Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip
Tip-enhanced Raman spectroscopy (TERS) has found wide applications and shown great potential in physics, chemistry and material science. In the traditional TERS configuration with the apex of a metal tip directly illuminated by a far-field laser beam, background excitation is inevitable due to the illuminating focal spot. Such an issue is quite severe in some cases like bulk material analysis. An effective way to avoid background excitation is to shine light on a grating on the tip body and transmit the grating-coupled surface plasmonic polaritons (SPPs) to focus at the tip apex. Although such a scheme has been applied in nano-Raman spectroscopy, the plasmonic nanofocusing mechanism has not been clearly understood.
The research group of Prof. Ting Mei and Wending Zhang from Northwestern Polytechnical University present a detailed analysis on mode evolution of grating-coupled SPPs on the conical metal tip based on the guided-wave theory and finite difference time domain (FDTD) method. It is shown that the grating-coupled SPPs experience mode evolution along the adiabatic tapered plasmonic waveguide and are finally transferred to the fundamental plasmonic guide mode TM01, which subsequently achieves nanofocusing at the tip apex. The analysis provides comprehensive understanding on the mechanism, which may help optimize the design and improve the performance of TERS with grating-assisted light coupling.
(a) Geometry of a conical silver tip with grating-assisted light coupling; (b) Effective indices neff in real part of guided SPP modes versus the radius of cylindrical silver guide R; (c) Electric field intensity distribution at the tip apex; (d) Transverse electric field intensity distribution at 1 nm below the tip apex
The nanophotonics research group in Shaanxi Key Laboratory of Optical Information Technology of Northwestern Polytechnical University was established in September 2013 by Professor Ting Mei. The team consists of 1 professor (Ting Mei), 1 associate professor (Wending Zhang) and 17 Ph.D./master students. The main research of the group includes light field manipulation with optical fiber and plasmonic structures, interaction of structured light and plasmonic structures, and nonlinear plasmonics. They have recently completed a series of works on (1) establishing a theoretical model of polarization dependent vector mode coupling and cascade vector mode coupling for the generation and regulation of structural optical field in optical fiber; (2) realizing the efficient generation and polarization conversion of cylindrical vector field in optical fiber, and on this basis, constructing optical fiber vector laser with dynamic conversion of polarization state; (3) realizing the generation of high-order vortex field and the regulation of topological charge in optical fiber, and on this basis, constructing the STED light source with precise controlling of wavelength and mode and highly coaxality; (4) achieving the generation of ultrafast vortex field in optical fiber and the nonlinear frequency conversion of vortex field; (5) obtaining a highly field enhanced nanofocusing source by utilizing a metallic fiber tip excited by the vector beam internally. Their works in the field of surface plasmonics further include local field enhancement in spectroscopy, plasmonic nanofocusing, plasmonic hot-electron effects, nonlinear enhancement effects, etc.
Lu F F, Zhang W D, Huang L G, Liang S H, Mao D et al. Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip. Opto-Electronic Advances 1, 180010 (2018).