Shape and size dependent nonlinear refraction and absorption in citrate-stabilized, near-IR plasmonic silver nanopyramids

Abstract

Using a combination of a mild stabilizer and a mild reductant, sodium citrate and hydrazine hydrate, anisotropic silver nanocrystals (NCs) were synthesized with tunable plasmon peaks at 550 nm, 700 nm, 800 nm, 900 nm and 1010 nm (the samples are named Ag-550, Ag-700, Ag-800, Ag-900 and Ag-1010, respectively). TEM investigations revealed that Ag-550 NCs were pentagonal nanoplates while the other four samples were nanopyramids with a pentagonal base with the edge length varying between 15 and 30 nm. The non-linear optical (NLO) properties of these NCs were studied by the Z-scan technique using the CW He–Ne laser (632.8 nm, 15 mW). The shape change from 2D nanoplates (Ag-550) to 3D nanopyramids (Ag-700) resulted in sign reversal of the non-linear refractive index, n₂, from a negative (−3.164 × 10⁻⁸ cm² W⁻¹) to a positive one (1.195 × 10⁻⁸ cm² W⁻¹). This corresponds to a change from a self-defocussing effect to a self-focussing one. Besides shape, the size effect is also prominently observed. Amongst nanopyramids, as the edge length increases, n₂ increases linearly and reaches a maximum of 3.124 × 10⁻⁸ cm² W⁻¹. Doubling the edge length from 15 nm to 30 nm resulted in 162% increase in n₂. On moving from Ag-550 to Ag-900 NCs, with the increasing plasmon wavelength, the non-linear absorption (NLA) coefficient increased exponentially to a high value of 8.52 × 10⁻⁴ cm W⁻¹. However, Ag-1010 showed 29% decrease in NLA which is attributed to twinning present in the crystal structure as seen in the HR-TEM images. Due to the tunable NLO properties, these anisotropic Ag NCs hold great potential for applications in optical limiting, switching and data storage.