A unique combination between structural parameters of collinearly arranged spherical particles is proposed as an effective plasmonic substrate for ultrahigh enhancement in hyper-Raman scattering signals. The suggested spherical trimer systems are mainly composed from two identical nanoparticle separated by a third alike shape resonator of different size. All the interacting plasmonic element are made from gold, arranged in 1D array and illuminated by a longitudinally polarized light. The optical properties, spatial distribution of nearfields and the surface charge densities were calculated numerically by FDTD tool. The enhancement factor of the hyper-Raman scattering, and the associated Raman shift were calculated theoretically from the optical response of the trimer. The extinction spectra of the heterotrimers demonstrate the excitation of two plasmonic modes, the first coupled band excited at a longer wavelength and is attributed to the in-phase coupling between the dipole moments induced in each of the three spherical resonators, the other hybrid mode observed in the shorter wavelength region and is resulted from the coupling between the dark mode excited in the intermediate particle and the bright band monitored in the bordered particles. The nearfields associated with the excitation of the two plasmonic modes are strongly localized and highly enhanced at the same intercoupling regions (hot spots) which optically match the excitation wavelength and the second-order stock condition. Through careful selecting of the relative size of the coupled nanoparticles and their coupling separation, the enhancement factor of hyper-Raman scattering signal can reach as high as 1 × 1013.
Keywords: Raman scattering; longitudinal mode; plasmonic coupling; spherical nanoparticles.
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