Interaction between ultrashort laser pulses and gold nanoparticles: nanoheater and nanolens effect

Theoretic and experimental results on the heating process and near field localization arising when gold nanoparticles are irradiated by ultrashort laser pulses are presented. The system under consideration consists of nanoparticles with radius of 20, 40, or 100 nm in vacuum or deposited on different substrates. Substrate materials with different dielectric properties are used to sense and visualize the nanoparticle heating and near electromagnetic field distribution. The theoretic analysis is based on two-temperature heat model for estimation of the nanoparticle temperature and Finite Difference Time Domain (FDTD) method for description of the near field distribution in the vicinity of the particles. It is found that at even moderate laser fluences, particle temperature can reach a value sufficient for bubble formation in biological tissues. The analysis of the near field distribution shows that when particle is deposited on substrate surface, the dielectric properties of the substrate define the localization and enhancement of the near field intensity. The efficiency of this process determines the contribution of particle heating or near field intensity enhancement in the surface modification process. The localization of the near field intensity in the vicinity of the contact point between the particle and substrate is proved experimentally for metal and silicon substrates, where the experimentally obtained surface modifications resemble the theoretically predicted intensity distribution.