Photothermal effects of nanoparticles in liquid media

Abstract

Many organic and inorganic materials like dyes, carbon based nanomaterials and inorganic nanoparticles were found to show strong interaction with laser light. Nanoparticles offer the advantage that they can be designed specifically with respect to their composition, size, shape, and surface chemistry, what gives unique properties to them. Thus, nanoparticles can be used for numerous technical applications as well as in biology and medicine, and they are good to investigate fundamental questions. We evaluate a variety of different kinds of nanoparticles suspended in various solvents regarding their nonlinear attenuation characteristics with respect to nanosecond laser pulses. Generally, in a first step the samples are characterized with respect to their linear optical properties by spectral absorption measurements, their particles' size and size distribution estimated by dynamic light scattering, and their structure analyzed by scanning electron microscopy. Subsequently, we measure the nonlinear attenuation of the samples along the optical axis using our standard experimental setup [1-3]. Additional nonlinear scattering measurements allow us to discriminate against nonlinear absorption and thus to learn about the involved physical processes leading to the nonlinear extinction. Thus by measuring nonlinear scattering and nonlinear attenuation we are able to estimate the nonlinear absorption of the various samples. In order to further improve the nanoparticle's absorption coefficient we have to identify the most important material properties influencing the extinction of the samples. That task is performed by means of a Principal Component Analysis (PCA). Due to the large amount of data the analyses are limited to nanosecond pulses at the wavelength of 532 nm. In parallel, we started to perform numerical calculations to simulate the attenuation of suspended nanoparticles caused by nonlinear scattering. Nonlinear scattering is an induced process resulting from local heating of the nanoparticle and its surrounding. To learn about the details of the thermal heating processes we use a finite elements method to solve the heat transport problem. The simulation provides insight into the temporal size evolution of induced scattering centers. Furthermore, it allows us to calculate the influence of Mie scattering on the attenuation process. We outline, that this method can be used to predict the dependency on various material properties. A comparison of simulated results with experimental ones can lead to a further understanding of the interaction between the different mechanisms involved [4].