In-situ characterization of Fs laser shaping of quasi-percolated Ag nanoparticle layers embedded in amorphous Al<inf>2</inf>O<inf>3</inf>

Abstract

Summary form only given. Metal nanoparticles (NPs) embedded in dielectric materials are artificial material systems with exciting optical properties. In particular, surface plasmon (SP) resonances observed in these materials, which are related to the size, shape and separation of the NPs, amongst other factors; enable the design of optical devices with a tailored spectral response, provided that a good control over the NP morphology is achieved during their synthesis. Among different fabrication methods, Pulsed Laser Deposition (PLD) is a powerful technique that allows producing single, multilayer and columnar NP structures [1,2]. However, an important drawback of PLD is the difficulty for producing NPs with well defined symmetry (spherical, oblate or prolate) and alignment [1]. In this work we have tackled this problem through fs-laser shaping of the produced nanostructures. Fs-laser irradiation has been shown to change the shape of silver NPs from spheres to ellipsoids in bulk materials [3], as well as to induce aligned structures upon optimum experimental irradiation conditions. In our experiment we apply this approach to nanostructures consisting in an ultrathin single layer of Ag NPs embedded in an amorphous Al2O3 matrix. The use of a-PLD allows controlling the size of the NPs as well as the thickness of the underneath and covering a-Al2O3 layers through the number of laser pulses on each target. In particular, we have selected the size of the NPs to be close to percolation, which leads to a very heterogeneous distribution of Ag NPs that are highly non-spherical and randomly oriented as it is shown in Figure 1.a.The experimental setup used for NP reshaping is based on an amplified fs-laser. The spatial beam intensity distribution at the sample plane was highly Gaussian. The polarization of the irradiation beam was selected to be perpendicular to the plane of incidence. A multipulse irradiation scheme was followed in all cases [3]. White light probing of the spectral sample transmission during irradiation allows in-situ monitoring of the shape transformation. The initially broad polarization-isotropic absorption band can be significantly narrowed and shifted in a controlled way to the blue spectral region by adjusting the laser fluence, and even transformed to show polarization anisotropy (Figure 1.b). Using the high spatial resolution (<;10 μm) of the optical probe technique, the dependence of the NP shape on the local fluence is studied on a single irradiated region.