We demonstrate experimentally the generation and near-field imaging of nondiffracting surface waves, plasmonic Airy beams, propagating on the surface of a gold metal film. The Airy plasmons are excited by an engineered nanoscale phase grating, and demonstrate significant beam bending over their propagation. We show that the observed Airy plasmons exhibit self-healing properties, suggesting novel applications in plasmonic circuitry and surface optical manipulation.

Airy beams represent an important class of nondiffracting waves which can be realized on a flat surface. Being generated in the form of surface-plasmon polaritons, such Airy plasmons demonstrate many remarkable properties: they do not diffract while propagating along parabolic trajectories, and they recover their shape after passing through obstacles. This paper reviews the basic physics of Airy plasmons in both paraxial and non-paraxial cases, and describes the experimental methods for generation of Airy surface waves on metal surfaces, including a control of their trajectories, as well as the interference of Airy plasmons and hot-spot generation. Many unusual properties of Airy plasmons can be utilized for useful applications, including plasmonic circuitry and surface tweezers. Picture: Observation of two colliding Airy plasmons.

We predict and demonstrate the generation of a plasmonic hot spot on the surface of a metal film by the interference of two Airy surface plasmons. We show that the position of the hot spot can be controlled by the distance between the excitation gratings as well as by the phase front of the initial excitation. The observed effect constitutes a planar analogy to Airy beam autofocusing and offers new opportunities for spatially resolved surface plasmon sensing and optical surface tweezers.

We study the polarization characteristics of light emission and collection in the near field by the tips of a Dual-SNOM (two scanning near-field optical microscopes) setup. We find that cantilevered fiber probes can serve as emitters of polarized light, or as polarization-sensitive detectors. The polarization characteristics depend on the fiber type used for tip fabrication. In Dual-SNOM measurements, we demonstrate mapping of different field components of the plasmonic dipole pattern emitted by an aperture probe.

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...