Plasmon management in index engineered 2.5D hybrid nanostructures for surface-enhanced Raman scattering

Abstract

Leaky mode resonance (LMR) and optical standing waves (SWs) generated in suitably illuminated vertically aligned supported nanowires (NWs) were demonstrated to enhance the plasmonic electric (E) field behavior of silver nanoparticles (AgNPs) dispersed on the wires. The combination of LMR and SW can significantly enhance the local plasmonic E field around high-density AgNPs. The results of the finite difference time domain (FDTD) calculations were experimentally verified by comparing the surface-enhanced Raman scattering (SERS) sensitivity, which is directly dependent on the E field, in AgNP-coated silicon, germanium and silver NWs. As LMR and SW are functions of the substrate optical index (n, k), an engineering of the indices predicted a (3.88, 0) theoretical value to maximize the plasmonic E-field on the hybrid scaffold at a given AgNP density. These SERS substrates were utilized for the detection of marine toxins, L-BMAA (2-amino-3-(methylamino) propionic acid hydrochloride) and Malachite green, which are used extensively in seafood. AgNP-decorated silicon NWs, whose index (3.88, 0.02) lies close to the theoretically predicted value, exhibit at least pico-molar sensitivity toward those marine toxins. A plasmon management strategy is developed that could assist in lowering the detection level of environmental toxins by SERS. Surojit Chattopadhyay at National Yang-Ming University, Taiwan, and his colleagues have developed sensitive molecular sensors based on silicon nanowires decorated with silver nanoparticles. Detecting small quantities of molecules is important for areas such as environmental sensing of toxins. The researchers utilize antenna effects in hybrid nanostructures that enhance surrounding light fields. In particular,the resonances between an incoming laser beam and the optical fields from the nanowires cause standing wave patterns. The plasmonic resonances in the small silver nanoparticles that are attached to the surface of the nanowires are enhanced by the optical waves and the antenna effect. These antenna-induced strong plasmons (say super-plasmons) considerably improves the detection of surrounding molecules. In initial experiments with toxic molecules, the researchers demonstrated an ultrahigh detection sensitivity that is close to the theoretical value. Irradiated vertically aligned supported 2.5 dimensional nanowires (NWs) show strong optical absorption due to the leaky mode resonance (LMR), and optical standing waves (SWs) created by waveguiding effects. Silver nanoparticles (AgNPs) have their plasmonic electric (E) field strengths modulated under the effect of the LMR and SW as a function of the optical indices (n, k) of the NWs. At (n=3.88, k=0), a theoretical maxima of the E-field was obtained, which matches closely with the Si (3.84, 0.02) system. The optimized hybrid system, AgNPs on silicon NW, demonstrates pico-femto molar detection of marine toxins by surface plasmon-aided SERS.