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Photoacoustic imaging techniques with gold nanoparticles as contrast agents have received a great deal of attention. The photoacoustic response of gold nanoparticles strongly depends on the far-field optical properties, which essentially depend on the dielectric constant of the material. The dielectric constant of gold not only varies with wavelength but is also affected by temperature. However, the effect of the temperature dependence of the dielectric constant on gold nanoparticles' photoacoustic response has not been fully investigated. In this work, the Drude-Lorentz model and Mie theory are used to calculate the dielectric constant and absorption efficiency of gold nanospheres in aqueous solution, respectively. Then, the finite element method is used to simulate the heat transfer process of gold nanospheres and surrounding water. Finally, the one-dimensional velocity-stress equation is solved by the finite-difference time-domain method to obtain the photoacoustic response of gold nanospheres. The results show that under the irradiation of a high-fluence nanosecond pulse laser, ignoring the temperature dependence of the dielectric constant will lead to large errors in the photothermal response and the nonlinear photoacoustic signals (it can even exceed 20% and 30%). The relative error of the photothermal and photoacoustic response caused by ignoring the temperature-dependent dielectric constant is determined from both the temperature dependence of absorption efficiency and the maximum temperature increase of gold nanospheres. This work provides a new perspective for the photothermal and photoacoustic effects of gold nanospheres, which is meaningful for the development of high-resolution photoacoustic detectors and nano/microscale temperature measurement techniques.

Accurate measurement of temperature and velocity fields in combustion and flow diagnostic plays an important role in analyzing the heat transfer mechanism and ensuring the safe operation of equipment. In the present work, considering the refraction effect of sound waves, temperature and velocity fields are reconstructed simultaneously based on the nonlinear acoustic tomography (NAT) by using the covariance matrix adaptation evolution strategy (CMA-ES) algorithm. In the forward problem, the fast two-point ray-tracing algorithm is introduced to track curved sound waves more efficiently. Due to the sparse measurement signals caused by the limited space and curved acoustic paths, Tikhonov regularization is introduced as the constraint of smoothing prior information to reduce the ill-posedness of the problem. The influence of velocity regularization parameter and temperature regularization parameter on simultaneously visualizing the temperature and corresponding velocity fields are investigated, respectively. Then the selection strategies of these two regularization parameters are given, which are proved to have high selection efficiency. On this basis, inhomogeneous temperature and velocity fields are reconstructed simultaneously by using the CMA-ES algorithm, simulated annealing, and genetic algorithms. By solving the inverse problem of the NAT, which is highly nonlinear and ill-posed, the feasibility and effectiveness of CMA-ES algorithm in the simultaneous reconstruction of velocity and temperature fields are confirmed. The proposed method for visualizing the temperature and velocity fields will supply key feedback for the combustion process control.

For enhancing the photoelectric performance of the GaAs photovoltaic cell, the non-dominated sorted genetic algorithm-II (NSGA-II) is employed to solve the multi-objective optimization problem, which consists of the thicknesses of active layer (GaAs) and three anti-reflection (AR) films (TiO2-HfO2-SiO2). Through maximizing (minimizing) the light absorption of photons with energies higher (lower) than the bandgap in the active layer directly and reducing the cost at the same time, the Pareto optimal solution is obtained. An increment in the absorptance, short circuit current and photoelectric conversion efficiency by 50%, 43.4% and 44.9% is achieved by the optimal GaAs solar cell due to excellent AR properties obtained by NSGA-II optimization. We systematically investigate the effects of carrier lifetime, surface recombination velocity, carrier mobility as well as doping concentration on the photovoltaic parameters, and acquire the limitations of these parameters. The strong robustness of the optimal GaAs solar cell enables it to be adapted to wide-angle sunlight incident, and the probability to achieve an absorptance of 0.963 is 97% even if there is a 20% machining error.

The plenoptic imaging technique provides a promising approach to the non-invasive three-dimensional measurement, especially for the high-temperature combustion diagnosis. We establish a light-field convolution imaging model for diffusion flame in this work, considering the radiation transfer process inside the diffusion flame and the light transfer process inside the focused plenoptic camera together. The radiation transfer process is described by the radiation transfer equation and solved by the generalized source multi-flux method. Wave optics theory is adopted to describe the light transfer process, combining Fresnel diffraction and the phase conversion of the lens. The flame light-field image is obtained by the light-field convolution imaging model and adopted as the measurement signal to reconstruct three-dimensional temperature field. The inverse problem of temperature reconstruction is solved by the least square QR decomposition method. The simulative temperature reconstruction work is conducted, including the inverse analysis, the uncertainty analysis, and the measurement noise influence. All the results show that the proposed measurement method is available to reconstruct three-dimensional temperature with satisfactory accuracy and acceptable uncertainty. Both symmetric and asymmetric distributed temperature fields are investigated, and the reconstructed results prove the validity and universality of the measurement method.

Based on the demand for ideal selective solar absorbers to harvest a full solar spectrum, we present an auspicious method, the multi-island genetic algorithm (MIGA), to optimize the configuration of a universal selective solar absorber, which can avoid the premature phenomenon and superior to traditional genetic algorithm. The selective solar absorber equipped with the set of optimal geometric parameters exhibits the solar absorptance greater than 0.932 with 96% probability and mid-infrared emittance lower than 0.058 at the same time considering unavoidable 20% fabrication uncertainties in the design variables. In addition, the absorber is a combination of great characteristics, such as polarization independence and angle insensitivity. The hybrid resonance modes of Wood’s anomaly, surface plasmon polariton and magnetic polariton are responsible for the high absorption performance. The MIGA method presented is proved to be an effective and robust tool in the optimization of micro-nano structures with manufacturability for thermal and energy applications.

Inverse estimation of the temperature/phase change-dependent refractive index of semitransparent media is studied in present work. The direct models (including the coupled radiation-conduction heat...

Standard plenoptic camera can be used to capture multi-dimensional radiation information of high temperature luminous flame to reconstruct the temperature distribution. In this study, a novel method for reconstructing three-dimensional temperature field is proposed. This method is based on the optical tomography combined with standard plenoptic camera. The flame projection information from different planes is contained in one radiation image. In this model, we introduced the effective concept of the nearest neighbor method in the frequency domain to strip the interference of redundant information in the projection and to realize three-dimensional deconvolution. The flame emission intensity received by the pixels on the charge-coupled device sensor can be obtained according to the optical tomographic model. The temperature distributions of the axisymmetric and non-axisymmetric flames can be reconstructed by solving the mathematical model with the nearest neighbor method. The numerical results show that three-dimensional temperature fields of high temperature luminous flames can be retrieved, proving the validity of the proposed method.

The boundary emissivity, thermal conductivity, and boundary time-varying heat flux in the participating media are retrieved in this work. In the forward model, the coupled radiation-conduction heat transfer in the participating medium is resolved by the finite volume method combined with discrete ordinates method. The inverse model utilizes the temperature signals at the appropriate position of the medium as the known information and uses the unscented Kalman filter (UKF) as an optimization tool to reconstruct the boundary emissivity, thermal conductivity, and boundary time-varying heat flux. It is found that when the emissivity, thermal conductivity, and boundary time-varying heat flux are reconstructed simultaneously, only the temperature information of two locations is required. The influence of the measurement noise, sampling interval, absorption coefficient, process noise covariance, measurement noise covariance on the accuracy and stability of the retrieval results is investigated in detail. All the reconstruction results indicate that the UKF technique is effective and robust for estimating the photothermal parameters and boundary condition of the radiation-conduction heat transfer problems.

Photoacoustic imaging (PAI) combines the advantage of optical and ultrasonic imaging, which has a high signal-to-noise ratio, and spatial resolution. To further improve the performance of PAI, gold nanorods can be utilized as exogenous contrast agents, and their size can be controlled. The size change of gold nanorods will change their absorption and heat transfer characteristics, and then affect their photoacoustic characteristics. Therefore, in the present work, the influences of absorption characteristics (absorption cross section and volume absorption coefficient) and heat transfer characteristics (specific surface area) of gold nanorods on photoacoustic response are studied by FDTD (FDTD Solutions, Lumerical) and FEM (Comsol Multiphysics). Results show that the increase of specific surface area will enhance the thermal coupling between gold nanorods and water, so as to improve the heat transfer and produce different photothermal responses without significantly affecting the photoacoustic quantum yield (As the specific surface area increases by about 10 times, the temperature increases by about 12 times, while the photoacoustic quantum yield increases by about 24%). On the contrary, absorption characteristics are decisive factors for both photothermal and photoacoustic responses. Therefore, in terms of improving photoacoustic quantum yield, when the size of gold nanorods changes, more attention should be paid to the improvement of the absorption characteristics rather than the specific surface area. In addition, increasing the specific surface area can significantly improve the photostability of gold nanorods, which is very important for practical applications.

This numerical study investigates the feasibility of simultaneous retrieval of particle size distribution (PSD) and optical constants of spheroids by optical spectroscopy. In this simulation the particles are considered as spheroid. The aspect ratio of an oblate and prolate spheroid is set as 0.8 and 1.2, respectively. At this constant aspect ratio, the particle’s orientation has been changed by altering its major semi-axis. Two continuous wavelength lasers are employed to irradiate the particle samples. Multi-angle and multi-wavelengths elastic forward scattering intensity and the spectral collimated transmittance are employed to measure signals. For forward scattering, the spheroid is effectively replaced by a sphere of an approximated radius and the modified Mie theory is employed to calculate the scattering intensity. For the collimated transmittance, the extinction efficiency of non-spherical particle is measured based on the extended anomalous diffraction approximation. The Log-Normal distribution is used to get the volume frequency distribution of the particles and the inverse process is done by using the improved quantum particle swarm optimization. Two different sets of optical constant (e.g., complex refractive index), semi-major axis of non-spherical particle and discrete rate are retrieved by the inverse simulation. The results show that, the proposed spectroscopic technique can retrieve PSD and optical constants of non-spherical particles simultaneously within the tolerable error limit less than 10%.

We theoretically demonstrate a near field thermostat based on the mechanism of three-body photon heat tunneling and tunability of graphene. The system consists of three separated parallel plates at different temperatures, with the hot and cold body (located at either side of the system) coated with a monolayer of graphene. As a unique two-dimensional material, graphene owns the tunable plasmonic dispersion relation which can be rapidly modulated by changing its chemical potential in an electronical manner. By changing the chemical potentials of graphene, the near field radiative heat flux (NFRHF) transporting in the system can be tuned accordingly. As a consequence, the equilibrium temperature of the intermediate body can be adjusted from the temperature of the hot body to that of the cold one. The results show that the equilibrium temperature can be adjusted from 326 K to 388 K in SiO2-SiO2-SiO2 configuration, which occupies about sixty percent of the temperature range with respect to the cold and hot body. This phenomenon implies that near field contactless temperature adjustment can be carried out by the proposed configuration. The underlying mechanism is attributed to the varied NFRHF with tunable surface plasmon polaritons of graphene. Moreover, three-body photon heat tunneling can benefit the regulation function of the system in the near field regime and an enhanced region for adjusting temperature is observed. An interesting phenomenon is observed that the mismatch between different materials raises the regulation ability to 82 K in SiO2-SiC-SiO2 configuration. The results obtained in this study could find significant applications in the field of near field thermal management and controlling temperature in the separation of micrometer and millimeter scale, especially paves a new way toward improving the performance of near field thermal transistor based on insulator-metal transition material.

The Reverse Monte Carlo coupled with Runge-Kutta ray tracing (RMCRKRT) method is developed to solve the radiative heat transfer problems in semitransparent media with graded index. Due to the fact the curved ray trajectory determined by Fermat principle almost can’t be expressed as the simple explicit functions except for a few special refractive index distributions. In RMCRKRT, the Runge-Kutta ray tracing technique is employed to obtain the ray trajectory numerical solution of graded index medium, and the Reverse Monte Carlo method is employed to solve the radiative heat transfer problems. The correctness and accuracy of RMCRKRT is validated by comparing with the benchmark numerical solutions. The diffuse, specular and bidirectional reflectance distribution function (BRDF) surfaces are considered, and the effects of diffuse, specular and BRDF surfaces on radiative heat transfer are investigated by using the RMCRKRT developed in this work. Calculation results show that significant differences exist in the radiative intensity under different boundary conditions. Therefore, BRDF should be used instead of diffuse reflection and mirror reflection in practical application.

The manipulation of microscale fluid has been widely used in biology, medicine, and chemistry. However, the traditional control systems are relatively large, complex, and costly. Optical driving micro- and nanofluid is a new trend of microfluidics, which combines the advantages of both optics and microfluidics in the micro-nano scale. In the present work, we investigated a method to drive microfluid by taking advantage of the localized surface plasmon resonance effect of gold nanoparticles, which can convert optical energy to fluid motion. First, numerical simulation was carried out to calculate the electromagnetic, temperature, and flow field around laser-irradiated gold nanoparticles. Then, the simplified heat source condition was verified. The nanoparticle array was regarded as heat source to induce convection flow. The influence of the spacing and number of nanoparticles in array was investigated. On this basis, the structural parameters of nanoparticle array that can be used to regulate the velocity of microfluidic were obtained.

An improved integral equation method based on radiation distribution factors (RDFIEM) is developed to solve the radiative intensity with high directional resolution in semitransparent media with graded refractive index (GRI). The radiation distribution factors (RDFs) are solved by using the reverse Monte Carlo curved ray tracing technique (RMCCRT). Based on the RDFs, the source terms along the curved ray path decided by the Fermat principle are predicted accurately. Then, the integral equation method (IEM) is employed to solve the radiative intensity through integral along the curved ray path. One-dimensional and two-dimensional test cases are done, and all the results show that the RDFIEM is not only flexible in treating directional radiation problems but also very accurate and efficient.

Optofluidic control is emerging as a promising tool in wide applications like pharmaceutics, chemistry, energy, and biology, due to the development of micro-fluidic chip. They usually require complex optical set-ups or special materials for working fluid or micro-channel. In the present work, a simple optofluidic control method using light illuminated gold nanobrick array in a distributary micro-channel was proposed. The plasmonic nanostructure acts like a microvalve which can be tuned continuously by manipulating the illumination light polarization state or intensity. Two different types of micro-channels were studied to examine the performance of the proposed method. It is found that for h-shaped micro-channel, the mass flow rate can be controlled by simply adjusting the intensity of the incident monochromatic light. As for the Y-shaped micro-channel, the manipulation of mass flow rate is achieved by changing the polarization state of the incident light. On this basis, the detailed influence factor of the light controlled microvalve is investigated to optimize its performance.

Due to the large computational time caused by complex computational process of the existing inversion algorithm, real-time reconstruction of high-magnitude time-dependent heat flux in graded index media is quite challenging. In this study, based on hybrid technology of the Kalman filter and recursive least-square estimator (KF-RLSE), the real time reconstructed high-magnitude time-varying heat flux on graded index media surface, and the measurement information comes from the opposite side of the media. The ideal participating media, which is assumed to be isotropic scattering, constant thermophysical properties, and opaque and diffuse gray boundary, is employed to verify the reliability and validity of the proposed. All the reconstruction results show that the KF-RLSE algorithm can effectively reconstruct the boundary heat flux regardless of the positive or negative gradient of the refractive index. When the refractive index of each position increases or reduces, the transient heat flux on the surface can still be predicted effectively and acceptably. Furthermore, effects of different parameters on the accuracy and stability of the estimated results are also investigated. The reconstructed results show that the time-dependent heat flux can still be effectively reconstructed even when the measurement noise does not match its covariance. Meanwhile, the accuracy of the reconstruction results improves with the decrease of measurement noise covariance when the measurement noise distribution is fixed in a curtain range.

The noninvasive reconstruction of optical parameter fields in participating phantom is investigated experimentally based on the measured time-resolved radiative signals, which are detected by the time-correlated single-photon counting system. The discrete ordinate method is employed to solve the forward transient radiative transfer equation to simulate the time-resolved radiative transfer in the physical phantom exposed to ultra-short pulse laser irradiation. On top of that, the sequential quadratic programming algorithm based on the generalized Gaussian Markov random field is applied to solve the inverse problem. To improve the computational efficiency, an adjoint equation technique is adopted to determine the gradient of objective function. Good agreement was obtained between the predicted optical parameter fields and realistic ones. All the reconstruction results demonstrate that the proposed reconstruction methods are proved to be accurate and efficient experimentally.

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed.

Corneal laser surgery is a widely used method for the treatment of ocular myopia, hyperopia, and astigmatism. Although it is a well-established technique, the photothermal properties of the cornea are often overlooked, causing unexpected changes in temperature during laser irradiation. Therefore, there is a need for further investigation of the temperature response of the cornea under laser irradiation. In the present work, a photothermal corneal numerical model is presented, assuming the stratification of the cornea with laser ablation in an uncoagulated layer, a coagulated layer, a dehydrating layer, a dried layer, and a carbonized layer. The modified Pennes' bioheat transfer equation and Lambert-Beer's law are applied to simulate heat transfer in the corneal tissue during laser irradiation. And the corneal dynamic photothermal parameters are considered in the proposed model. The central surface temperature, the boundary and thickness of each layer, and the thermal damage during laser irradiation are investigated. From the model, it was found that in the steady-state process, the thickness of the coagulated layer was 2.6, 14.4, and 52.4 times larger than that of the dehydrating layer, the dried layer, and the carbonized layer, respectively. The thickness of the corneal thermal damage gradually increased, and reached a peak of 0.196 mm at about 18.2 ms. Subsequently, it sharply decreased by 0.01 mm before stabilizing. On this basis, the influence of laser intensity is investigated. The parametric investigation and analysis presented provide a theoretical basis for corneal laser surgery, which can be used to improve our understanding of laser-tissue surgery.

In this letter, based on the quantum Hall regime of magneto-optical graphene, we have theoretically investigated the coupling of magnetoplasmon polaritons (MPP) to surface phonon polaritons (SPhPs) by investigating the radiative heat transfer between two graphene-coated SiO2 slabs. By applying an external magnetic field, the separated branches of intraband and interband MPP can both couple with SPhPs to form tunable modes, which remould the energy transport of the system. The heat transfer mechanism is completely changed from enhancement to attenuation due to the strong coupling, and the thermal stealthy is realized for the graphene. The letter has great significance for the graphene-based magneto-optical devices., 4 pages, 4 figures

Nowadays, the requirement for achieving dynamic radiative cooling is more and more intense, so a cooling system is proposed and developed to meet the demand in this paper. This cooling system is composed of a filter and a periodic trapezoidal VO

In this work, we have predicted a giant thermal magnetoresistance for the thermal photon transport based on the tunable magnetoplasmon of graphene. By applying an external magnetic field, we find that the heat flux can be modulated by approximately three orders of magnitude. Accordingly, negative and giant relative thermal magnetoresistance ratios are both achieved for magnetic fields with a maximum strength of 4 Tesla. This effect is mainly caused by the suppression and enhancement of scattering interactions mediated by graphene magnetoplasmon. Specifically, it has never been achieved before for nanoparticles, which have no response to magnetic fields. The effect is remarkable at these reasonable strengths of fields, and thus has considerable significance for the real-life applications. It is also expected to enable technological advances for the thermal measurement-based magnetic sensor and magnetically thermal management., 14 pages, 4 figures

International audience; In this Letter, active control of near-field radiative heat transfer (NFRHT) between two isotropic materials is realized by a coating-twisting method. The two slabs are coated with graphene gratings, and then the NFRHT can be not only enhanced but also weakened, by tuning the twisted angle between the two gratings. The physical mechanism is attributed to the modes coupled by the graphene gratings and the isotropic material, which can vary with the twisted angle. The proposed method is also applicable for other kinds of anisotropic films and may provide a way to realize high-precision nanoscale thermal management, nimble thermal communications, and thermal switch.

In this paper, magnetoplasmonic manipulation of near-field radiative heat transfer (NFRHT) is realized using two twisted graphene gratings. As a result of the quantum Hall regime of magneto-optical graphene and the grating effect, three types of graphene surface plasmon polaritons (SPPs) modes are observed in the system: near-zero modes, high-frequency hyperbolic modes, and elliptic modes. The elliptic SPPs modes, which are caused by the combined effect of magnetic field and grating, are observed in the graphene grating system for the first time. In addition, the near-zero modes can be greatly enhanced by the combined effect grating and magnetic field, rendering graphene devices promising for thermal communication at ultra-low frequency. In particular, the near-zero modes result in a unique enhancement region of heat transfer, no matter for any twisted angle between gratings. The combined effect of grating and magnetic field is investigated simultaneously. By changing the strength of magnetic field, the positions and intensities of the modes can be modulated, and hence the NFRHT can be tuned accordingly, no matter for parallel or twisted graphene gratings. The magnetic field endows the grating action (graphene filling factors and twisted angles) with a higher modulation ability to modulate the NFRHT compared with zero-field. Moreover, the modulation ability of twist can be tuned by the magnetic field at different twisted angles. In sum, the combined effect of magnetic field and grating provides a tunable way to realize the energy modulation or multi-frequency thermal communications related to graphene devices.

The photothermal characteristics of semi-transparent medium are the basis for describing the transmission process of photothermal radiation. Therefore, the related measurement researches have important application value for the non-destructive testing of high-tech module. In the present work, an infrared thermal wave radar imaging (TWRI) technique combined with sequential quadratic programming (SQP) algorithm was proposed for simultaneous reconstruction of the photothermal property distributions in 2D semi-transparent materials containing defects. The Fourier transform, Hilbert transform, and the Chirp lock-in correlation algorithm were applied and the results of the amplitude and phase information of the temperature signals were compared, so as to identify the position of internal defects, which reduced the number of parameters needed to reconstruct. Then, the SQP algorithm was introduced to simultaneously reconstruct the absorption coefficient and thermal conductivity coefficient distributions in the medium. The TWRI-SQP technique absorbs the advantages of fast identification of defects by TWRI technique and the accurate reconstruction of the photothermal properties of the semi-transparent materials by SQP algorithm. The number of internal defects, geometric dimensioning, shape, and photothermal properties can be reconstructed accurately. The results show that this method is practical and robust to detect the photothermal properties of 2D semi-transparent materials with defects.

Temperature and concentration distributions are essential parameters for understanding physicochemical processes in the in-cylinder/in-chamber engine and developing efficient combustion technology. However, due to the limited measurement space in the combustion chamber and the strong vibration and flame emission during the operation of the equipment, it is difficult to measure the multi-physical field by the traditional measurement technology. Tomographic laser absorption reconstruction based on laser absorption spectroscopy is a promising diagnostic for mapping the temperature and concentration images because it reduces the need for optical access. Since the inherently ill-posedness of the tomographic inverse problem leads to artifacts, low accuracy, poor robustness, and inability to visualize dynamic flow boundaries in the measured temperature and concentration images, the reconstructed algorithm must contain additional information to promote the presumed solution attributes. To address these problems, we propose a novel algorithm called Gaussian parametric level set method coupled with regularized Landweber to obtain the posterior topological structure of the turbulent reaction flow, which can compensate for the lack of a priori knowledge of the tomographic inverse problem. Numerical validation and algorithm comparison are performed using absorption spectra of water vapor at 7153.748 cm−1 and 7154.353 cm−1 and multi-angle parallel beam arrangement. The proposed approach significantly improves reconstructed quality and has excellent robustness. Even if the sparsely sampled data contains 10% random noise, the root mean square error is still within 0.12 in the temperature range 296 - 1600 K, approximately 50% of the error of the classical tomography methods. And the standard deviation is about 10% of the reconstructed results of Landweber. The proposed new method can obtain clear dynamic flow field boundaries, eliminate artifacts, improve measurement accuracy and robustness, and its applicability in harsh environments will benefit the supervision of combustion equipment, fuel utilization, pollutant control and optimal design of new combustion systems.

Estimation of space-dependent thermal conductivity and absorption coefficient in a one-dimensional participating medium was investigated in this study. For the forward problem, discrete ordinate method was used to solve the radiative transfer equation. The conduction-radiation energy equation was decoupled by the radiative source term and solved by the finite difference method. Lie-group shooting method (LGSM) was introduced to retrieve the space-dependent thermal conductivity and absorption coefficient from the coupled optical (radiative intensity profile) and thermal (temperature profile) information. LGSM was employed to solve the inverse problem of combined conduction-radiation heat transfer for the first time. This method does not require priori information about the functional form of thermal properties and initial guesses can be selected randomly. Numerical examples of the thermal conductivity and absorption coefficient under different distributions were examined to show that the new approach is highly accurate and efficient, even for identifying highly discontinuous and oscillatory parameters. The estimation was conducted in the presence of the random measurement noise. However, the estimated results still showed high accuracy, robustness and low susceptibility to the shape of property distributions.

A numerical model concerning the phase transition and temperature distribution of matrix media around plasmonic nanoparticle assemblies under illumination of monochromatic light is investigated. The critical intensity of incident light for isolated nanosphere, nanodisk, and nanorod to melting the surrounding ice is obtained. For nanospheres and nanodisk dimers, enhancement of photothermal conversion (PTC) is observed. Besides, the phase transition of matrix media is not only influenced by PTC, but also the configuration of nanoparticle assemblies. In particular, we demonstrate that for nanorod dimers arranged end-to-end and side-to-side, the dissipation of light for two nanorods are the same in every polarization angles. The change of polarization angle only influence the overall light-to-heat conversion of the dimer. However, for L geometry nanorod dimer, the dissipation of light are different for the two nanorods. This present a way to manipulate and precisely control of temperature distribution and phase transition in nanoscale by tuning the light polarization angles.

A hybrid algorithm of extended Kalman filtering and recursive least-square estimator (EKF-RLSE) is developed to solve the nonlinear inverse heat conduction-radiation problems in participating medium. By measuring the time-resolved temperature signals from a thermocouple installed on the other boundary of the participating medium, the proposed EKF-RLSE algorithm can realize real-time retrieval of the time-varying heat flux on the medium surface. The influence factors of the sampling interval, measurement and process covariance, forgetting factor, and medium thickness on the reconstruction accuracy and stability of the EKF-RLSE are investigated by numerical experiments. All the estimated results show that the present algorithm is robust and accurate in real-time estimating transient heat flux. If the nonlinear coupled conduction and radiation heat transfer problems is regard as the linear problems, the unknown time-dependent heat flux predicted by the KF-RLSE are also illustrated in this work.

A combined lock-in thermography (LIT) technique with sequential quadratic programming (SQP) algorithm is proposed for simultaneous reconstruction of the optical and thermal property distributions in a 2D semitransparent medium. The LIT technique is applied to identify the positions of inclusions in media, in which the discrete correlation method is employed to extract the amplitude and phase angle of thermal wave signals. The SQP algorithm is employed as the main optimization method to simultaneously reconstruct the absorption coefficient, scattering coefficient, and thermal conductivity distributions in the medium. The LIT-SQP technique combines the advantages of rapid identification of inclusion position of the LIT technique and accurate reconstruction of inclusion properties of the SQP algorithm. The inclusions of various positions, shapes, numbers, and sizes are investigated. All the inclusions can be accurately reconstructed by the proposed LIT-SQP technique, which is proved to be more efficient and accurate than the LIT technique and the SQP algorithm alone.

PEGylation is widely applied as a surface modification method for nanoparticles in biomedical applications to improve their biological properties, including biocompatibility and immunogenicity. In most of its biomedical applications, nanoparticles are served as optical or thermal contrast agents. Therefore, the impact of poly (ethylene glycol) (PEG) coating thickness on the optical and thermal properties of nanoparticles needs to be further investigated. In the present work, we studied two kinds of commonly used nanoparticles, including nanosphere and nanorod. The temperature and electric fields are obtained for nanoparticles with different PEG coating thicknesses. It is found that the change of PEG coating thickness on gold nanospheres only has impact on the absolute value of maximum absorption and scattering efficiencies, which barely influences the LSPR wavelength λmax and other optical and thermal characteristics. In contrast, for nanorod, the maximum efficiencies are barely influenced by the variation of PEG coating thickness. On the other hand, the localized surface plasmon resonance wavelength has an evident red shift with the increasing of PEG coating thickness. The maximum absorption efficiency is a way to evaluate the energy dissipation rate, which decides the scale of the heat source induced by nanoparticles. These findings are crucial for the accurate prediction of optical and thermal properties of nanoparticles in biomedical application. The present work also presents a possible way to manipulate the optical and thermal behaviors of nanoparticles in the application of biomedicine without changing the morphology of nanoparticles.

Thermal therapy is a very promising method for cancer treatment, which can be combined with chemotherapy, radiotherapy and other programs for enhanced cancer treatment. In order to get a better effect of thermal therapy in clinical applications, optimal internal temperature distribution of the tissue embedded with gold nanoparticles (GNPs) for enhanced thermal therapy was investigated in present research. The Monte Carlo method was applied to calculate the heat generation of the tissue embedded with GNPs irradiated by continuous laser. To have a better insight into the physical problem of heat transfer in tissues, the two-energy equation was employed to calculate the temperature distribution of the tissue in the process of GNPs enhanced therapy. The Arrhenius equation was applied to evaluate the degree of permanent thermal damage. A parametric study was performed to investigate the influence factors on the tissue internal temperature distribution, such as incident light intensity, the GNPs volume fraction, the periodic heating and cooling time, and the incident light position. It was found that period heating and cooling strategy can effectively avoid overheating of skin surface and heat damage of healthy tissue. Lower GNPs volume fraction will be better for the heat source distribution. Furthermore, the ring heating strategy is superior to the central heating strategy in the treatment effect. All the analysis provides theoretical guidance for optimal temperature control of tissue embedded with GNP for enhanced thermal therapy.

An accurate and stable identification technique is developed to retrieve the optical constants and particle size distributions (PSDs) of particle system simultaneously from the multi-wavelength scattering–transmittance signals by using the improved quantum particle swarm optimization algorithm. The Mie theory are selected to calculate the directional laser intensity scattered by particles and the spectral collimated transmittance. The sensitivity and objective function distribution analysis were conducted to evaluate the mathematical properties (i.e. ill-posedness and multimodality) of the inverse problems under three different optical signals combinations (i.e. the single-wavelength multi-angle light scattering signal, the single-wavelength multi-angle light scattering and spectral transmittance signal, and the multi-angle light scattering and spectral transmittance signal). It was found the best global convergence performance can be obtained by using the multi-wavelength scattering–transmittance signals. Meanwhile, the present technique have been tested under different Gaussian measurement noise to prove its feasibility in a large solution space. All the results show that the inverse technique by using multi-wavelength scattering–transmittance signals is effective and suitable for retrieving the optical complex refractive indices and PSD of particle system simultaneously.

This study proposes an inverse heat transfer process to estimate the time-dependent melting thickness and input heat flux in participating media. The inversion algorithm comprises Kalman filter technique and recursive least squares estimator (KF-RLSE) for on-line and real time predict the input heat flux and position of the solid–liquid phase front from the temperature history by using a sensor mounted on the surface. The KF technique is employed to generate a regression equation between the biased residual innovation and the unknown thermal coefficient. The recursive least squares estimator is utilized in the regression equation to predict time-varying phase front position and input heat flux. Based on simulation results, the proposed algorithm exhibits good reconstruction capability. The influences of forgetting factor, process noise, measurement noise, and absorption coefficient on the stability and precision of the retrieval results are also analyzed. The accuracy of the estimation increases with decrease of the distance between the acting surface of input heat flux and the sensor location. Furthermore, the present algorithm cannot accurately determine the abrupt heat flux with large time step.

A hybrid stochastic particle swarm optimization (SPSO) and sequential quadratic programming (SQP) algorithm is proposed to estimate the space-dependent absorption coefficient (κa) and scattering coefficient (κs) fields simultaneously. The retrieval results of SQP algorithm are highly affected by the initial guess, and thus, SPSO is adopted to obtain the initial value of SQP because of its global search capability and lesser dependence on initial guess. The regularization term based on the generalized Gaussian Markov random field model is employed to overcome the ill-posed characteristic of the inverse problem. The inverse coupled radiation-conduction problem is solved in a one-dimensional participating medium with uniform refractive and graded refractive indices exposed to a pulse laser, respectively. All the results show that the separate space-dependent κa and κs fields can be estimated simultaneously and accurately from the coupled optical and thermal signals.

High-directional resolution radiation intensity (RI) can provide substantial measurement information inside the participating medium, such as the distribution of temperature and physical properties. Therefore, efficiently and accurately solving the radiative transfer equation (RTE) to obtain RI in any direction is the key and challenge of target-detection and inverse-radiation problems. In our previous works [ 1 , 2 ], the integral equation method based on the radiation distribution factor (RDFIEM) was proposed to accurately obtain an arbitrary directional RI. To overcome the inefficiency of the RDFIEM in building a radiation distribution factor (RDF) database from the time-consuming reverse Monte Carlo (RMC) method, a method of equation-solving RDFIEM (ES-RDFIEM) was improved and developed to obtain the solution of RTE in a three-dimensional (3D) anisotropic scattering medium in this work, which can effectively avoid the stochastic ray-tracing process of the traditional RMC method and obtain the value of RDF by solving linear equations directly. The mathematical principles and formulae of ES-RDFIEM are introduced and deduced in detail, whose core idea is to suppose a specified element with a unit blackbody emission, whereas the remaining elements have no energy emission. Subsequently, the linear equations for the RDF, which are only concerned with the physical properties and geometric factors of the radiative system, can be constructed and solved. The computational accuracy and efficiency of ES-RDFIEM and RMC in several cases with different parameters were comprehensively compared. The results showed an excellent agreement between the RDF values and the RI calculated by the two methods. The computational efficiency of ES-RDFIEM was significantly improved compared to RMC, which was almost unaffected by radiation properties.

Once the geometry and physical properties of the radiation system are determined, the integral equation method based on radiation distribution factor (RDFIEM) can be effectively used to calculate the directional radiation intensity with high resolution. However, there exists an inherent shortcoming of low efficiency for statistical ray sampling since the radiation distribution factors (RDFs) are obtained by reverse Monte Carlo (RMC) method. For overcoming this deficiency, an efficient semi-analytical equation-solving RDFIEM (ES-RDFIEM) method is proposed to solve the RDFs directly. In this paper, the principles and formulas of ES-RDFIEM in 1D radiation system with anisotropic scattering medium are developed in detail. Benefit from that the RDF is only related to the geometry and radiation properties of the medium, it can be assumed that only a certain element n has unit blackbody radiation intensity, while the remaining elements do not emit energy. In this way, a semi-analytical set of linear equations can be constructed to solve the RDF matrix directly. Compared with RMC, multiple cases with different radiative parameters are adopted to examine the performance of ES-RDFIEM. Numerical results show that ES-RDFIEM is in good agreement with RMC in most conditions, especially in isotropic scattering or weak scattering of anisotropic scattering. In addition, ES-RDFIEM can effectively avoid the random errors produced by RMC, making the result of ES-RDFIEM smoother than RMC. More importantly, it only takes a few CPU times for computing all the RDF values by ES-RDFIEM, which is one or two orders of magnitude less than that of RMC, and its computational efficiency is almost independent of the radiation properties, showing an excellent efficiency.

An efficient equation-solving integral equation method (ES-RDFIEM) is proposed to calculate the radiation distribution factor (RDF), which was required to be solved by the time-consuming ray tracing with reverse Monte Carlo method (RMC) in the past. Since the RDF is only related to the radiative properties and geometry of the medium, it can be assumed that the blackbody radiation intensity of a certain element n is 1, while there is no emission from the remaining elements of the radiation system. In this way, the RD(j, n) (j = 1, 2,…, N) can be obtained by solving N × N equations based on the equivalent relationship of incident radiation. As a result, the stochastic statistical process of the RMC can be effectively avoided. The numerical results show that the RDF and radiation intensity calculated by ES-RDFIEM and RMC are highly consistent for one-dimension and two-dimension emitting, absorbing, isotropic scattering medium with transparent boundary. The CPU time for ES-RDFIEM to calculate the RDFs of the whole radiation system is much shorter than RMC under the same condition, which, in addition, does not increase with the increase of scattering albedo or optical thickness. With the same calculation accuracy as RMC, ES-RDFIEM has the significant advantage of high efficiency, which shows great potential in solving inverse radiation problems.

A novel bio-inspired swarm algorithm, social spider optimization (SSO), is introduced to solve the inverse transient radiation and coupled radiation–conduction problems for the first time. Based on the original model, five improved SSO (ISSO) algorithms are developed to enhance search ability and convergence velocity. The sensitivity analysis of measured signals with respect to the physical parameters of the medium are described. After which, the SSO and ISSO algorithms are applied to solve the inverse estimation problems in a one-dimensional participating medium. Two cases concerns radiative transfer problems are investigated, in which the radiative source term, extinction coefficient, scattering albedo, and scattering symmetry factor are reconstructed. Furthermore, the coupled radiation–conduction heat transfer model is considered and the main parameters such as the conduction–radiation parameter, boundary emissivity, and scattering albedo are retrieved. All retrieval results show that SSO-based algorithms are robust and effective in solving inverse estimation problems even with measurement errors. Findings also show that the proposed ISSO algorithms are superior to the original SSO model in terms of computational accuracy and convergence velocity.

The hybrid social spider optimization (SSO) algorithm is employed to estimate the thermophysical properties of phase change material for the first time. The enthalpy formulation of governing energy equation is solved by using the finite volume method and the radiative transfer equation is computed by using the discrete ordinate method. Two hybrid algorithms, namely the differential evolution-SSO (DE-SSO) and simplex method-SSO (SM-SSO) algorithms, are proposed to improve the search ability and computation efficiency of the inverse technique. The scattering albedo, boundary emissivity, conduction-radiation parameter, and Stefan number are estimated simultaneously. All the results demonstrate that the hybrid SSO algorithms are robust and effective for solving simultaneous estimation tasks even with measurement noise and DE-SSO algorithm achieves the best performance in terms of computational accuracy and convergence velocity. Furthermore, the effect of population size on the reconstruction results is also investigated and a suggested value is given.

Accurate measurement of thermal and optical properties of semitransparent media is important in the control of temperature distribution in the production and fabrication process. In this study, a s...

The investigation of nanoparticle's optical properties is crucial for their biological and therapeutic applications. In the present work, a promising type of gold nanoparticle, the triangular prism which was reported to have multipolar surface plasmon peaks, was studied. The Plasmon ruler effect of nanotriangle dimers was observed and investigated for the first time. Well-defined trends of the extinction spectra maxima, which have a linear correlation with the triangle edge length, for lower order extinction corresponding to in-plane mode, were observed. On this basis, the optical property of nanotriangle dimers with different arrangements, including two nanotriangles aligned side-by-side, bottom-to-bottom, and in line, were studied. For the side-by-side arrangement, an additional peak was generated on the red shift side of the peak corresponding to dipole mode. When the distance between two prisms was scaled by the triangular side length, the relative plasmon shift can be approximated as an exponential function of the relative offset distance. Moreover, for dimers with nanotriangles arranged in line, there was a global blue shift of the extinction spectra with the approaching of two particles, including the higher order mode extinction. An interesting phenomenon was found for dimers with two nanotriangles aligned bottom-to-bottom. The resonance band split into two bands with the decreasing of the offset distance.

This paper presents a non–intrusive inverse heat transfer procedure for predicting the time–varying heat flux on the surface of semitransparent media. Inverse radiation–conduction problems were analyzed using a Kalman filter coupled with a recursive least–square estimator (KF–RLSE). Since its unique capability to make fast predictions, KF–RLSE can be easily integrated to existing real–time control systems of industrial facilities. The performance of KF–RLSE was examined thoroughly in a series of numerical simulations in two semitransparent materials (i.e. the glass with black coating and the ceramic of zirconium dioxide ZrO 2 ) to extract the time–varying surface heat flux on–line from the measured temperature history at boundary. Results showed that the proposed method can predict the unknown boundary flux with an acceptable error. The influence of different parameters on the accuracy and stability of the predicted heat flux was also investigated. Results indicated that the sensor location, process noise covariance and absorption coefficient exerted stronger effects on retrieval results compared with other parameters.

A reconstruction scheme for the fluorescence tomography is investigated based on the time-domain radiative transfer equation (TD-RTE). Two coupled TD-RTEs, which can provide considerable measurement data, are used as the forward model and solved by the discrete ordinate method. The sequential quadratic programming (SQP) is employed to build the reconstruction scheme for solving the inverse problem. The gradient of objective function is calculated efficiently by the adjoint equation technique. Considering the ill-posed nature of the inverse problem, the regularization term based on the generalized Gaussian Markov random field (GGMRF) model is adopted to enhance the reconstructed image. Influence of the initial guess, contrast, noisy data, and shape of the fluorescent target are analyzed. Simulated results show that the proposed algorithm performs efficiently and accurately on reconstructing the distribution of the fluorescence yield.