Study of optical nonlinearities in laser ablation produced gold nanoparticles through the Z-scan technique

Gold nanoparticles have attracted attention of researchers due to certain unique features like biocompatibility, stability during bioconjugation with other metals or semiconductor materials, expression of surface plasmon resonance phenomenon etc. In the current work, we subject laser ablation produced gold nanoparticles to the study of optical nonlinearities by employing 120 fs, 84 MHz ultrafast laser pulses at 800 nm wavelength. After confirmation of particle size and shape is carried out by Transmission Electron Microscopy, the optical nonlinear absorption and refraction parameters are measured via the popular Z-scan technique. Excitation intensity is varied from 6.23 GW/cm² upto 23.37 GW/cm² for open aperture measurements and from 3.39 GW/cm² to 23.37 GW/cm² for closed aperture measurements. The open aperture results show significant nonlinear absorption effect through an initial two-photon absorption and a subsequent multi-photon absorption phenomenon. The transition from saturable absorption to reverse saturable absorption takes place at an excitation threshold of 17.14 GW/cm². The closed aperture measurements depict a fairly symmetric response upto an excitation intensity of 15.58 GW/cm² after which asymmetry initiates. This could be possibly due to the dominance of nonlinear absorption over nonlinear refraction. The coefficients of nonlinear absorption and nonlinear refraction are in the range of 10^-3 cm/GW and 10^-5 cm/GW respectively. An understanding of these optical nonlinearities enhances their application for biomedical diagnostics, and optical switching and limiting.