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...
Photoluminescence (PL) spectroscopy has proven to provide deep insights into the optoelectronic properties of monolayer . Herein, a corresponding study is conducted on the excitonic properties of mechanically exfoliated monolayer under multivariate physical and chemical stimuli. Specifically, midgap exciton states that originate from lattice defects are characterized and they are compared to existing models. Through statistical data analyses of substrate‐, temperature‐, and laser‐power‐dependent measurements, a PL enhancement is revealed through physisorption of water molecules of the controversially discussed excited‐state A biexciton (). In addition, analyses of monolayer on gold substrates show that surface roughness does not account for changes in doping level within the material. Also, a shift in the electron–phonon coupling properties that arises from thin films of water that are physisorbed on top of the samples is reported.
In this study, we explore analytically and experimentally long- and short-range surface plasmon polariton (LR-SPP and SR-SPP, respectively) modes in gold wedges. Especially, we aim to observe the 2-dimensional confinement of the electromagnetic field in gold wedges as it could enhance the light-matter interaction by offering a local density of states which depends on the propagation constant, consequently on the wedge height. The LR-SPP mode can propagate over a long distance, but the real part of the propagation constant remains relatively insensitive to the decreasing wedge height. This mode also experiences cut-off at a wedge height of about 50 nm in our experimental condition. Meanwhile, the SR-SPP mode has a large propagation constant that increases further with decreasing wedge height. As a result, the effective wavelength of the mode shrinks confining the electromagnetic wave longitudinally along the propagation direction in addition to enhancing the transverse confinement of SR-SPP. In the experiment, we use gold wedges with different edge heights to excite each SPP mode individually and image the electromagnetic near field by using a pseudo-heterodyne scattering scanning near-field optical microscope. By imaging the LR-SPP mode field, we demonstrate that the theoretical and measured values of the effective wavelength agree quite well. By using short wedges, we measure the SR-SPP mode field and demonstrate that the effective wavelength decreases to 47% in about half a micrometer of propagation distance. This corresponds to a 3.5 times decrease of the vacuum wavelength or an effective index of 3.5. It is important to note that this value is, by no means, the limit of the electromagnetic field’s longitudinal confinement in a gold wedge. Rather, we were only able to measure the electromagnetic field up to this point due to our measurement limitations. The electromagnetic field will be propagating further, and the longitudinal confinement will increase as well. In conclusion, we measured the SR-SPP in a gold wedge and demonstrate the electromagnetic field confinement in the visible spectrum in gold wedges.
In this study, we investigate analytically and experimentally the roles of quasi-linearly polarized (LP), hybrid, plasmonic and photonic modes in optical detection and excitation with aperture tips in scanning near-field optical microscopy. Aperture tips are tapered and metal-coated optical fibers where small circular apertures are made at the apex. In aperture tips, there exist plasmonic modes that are bound at the interface of the metal cladding to the inner dielectric fiber and photonic modes that are guided in the area of the increased index in the dielectric fiber core. The fundamental photonic mode, although excited by the free-space Gaussian beam, experiences cutoff and turns into an evanescent mode. The photonic mode also becomes lossier than the plasmonic mode toward the tip aperture, and its power decay due to absorption and reflection is expected to be at least 10 −9 . In contrast, the fundamental plasmonic mode has no cutoff and thus reaches all the way to the tip aperture. Due to the non-adiabaticity of both modes' propagations through the taper below a core radius of 600 nm, there occurs coupling between the modes. The transmission efficiency of the plasmonic mode, including the coupling efficiency and the propagation loss, is expected to be about 10 −6 that is at least 3 orders of magnitude larger than that of the photonic mode. Toward the tip aperture, the longitudinal field of the photonic mode becomes stronger than the transverse ones while the transverse fields always dominate for the plasmonic mode. Experimentally, we obtain polarization resolved images of the near-field at the tip aperture and compare with the x-and y-components of the fundamental quasi-LP plasmonic and photonic modes. The results show that not only the pattern but also the intensity ratios of the x-and y-components of the aperture near-field match with that of the fundamental plasmonic mode. Consequently, we conclude that only the plasmonic mode reaches the tip aperture and thus governs the near-field interaction outside the tip aperture. Our conclusion remains valid for all aperture tips regardless of the cladding metal type that mainly influences the total transmission efficiency of the aperture tip.
Having virtues from plasmons and scanning probe microscopy (SPM), plasmonic tips employ radially polarized conical plasmons and create hot-spots at their apexes. Plasmonic tips are tapered and fully metal-coated vortex fibers that have M-shaped refractive index profiles. Vortex fibers allow the radially polarized mode to propagate over a long distance with high modal purity. When the fiber mode reaches the tapered region, it resonantly excites the plasmon mode at a metal-dielectric outer interface. In this paper, we study the plasmonic tip's behavior in liquids both theoretically and experimentally. By adiabatically tapering the vortex fiber, the radially polarized mode gets confined from the fiber with a diameter of 115 μm down to the tapered part with a diameter of 1.42 μm as a waveguide mode. In this region, the plasmon mode gets excited thus reaches the apex with a diameter of 200 nm. Our calculations show that the plasmon coupling efficiency increases in liquids due to two competing processes: a significant increase of the interaction region and slight decrease of the penetration depth of fields in metal. By choosing a liquid that either allows or forbids the phase-matching, we demonstrate that the plasmon coupling efficiency can increase or vanish. Due to the wetting effect, a tapered liquid-layer forms over the tip like an additional waveguide and allows resonant coupling of fiber modes to the liquid layer
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