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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.

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.

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.