We present a detailed analysis on mode evolution of grating-coupled surface plasmonic polaritons (SPPs) on a conical metal tip based on the guided-wave theory. The eigenvalue equations for SPPs modes are discussed, revealing that cylindrical metal waveguides only support TM 01 and HE m1 surface modes. During propagation on the metal tip, the grating-coupled SPPs are converted to HE 31 , HE 21 , HE 11 and TM 01 successively, and these modes are sequentially cut off except TM 01 . The TM 01 mode further propagates with drastically increasing effective mode index and is converted to localized surface plasmons (LSPs) at the tip apex, which is responsible for plasmonic nanofocusing. The gap-mode plasmons can be excited with the focusing TM 01 mode by approaching a metal substrate to the tip apex, resulting in further enhanced electric field and reduced size of the plasmonic focus.
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.
A flat slow-light band over a wide frequency range is obtained in the hetero-MIM (metal-insulator-metal) waveguide with zero group velocity dispersion (GVD). The zero GVD originates from dispersion compensation by the photonic mode and the plasmonic mode, the mechanism of which does not exist in the homo-MIM structure. By changing dielectric permittivity of the insulator or the difference of two different metallic plasma frequencies, the group index and the bandwidth can be tuned. The dispersionless slow light characteristic in the hetero-MIM waveguide may be useful in the new design of plasmonic devices.
Plasmonic waveguides with an insulator core sandwiched between hyperbolic metamaterials (HMMs) claddings, i.e. HIH waveguide, are investigated for achieving wide slow-light band with adjustable working wavelength. The transfer matrix method and the finite-difference-time-domain simulation are employed to study waveguide dispersion characteristics and pulse propagation. By selecting proper silver filling ratios for HMMs, the hetero-HIH waveguide presents a slow-light band with a zero group velocity dispersion wavelength of 1.55 μm and is capable of buffering pulses with pulse width as short as ∼20 fs. This type of waveguides might be applicable for ultrafast slow-light application.
The plasmonic characteristics of a periodic array of cavities in a silicon substrate are investigated for hot-electron photodetection. Resonances of cavity surface plasmons bound to air cavities and silicon cavities, and resonance of Bragg-surface plasmon polaritons are illustrated by the map of metal absorption. Hybrid modes formed with combination of these modes can strongly enhance absorption in metal and be exploited to optimize hot-electron photodetectors for single-band and dual-band detection at optical communication wavelengths.
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