Imaging slit-coupled surface plasmon polaritons using conventional optical microscopy.

We develop a technique that now enables surface plasmon polaritons (SPPs) coupled by nano-patterned slits in a metal film to be detected using conventional optical microscopy with standard objective lenses. The crux of this method is an ultra-thin polymer layer on the metal surface, whose thickness can be varied over a nanoscale range to enable controllable tuning of the SPP momentum. At an optimal layer thickness for which the SPP momentum matches the momentum of light emerging from the slit, the SPP coupling efficiency is enhanced about six times relative to that without the layer. The enhanced efficiency results in distinctive and bright plasmonic signatures near the slit visible by naked eye under an optical microscope. We demonstrate how this capability can be used for parallel measurement through a simple experiment in which the SPP propagation distance is extracted from a single microscope image of an illuminated array of nano-patterned slits on a metal surface. We also use optical microscopy to image the focal region of a plasmonic lens and obtain results consistent with a previously-reported results using near-field optical microscopy. Measurement of SPPs near a nano-slit using conventional and widely-available optical microscopy is an important step towards making nano-plasmonic device technology highly accessible and easy-to-use.

High-throughput diffraction-assisted surface-plasmon-polariton coupling by a super-wavelength slit.

We propose a novel SPP coupling scheme capable of high SPP throughput and high SPP coupling efficiency based on a slit of width greater than the wavelength, immersed in a uniform dielectric. The dispersive properties of the slit are engineered such that the slit sustains a low-loss higher-order waveguide mode just above cutoff, which is shown to be amenable to wavevector matching to the SPP mode at the slit exit. The SPP throughput and SPP coupling efficiency are quantified by numerical simulations of visible light propagation through the slit for varying width and dielectric refractive index. An optimal SPP coupling configuration satisfying wavevector matching is shown to yield an order-of-magnitude greater SPP throughput than a comparable slit of sub-wavelength width and a peak SPP coupling efficiency ≃ 68%. To our knowledge, this is the first investigation of coupling between higher-order waveguide modes in slits of super-wavelength width and SPP modes.

Enhancing the efficiency of slit-coupling to surface-plasmon-polaritons via dispersion engineering.

We describe a simple method for enhancing the efficiency of coupling from a free-space transverse-magnetic (TM) plane-wave mode into a surface-plasmon-polariton (SPP) mode. The coupling structure consists a metal film with a dielectric-filled slit and a planar, dielectric layer on the slit-exit side of the metal film. By varying the dielectric layer thickness, the wavevector of the SPP mode on the metal surface can be tuned to match the wavevector magnitude of the modes emanating from the slit exit, enabling high-efficiency radiation coupling into the SPP mode at the slit exit. An optimal dielectric layer thickness of approximately 100 nm yields a visible-frequency SPP coupling efficiency approximately 4 times greater than the SPP coupling efficiency without the dielectric layer. Commensurate coupling enhancement is observed spanning the free-space wavelength range 400 nm < or = lambda(0) < or = 700 nm. We map the dependence of the SPP coupling efficiency on the slit width, the dielectric-layer thickness, and the incident wavelength to fully characterize this SPP coupling methodology.

A plasmonic random composite with atypical refractive index.

We present a material composite consisting of randomly oriented elements governed by non-resonant interactions. By exploiting near-field plasmonic interaction in a dense ensemble of subwavelength-sized dielectric and metallic particles, we reveal that the group refractive index of the composite can be increased to be larger than the effective refractive indices of constituent metallic and dielectric parent composites. These findings introduce a new class of engineered photonic materials having customizable and atypical optical constants.

Electron-spin-dependent terahertz light transport in spintronic-plasmonic media.

In this Letter, we demonstrate that electron spin can influence near-field mediated light propagation through a dense ensemble of subwavelength bimetallic ferromagnetic/nonmagnetic microparticles. In particular, we show that ferromagnetic particles coated with nonmagnetic metal nanolayers exhibit an enhanced magnetic field controlled attenuation of the electromagnetic field propagated through the sample. The mechanism is related to dynamic, electromagnetically induced electron spin accumulation in the nonmagnet. The discovery of an electron spin phenomenon in the light interaction with metallic particles opens the door to the marriage of spintronic and plasmonic technologies and could pave the way for the development of light-based devices that exploit the electron spin state.

Terahertz Time-Domain Investigation of Axial Optical Activity from a Sub-wavelength Helix.

Chiral media are characterized by preferential interaction with either left- or right- circularly polarized radiation, whereupon an optically active medium and its enantiomorph possess rotary powers of opposing sign due to mirror handedness of their micro- or nano-structures. Here, we report on the first time-resolved investigations of few-cycle pulse propagation along the axis of a sub-wavelength size helix. Time-resolved measurements of the electric field pulse scattered from the helix enable temporal discriminations of transient scattering mechanisms within the helix. Our main finding is that polarization circularization associated with axial propagation through the helix is non-instantaneous, and requires several picoseconds to develop before reaching steady state values. Using a 3D FDTD model, we describe the field and Poynting vector dynamics within the helix leading to steady state polarization circularization. Our conclusions not only support the established picture that optical activity arises from multiple scattering within the helical structure, but also show that this operative mechanism requires a finite time to induce steady state polarization circularization.

Photonic anisotropic magnetoresistance in dense co particle ensembles.

In this Letter, we experimentally show that the amplitude and arrival time of the terahertz optical transmission through dense ensembles of subwavelength-size ferromagnetic particles is strongly dependent on the orientation of an externally applied magnetic field. The attenuation and delay have the same magnetic field orientation dependence as the electrical anisotropic magnetoresistance inherent to bulk ferromagnetic metals. We envision the application of this magnetic effect in terahertz photonic devices.

Coherent plasmonic enhanced terahertz transmission through random metallic media.

We experimentally show coherent, enhanced terahertz transmission through dense 3D random metallic media having subwavelength heterogeneity. Preservation of the incident polarization state and strong dispersion of the transmitted radiation indicate that the enhanced transmission is due to delocalized plasmonic propagation over distances 5 orders of magnitude greater than the skin depth. The experimental observations are supported by numerical finite difference time-domain simulations.