Light Absorption Analysis and Optimization of Ag@TiO2 Core-Shell Nanospheroid and Nanorod.

For photothermal therapy of cancer, it is necessary to find Ag @TiO2 core-shell nanoparticles that can freely tune the resonance wavelength within the near-infrared biological window. In this paper, the finite element method and the size-dependent refractive index of metal nanoparticles were used to theoretically investigate the effects of the core material, core length, core aspect ratio, shell thickness, refractive index of the surrounding medium, and the particle orientation on the light absorption properties of Ag@TiO2 core-shell nanospheroid and nanorod. The calculations show that the position and intensity of the light absorption resonance peaks can be freely tuned within the first and second biological windows by changing the above-mentioned parameters. Two laser wavelengths commonly used in photothermal therapy, 808 nm (first biological window) and 1064 nm (second biological window), were selected to optimize the core length and aspect ratio of Ag@TiO2 core-shell nanospheroid and nanorod. It was found that the optimized Ag@TiO2 core-shell nanospheroid has a stronger light absorption capacity at the laser wavelengths of 808 nm and 1064 nm. The optimized Ag@TiO2 core-shell nanoparticles can be used as ideal therapeutic agents in photothermal therapy.

Multiplexed rectangular dielectric gratings with multiple narrow-band refractive index filtering and sensing.

We propose a low-loss compound structure consisting of a multiplexed rectangular dielectric grating and a waveguide layer, which can function as multi-band optical filters and sensors in TE and TM polarization by utilizing the resonant mode of the waveguide (WG) and the hybrid SP, respectively. By manipulating the parameters and subsequently constraining the local density of multi-resonant modes to several distinct resonant wavelengths, we propose a novel category of highly sensitive refractive index sensing platforms. Spectral shifts ranging from 110 to 131 nm/RIU with FOM of (22, 26.2)/RIU under TE polarization and 80 to 114 nm/RIU with FOM of (5.7, 8.1)/RIU under TM polarization can be accurately discerned for multiple individual analytes across a broad spectral range. The proposed structures offer enhanced flexibility in the design of structures across a wide spectral range, catering to various potential applications in multi-band optical filters, sensors, and photodetectors.

Inversion of the Complex Refractive Index of Au-Ag Alloy Nanospheres Based on the Contour Intersection Method.

The contour intersection method is a new method used to invert the complex refractive index of small particles. Research has yet to be reported on using this method to invert the complex refractive index of nanoparticles. This paper reports the feasibility and reliability of the contour intersection method in the inversion of the complex refractive index of nanoparticles using Au-Ag alloy nanospheres. The Mie theory and the size-dependent dielectric function are used to calculate the light scattering and absorption efficiency of Au-Ag alloy nanospheres corresponding to the complex refractive index. The complex refractive index of the particles is obtained by inversion with the contour intersection method. The backscattering efficiency constraint method is used to determine the unique solution when multiple valid solutions from the contour intersection method appear. The effects of the Au component percentage, particle size, and measurement errors on the inversion results are quantitatively analyzed. Finally, the inversion accuracy is compared and analyzed with the traditional iterative method. The results show that as long as the light scattering efficiency, light absorption efficiency, and backscattering efficiency of Au nanospheres can be measured, the accurate complex refractive index can also be calculated by inversion using the contour intersection method. The accuracy of the inversion results can be ensured when the measurement error is less than 5%. The results of inversion using the contour intersection method are better than those of the iterative methods under the same conditions. This study provides a simple and reliable inversion method for measuring the complex refractive index of Au-Ag alloy nanospheres.

PyMieLab_V1.0: A software for calculating the light scattering and absorption of spherical particles.

Light scattering and absorption by small particles are widely used in fields such as biomedicine, information technology, and energy technology. However, their theoretical study requires not only a high level of knowledge in electromagnetism but also a high level of computer programming skills. To solve this problem, a software called PyMieLab (https://gitlab.com/Climb12/pymielab.git) for calculating the light scattering and absorption of spherical particles has been developed based on Mie theory. This software is interactive, versatile, visual, flexible, and scalable. It has a friendly graphical user interface and can be used as a numerical simulation platform for scientific research, as well as provides a rich database of particle refractive indices. Moreover, it offers a reliable research platform for discovering new optical properties of specific materials and exploring materials with better optical properties in related fields. This paper describes in detail the theoretical basis, the graphical user interface, the calculation functions, the operational and procedural processes, the features, and the numerical verification of the software. It illustrates the application value of the software with two simulation examples.

Retrieval of Size Distribution and Concentration of Au-Ag Alloy Nanospheroids by Spectral Extinction Method.

In order to monitor the synthesis processes or characterize nanoparticles for application, a new method that allows in situ determination of the two-dimensional size distribution and concentration of Au-Ag alloy nanospheroids, based on their extinction spectrum, is developed. Non-negative Tikhonov regularization and T-matrix method were used to solve the inverse problem. The effects of the two-dimensional size steps, wavelength range, and measurement errors of extinction spectrum on the retrieval results were analyzed to verify the feasibility and accuracy of the retrieval algorithm. Through comparative analysis, the size steps and wavelength range that make the retrieval error smaller are found. After adding 0.1% random noise to the extinction spectrum, a small variation in the retrieval error of the mean size is observed. The results showed that the error of the mean size is smaller than 2% and the error of the concentration is smaller than 3%. This method is simple, fast, cheap, nondestructive, and can be done in situ during the growth process of nanoparticles.

Optical absorption analysis and optimization of gold nanoshells.

Gold nanoshells, consisting of a nanoscale dielectric core coated with an ultrathin gold shell, have wide biomedical applications due to their strong optical absorption properties. Gold nanoshells with high absorption efficiencies can help to improve these applications. We investigate the effects of the core material, surrounding medium, core radius, and shell thickness on the absorption spectra of gold nanoshells by using the light-scattering theory of a coated sphere. Our results show that the position and intensity of the absorption peak can be tuned over a wide range by manipulating the above-mentioned parameters. We also obtain the optimal absorption efficiencies and structures of hollow gold nanoshells and gold-coated SiO(2) nanoshells embedded in water at wavelengths of 800, 820, and 1064 nm. The results show that hollow gold nanoshells possess the maximum absorption efficiency (5.42) at a wavelength of 800 nm; the corresponding shell thickness and core radius are 4.8 and 38.9 nm, respectively. They can be used as the ideal photothermal conversation particles for biomedical applications.