Quasi-uniform excitation source for cascade enhancement of SERS via focusing of surface plasmons

A novel surface-enhanced Raman scattering (SERS) excitation source based on focusing of surface plasmons around the center hole of a metal disk for cascaded enhancement is put forward and studied theoretically. The device offers intense SERS excitation with quasi-uniformity and horizontal polarization over a comparatively large hole through the combination of electromagnetic field focusing and hole plasmon resonance. As revealed by finite-difference time-domain (FDTD) method, the intensity spectra and the characteristics of the near field for the wavelength range of 650-1000nm exhibit a number of enhancement modes. Electric field intensity of the optimal mode enhances the SERS signal inside the hole by over four orders. An analytical model was also developed to gain precise interpretation on FDTD results. Our model also reveals the possibility of achieving eight orders of enhancement by optimizing the scale of the disk. In addition to generation of highly optimized hot spots, the large active hole also offers potential applications in fluorescence enhancement and nonlinear spectroscopy.