Optimization of electric field enhancement of Ag@SiO2 trimer nanospheres by finite difference time domain method

Abstract

Plasmonic properties of trimer nanocluster depend on their geometric configuration. In this work, local electric field enhancement in the junction region of the trimer Ag@SiO2 core-shell is investigated using three-dimensional electromagnetic simulation based on finite-difference time domain (FDTD) method. Plasmonic wavelength dependence for the two orthogonal polarizations is analyzed for different geometric configurations by gradually breaking the symmetry of the trimer nanospheres. The highly symmetric trimer in the form of equilateral triangle pattern possesses highly intense hotspots with the resonance wavelengths that are degenerate in nature. Breaking of this mode degeneracy occurs when the symmetric triangular pattern of the trimer is gradually transformed to a linear chain arrangement. Trimer core shell with linear chain symmetry exhibits the highest electromagnetic field enhancement which is nearly four times greater than the bare metal trimer but is very sensitive to polarization direction. The localized surface plasmon resonance (LSPR) is red shifted and blue shifted under longitudinal and transverse polarization, respectively. The LSPR shift and the electric field enhancement of the trimer nanocluster upon symmetry breaking under different polarization is compared and is explained using plasmon hybridization theory. The large polarization insensitive electric field enhancement exhibited by the symmetry core-shell may be employed for surface enhanced Raman spectroscopy based chemical and biological molecule detection.