields using a micromagnetic approach and focus on the analysis of the energetics of the reversal process. A strong thickness dependence of the chirality reversal in the nanodiscs is found that emanates from the anisotropy of the demagnetizing fields. Our results indicate that chiral switching of the magnetic moments in thin discs can give rise to a transient vortex-antivortex lattice not observed in thicker discs. This difference in the chirality reversal mechanism emanates from profoundly different energy barriers to overcome in thin and thicker discs. We also report the polarity-chirality correlation of a vortex that appears to depend on the aspect ratio of the nanodiscs.

Passive radiative cooling is a novel concept and is likely to be important for building and industrial energy efficiency efforts, as it will significantly contribute to the reduction of thermal management costs for electronic equipment. In most studies, radiative cooling devices are not considered for their colors; although to find a large number of users, it must be attractive to designers or architects, who usually pay significant attention to aesthetic and decorative aspects of paints. Since the majority of the coatings reported in the literature are white, there is also a need to develop color-coordinated paints and coatings. Here, we propose an approach for designing a simple structure of colored radiative cooling devices assisted by a plasmonic structures. We show that they can be tuned as desired to produce different hues of colored coatings, while maintaining adequate radiative cooling power. To demonstrate the conflicting functions of color display and radiative cooling performance, we use a bowtie nanoantenna as a color-displaying structure to investigate how the structural factors affect the cooling performance and color display accordingly. We show that periodic high index-low index alternating layers (SiO2–TiO2) on top of a thin silver layer cause broadband reflection in visible and near-infrared spectrums, while to achieve narrowband absorption in the visible region, which leads to the desired colorization, the bowtie nanoantenna is utilized. We report that by changing the structural parameters of a nanoantenna, the resonance peaks are controlled to yield a narrowband absorption in the visible spectrum to create different colors. Moreover, our results indicate that although adding coloration structure to a conventional radiative cooling system reduces the cooling power by around 30%, it is still reasonable high, around 60 W/m2, and is still suitable to be used for daytime radiative cooling where control over the color is needed. Acceptable cooling power while ability to control the coloration make the proposed colored radiative cooling a potential candidate to be used in various applications, both in high end buildings or for thermal management of electronic equipment.

While there is an established literature on the effect of geometric parameters and material properties on the sensitivity of tapered fiber refractive index sensors, the impact of these parameters has been largely overlooked at sensor at diameters comparable to wavelength. Here, we investigate the effect of geometric parameters and materials properties at dimensions comparable to wavelength using full-wave solutions of Maxwell’s equations. Our results indicate that to achieve the maximum sensitivity for the tapered fiber sensors, the diameter should be closer to operational wavelength. For diameters less than the operational wavelength, sensitivity reduces instead of further increasing, a phenomenon opposite to what was predicted by ray-tracing based tapered fiber models. Another important finding of this study is the effect of optical loss constant on the sensitivity, a parameter often overlooked in the literature. As the optical loss constant was varied from 0 to 0.04 for each iteration of RI variation range, a linear decrease in sensitivity from 0.12 trans. (A.U)/RIU to 0.05 trans. (A.U)/RIU.

Vanadium dioxide (VO2) has attracted interest due to its phase transition from the insulating to the metallic states for potential use in a variety of optical and photonic applications. Here, we propose a VO2/Si core-shell structure to improve switching in band-selective reflection properties of the composite. Mie scattering formulation is used to analyze the structure before and after phase transition to show the impact of resonance quality on the mid-infrared light back-scattering. After investigating the effect of various VO2 nano-sphere radius sizes in both phases on light reflectivity, a VO2/Si core-shell structure is proposed to boost reflectivity and improve light controllability. Randomly distributed nanoparticles are studied to illustrate how these composites have similar behavior to their deterministic-distributed counterpart. Our results indicate that up to two-thirds of incident light power can be controlled by embedding proper core-shells in a polymer host material.

Optical coatings composed of thermomechanically superior materials with high reflectance in the broadband spectrum have recently attracted attention. Tungsten (W), among other VIB group materials, suits well for applications that require high thermomechanical stability but suffer from low reflectivity below 1 mu m. This paper proposes a bio-inspired multilayer structure that lowers Tungsten's absorption over a broadband spectrum. The proposed design mimics the patterns observed on Morpho butterfly wings and reduces the absorption of W over a broad spectrum. At the initial stage, the impedance mismatch method is implemented to optimize the number and thickness of the homogeneous multilayers over the broadband spectrum. Then, the proposed pattern is incorporated into the homogenous structure. The effect of inhomogeneities on the transverse directions is investigated. The resulting spectral characteristics of both homogeneous and inhomogeneous structures are analyzed by wave impedance analysis. The total optical path was extracted with this analysis. Using this analysis, the mechanisms that trigger sharp reflection dips in homogenous layers are uncovered. S-parameters of the inhomogeneous structures are extracted and evaluated. Our findings indicate that the increased rate of change of the S-parameters phase in inhomogeneous structures is responsible for increased dips over the spectrum. (c) 2021 Elsevier Ltd. All rights reserved.

Although black silicon is utilized in a wide range of applications due to its broadband spectral emission and absorption, the underlying electromagnetic mechanisms are not well explored. In this study, the underlying phenomena that are responsible for these enhanced spectral features are investigated. The absorption spectra of the black silicon with random textures are analyzed, and the electromagnetic mechanisms that drive elevated absorption are explored. Our findings reveal that two separate electromagnetic phenomena occur in the textures, effective wavelength matching and waveguide modes. Detailed analysis reveals that the occurrence condition of those phenomena is highly dependent on the dimensions of the textures in the transverse direction. The effect of the texture dimensions and doping concentration both on absorption characteristics and physical phenomena is analyzed in detail. The findings of this study explain the absorption mechanisms of black silicon observed in experimental studies, which can lead to designer materials with rough surfaces for the desired spectral emissivity.