Highly efficient plasmonic nanofocusing on a metallized fiber tip with internal illumination of the radial vector mode using an acousto-optic coupling approach

Abstract

Tip-based plasmonic nanofocusing, which delivers light into a nanoscale region and achieves localized electromagnetic (EM) field enhancement beyond the diffraction limit, is highly desired for light-matter interaction-based super-resolution imaging. Here, we present the plasmonic nanofocusing at the apex of a silver (Ag)-coated fiber tip with the internal illumination of a radial vector mode (RVM) generated directly in an optical fiber based on an acoustically-induced fiber grating (AIFG). As illustrated by theoretical calculation, a picture of the nanofocusing plasmonic tip given by analyzing the mode conversion process that the surface plasmon polariton (SPP) mode excited via the radial polarization optical mode can propagate to the apex of the plasmonic tip for nanofocusing because it is not cut off as the tip radius decreases; while the SPP mode which transited from the linear polarization optical mode cannot propagate to the tip apex for nanofocusing because it is cut off as the tip radius decreases. The electric field intensity enhancement factor $|{\rm{E}}_{{\rm{apex}}}^{\rm{2}}|/|{\rm{E}}_{{\rm{input}}}^{\rm{2}}|$ of a plasmonic tip with a tip radius of 20 nm was calculated to be ~2 × 103. Furthermore, the electric field enhancement characteristic at the tip apex was also experimentally verified by using surface-enhanced Raman spectroscopy (SERS). The Raman scattering intensity was observed to be ~15 times as strong as that with internal illumination using the linear polarization mode (LPM), revealing their significantly different nanofocusing characteristics. A Raman sensitivity of 10−14m was achieved for the target analyte of malachite green (MG), denoting significant electric field enhancement and effective plasmonic nanofocusing. The energy conversion efficiency of the radial polarization optical mode to the corresponding SPP mode at the tip apex was measured to be ~17%. This light delivery technique can be potentially further exploited in near-field microscopy with improved resolution and conversion efficiency.

Numerical Investigation of Multifunctional Plasmonic Micro-Fiber Based on Fano Resonances and LSPR Excited via Cylindrical Vector Beam

“…Figure 3b,c exhibit the electric-field intensity distribution of plane β excited by a pair of strictly degenerate vector modes with the polarization direction parallel to x-and y-axis The electric field enhancement can be notably influenced by illumination modes for the reason of the polarization-dependent excitation of localized surface plasmon modes [33][34][35]. Our previous work [36][37][38]49,50] demonstrates that cylindrical vector beams (CVB), including APB and RPB, are helpful to improve the field enhancement for their special polarization distribution on the cross-section of the light beam. Here, APB and RPB are adopted to excite the side-face configuration and end-face configuration, the simulation model of which is shown in Figure 4a,d, respectively.…”

Generation of polarization and phase singular beams in fibers and fiber lasers

“…61,95,133 Furthermore, we have theoretically and experimentally presented the nanofocusing characteristic of several metal-coated fiber tips under RPB excitation in the visible band, providing an effective guideline for designing the background-free tip-enhanced Raman spectroscopy system. [153][154][155] Combining the azimuthal polarization characteristics of APB with the spatial symmetry characteristics of the silver nanoprism arrays, 156 the sensitivity of surface-enhanced Raman spectroscopy has been enhanced by a factor of 3.3 × 10 7 . In addition, in the fields of fiber communications, Ryf et al demonstrated mode-division multiplexing based on six CVBs or VBs that carried a 40 Gb∕s signal over 96 km in few-mode fiber, 157 and it significantly increases the transmission capacity of fiber communication systems.…”

Numerical calculation of the light propagation in tapered optical fibers for optical neural interfaces

“…17,18 Several studies demonstrate that the use of plasmonic structures surrounding the fiber could help confine and concentrate light at the nanoscale. [19][20][21][22] Also, structures made of high-index dielectrics 23 or metals 24 have been shown to enable sub-diffraction light confinement. These studies usually assume single-mode optical fibers and therefore do not consider the modedivision demultiplexing of guided light in tapered optical fibers.…”