Search

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Our findings confirm that second-harmonic microscopy is a robust and non-destructive method for probing the crystallographic orientation of gold, and can serve as a guideline for enhancing nonlinear response in plasmonic systems.

The absorption and scattering resonances of metal nanostructures are often assumed to be defined by the same condition of localized surface plasmon resonance. Using an electrostatic approximation, we demonstrate that the absorption and scattering cross sections of spherical nanoparticles reach their maxima at different wavelengths, which in turn differ from that defined by the Fröhlich condition (FC). These deviations from the FC originate from and are proportional to the material absorption. Our results provide the design guidelines for maximizing absorption and scattering of spherical nanoparticles and are thus of special importance for applications where the efficiency of radiation absorption or scattering is crucial.

Optical properties of a single ultrasharp groove of subwavelength width cut in an otherwise flat metal surface are examined theoretically. We calculate optical extinction, scattering, and absorption cross-section spectra for a wide range of groove profiles, establishing several fundamental trends. As grooves are made sharper, amplitudes of oscillations in cross-section spectra and their period decrease, while the absorption level increases, leading eventually to efficient broadband (nonresonant) absorption. Oscillations in scattering spectra are generally more pronounced than those in corresponding absorption spectra. For ultrasharp grooves, oscillations in all spectra can be suppressed by increasing the groove depth. Finally, the level of absorption relative to that of scattering increases as the top groove width decreases, a trend that is analogous to that found when decreasing the size of metal nanoparticles.

We analytically describe light transmission through a single subwavelength slit in a thin perfect electric conductor screen for the incident polarization being perpendicular to the slit, and derive simple, yet accurate, expressions for the average electric field in the slit and the transmission efficiency. The analytic results are consistent with full-wave numerical calculations and demonstrate that slits of widths ∼100  nm in real metals may feature nonresonant (i.e., broadband) field enhancements of ∼100 and transmission efficiency of ∼10 at infrared or terahertz frequencies, with the associated metasurface-like array of slits becoming transparent to the incident light.

We consider a simple configuration for realizing one-dimensional slow-light metamaterials with large bandwidth-delay products using stub-shaped Fabry-Perot resonators as building blocks. Each meta-atom gives rise to large group indices because of a classical analog of the dressed-state picture of electromagnetically induced transparency. By connecting up to eight meta-atoms, we find bandwidth-delay products over unity and group indices approaching 100. Our approach is quite general and can be applied to any type of Fabry-Perot resonators and tuned to different operating wavelengths.

Surface plasmon polaritons (SPPs) are locally excited at silver surfaces using (∼100) nanometer-sized nanodiamonds (NDs) with multiple nitrogen-vacancy (NV) centers (∼400). The fluorescences from an externally illuminated (at 532 nm) ND and from nearby NDs, which are not illuminated but produce out-of-plane scattering of SPPs excited by the illuminated ND, exhibit distinctly different wavelength spectra, showing short-wavelength filtering due to the SPP propagation loss. The results indicate that NDs with multiple NV centers can be used as efficient subwavelength SPP sources in planar integrated plasmonics for various applications.

Retardation-based stripe antennas due to the excitation of spoof surface plasmons on a corrugated metal stripe are proposed and numerically studied in the terahertz regime, revealing sharp Fabry-Perot resonances in scattering cross-section spectra with strongly enhanced local fields. The order of the resonance exhibiting the sharpest scattering cross section and strongest field enhancements (FEs) is found to coincide with the number of grooves, due to the hybridization of the antenna resonance with the individual groove resonance. The proposed (spoof surface plasmon-based) antennas with narrow resonances and large FE open up new possibilities for metamaterial design and seem very promising for sensing applications in the terahertz frequencies.

We report a novel design of a compact wavelength add-drop multiplexer utilizing dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs). The DLSPPW-based configuration exploits routing properties of directional couplers and filtering abilities of Bragg gratings. We present practical realization of a 20-μm-long device operating at telecom wavelengths that can reroute optical signals separated by approximately 70 nm in the wavelength band. We characterize the performance of the fabricated structures using scanning near-field optical microscopy as well as leakage-radiation microscopy and support our findings with numerical simulations.

Within our recently proposed generalized nonlocal optical response (GNOR) model, where nonlocal response is included by taking into account both convective and diffusive currents of the conduction electrons, we revisit the fundamental problem of an optically excited plasmonic dimer. We consider the transition from separated dimers via touching dimers to finally overlapping dimers. In particular, we focus on the touching case, showing a fundamental limit on the hybridization of the bonding plasmon modes due to nonlocality. Using transformation optics, we determine a simple analytical equation for the resonance energies.

Using the electrostatic approximation, we analyze electromagnetic fields scattered by sharp conical metal tips, which are illuminated with light polarized along the tip axis. We establish scaling relations for the scattered field amplitude and phase, and verify the validity with numerical simulations. Analytic expressions for the wavelength at which the scattered field near the tip changes its direction and for the field decay near the tip extremity are obtained, relating these characteristics to the cone angle and metal permittivity. The results obtained have important implications for various tip-enhanced phenomena, ranging from Raman and scattering near-field imaging to photoemission spectroscopy and nano-optical trapping.

The presence of a metal nanoparticle (MNP) near a quantum dipole emitter, when a localized surface plasmon mode is excited via the resonant coupling with an excited quantum dipole, dramatically changes the relaxation dynamics: an exponential decay changes to step-like behavior. The main physical consequence of this relaxation process is that the emission, being largely determined by the MNP, comes out with a substantial delay. A large number of system parameters in our analytical description opens new possibilities for controlling quantum emitter dynamics.

We revisited the fundamental problem of bending loss calculation in optical waveguide theory using transformation optics (TOs). Due to the fact that TOs is based on the form invariance property of Maxwell equations, this new approach provides more accurate calculation of waveguide bending loss compared to the conventional refractive index conformal mapping method, especially for small bending radii typical for plasmonic waveguides or photonic waveguides with high-index contrast. We believe our results provide a simple yet reliable way of bending loss calculation for highly confined optical (including plasmonic) waveguides.

We report on the experimental demonstration of detuned-resonator induced transparency in the near-infrared (~800 nm) using two detuned racetrack resonators side-coupled to a bus waveguide. Both resonators and the bus waveguide are in the form of dielectric-loaded surface plasmon polariton waveguides. Leakage radiation microscopy imaging is employed to measure transmission spectra, featuring local maxima at intermediate wavelengths with asymmetrical profiles that are found in good agreement with full-wave numerical simulation results.

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.

We demonstrate, both numerically and experimentally, that metal-insulator-metal configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus supporting gap-surface plasmon resonances, can be designed to function as reflective broadband half-wave plates. Using gold as the metal, the constructed wave plates in the near-infrared regime show scalability, bandwidth of ~20% of the design wavelength, and theoretical reflectivity above 85%, while a reflectivity of ~50% is experimentally measured.

Giant field enhancement and field singularities are a natural consequence of the commonly employed local-response framework. We show that a more general nonlocal treatment of the plasmonic response leads to new and possibly fundamental limitations on field enhancement with important consequences for our understanding of surface-enhanced Raman spectroscopy (SERS). The intrinsic length scale of the electron gas serves to smear out assumed field singularities, leaving the SERS enhancement factor finite, even for geometries with infinitely sharp features. For silver nanogroove structures, mimicked by periodic arrays of half-cylinders (up to 120 nm in radius), we find no enhancement factors exceeding 10 orders of magnitude (10(10)).

Using the effective-index approximation we show that touching spherical metal surfaces form a broadband omnidirectional absorber of gap surface plasmons (GSP), concentrating all GSP waves travelling within a certain radius at the point of contact (at which the field intensity tends to infinity even in the presence of metal absorption) and representing thereby a two-dimensional analogue of an optical black-hole realized without use of metamaterials. The developed wave analysis is supplemented with the geometrical optics (adiabatic) description providing explicit expressions for the critical radius (radius of the event horizon) and buildup of field enhancement along ray trajectories.

We report on the realization of long-range dielectric-loaded surface plasmon polariton waveguides (LR-DLSPPWs) consisting of straight and bent subwavelength dielectric ridges deposited on thin and narrow metal stripes supported by a dielectric buffer layer covering a low-index substrate. Using imaging with a near-field optical microscope and end-fire coupling with a tapered fiber connected to a tunable laser at telecommunication wavelengths (1425-1545 nm), we demonstrate low-loss (propagation length ∼500 μm) and well-confined (mode width ∼1 μm) LR-DLSPPW mode guiding and determine the propagation and bend loss.

We demonstrate that a pair of perpendicular electrical dipolar scatterers resonating at different frequencies can be used as a metamaterial unit cell to construct a nanometer-thin retarder in reflection, designing nanocross and nanobrick plasmonic configurations to function as reflecting quarter-wave plates at ~1520 and 770 nm, respectively. The design is corroborated experimentally with a monolayer of gold nanobricks, transforming linearly polarized incident radiation into circularly polarized radiation at ~780 nm.

Using the analytical description of channel plasmon polaritons (CPPs) developed recently [Opt. Lett.34, 2039 (2009)], we consider the adiabatic CPP nanofocusing in V grooves with linearly decreasing groove angles, deriving closed-form expressions for local field enhancements. Calculations at telecom wavelengths predict the possibility of reaching the intensity enhancements exceeding approximately 1200 for realistic groove tapers cut into gold. Our conclusions are in good agreement with the results of previous investigations.

Using the effective-index approximation along with the explicit expression for the propagation constant of gap surface plasmons established recently [Opt. Express16, 2676 (2008)], we derive closed-form expressions for channel plasmon polariton (CPP) modes supported by V grooves cut into metal. The CPP field distributions and mode characteristics are calculated for the air-gold configuration. The results obtained are found to be in good agreement with the results of previous numerical simulations.

We consider wavelength-selective splitting of radiation using directional couplers (DCs) formed by dielectric-loaded surface-plasmon-polariton waveguides (DLSPPWs). The DCs were fabricated by depositing sub-wavelength-sized polymer ridges on a gold film using large-scale UV photolithography and characterized at telecommunications wavelengths with near-field microscopy. We demonstrate a DLSPPW-based 45-microm-long DC comprising 3 microm offset S bends and 25-microm-long parallel waveguides that changes from the "through" state at 1500 nm to 3 dB splitting at 1600 nm, and show that a 50.5-microm-long DC should enable complete separation of the radiation channels at 1400 and 1620 nm. The DC performance is found to be in good agreement with full vectorial three-dimensional finite-element simulations.

We present the results of optical characterization of metal V-groove waveguides using scanning near-field microscopy, showing broadband transmission with subwavelength confinement and propagation lengths exceeding 100 microm. An updated fabrication method using a combination of UV and nanoimprint lithography is presented. The developed approach is mass-production compatible, adaptable to different designs, and offers wafer-scale parallel fabrication of plasmonic components based on profiled metal surfaces.

We theoretically study channel plasmon-polaritons (CPPs) with a geometry similar to that in recent experiments at telecommunication wavelengths [Bozhevolnyi et al., Nature 440, 508 (2006)]. The CPP modal shape, dispersion relation, and losses are simulated by using the multiple multipole method and the finite difference time domain technique. It is shown that, with an increase of the wavelength, the fundamental CPP mode shifts progressively toward the groove opening, ceasing to be guided at the groove bottom and becoming hybridized with wedge plasmon-polaritons running along the groove edges.

Fabrication of dielectric optical components for surface plasmon polaritons (SPPs) by two-photon polymerization (2PP) is studied. This direct-write femtosecond laser technology provides a low-cost and flexible method for the fabrication and investigation of plasmonic structures and optical components. Using the 2PP technique, we fabricated narrow dielectric ridges with dimensions as small as 150 nm on metal surfaces. SPP excitation with the laser-fabricated structures and guiding along them are demonstrated.

Propagation of surface plasmon polaritons (SPPs) excited in the wavelength range 720-830 nm at a corrugated gold-film surface with areas of 150-nm-wide and 45-nm-high scatterers arranged in a 380-nm-period triangular lattice containing line defects is investigated by use of near-field optical microscopy. We demonstrate that the SPP at 740-750 nm propagates along 2.2-microm -wide and 16-microm -long line defects with ~50% loss, whereas its propagation along narrower line defects is strongly damped and in periodically corrugated areas is inhibited. We observe significant deterioration of these effects for both longer and shorter wavelengths and conclude that the SPP guiding occurs as a result of the SPP bandgap effect in the structures.

It is generally accepted that measurement of the contrast of the intensity interference pattern formed by two counterpropagating evanescent waves can be used to characterize the resolving power of a collection near-field microscope. We argue that, if the light collected by a fiber probe propagates toward a detector in the form of guided fiber modes, then this perception is wrong. Using symmetry arguments and simple calculations, we show that, if an extended fiber probe is axially symmetric and oriented perpendicular to the propagation direction, then the measured contrast should always be equal to the contrast of the interference pattern.

Using degenerate four-wave mixing in an Fe:LiNbO(3) 0.04-wt. % crystal and an external-reflection near-field optical microscope, we have achieved phase conjugation of light emitted by a fiber tip. We observe that the phase-conjugated light at a wavelength of 633 nm can reach a power of ~0.1 nW and produce a 180-nm-wide spot image in the near-field microscope. This is the first direct demonstration, to our knowledge, of the phase conjugation of near-field components of optical fields.