Speaker
Description
Localized surface plasmon resonance (LSPR) in novel metal embedded nanocomposites have attracted considerable interest due to their tunable optical properties with applications in photonics and sensing [1]. The light induced coupling between photons and oscillating conduction electrons (SPR) governs the optical response in nanoparticles (NPs) embedded in dielectrics. Furthermore, light-matter interaction transforms the NPs (size, shape, volume concentration) leading to localized effect on structural coloration. In this context, femtosecond laser irradiation has emerged as a versatile technique for tailoring metal nanocomposites through high peak intensity enabling energy coupling via non-linear effects, resulting in reorganizing or reshaping nanoparticles [2].
In this work, we present a dual laser-based strategy which combines nanocomposite synthesis optimization along with fs laser processing. As a first step, Ag based nanocomposite samples were fabricated by the technique of alternate pulsed laser deposition (a-PLD) with an excimer laser (λ= 193 nm, = 20 ns) [3]. By systematically tuning deposition parameters, we obtain near-coalescent Ag-thin films of less than 6 nm thickness on soda lime glass, as characterized by Rutherford backscattering Spectrometry (RBS) measurements. Variation in the dielectric (Al2O3) top cover-layer thickness (from 35 to 240 nm) is introduced to obtain different spectral responses (so different colour response), as shown in the spectra and insets in Figure 1(a)).
Femtosecond laser processing (λ= 515 nm, = 300 fs) of large areas (1.5 × 1.5 mm2) on the nanocomposite samples by varying fluence at constant repetition rate (500 kHz) shows the possibility to tune colours both in reflectivity and in transmission. In Figure 1(b-d) we show the results obtained in the sample with 240 nm dielectric cover layer. In reflectivity (Figure 1(b)), we obtained both yellow and turquoise colours, indicating respectively a red-shift and blue-shift with respect to the original greenish colour. In transmission (Figure 1(c-d)), colour changes are obtained when exceeding a fluence threshold, (F ≥ 55 mJ/cm2, beyond the bleaching limit). Noteworthy, the colouring shows an anisotropic behaviour depending on the direction of the while light illumination polarization with respect to the laser polarization (either parallel or perpendicular), which is consistent with a laser-induced reorganization and shaping of the nanoparticles. Complementary TEM measurements provide additional information about the underlying processes.