Thermal Mirror Research Articles

Resonant-mode metasurface thermal super mirror by deep learning-assisted optimization algorithms

A “super-mirror” having ultrahigh infrared reflectance is achieved by an optimized photonic contrast grating metasurface. Finding ways to achieve this exceptional performance can be enabled by implementing global optimization and machine learning elements, such as Bayesian optimization and genetic algorithm. Here, we acquired an optimized grating design made of high-index germanium, which excites resonances that result in ultralow emittance at certain wavelengths. Our optimizations assisted in the discovery of hybridized coupling of Fabry-Pérot modes and guided modes in a monolithic microscale multilayered coating. We demonstrate constraints in the given geometric variable ranges improves the overall performance of algorithms. We also show the enhanced performance of a deep learning Feedforward Neural Network, which is implemented as the inverse design using the network trained with dataset obtained from Bayesian optimization and Genetic Algorithm approaches. The performance of the Feedforward Neural Network-assisted design produced normal emissivity difference by only +3.5 %, with lower sensitivity to grating dimensional parameter variations. The improvement is achieved by predicting and better understanding of the optical physics of resonant gratings. The proposed few-layer grating coating can be applied to space components, enclosures, and vessels to suppress thermal radiative heat loss.

Graphene-enhanced phase change material systems: Minimizing optical and thermal losses for solar thermal applications

Photothermal utilization systems with nano-enhanced phase change materials are encouraging due to their high cost-effectiveness, tunability, and sustainability. However, the optical and thermal losses, which have failed to be quantified and suppressed in most previous work, give rise to an intractable challenge for practical applications. Herein, we comprehensively quantify the energy flow during the photothermal conversion and storage processes and propose various strategies to suppress the optical and thermal losses for boosting solar energy utilization via experiments and numerical simulations. The results indicate that a 30 % optical loss is suppressed by introducing graphene into the phase change composite (PCC), leading to a 5.5 % increase in photothermal storage (PTS) efficiency. Compared with the graphene-doped one, the PCC achieves a 5.6 % reduction in conductive thermal loss and a 2.2 % enhancement in PTS efficiency via optimizing insulation materials. Besides, the convective loss is suppressed by employing a transparent aerogel cover, resulting in a 4.3 % increase in PTS efficiency. Furthermore, the radiant thermal loss is reduced by 21.2 % with a thermal mirror due to the reflection of infrared radiation, and thus the PTS efficiency is boosted by 9.5 %. This work's findings offer new insight into optical and thermal management for efficient solar thermal utilization.

Thermal effect of high-power laser propagation in complex channels and its mitigation

We investigate the channel thermal effect resulting from the gas thermal blooming effect and mirror thermal deformation on the beam quality, and we propose a mitigation measure for blowing purge gases. Without gas blowing, the gas thermal blooming effect opposes the influence of mirror thermal deformation on the beam phase, resulting in significant phase distortion and offset. Noticeable distortion and divergence of the laser spot was also observed. After blowing purge gases, the beam phase was primarily determined by the mirror thermal deformation. Simultaneously, the mean-squared beam width decreased uniformly, improving the symmetry and energy concentration of the laser spot. However, the laser spot tended to stabilize when the gas velocity exceeded a certain threshold.

Three-dimensional measurement of laser-induced thermal mirror dynamics using photothermal vortex interferometer with azimuthal phase spectra analysis

Measurement of the thermal mirror caused by laser heating provides direct access to the optical, thermophysical, and mechanical properties of solid materials. However, the nanoscale time-dependent three-dimensional (3D) thermal mirror hinders the existing photothermal techniques to obtain the thermal mirror dynamics without sacrificing the spatial profile of the thermal mirror. To fill the gap of 3D measurement of laser-induced thermal mirror dynamics, a photothermal vortex interferometer (PTVI) is proposed. The PTVI produces a multi-ring petal-like interferogram by the coaxial coherent superposition of the high-order conjugated Laguerre-Gaussian beams. The nanoscale 3D thermal mirror with a rotationally symmetric surface profile can be encoded into different azimuthal phase shifts at the different radii of the interferogram. An azimuthal phase spectra analysis is proposed to extract the azimuthal phase shifts from the continuously captured interferograms, enabling reconstruction of the 3D thermal mirror dynamics. The experimental results verified that the PTVI can measure the surface deformation due to the thermal mirror at an accuracy of picometer scale, and the optical and thermophysical properties of an optical glass sample can be determined with the assistance of the thermophysical model of the 3D thermal mirror dynamics. The approach can be an effective tool for characterization of solid materials.

Effect Of Thermal Stimulation And Mirror Therapy On Hand Functions In Patients With Stroke: An Interventional Study


 Background: Thermal stimulation can be used as an intervention to enhance sensory function. Activated areas by thermal stimulation are greater than those by tactile or mechanical stimulation. Mirror therapy creates the illusion of normal movement with the mirror image of the unaffected upper extremity. Thermal stimulation provides somatosensory stimulation that causes the forced use of the involved extremity as a protective response to heat which provokes volitional/reflexive motor activity. Somatosensory stimulation may induce brain plasticity.
 Purpose: This study was carried out to determine the effect of thermal stimulation and mirror therapy in the sensorimotor and functional recovery of hand in stroke patients with hand impairment.
 Methods: 18 stroke survivors were randomly assigned to Thermal Stimulation rehabilitation treatment and Mirror Therapy rehabilitation group (30 minutes daily for 3 weeks). Three measures, including Brunnstorm stage, Wolf motor Function Test, and Revised Nottingham Sensory Assessment scale was used as an outcome measure to assess the motor and sensory recovery.
 
 
 
 Results: The Thermal Stimulation group showed a potential trend for improved recovery stage as measured by Brunnstorm staging for motor recovery of hand as compared to the Mirror therapy group. The Thermal Stimulation group also showed a statistically significant and large improvement in tactile, proprioception and stereognosis score as measured by Revised Nottingham sensory assessment scale whereas Mirror Therapy group showed significant improvement in stereognosis score alone.
 
 
 
 Conclusion: Thermal stimulation has more positive effect in the sensorimotor and functional recovery of hand in acute stroke patients with hand impairment within 3 weeks in comparison to Mirror Therapy. Thermal stimulation protocol should be added in the clinical setting of rehabilitation in stroke patients with hand impairment on the basis of the result of this study.

Macro and micro methodologies for thermomechanical mirror buckling of freestanding graphene membranes

The thermal mirror buckling behavior of freestanding graphene, which is a particular phenomenon observed in scanning tunneling microscope experiments, has not been analyzed systematically in the existing literature. However, because loads imposed by the scanning tunneling microscope tip are both mechanical and thermal, it is appropriate to regard this behavior as thermomechanical mirror buckling. The mirror buckling behavior is related to geometrical characteristics of a model, such as structure curvature. Therefore, appropriate analytical methods should be determined before examining the deep-seated origin of mirror buckling of graphene membranes. In this study, three methods are used separately in both the macroscale and microscale analyses, namely, continuum mechanics, molecular structural mechanics, and molecular dynamics, to analyze the mirror buckling of graphene. A comparison of the theoretical analysis and experimental results shows that, of the three methods, the molecular dynamics method is the most effective and suitable method for examining the thermomechanical mirror buckling behavior of freestanding graphene. The effects of the load magnitudes and graphene membrane radius on the deflection are also analyzed.

Structural and thermal properties of Yb:CaBTeX glasses as a function of TeO2 content

Yb3+-doped Calcium Borotelurite glasses were synthesized. The structural and thermal properties were investigated by X-ray diffraction, density, Fourier-transform infrared, Raman, Vickers microhardness, differential thermal analysis, specific heat, and thermal expansion. The thermal mirror was used to determine thermal diffusivity and thermal conductivity. The results were discussed as a function of tellurium oxide content. There were an increase in density and a decrease in Vickers microhardness due to the substitution of CaO, and B2O3 for TeO2. By Fourier-transform infrared spectra, the fraction of tetrahedral borons increase up to 31% mol of TeO2, however, for 54% there was a decrease, indicating a reduction of BO4 and the formation of BO3 units. The thermal expansion coefficient and thermal stability increased with TeO2 content, while there was a reduction for specific heat and thermal diffusivity. The results indicated that Yb:CaBTeX glasses have good thermal and structural properties with potential for applications in optical devices.

Möbius mirrors

An accelerating boundary (mirror) acts as a horizon and black hole analog, radiating energy with some particle spectrum. We demonstrate that a Möbius transformation on the null coordinate advanced time mirror trajectory uniquely keeps invariant not only the energy flux but the particle spectrum. We clarify how the geometric entanglement entropy is also invariant. The transform allows generation of families of dynamically distinct trajectories, including -symmetric ones, mapping from the eternally thermal mirror to the de Sitter horizon, and different boundary motions corresponding to Kerr or Schwarzschild black holes.

Three-Dimensional, Time-Dependent Analysis of High- and Low-Q Free-Electron Laser Oscillators

Free-electron lasers (FELs) have been designed to operate over virtually the entire electromagnetic spectrum, from microwaves through to X-rays, and in a variety of configurations, including amplifiers and oscillators. Oscillators can operate in both the low and high gain regime and are typically used to improve the spatial and temporal coherence of the light generated. We will discuss various FEL oscillators, ranging from systems with high-quality resonators combined with low-gain undulators, to systems with a low-quality resonator combined with a high-gain undulator line. The FEL gain code MINERVA and wavefront propagation code OPC are used to model the FEL interaction within the undulator and the propagation in the remainder of the oscillator, respectively. We will not only include experimental data for the various systems for comparison when available, but also present, for selected cases, how the two codes can be used to study the effect of mirror aberrations and thermal mirror deformation on FEL performance.

Energy-saving window systems: types, development, comparisons and perspectives

The tendency to use natural resources is decreasing; energy conservation and ecosystems are taking the first positions. Therefore, the use of glass finishing of buildings has become a starting point in the search for new technologies to save energy use, reduce the need for air conditioning and additional lighting. The developed smart glass is an advanced type of glass that can change transparency depending on different conditions, or convert sunlight into electricity that can be used for human needs. The work examines the appearance of the first types of smart windows, and further development of smart windows using various films, chemical and organic compounds. The prospect of using smart glass to improve natural lighting in various fields and the future possibilities of related technologies by comparing several types of smart glass. The main types of smart windows are presented: photochromic window, thermochromic window, electrochromic window, liquid crystal windows, windows based on suspended particles, window using a "thermal mirror". On the basis of the given characteristics of various technologies of smart glass, its advantages and disadvantages, as well as the associated difficulties, which should be resolved in the future, are revealed. The existing classification of smart windows by the type of use of films in them and their advantages and disadvantages of using. The main newest technologies of absorption of sunlight by the surface of the window and the conversion of light into electricity, the problem of achieving transparency by glass, and the tendency of solving this issue are described. On specific examples, the areas of application of glass with controlled transparency are considered, and the economic indicators obtained when it is used in buildings. The technology of using solar panels in the form of blinds and their influence on providing the building with energy is considered. improvement of some of its optical properties, contributed to a decrease in its cost, which leads to an increase in the availability of this material.

SUNNY HOUSE WITH A VEGETARIAN

The relevance of passive energy saving technologies in energy efficient low-rise construction in Russia is indicated. The definition of a passive house and its feature is given. Indicated that an attractive source of energy for heating the house is the energy of the sun. The definition of a solar house is given. The requirements for a solar passive house during its design are described: compact form of the house, optimal orientation of the house to the cardinal points, differentiation of glazing at home, passive use of solar energy, etc. It is noted that the most common system of passive heating of a house is to heat insulated glazed volume between nature and internal space of the house (vegetarian). The definition of a vegetarian is given, its design, features and advantages are described. Considered and analyzed various ways of heating solar houses from a vegetarian: a semi-direct, indirect, thermosiphon system with heating and circulation of warm air around the house. The classification of solar houses is discussed depending on the architectural solution for the placement of the vegetarian: a detached house with a vegetarian; a house with a vegetarian adjoining its main living space; a house located with a vegetarian under a common roof; a house with a vegetarian built into its living volume, a house with a “double shell”. The following types of vegetarians are listed: attached to an existing house, built into the house or being a “second shell” for the house. Practical recommendations for optimal work of a vegetarian are given: the need for special glazing (thermal mirror), protection from sunlight in the summer. The conclusion is made about the prospects of solar houses with a vegetarian due to the clear advantages of the passive heating system of the house and a high architectural and aesthetic level.

Heat coupling effect on photothermal detection with a moving Gaussian excitation beam.

A strong photothermal response is beneficial to the measurement of optical and thermal properties of optical materials using the laser-induced thermal mirror method. A highly sensitive asymmetrical thermal mirror method was recently proposed by employing a moving Gaussian excitation beam [Appl. Phys. Lett.114, 131902 (2019)APPLAB0003-695110.1063/1.5080163]. However, the heat transfer across the interface between the thermodynamic system and the surroundings is ignored, which will lead to an error in the absolute measurement of the material properties. To address the problem, we present a theoretical and experimental study of heat transfer within the heated sample and out to the air coupling fluid in the photothermal detection with a Gaussian excitation beam moving at a constant velocity. We analyze the dynamic temperature fields inside the sample and in the surrounding air, and the phase shifts induced by the thermoelastic displacement of the sample (thermal mirror) and the refractive index gradient of air (thermal lens), as well as the diffracted intensity profiles of the probe beam in the detection plane. The experiments are implemented under normal pressure and vacuum, respectively, for a fused silica glass-air heat coupling system to verify the theoretical model. The experimental results show that the thermal lens, due to the heat coupling effect, introduces a signal deviation approximately 4.2% of the total photothermal signal, which is close to the theoretical result of 5%.

Design and analysis of a high accuracy bidirectional thermal deformable mirror

To realize an efficient, low-cost, and two-way heat-driven deformable mirror, a deformable mirror-integrated structure using a thermoelectric cooler is proposed. The bidirectional heat-driven stroke, thermal field distribution, and deformation were obtained by finite element analysis. At the same time, the error correction effect of the deformed mirror is verified by experiments: the mirror peak value is corrected from 7.087 to 0.386 μm and the root-mean-square value is corrected from 1.347 to 0.081 μm. For four special aberrations—convex divergence, concave divergence, 45-deg astigmatism, and 90-deg astigmatism—the correction error can be controlled below 5%. More importantly, the cost of the deformed mirror structure designed is under 10,000 yuan; the cost is greatly reduced compared to other deformed mirror structures.

Comparison of photothermal responses under spatial and temporal modulations of CW Gaussian beam excitation: A numerical study

Obtaining stronger photothermal response of optical materials is of great interest in the measurement of optical losses using the photothermal tools, in particular the thermal mirror. In our previous work, we experimentally demonstrated a more effective method of thermal mirror detection employing the spatial modulation (SM) of a continuous wave Gaussian beam excitation instead of the conventional temporal modulation (TM). A complete analytical model is presented to describe the laser-induced three-dimensional dynamic thermal fields in a semi-infinite sample and the time-dependent thermal mirror due to the thermoelastic response of the material as well as the evolution of the intensity distribution of a probe beam after interacting with the thermal mirror, in both the SM and TM schemes under the low absorption and exponentially-decaying absorption cases. With the model, we obtained the variations of temperature, surface displacement, and diffraction intensity giving rise to the photothermal response of the material. The results of the comparison between the two schemes reveal that the SM scheme is more sensitive than the TM scheme originating from a large variation range of the temperature in the SM scheme. We also explained the complex relation between the surface displacement and the diffraction intensity of the probe beam at the center of the detection plane on the basis of the phase shift caused by the thermal mirror. The presented model and the results of comparison are of great importance to gain a deeper insight into the photothermal characterization in both the SM and TM schemes.

Photothermal Mirror Method for the Study of Thermal Diffusivity and Thermo-Elastic Properties of Opaque Solid Materials

We have carried out the theoretical and experimental time evolution and amplitude study of the photothermal mirror signal generated by focusing a laser beam on the surface of a suite of solid samples. Based on a theoretical model that resolves the thermal diffusivity equation and the equation for thermo-elastic deformations simultaneously, we have calculated the transient time evolution and amplitude of the signal. We observe the same time evolution pattern for samples as diverse as glass, quartz, metals, and synthetic ceramic oxides. The data have yielded a linear dependence between the time build-up of the thermal mirror and the inverse of the thermal diffusivity for all the samples. For moderate power levels, we also observe a linear behavior between the stationary value of the signal and the thermally induced phase shift value. From the calibration curves, we have determined the thermally induced phase and the thermal diffusivity coefficients of two prospective nuclear reactor control rod materials, dysprosium titanate ( $$\hbox {Dy}_{2}\hbox {TiO}_{5}$$ ) and dysprosium dititanate ( $$\hbox {Dy}_{2}\hbox {Ti}_{2}\hbox {O}_{7}$$ ) to be $$D = (7.0 \pm 0.4) \times 10^{-7} \mathrm{m^{2}\cdot s^{-1}}$$ .

Accessing thermo-mechanical properties of semiconductors using a pump-probe surface displacement method

Description of the physical mechanism leading to laser induced thermal and electronic effects in semiconductors is crucial in both basic research and technological applications. In this paper, we present a thermal mirror technique to study the thermo-mechanical properties of semiconductors. A detailed theoretical investigation is presented, and the dominant effects are described in terms of the physical properties of the material. The effect of heat coupling between the sample and the surrounding fluid was taken into account and considerations on the time and spatial approximations to the photogenerated carriers profile were used to simplify the theoretical model. These approximations were then compared to numerical models and the results hold for high recombination rate semiconductors. Experiments were performed to validate the theoretical model, and the thermal diffusivity and photogenerated heat in the sample were determined. The values obtained for these properties were found to be in good agreement with literature.

Processing of thermal mirror signals under continuous wave excitation

The main features of the photothermal mirror signals arising under the continuous wave excitation were analyzed in terms of a model that takes account of thermal, mechanical, and diffraction effects. Formulae to describe the initial slope and stationary value of the signal were derived and compared with the numerical simulation results. We suggested an approach to processing the thermal mirror signals based on exploiting the initial slope and stationary value. The method was verified using numerical simulation and experimental results. We compared the method performance with the conventional approach using thermal mirror signals excited in the luminescent glasses. It was shown that the developed technique has an essentially lower computational cost, while offering a comparable level of accuracy.

Analysis of the Thermo-Reflectivity Coefficient Influence Using Photothermal Pump-Probe Techniques.

Recent improvements in the modeling of photo-induced thermo-optical-mechanical effects have broadened the application of photothermal techniques to a large class of solids and fluids. During laser excitation, changes in optical reflectivity due to temperature variation may affect the photothermal signal. In this study, the influence of the reflectivity change due to heating is analyzed for two pump-probe photothermal techniques, thermal lens and thermal mirror. A linear equation for the temperature dependence of the reflectivity is derived, and the solution is tested using optical properties of semi-transparent and opaque materials. For semi-transparent materials, the influence of the reflectivity change in photothermal signals is less than 0.01%, while for opaque materials it is lower than 3%.

Design and Measurement of Radiowave Transmissive Thermal Control Mirror

This paper describes a new flexible thermal control mirror: controlled optical surface films. Controlled optical surface film exhibits high reflectivity to solar radiation, high infrared emissivity, and high transmittance to radio waves. The target of radiowave transmittance is Ge-coated Kapton, which is used on antenna covers; the target of thermooptical properties is the optical solar reflector. Controlled optical surface film controls spectral reflectance and absorptance in wavelengths ranging from 0.26 to with a dielectric multilayer. This multilayer is designed using a genetic algorithm method. The solar absorptance of the latest controlled optical surface film is 0.09, and its total hemispherical emittance is 0.77 at 300 K. These properties are similar to those of the optical solar reflector. In addition, there is little radiowave attenuation in this controlled optical surface film, and its radiowave transmittance is similar to that of Ge-coated Kapton.

Critical review of ionic liquids for the pretreatment of lignocellulosic biomass

Ionic liquids have been the subject of active research over the course of the last decade and have in the past been touted as one of the most promising technologies for revolutionising the chemical and petro-chemical industries. The sheer abundance of potential ionic liquid structures coupled with their tuneable physico-chemical properties has endeared ionic liquids to the scientific community across a broad range of disciplines with potential applications that include pharmaceuticals, electrolytes, thermal energy storage media and liquid mirror telescopes. Within the context of a biorefinery for the production of biofuels and other bio-based products from renewable resources, the unique abilities of some ionic liquids to selectively dissolve biomass components or whole native biomass have been demonstrated. This ability has sparked extensive investigations of ionic liquids for the pretreatment of different biomass types, particularly for the production of cellulosic biofuels. However, the esoteric nature of ionic liquids persists and constructing a fundamental framework for correlating ionic liquid structures with useful applications remains a significant challenge. In addition to the above, the more practical challenges of toxicity, high costs, high viscosities, low solids loading and complex recycling are key factors hindering the wide-scale uptake of ionic liquids as pretreatment solvents in a commercial biorefinery. This critical review provides insights from academic studies and the implications thereof for elevating ionic liquids from the status of ‘promising’ to ‘commercialisable’ in the pretreatment of biomass. It is vital that key hurdles for the commercialisation of ionic liquids in the form of high costs, high viscosities, poor water tolerance, toxicity, low solids loading and recovery/recycling be addressed.