Plasmonic Tip Based on Excitation of Radially Polarized Conical Surface Plasmon Polariton for Detecting Longitudinal and Transversal Fields

Abstract

We study experimentally the excitation of the radially polarized conical surface plasmon polariton (SPP) in a fully metalcoated conically tapered M-profile fiber which works as a "plasmonic tip" for the scanning near-field optical microscope (SNOM). This structure extends the Kretschmann configuration to the conical geometry. In this plasmonic tip, the radially polarized waveguide mode, propagating inside the fiber, resonantly excites the radially polarized SPP on the metal surface, which consequently gets confined at the apex where the field oscillates longitudinally along the tip axis. We also demonstrate the reverse process, where a longitudinal field excites the radially polarized SPP mode which then resonantly excites the radially polarized waveguide mode. This plasmonic tip combines the advantageous properties of near-field optical probes. Though, it has the shape of an apertureless SNOM tip, it can simplify the detection/excitation procedure and suppresses the background signal by its fiber-based design. Unlike the sharp apertureless SNOM tips that detects only the longitudinal field component or aperture SNOM tips that detect mostly the transversal component, the plasmonic tip detects both longitudinal and transversal field in collection mode and backward-scattering mode, respectively. The plasmonic tip, with further improvements, can become an advanced tool in SNOM due to its ability for background-free near-field detection, ease of operation, and higher conversion efficiency from far-field to near-field than conventional tips. KEYWORDS: plasmonic tip, radially polarized conical SPP, radially polarized fiber mode, longitudinal and transversal field, SNOM, M-profile fiber T he biggest challenge for studying the optical properties of nano-objects arises in efficient delivery and detection of light to and from nanoscale regions. Surface plasmon polaritons (SPPs) help to overcome this limit. 1,2 It enables strong confinement and enhancement of electromagnetic energy below the diffraction limit of light in a variety of structures, particularly in tapered metallic structures with sharp edges or tips. 3,4 Being hybrid electromagnetic waves, SPPs comprise properties of transverse (photon) and longitudinal (plasmon) waves. When propagating in tapered metallic structures toward the sharp edges or apexes, longitudinal component of SPPs field becomes more pronounced. 5−14 This leads to a decrease of the wavelength, thus, allowing it to get localized at the tip apex and resulting in a highly confined and enhanced field at the apex with a strong longitudinal component. 5,7−9 This phenomenon is called SPP superfocusing, and in a metallic cone, it takes place only for the radially polarized SPP mode. 7,8 Tip-enhanced microscopy technique, which provides the highest spatial optical resolution among the optical detection methods, takes advantage of SPP localization at a nanoscale apex of a conical metallic tip. 15−23 The localization can be achieved either by exciting SPPs on a tip shaft 21−23 or simply by placing a tip...

Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip

“…The eigenvalue equation of HE mn modes for cylindrical metal waveguides is presented. During propagation on the metal tip, the grating-coupled SPPs can be gradually coupled to HE 31 , HE 21 , HE 11 and TM 01 modes, and these modes are sequentially cut off except TM 01 . The TM 01 mode further propagates with drastically increasing effective mode index and thus is converted to LSPs at the tip apex, which is responsible for plasmonic nanofocusing.…”

“…4(c). At the same time, the n eff of grating-coupled SPPs is gradually close to that of the HE 21 mode, so part of them is further coupled to the HE 21 mode, as shown in Fig. 4(d), and the HE 21 mode can propagate along the interface of the tip.…”

“…The key to achieving plasmonic nanofocusing is the design of metal nanostructures with excellent localized surface plasmon resonance (LSPR) effect, such as wedges 16 , tetrahedrons 17 , grooves 18 , nanoparticles 19 , etc. Besides, it is noteworthy that noble metallic tips can combine electric field enhancement and spatial confinement characteristics to generate nano-confined light source 20,21 . Conical metal tips have been commonly adopted for TERS in the configuration of scanning probe microscopy 22 .…”

Highly Efficient Excitation of Surface Plasmons Using a Si Gable Tip

“…an SPP mode). Metal‐coated dielectric tips have also been used to excite SPP at the metal/dielectric interface with relatively low excitation efficiency (∼ 0.3%) . Such excitation efficiency depends on several parameters such as the shape of the tip, the distance of the tip from the interface and the refractive index of the tip, as well as the medium surrounding it.…”

Optimization of a Plasmon Enhanced Field Emitter Array Using a Nano-Tip-Based Plasmonic Double-Gate Structure

“…The sub-wavelength apertures act as plasmonic waveguides, and the concept of EOT can be applied to a NPDS to enhance the near-field efficiency on a nano-tip array. With near-field enhancements in metallic nano-tips, which have been investigated by various theoretical and experimental studies [14]- [16], localized SPs can secure field enhancement on the nano-tip apex and improve the electron yield of SP-enhanced NPDS. This effect can also be applied to field emitter arrays (FEAs) that can demonstrate an order-ofmagnitude enhancement in brightness with low emittance characteristics [17]- [20].…”