Jeff Wuenschell

Blue-shift of surface plasmon resonance in a metal nanoslit array structure

The adsorption of a self-assembled monolayer of molecules on a metal surface commonly causes a red-shift in its surface plasmon resonance. We report that the anomalous dispersion of surface plasmons in a Au nanoslit array structure can cause a blue-shift of optical transmission upon adsorption of a non-absorbing self-assembled monolayer of molecules. We develop a simple model that explains the blue-shift observed in the transmission spectra with monolayer adsorption in terms of the interplay of anomalous dispersion and the cavity resonance of surface plasmons in the nanoslit array.

High-sensitivity surface plasmon resonance spectroscopy based on a metal nanoslit array

We have chemically modified metal nanoslit array surfaces with alkanethiol self-assembled monolayers and have characterized the resulting spectral shift of optical transmission. Adsorption of a self-assembled monolayer (1.5nm thick) on a silver nanoslit array (slit width of 30–50nm and grating period of 360nm) is found to cause an 11nm redshift of the main transmission peak. Strong confinement of optical fields in the narrow slit region allows sensitive transduction of surface modification into a shift of surface plasmon resonance wavelength.

Surface plasmon dynamics in an isolated metallic nanoslit.

We present an analytical study of the dynamic interplay among surface plasmon polarization charges, electromagnetic fields, and energy flow in the metal/dielectric interface and metal nanoslit structure. Particular attention is given to the regime where the energy flow in the metal side is significant compared to that in the dielectric side. The study reveals that a vortex-like circulation of energy is an intrinsic feature of surface plasmon propagation supported by a metal/dielectric interface, and, in general, a vortex can form when the permittivity and permeability values of the materials involved satisfy the following condition: {(epsilon(m)/epsilon(d)) < -1 and (mu(m)/mu(d)) > -1} or {(epsilon(m)/epsilon(d)) > -1 and (mu(m)/mu(d)) < -1}.

Excitation and Propagation of Surface Plasmons in a Metallic Nanoslit Structure

We provide an analysis of optical interactions with a nanoslit structure formed on a metal film. In particular we analyzed the excitation and coupling of surface plasmons (SPs) at the input and output apertures of the nanoslit as well as the propagation of light in the nanoslit waveguide structure. In the input coupling case, an incident light (TM polarized) is found to induce polarization charges on the surface around the aperture. These charges generate the normal component of electric field on the incident surface around the corner. This results in an inward flow (funneling) of incident power into the slit. At the output side, the slit corners again play a critical role, exciting SPs on the exit surface, which propagate away from the slit, and also allowing for decoupling of the SP power into free-space radiation. In the case of narrow slits, the incident optical power is carried exclusively via the SP mode that has an antisymmetric distribution of polarization charges on the slit walls. The materials required to support a surface-bound wave are also discussed, taking into account the dielectric loss of metal.

Near- to far-field imaging of free-space and surface-bound waves emanating from a metal nanoslit

The authors report the radiation pattern (radial and angular distribution of light intensity) of a silver nanoslit measured in the near- to far-field regimes. In most far fields, the 1∕r dependence of intensity distribution, expected from a cylindrical wave emanating from a line source, is clearly observed. The glancing angle regime is found to be governed by the presence of surface plasmons, showing higher intensity closer to the metal surface. From the radiation patterns measured with a tilted-probe, radial-scan method, a branching ratio is quantitatively determined for the free-space radiation and surface plasmon components, emerging from the nanoslit.

Near- to far-field imaging of phase evolution of light emanating from a metal nanoslit

We report near- to far-field measurement of optical wavefronts emanating from a nanoslit formed in a thin (50 nm thick) Ag film. The evolution of optical phases is imaged using a self-interference technique in conjunction with a scanning probe method. The phase relationship of the slit-transmitted waves with respect to the direct transmission through the thin metal film is quantitatively established. The singular-phase points resulting from the interplay of slit diffraction and surface plasmons are identified in the intermediate-field region.

Edge trapping of exciton-polariton condensates in etched pillars

In this letter, we present a study of the condensation of exciton-polaritons in large etched pillar structures that exhibit shallow edge trapping. The ≈100 μm × 100 μm pillars were fabricated using photolithography and a BCl3/Cl2 reactive ion etch. A low energy region emerged along the etched edge, with the minima ≈7 μm from the outer edge. The depth of the trap was 0.5–1.5 meV relative to the level central region, with the deepest trapping at the corners. We were able to produce a Bose-Einstein condensate in the trap near the edges and corners by pumping non-resonantly in the middle of the pillar. This condensate began as a set of disconnected condensates at various points along the edges but then became a single mono-energetic condensate as the polariton density was increased. Similar edge traps could be used to produce shallow 1D traps along edges or other more complex traps using various etch geometries and scales.

Plasmon dynamics in a metal nanoslit

We present an analytical study of the dynamic interplay among surface plasmon polarization charges, electromagnetic fields, and energy flow in the metal/dielectric interface and metal nanoslit structure. Particular attention is given to the regime where the energy flow in the metal side is significant compared to that in the dielectric side. The study reveals that a vortex-like circulation of energy is an intrinsic feature of surface plasmon propagation supported by a metal/dielectric interface, and, in general, a vortex can form when the permittivity and permeability values of the materials involved satisfy the following condition: {(<i>epsiv_m </i>/<i>epsiv_d </i>) < -1 and (<i>mu_m </i>/<i>mu_d </i>) > -1} or {(<i>epsiv_m </i>/<i>epsiv_d </i>) > -1 and (<i>mu_m </i>/<i>mu_d </i>) < -1}.

Plasmonic phenomena in metal nanoapertures and chip-scale instrumentation for biochemical sensing

Plasmonic interactions in the metallic nanoaperture array allow for rich phenomena, such as concentration and channeling of light in the subwavelength scale structures. We present high-sensitivity surface plasmon resonance spectroscopy based on a metal nanoslit array. Strong confinement of optical fields in the slit region allows sensitive transduction of surface modification into a shift of surface plasmon resonance wavelength. A metal nanoslit array is also designed to provide spectral filtering in a fashion that is highly scaleable in physical dimension and channel capacity. A spectral sensing technology is presented that can shrink a spectrometer down to a chip-scale, yet offering high sensitivity (~lambda/100) in a broad spectral range (visible to NIR). Overcoming the limitations of diffractive optics, the plasmonics technology is expected to revolutionize the biomedical instrumentation area with its unique capability in spectroscopy, imaging, and sensing and manipulation of biochemicals.