Session T10: Focus Session: Optical Properties of Nanostructures V: Plasmonics and Metamaterials

Metal films with arrays of subwavelength holes show infrared spectral and plasmonic sensitivity to material in the holes

Experiments and FDTD calculations show that infrared (IR) absorption spectroscopy and IR transmission resonances of metal films with arrays of subwavelength holes (mesh) are more sensitive to material in the holes as opposed to material on the front or back surface of the mesh. The basic optical physics of the transmission resonances will be discussed including determinations of front-back coupling and surface plasmon (SP) dispersion curves. Applications including enhanced IR absorption spectroscopy of nanocoatings, catalytic reactions, and individual dust particles in the 1-5 micron diameter range will be discussed. Finally, the interaction of an IR SP-mediated resonance with a vibration of molecules in a mesh coating will be presented.   

Influence of the dielectric environment on periodic hole arrays

The influence of various dielectric environments, surrounding periodic hole arrays in optically thick metal films, was studied experimentally. The transmittance (T) and reflectance (R) at nearly normal incidence, were measured using a microscope photometer in the near infrared region and Bruker IFS 113v in the mid-infrared region. The metal films are fabricated on substrates with different refractive indices for the two spectral ranges. The refractive index of the material on the other side, can be either that of air or of another dielectric. For large differences in refractive index, between the two sides, the spectra contains separate resonances. These resonances shift in frequency as the refractive indices match better and coincide with the existing ones in the case of good match, enhancing them in amplitude and linewidth. The experimental results are compared to theoretical predictions.   

Controlling the polarization of light transmitted through metallic bilayers of subwavelength apertures

Periodic apertures in a metal film exhibit extraordinarily high optical transmission at wavelengths where surface excitations are at resonance with the incident light. The evanescent fields created by these subwavelength structures can channel the optical energy to specific locations, resulting in strong and localized field enhancements. We fabricated two metal films patterned with subwavelength slit arrays in close proximity and tailored the evanescent field coupling between them to achieve new functionalities. At the resonant wavelength, TM polarized light is transmitted efficiently with its transmitted magnitude and phase being strongly coupled to the lateral shift between the two layers. Transmission of TE polarization is smaller by several orders of magnitude. We present both numerical simulations and experimental data to demonstrate that by exploiting these properties a novel polarization rotator can be manufactured using a planer process offering the potential to dramatically reduce system cost and size.   

Perfect coupling of light to surface plasmons with ultra-narrow linewidths

We examine the coupling of electromagnetic waves entering a thin silver film that forms an oscillatory grating embedded between two otherwise uniform semi-infinite half spaces having identical dielectric constants. On reducing the grating period from the long wavelength limit we encounter signatures in the transmission, T, and reflection, R, coefficients associated with: 1) the symmetric surface mode, 2) the anti-symmetric surface mode, and 3) electromagnetic diffraction tangent to the grating; the first two can be regarded as generalized (plasmon) Woods anomalies while the third is the first-order conventional (electromagnetic) Woods anomaly. The energy density at the film surface is enhanced for wavelengths corresponding to these three anomalies, particularly for the antisymmetric plasmon mode in thinner films. When exciting with two waves entering from opposite directions we find that by adjusting the grating oscillation amplitude and fixing the relative phase of the incoming waves to be even or odd, T+R can be made to vanish for one or the other of the plasmon modes; this corresponds to perfect coupling (impedance matching) between the incoming light and these modes.   

Plasmon Hybridization of a thin metallic film

We investigate the surface plasmon modes of a thin metallic film using the Plasmon Hybridization method and solving Maxwell's equations. In the electrostatic limit, we show that the high energy plasmon mode is the antibonding mode in which surface charges are antisymmetrically distributed, and the low energy mode is the bonding mode in which surface charges are symmetrically distributed. In the thin film, secondary charges which are induced from the primitive plasmons on the other film surface play an important role to determine which plasmon mode has the higher or lower energy. Furthermore, we discuss how the secondary charges affect the propagation length of the surface plasmon by calculating the imaginary parts of the surface plasmon wave vectors.   

Plasmon coupling in nanoparticle rings

A surface plasmon is excited when the conduction band electrons of a metal oscillate coherently in phase with incoming excitation light. Plasmons can exist and propagate along structures that are smaller than the diffraction limit of light, the parameter which currently dictates the minimum size of optical interconnects. In addition to exploiting plasmons on continuous structures like thin films and nanowires for waveguiding, arrays of nanoparticles also pose potential for waveguiding. We have characterized the plasmon coupling of self-assembled rings of 40 nm gold nanoparticles functionalized with polystyrene using dark-field scattering microscopy and spectroscopy. Comparing images and spectra from the rings to those of single particles together with correlating images acquired by dark-field and SEM microscopy, we observe redshifted coupled plasmon modes that show a strong polarization dependence. In particular, segments of the ring aligned parallel to the axis of detected polarization display higher order longitudinal plasmon modes, similar to those observed for a long rod.   

Evidence for Surface Plasmon Standing Waves in Ag Nanostructure Arrays

We report on simulations of the near field for arrays of Ag nanowires and nanocolumns excited by plane waves of light at normal incidence. The results show a systematic variation of the local electric field with spatial period and incident polarization, which is confirmed experimentally [1]. The distribution of the local field and the dependence of local field intensity versus spatial period and polarization indicate that the excitations in the Ag nanostructres are surface plasmon standing waves. \\[0pt] [1] S.H.Guo, J.J. Heetderks, H.-C. Kan and R.J. Phaneuf, Optics Express 16, 18417 (2008).   

Transforming Light with Metamaterials

Metamaterials are expected to open a gateway to unprecedented electromagnetic properties and functionality unattainable from naturally occurring materials, thus enabling a family of new ``meta-devices.'' We review this new emerging field and significant progress in developing metamaterials for the optical part of the spectrum. Specifically, we describe recently demonstrated artificial magnetism across the whole visible, negative-index in the optical range, and promising approaches along with challenges in realizing optical cloaking. A new paradigm of engineering space for light with transformation optics will be also discussed.   

Analytic LC model for plasmonic resonances in nano-structured split-ring resonators

We systematically investigate the plasmonic resonant behavior of metallic nano-structured split-ring resonators as a function of the size of the split gap, substrate permittivity, metal skin depth, and sample height. We performed simulations of the structures and examine the E-field and current density maps. We present a simple, analytic LC model to describe the lowest order resonance and its dependence on the aforementioned parameters.   

Angular Dependence of Resonances from Rod Pairs and U-Shapes

We examine and compare the angular dependence of electric and magnetic resonances from rod pairs and U-shapes made of Ag with the long arms placed both horizontally and vertically. We discuss the results in terms of photonic band structure effects. In addition, we observe that the splitting of the higher order mode for vertical U-shapes in the s-polarization behave as symmetric and anti-symmetric modes in which the modes red and blue shift, respectively, as observed previously. However, both horizontal U-shapes and rod pairs also show a split for the p-polarization in which both modes red shift. We discuss the reasons for this behavior using both experimental results and simulations.   

Longitudinal elliptically polarized electromagnetic waves in anisotropic magnetoelectric split ring composites

We study the propagation of plane eletromagnetic waves through different systems consisting of arrays of split rings of different orientations. We find a mode such that the electric field becomes elliptically polarized with a component in the \textbf{longitudinal} direction (i.e., parallel to the wave vector). Even though the group velocity and the wave vector are parallel, the Poynting vector possesses a component perpendicular to the wave vector. The speed of light can be real even when the product $\epsilon\mu$ is negative. Other novel properties are explored.   

Natural negative refraction index in polycrystalline Fe and Ni at optical frequencies

Analysis of the photon-magnon interactions in Fe and Ni, 3d transition-metal ferromagnetics, demonstrating the coupling between the incident light and high-frequency spin waves with energy ( 0.2 -- 0.35 ) eV is presented. As a consequence, these metals in their polycrystalline form with nanoscale grains are found to possess a negative refraction index at optical frequencies, close to the high-frequency ferromagnetic resonance. The effect is due to the coexistence of the spin wave mode with the plasmonic mode, and both modes are activated by the e.m. field of the light, with simultaneous permittivity and permeability responses within some frequency band.   

Propagating and Localized Surface Waves in Metamaterial Stacks

We demonstrate the interference effect between propagating and localized surface modes of electromagnetic wave in metamaterial stacks, which leads to a transmission extremum. When radiation is incident on a metal surface perforated with an array of ring-shaped subwavelength apertures, the phase difference between the propagating surface Bloch wave and the localized surface wave can be tailored by the geometrical parameters of the array so as to affect the shape of the transmission spectrum. Above the resonant frequency of the aperture, interference between the surface waves leads to a minimum in the transmission spectrum, whereas below it, the interference leads to a maximum. While in multiple metamaterial stacks with hole arrays, the coupling of surface electromagnetic wave yields a new resonant mode with increasing quality factor of the transmission peak. We suggest that these features provide flexibility in engineering surface wave-based all-optical devices. Reference: Y. J. Bao, R. W. Peng, D. J. Shu, Mu Wang, X. Lu, J. Shao, W. Lu,and N. B. Ming, Phys. Rev. Lett. (2008) 101, 087401.