Elastic ͑in-plane͒ scattering of surface plasmon polaritons ͑SPP's͒ is modeled by considering isotropic pointlike scatterers whose responses to the incident SPP field are phenomenologically related to their effective polarizabilities. Numerical simulations of single, double, and multiple scattering are presented for randomly situated scatterers showing the interplay between different orders of scattering and localization phenomena. Correlation between the scattering regimes and spatial Fourier spectra of the corresponding SPP intensity distributions is considered. Various optical microcomponents ͑e.g., straight and curved micromirrors͒ formed by sets of point scatterers are also simulated, and the stability and dispersion of their characteristics are investigated. The appropriate experimental results for SPP scattering by both random and specially configured sets of microscatterers are reported for two excitation wavelengths ͑594 and 633 nm͒ and different metal ͑silver and gold͒ films. The near-field optical images obtained are related to the calculated SPP intensity distributions demonstrating that the model developed can be successfully used in studies of SPP elastic scattering, e.g., to design the microcomponents for SPP's.

Second harmonic (SH) scanning optical microscopy in reflection is used to image the gold film surface covered with randomly placed scatterers. SH images obtained with a tightly focused tunable (750-830 nm) laser beam show small (approximately 0.7 microm) and very bright (approximately 10(3) times the background) spots, whose locations depend on the wavelength and polarization of light. Comparing SH and fundamental harmonic (FH) images, we conclude that the localized SH enhancement occurs due to the overlap of FH and SH eigenmodes. The probability density function of the SH signal is found to follow the power-law dependence.

Direct observation of localized dipolar excitations on rough nanostructured surfacesUsing a photon scanning tunneling microscope ͑operating alternatively at the wavelengths of 594 and 633 nm͒ with shear-force feedback we image the topography of silver colloid fractals simultaneously with a near-field intensity distribution. We observe that near-field optical images exhibit spatially localized ͑within 150-250 nm͒ intensity enhancement by one to two orders of magnitude. These bright light spots are found to be sensitive to the light wavelength, polarization, and angle of incidence. We relate the observed phenomenon to the localization of resonant dipolar excitations in random nanostructured aggregates.

Imaging of surface plasmon polaritons (SPP's) with a photon scanning tunneling microscope (PSTM) combined with shear force feedback is experimentally investigated. A contrast correction factor, that accounts for spatial frequency filtration performed by an uncoated fiber tip of the PSTM (when detecting the SPP intensity distribution), is introduced and evaluated from the measurements of a standing wave interference pattern formed by two counterpropagating evanescent waves, that are generated by total internal reflection of light beams inside a glass prism. Influence of propagating field components stemming from inelastic SPP scattering on the resultant intensity distributions and, consequently, on the near-field optical images obtained with the PSTM is discussed. Optical images taken at different tip-surface distances are used to evaluate the contribution from propagating field components in near-field optical images taken with shear force feedback. The approach developed is applied to experimental studies of elastic scattering of SPP's excited at the wavelength of 633 nm along smooth and rough surfaces of gold and silver films.

We demonstrate experimentally generation of diffraction-free plasmonic beams with zeroth-and first-order Bessel intensity profiles using axicon-like structures fabricated on gold film surfaces and designed to operate at a wavelength of 700 nm. The central beam features a very low divergence (∼8π mrad) for a narrow waist of the order of one wavelength and the ability to self reconstruct, which are the main signatures of diffraction-free beams. © 2013 Optical Society of America OCIS codes: 240.6680, 250.5403, 050.1940. The unique properties of surface plasmon polaritons (SPPs), such as subwavelength confinement and surface field enhancement, have led to intensive investigations (both fundamental and applied) in this field, commonly known as plasmonics, with a significant portion being devoted to the design and fabrication of micro-and nano-optical components for SPP manipulation [1]. Although SPPs can be confined to relatively small transverse regions, using for example dielectric-loaded SPP waveguides [2], the inherent increase of effective index results in a decrease of propagation length. On the other hand, SPP beams that propagate along unstructured metal surfaces exhibit generally a longer propagation length, but exhibit weak localization and diverge quickly. A possible solution to overcome this tradeoff between confinement and propagation could be the generation of diffraction-free plasmonic beams (DF-SPPBs). Diffraction-free beams are characterized by a field distribution, whose transverse intensity profile is invariant along the propagation direction and has the ability to reconstruct itself if an obstacle is placed along the propagation axis. At the same time, the beam waist can be rather small, i.e., of the order of one wavelength. Such an ideal beam cannot be realized experimentally, since it requires infinite energy and an infinite spatial extent. Nevertheless, quasi-diffraction-free beams can be generated to exist in a spatially limited region. Durnin et al. [3] studied for the first time these kinds of beams, namely, the Bessel beams. In recent years, other diffraction-free beams, such as Mathieu [4] and Airy beams [5] have also been explored.The above-mentioned beams are solutions to the freespace three-dimensional Helmholtz equation (no boundaries, guiding surfaces, or nonlinear media). Therefore, the very existence of two-dimensional (2D) diffractionfree solutions is not straightforwardly warranted. In fact, it was shown that the Airy beam is the only possible diffraction-free solution to the 2D paraxial wave equation [6].In this context, plasmonic (SPP-based) Airy beams have been studied theoretically [7,8] and demonstrated experimentally [9][10][11][12]. At the same time, recent numerical studies in photonic crystals [13] demonstrated that diffraction can be tailored to generate 2D diffractionfree beams that resemble the profile of a Bessel function and present the self-healing property. Furthermore, a similar numerical approach was employed to model the generation of DF-SPPBs with axicon-sh...