Abstract
Abstract Metallic plasmonic probes have been successfully applied in near-field imaging, nanolithography, and Raman enhanced spectroscopy because of their ability to squeeze light into nanoscale and provide significant electric field enhancement. Most of these probes rely on nanometric alignment of incident beam and resonant structures with limited spectral bandwidth. This paper proposes and experimentally demonstrates an asymmetric fiber tip for broadband interference nanofocusing within its full optical wavelengths (500–800 nm) at the nanotip with 10 nm apex. The asymmetric geometry consisting of two semicircular slits rotates plasmonic polarization and converts the linearly polarized plasmonic mode to the radially polarized plasmonic mode when the linearly polarized beam couples to the optical fiber. The three-dimensional plasmonic modulation induces circumference interference and nanofocus of surface plasmons, which is significantly different from the nanofocusing through plasmon propagation and plasmon evolution. The plasmonic interference modulation provides fundamental insights into the plasmon engineering and has important applications in plasmon nanophotonic technologies.
Highlights
Surface plasmon polaritons (SPPs), the collective oscillation of electrons at the metal–dielectric interface, can intrinsically carry optical energy into the nanoscale without suffering from the diffraction limits [1, 2]
This paper proposes and experimentally demonstrates an asymmetric fiber tip for broadband interference nanofocusing within its full optical wavelengths (500–800 nm) at the nanotip with 10 nm apex
These results further demonstrate that the experiment results are correct and the transmitted spectrum is significantly affected by the spectral response of the single-mode optical fiber
Summary
Surface plasmon polaritons (SPPs), the collective oscillation of electrons at the metal–dielectric interface, can intrinsically carry optical energy into the nanoscale without suffering from the diffraction limits [1, 2]. The combination of SPP mode modulation and tapered metallic waveguides has led to many optical probes with excellent performance in near-field scanning optical microscopy (NSOM) and tip-enhanced Raman spectroscopy [25, 26] In these applications, the metal-coated fiber tip [7, 27, 28] and nanograting-coupled tapered probes [29, 30] can realize nanofocusing with a large near field enhancement illuminated by radially polarized light because of the constructive interference of SPP waves [20, 25,26,27,28,29,30]. 2 Results and discussion processed on the opposing facets of the metallic fiber tip by focused ion beam method to enable nanofocusing with linearly polarized light ( see Supplementary Figures S1 and S2), and they have an asymmetric degree of H = 935 nm (phase delay) that conforms to the circumference interference condition of SPP waves, determined by Eq (1), causing plasmons excitation and modulation. The terms εm and εair represent the dielectric constants of the Au film and surrounding medium respectively
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