The effects of Ag and Au nanoparticles for resonant excitations on the spectral enhancement of nanoparticle enhanced laser-induced breakdown spectroscopy

Abstract

We have investigated the spectral emission of Cu in an aqueous solution with Ag and Au nanoparticle (NP) enhanced laser-induced breakdown spectroscopy. Solutions of metal NPs and CuSO4 are prepared through the liquid–solid conversion process. Optical responses of Ag and Au NPs are simulated by a numerical calculation of the Maxwell equation using Yee unit cell, i.e., finite difference time domain. Absorption spectra of Ag and Au NPs are calculated, showing the near and out of resonant modes of localized surface plasmon (LSP) with 532 and 1064 nm light, respectively. A stronger field enhancement factor for Ag NPs can be achieved with 532 nm light due to the near resonant excitation of LSP and inter-band transition of electrons in Au. Strongest element emission can be achieved with Ag NPs under the illumination of 532 nm laser pulse, consistent with the simulated electric field enhancement factor. The spectral intensity can be improved by one magnitude of order with the resonant excitation of LSP for Ag NPs than that without metal NPs. The enhanced spectral emission is mainly caused by massive electrons emitted through field emission with 532 nm laser pulse and multiphoton photoemission process with 1064 nm laser pulse, which is judged by the Keldysh parameter. Electron densities in the regime of 2 × 1017 cm−3 are calculated by Stark broadening at Hα-line 656.27 nm with all the substrates. These findings reveal the relationship between the resonant mode and spectral enhancement, which is of great value for trace element detection in liquid.