Influence of the temperature-dependent dielectric constant on the photoacoustic effect of gold nanospheres.

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.

Dependence of the Nonlinear Photoacoustic Response of Gold Nanoparticles on the Heat-Transfer Process.

Photoacoustic (PA) imaging using the nonlinear PA response of gold nanoparticles (GNPs) can effectively attenuate the interference from background noise caused by biomolecules (e.g., hemoglobin), thus offering a highly potential noninvasive biomedical imaging method. However, the mechanism of the nonlinear PA response of GNPs based on the thermal expansion mechanism, especially the effect of heat-transfer ability, still lacks quantitative investigation. Therefore, this work investigated the effect of heat-transfer ability on the nonlinear PA response of GNPs using the critical energy and fluence concept, taking into account the Au@SiO2 core-shell nanoparticles (weakened heat transfer) and gold nanochains (enhanced heat transfer). The results showed that the stronger the heat transferability, the smaller the critical energy, indicating that the nonlinear PA response of different nanoparticles cannot be contrasted directly through the critical energy. Moreover, the critical fluence can directly contrast the proportion of nonlinear components in the PA response of different GNPs as governed by the combined effect of heat transferability and photothermal conversion ability.

Simultaneous determination of primary particle size distribution and thermal accommodation coefficient of soot aggregates using low-fluence LII.

For the ill-posed inverse problem of LII-based nanoparticle size measurement, recovered primary particle size distribution (PPSD) is sensitive to the uncertainty of LII model parameters. In the absence of reliable prior knowledge, the thermal accommodation coefficient (TAC) and fractal-dependent shielding factor are often required to be inferred simultaneously with the PPSD. In the simplified LII model for low fluence regime, TAC and fractal-dependent shielding factor are combined to define a new fractal-dependent TAC. The present study theoretically verified the feasibility of inferring PPSD and fractal-dependent TAC from the normalized LII signals. Moreover, the inversion is independent of prior knowledge of most full LII model parameters, which is attributed to low laser fluence, normalized signal, and fractal-dependent TAC.

Radiative thermal switch driven by anisotropic black phosphorus plasmons.

Black phosphorus (BP), as a two-dimensional material, has exhibited unique optoelectronic properties due to its anisotropic plasmons. In the present work, we theoretically propose a radiative thermal switch (RTS) composed of BP gratings in the context of near-field radiative heat transfer. The simply mechanical rotation between the gratings enables considerable modulation of radiative heat flux, especially when combined with the use of non-identical parameters, i.e., filling factors and electron densities of BP. Among all the cases including asymmetric BP gratings, symmetric BP gratings, and BP films, we find that the asymmetric BP gratings possess the most excellent switching performance. The optimized switching factors can be as high as 90% with the vacuum separation d=50 nm and higher than 70% even in the far-field regime d=1 µm. The high-performance switching is basically attributed to the rotatable-tunable anisotropic BP plasmons between the asymmetric gratings. Moreover, due to the twisting principle, the RTS can work at a wide range of temperature, which has great advantage over the phase change materials-based RTS. The proposed switching scheme has great significance for the applications in optoelectronic devices and thermal circuits.

Periodic trapezoidal VO2-Ge multilayer absorber for dynamic radiative cooling.

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 VO2-Ge multilayer absorber (VGMA). The filter on the top enables the VGMA to reflect most of the solar irradiation at daytime and the absorptance or emittance of the VGMA is very different in the spectrum band of 8-13 µm for insulating and metallic VO2 due to the phase transition characteristic of VO2. With this cooling system, close-to-zero absorptance in the range of 0.3-2.5 µm and high (low) absorptance from 8 to 13 µm are achieved for metallic (insulating) VO2. Based on changing the temperature and absorptivity or emissivity simultaneously, radiative heat can be transferred dynamically to the outer space. When VO2 is in the insulating phase, the absorption mechanism of the absorber is magnetic resonance and surface plasmon polariton resonance, and broadband high absorptivity is achieved by exciting slowlight waveguide mode at broadband wavelengths when VO2 is in metallic phase. The spectral absorptance characteristics of the absorber in the two phase states are investigated as a function of the layer number and the incident angle of the electromagnetic waves. The results show that the absorber designed is insensitive to the incident angle. Moreover, the net cooling power of the VGMA of metallic VO2 is instantly 4 times more than that of insulating VO2 once the phase change temperature is reached. This work will be beneficial to the advancement of dynamic radiative cooling.

Efficient optical parameter mapping based on time-domain radiative transfer equation combined with parallel programming.

A two-dimensional optical parameter mapping based on the time-domain radiative transfer equation (TD-RTE) is studied in this work. The finite element method with structured and unstructured grids is employed to solve TD-RTE and OpenMP parallel technology is employed to improve the computing efficiency. The sequential quadratic programming algorithm is used as a powerful optimization method to reconstruct absorption and scattering parameter fields and the maximum a posteriori estimation is employed by introducing the regularization term into the objective function to improve the ill-posed inverse problem. In addition, the effects of measurement errors on reconstruction accuracy are investigated thoroughly. All the simulation results demonstrate that the reconstructed scheme we developed is accurate and efficient in optical parameter mapping based on TD-RTE.

Near-field radiative heat transfer in multilayered graphene system considering equilibrium temperature distribution.

In the present work, the near-field radiative heat transfer of a multilayered graphene system is investigated within the framework of the many-body theory. For the first time, the temperature distribution corresponding to the steady state of the system is investigated. Unique temperature steps are observed near both boundaries of the system, especially in the strong near-field regime. By utilizing the effective radiative thermal conductance, the thermal freedom of heat flux in different regions of the system is analyzed quantitatively, and the cause of various temperature distributions is explained accordingly. To characterize the heat transfer ability of the whole system, we evaluate the system with two heat transfer coefficients (HTC), transient heat transfer coefficient (THTC), and steady heat transfer coefficient (SHTC). A unique many-body enhancement is observed, which causes a red-shift of resonance peak corresponding to graphene surface plasmon polaritons. Furthermore, a three-body enhancement of SHTC emerges thanks to the relay effect and the complexity of the system. The regime of heat transport can be tuned by changing the chemical potentials of graphene and undergoes a transition from diffusive to quasi-ballistic transport in the strong near-field regime.

Optimal temperature control of tissue embedded with gold nanoparticles for enhanced thermal therapy based on two-energy equation model.

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.

Application of the sequential quadratic programming algorithm for reconstructing the distribution of optical parameters based on the time-domain radiative transfer equation.

Sequential quadratic programming (SQP) is used as an optimization algorithm to reconstruct the optical parameters based on the time-domain radiative transfer equation (TD-RTE). Numerous time-resolved measurement signals are obtained using the TD-RTE as forward model. For a high computational efficiency, the gradient of objective function is calculated using an adjoint equation technique. SQP algorithm is employed to solve the inverse problem and the regularization term based on the generalized Gaussian Markov random field (GGMRF) model is used to overcome the ill-posed problem. Simulated results show that the proposed reconstruction scheme performs efficiently and accurately.

Fast method of retrieving the asymmetry factor and scattering albedo from the maximum time-resolved reflectance of participating media.

This research presents a parametric study of the time-resolved hemispherical reflectance of a semi-infinite plane-parallel slab of homogeneous, nonemitting, absorbing, and anisotropic scattering medium exposed to a collimated Gaussian pulse. The one-dimensional transient radiative transfer equation was solved by using the finite volume method. The internal reflection at the medium-air interface caused by the mismatch of the refractive indices was considered. In particular, this work focused on the maximum diffuse hemispherical reflectance. Three different optical regions were identified according to the dimensionless pulsewidth ßctp. The correlation between the normalized maximum hemispherical reflectance and ßctp was conformed to the Boltzmann function. The coefficients in the correlating functions of the match and mismatch refractive index cases were fitted as polynomial fitting functions of the single scattering albedo ω and Henyey-Greenstein asymmetric factor g. Thus, ω and g can be simultaneously reconstructed by the semi-empirical correlations without solving the forward model. In conclusion, the proposed method can potentially retrieve the asymmetry factor and single scattering albedo of participating media accurately and efficiently.