Abstract
At resonance, two coupled metallic nanorods generate highly enhanced and localized optical field in the gap that promotes light-matter interaction. For coupled heterogeneous nanorods, the highly enhanced field in the gap results in very efficient non-linear optical signal generation at the nanoscale like four-wave mixing. One of the important applications is that if bright mode of short nanorod and the dark mode of long nanorod are turned to spectrally overlap, the resulting asymmetric gap antennas can exhibits asymmetric Fano-like sharp dips in the resonance spectrum, which is of great interest in nano-sensing. So far, well-defined Fano resonators are mostly fabricated using cost inefficient top-down methods, such as electron beam lithography. Our goal is to use click reaction which gives almost 100% product yield to link nanorods with different aspect ratios. Since the nanorods are covered with CTAB bilayer before pre-treatment we have to partially remove the surfactant bilayer in order to bind our linker molecules to the surface of gold nanorods. Here we removed the CTAB surfactant by using acetonitrile. We found that the adsorption of CTAB on Au is facets-dependent and the ratio of ACN to water plays a major role is controlling the partial removal of CTAB bilayer. In order to synthesize head-to-head linkage of nanorods, we have to precisely control the ACN/H2O so that only the ends of nanorods expose and the linker molecules attach only to the exposed ends. In such a way, we generate chains of hetero-nanorods with extremely small gap that ensures a strong coupling and facilitates the asymmetric line shape as well as non-linear signal generation. We have carried out time-dependent spectral analysis on the solution and performed finite-difference time-domain simulations to verify the resonances. Our method is simple, low cost and the product can be used for nano-sensing and nanoscale nonlinear signal generation.