publication detail
Nonlinear plasmonic nanostructures for quadratic nonlinear optics
AUTHORS
Document type
Conference papers
Résumé
The increasing interest in metallic nanoparticles (NP) is motivated by their potential applications in photonics, information storage, detection systems, therapeutics, diagnostics, photovoltaic and catalysis. Their size and shape (nanospheres, nanorods, nanostars…) affect significantly their physical and chemical properties, especially their optical response. These nano-objects display high quadratic nonlinear optical responses, which strongly depend on nanoparticle geometry and surface state. We report here a systematic investigation of the influence of nanoparticle surface area and symmetry on hyperpolarizability β values, as measured via the Harmonic Light Scattering technique in colloidal NP solutions. Here we use a fundamental laser source at 1.064 nm, the second harmonic wavelength at 532 nm coming close to plasmonic resonances of Au and Ag nanoparticles. First we investigate the influence of surface area of noble metal nanospheres and nanorods on their β values. Gold and silver nanospheres, with diameters ranging from 3 to 80 nm have been characterized in order to explore the limits of validity of the purely dipolar origin of β values. Moreover, by studying gold and silver nanorods with different surface areas, we will evidence the predominance of these surface effects over shape factors, leading us to revisit previous studies on the NLO properties of these nanorods . Second, we have synthesized and characterized platinum “nanoflowers” (PtNFs) with different controlled sizes. These PtNFs display exceptionally strong first hyperpolarisabilities, higher than those of gold nanoparticles of similar size. Particle surface corrugation is shown to be the dominant factor governing these exceptionally high β values, with a very limited contribution from plasmonic effects that are negligible in the visible range. In a third part, a single – step growth of gold nanoprisms (GNPs) was achieved by seedless growth. Their second harmonic response has been investigated experimentally and theoretically with edge length ranging from 40 to 116 nm and for different curvature radii at corners . Their experimental β values are found to display a linear dependence with the surface area as reported for other metallic nanoparticle shapes, and also a strong influence of their corner sharpness on the corresponding nonlinearities. For this purpose, we have defined a quantitative sharpness factor depending on the external triangle side length Lext and the triangle corner curvature radius R according to : SF = (Lext- 2R)/Lext. Systematic calculations performed on gold nanoprisms with various edge lengths and corner radii do confirm this trend. Therefore, the influence of corner sharpness seems to dominate over centrosymmetry breaking. These results shed new light on the SHG properties of acentric, sharp corner gold nanoparticles, and open the way to the investigation of various noble metal nanopolyhedra (e.g. nanocubes) to develop new families of highly nonlinear metallic nanostructures making use of an increased number of facets and of the sharpness of related geometric singularities