Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A16: Focus Session: Nano-optical Plasmonics |
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Sponsoring Units: DMP Chair: Gary Wiederrecht, Argonne National Labs Room: LACC 404A |
Monday, March 21, 2005 8:00AM - 8:12AM |
A16.00001: Surface Plasmon Rainbow Jets Alexandre Bouhelier, Gary Wiederrecht A new method for optically exciting and visualizing surface plasmons in thin metal films is described. The technique relies on the use of a high numerical aperture objective lens to locally launch surface plasmons with an area much smaller than their lateral decay length. We visualize directly the intensity distribution of the surface plasmons by detecting the intrinsic lossy modes associated with plasmon propagation in thin films. Our approach allowed us to excite simultaneously a broad spectral continuum of surface waves and to describe for the first time surface plasmon rainbow jets. We quantified the attenuation of the jet as a function of wavelength and film thickness and compared it to the different propagation damping mechanisms. We demonstrated the influence of the interface on the surface plasmon propagation length and demonstrated surface plasmon spectral filtering using molecular excitonic adsorbates. We will discuss the potential of the technique to pump-probe, plasmon-based interface spectroscopy. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A16.00002: Subwavelength Focusing and Guiding of Surface Plasmon Polaritons U. Welp, V. Vlasko-Vlasov, L. Yin, A. Rydh, J. Pearson, S.-H. Chang, S.K. Gray, G.C. Schatz, D.E. Brown, C.W. Kimball It is found experimentally that subwavelength holes in thin metal films are versatile sources for the launching of surface plasmon polaritons (SPP). We use near field scanning optical microscopy to image the evanescent electromagnetic fields around individual holes and in hole arrays. For an arc-shaped hole array fabricated with focused ion beam machining into a Cr/Ag bilayer we show that SPP can be focused to a spot with subwavelength width. Finite-difference time-domain calculations give a quantitative account of the observed SPP patterns. Furthermore, we show that the high SPP intensity in the focal spot can be launched onto a 250 nm wide metal strip guide. This work was supported by the U.S. Department of Energy under Grant Nos. W-31-109-ENG-28, DEFG02-91-ER45439 and DEFG02-03-ER15487. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A16.00003: Sub-wavelength confinement and the diffraction limit for surface plasmon waveguides Rashid Zia, Anu Chandran, Mark Selker, Mark Brongersma Surface plasmon-polaritons have received much attention for their ability to guide electromagnetic energy at optical frequencies. Unlike dielectric waveguides which confine volume electromagnetic waves to an optically dense core via index contrast, these surface electromagnetic waves are coupled to localized charge density oscillations along metal-dielectric interfaces. Consequently, theoretical and experimental works to date have highlighted the differences between the confinement provided by dielectric and plasmonic waveguides. Here, we present a series of related computational and experimental studies directed at illustrating the similarities of dielectric and plasmonic waveguides. Beginning with near-field images of confined plasmon propagation obtained by Photon Scanning Tunneling Microscopy (PSTM), we will discuss the limitations on confinement in plasmonic waveguides. These images will be interpreted by comparison with three-dimensional numerical solutions for the guided polariton modes. Vertical and lateral localization along finite width interfaces will be contrasted, and power density profiles investigated. The implications for the diffraction limited size of surface plasmon modes will be discussed, and ideal geometries for power concentration highlighted. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A16.00004: Optical response of structured noble-metal nanoparticle aggregates Jun Jun Xiao, K. W. Yu Interactions of light with subwavelength structures open new avenues of controlling light for many applications. The optical responses of structured array of noble-metal nanoparticle aggregates immersed in a glass matrix are investigated theoretically, motivated by the recent experimental observation of the splitting of the surface plasmon bands in silver arrays. To capture the strong electromagnetic coupling between the approaching particles in a silver aggregate, the spectral representation of the multiple image formula has been used, and a semiclassical description of the silver dielectric function is adopted from the literature [1]. The splitting of plasmon resonance band of the incident longitudinal and transverse polarized light is found to be strongly dependent on the particle diameter and their separation. Our results are shown in accord with the recent experimental observation. Moreover, a large redshift for the longitudinal polarization can be reproduced. The reflectivity spectrum is further calculated for a dilute suspension of dimer and chain arrays. \newline [1] J. J. Xiao, J. P. Huang, K. W. Yu, Phys. Rev. B, in press. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A16.00005: Omni-directional light emission via surface plasmons from a metal-polymer-metal structure John S.Q. Liu, Mark L. Brongersma Light extraction from LEDs is an important efficiency problem. Planar metallic microcavities have been used in the past to allow facile electrical excitation and obtain resonantly enhanced emission. In general this emission is only enhanced in a narrow angle range, while for some applications a wide emission angle is desirable. We will demonstrate that by using a properly designed, planar, metallic microcavity it is possible to enhance the free space emission via low momentum surface plasmons that lie above the light line. Additionally, for an optimized cavity thickness this enhanced emission can be observed at a well-defined frequency for all angles due to a nearly flat surface plasmon dispersion relation, hence the term omni-directional emission. This effect is predicted through simulations of dipole emission and verified in photoluminescence experiments using gold as the metal and a light-emitting polymer (DOW BP79) as the active medium. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A16.00006: Plasmon Waveguides: Balancing Propagation, Localization, and Loss below the Diffraction Limit Jennifer Dionne, Luke Sweatlock, Harry Atwater, Albert Polman On subwavelength scales, photon-matter interactions are limited by diffraction. Circumventing this diffraction limit is now a principle focus of integrated nanophotonics. Here, we present studies of surface plasmon (SP) waveguides exhibiting both long-range propagation and spatial confinement of light with lateral dimensions of less than 10 percent of the free-space wavelength. Attention is given to characterizing the dispersion relations, mode profiles, wavelength dependent propagation, and energy density decay in metallodielectric waveguides comprised of silicon dioxide/Ag/silicon dioxide and Ag/silicon dioxide/Ag structures with waveguide thicknesses ranging from 12nm-50nm. Numerical dispersion analysis indicates the presence of three distinct SP branches, including the existence of modes in the plasmon bandgap. For bound modes in Ag waveguides, near-IR propagation lengths exceed centimeter scales only at the expense of confinement. However, enhanced propagation is observed at shorter wavelengths despite notable field localization in the metal. Likewise, for silicon dioxide SP waveguides, propagation lengths exceed tens of microns with fields confined to within 30 nanometers of the structure. Applications of both short and long-wavelength plasmons to photonic waveguiding will be discussed, and utilization of such results for integrated plasmonic applications will be explored. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:48AM |
A16.00007: Strongly coupled plasmon excitations in nanostructures and device applications Invited Speaker: Since the 16th century, optical device size and performance has been limited by diffraction. However the diffraction limit can be circumvented via design of ``plasmonic'' device components with spatial confinement of light at dimensions less than 10{\%} of the free-space wavelength. Achieving control of light-material interactions at nanoscale dimensions requires structures that guide electromagnetic energy with a lateral mode confinement below the diffraction limit. Electromagnetic energy can be guided below the diffraction limit along ultrathin metallic stripes and subwavelength scale chains of closely spaced metal nanostructures via non-radiating surface plasmons. Recent experiments confirmed that strongly coupled collective plasmonic modes in metal nanostructures enable electromagnetic energy transport over distances of about 0.5 $\mu $m in plasmon waveguides. The emission rate from active dipole emitters such as semiconductor nanocrystals can also be enhanced by coupling into metallic nanostructures. Thus there appears to be no fundamental scaling limit to the size and density of photonic devices, and ongoing work is aimed identifying important device performance criteria in the subwavelength size regime. Ultimately it may be possible to design an entire class of subwavelength-scale optoelectronic components that form building blocks of an optical device technology scaleable to molecular dimensions, with imaging and spectroscopy applications. \newline \newline Co-authors: Luke Sweatlock, Jennifer Dionne, and Julie Biteen, California Institute of Technology [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A16.00008: Tuning the Nanooptics of Metallic Nanorods: End Effects Garnett Bryant, Javier Aizpurua Understanding the nanooptics of metallic nanoparticles is critical for developing their use as sensors and nanohighways for directed excitation transport. End effects provide a new means to tailor nanorod response. We calculate the optical response of nanorods focusing on end effects and show that rod termination critically determines the position and width of plasmon resonances. Rods with hemispherical ends exhibit broad response typical for dipolar plasmonic charge oscillation along the rod. Rods with inverted, concave ends exhibit much narrower, cavity-like resonances unlike typical plasmon resonances. However, near-field enhancement is not dramatic for rods with two concave ends. Large near-field enhancement and narrow resonances can be achieved simultaneously for the near-field at the concave end of a rod with one concave end and one hemispherical end. Multiple narrow resonances can occur for large rods and deep cavities at the rod ends. These sharp resonances are seen both in the far-field and the near-field response. [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A16.00009: Optical propagation via dipolar coupling in metal nanoparticle chains Willes H. Weber, George W. Ford Electromagnetic propagation in metal nanoparticle chains offers the potential for nano-sized integrated optical circuits. Dispersion relations for dipolar modes propagating along such a chain are calculated by solving the full Maxwell equations, including radiation damping. The nanoparticles are treated as point dipoles, which means the results are valid only for $a$/$d \quad \le $ 1/3, where $a$ is the particle radius and $d$ the spacing.$^{1}$ The discrete modes for a finite chain are first calculated, then these are mapped onto the dispersion relations appropriate for the infinite chain. Computed results are given for a chain of 50-nm diameter Ag spheres spaced by 75 nm.$^{2}$ We find large deviations from previous quasistatic results:$^{3}$ Transverse modes interact strongly with the light line. Longitudinal modes develop a bandwidth more than twice as large, resulting in a group velocity that is more than doubled. All modes for which $k_{mode} \quad \le $ \textit{$\omega $/c} show strongly enhanced decay due to radiation damping. These features are consistent with recent calculations by Citrin.$^{4}$ $^{1}$ S. Y. Park and D. Stroud, Phys. Rev. B \textbf{69}, 125418 (2004). $^{2}$ W. H. Weber and G. W. Ford, Phys. Rev. B \textbf{70}, 125429 (2004). $^{3}$ M. L. Brongersma, J. W. Hartman, and H. A. Atwater, Phys. Rev. B \textbf{62}, 16356 (2000). $^{4}$ D. S. Citrin, Nano Lett. \textbf{4}, 1561 (2004). [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A16.00010: Adiabatic Nanoplasmonic Energy Concentration Mark Stockman We predict that propagation of surface plasmon polaritons (SPPs) toward a sharp edge of smoothly (adiabatically) graded metallic layer causes their slowing down and asymptotic stopping. This is accompanied by a concentration of electromagnetic energy and enhancement of local optical fields. As such a nano-concentration effect , we consider the adiabatic energy concentration in a conic nanoplasmonic waveguide ..[1]. In this case, SPPs are created in the $m=0$ plasmonic state at the wide (microscopic) edge of the system and propagate toward the tip of the conic waveguide. As local radius $R$of the cone waveguide decreases, both the phase and group velocity tend to zero $\propto R$. This asymptotic stopping lead to the accumulation of SPPs at the tip and their adiabatic transformation to the standing surface plasmons. For silver, it is possible to have local optical intensity at the tip increased by three orders of magnitude. We also discuss two- dimensional focusing nanoplasmonic waveguides. In conclusion, we will discuss the many prospective application of this effect. [1]. M. I. Stockman, \textit{Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides}, Phys. Rev. Lett. \textbf{93}, 137404-1-4 (2004). [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A16.00011: Green-function theory of confined plasmons in coaxial cylindrical geometries: finite magnetic field M.S. Kushwaha, B. Djafari-Rouhani We report on a theoretical investigation of the plasmon propagation in the coaxial cylindrical geometries using Green's function (or response function) theory in the presence of an applied axial magnetic field ($\vec{B}\parallel \hat{z}$). Green's function theory generalized to be applicable to such quasi-one dimensional (1D) systems enables us to derive explicit expressions for the corresponding response functions (associated with EM fields), which can in turn be used to compute numerous physical properties of the system under consideration. As an application, we present several illustrative examples on the dispersion characteristics of the confined and extended magnetoplasmons in the single- and double-interface structures. These dispersive modes are also substantiated through the computation of local as well as total density of states (DOS). It is found that, unlike the zero-field case, the magnetoplasma propagation is nonreciprocal with respect to the sign of the index $m$ of the Bessel functions involved. We also briefly clarify some delusive traces of the edge magnetoplasmpons for a plasma shell embedded between two identical or unidentical dielectrics. Our theoretical framework can also serve as a powerful technique for studying the intrasubband plasmons and magnetoplasmons in the emerging mutiple-walled carbon nanotubes. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A16.00012: Plasmonic resonances, scattering cross section, and electromagnetic forces between two coupled Au nanowires Klaus Halterman, Merle Elson, Surendra Singh We compute the electromagnetic response of two infinitely long silver nanowires, each with a square cross section, when illuminated by a plane wave with the electric vector perpendicular to the axis of the wires. The wire dimensions are on the order of $0.1 \lambda$. Of particular interest is when the incident wavelength excites plasmonic resonances in the wires and the associated field enhancement. We focus on (a) the scattering cross section and the (b) forces exerted on the wires both directly by the plane wave and by mutual coupling between the wires. [Preview Abstract] |
Monday, March 21, 2005 10:48AM - 11:00AM |
A16.00013: Surface Plasmons in Anisotropic Metal-Dielectric Structures Arkadii Krokhin, Kemal Bagci, Arup Neogi We study the effects of anisotropy of a dielectric substrate and a metal film on the dispersion relation and range of propagation of surface plasmons. The substrate is considered to be a uniaxial crystal with its axis perpendicular to the metal surface. The dielectric constants of the substrate are $\varepsilon _{\vert \vert } $ (in the plane of propagation) and $\varepsilon _\bot $ (in the perpendicular direction). The metal film is characterized by a complex dielectric tensor with isotropic real negative part ${\varepsilon }'(\omega )$and anisotropic positive imaginary part ${\varepsilon }''_{ik} (\omega )$. The latter has two different components${\varepsilon }''_{\vert \vert } (\omega )$ and ${\varepsilon }''_\bot (\omega )$. Anisotropy of the dissipative part of the dielectric tensor is due to the surface channel of electron scattering, leading to the lower ac conductivity of the thin film in the direction perpendicular to the metal surfaces. We show that the substrate with $\varepsilon _\bot >\varepsilon _{\vert \vert } $ gives rise to larger propagation length of the surface plasmon. Thus from the point of view of efficiency of the plasmonic devices anisotropic \textit{negative} uniaxial crystals are preferential. In this case the decay length of the plasmon field in the substrate also increases. This decay length is an important characteristic of the sub-wavelength optical resolution. [Preview Abstract] |
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A16.00014: Near-field study of the perfect lens at visible wavelengths Pieter Kik, Grady Webb-Wood It has been predicted that thin metal films can be used to generate images with a spatial resolution better than the diffraction limit via the local excitation of surface plasmons [1]. Such near-field focusing could have applications in optical data storage and nanofabrication. We present near-field scanning optical microscopy (NSOM) experiments that clearly demonstrate frequency dependent focusing using a near-field lens. The `perfect lens' is fabricated by depositing a 50nm thick gold layer onto a 50nm thick silicon nitride membrane. Focusing is detected by monitoring the interference between light emitted from a nanoscale object (the aperture of an NSOM tip) and radiation scattered by Pt nanoparticles placed in the image plane behind the lens. NSOM scans performed at wavelengths in the range 468nm-676nm reveal the role of surface plasmons in the imaging process. The measured frequency dependent image resolution is compared with numerical simulations based on the Finite Integration Technique. [1] J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2001) [Preview Abstract] |
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