Range Surface Plasmon Research Articles

Plasmonic Notch Filter Design Based on Long-range Surface Plasmon Excitation Along Metal Grating

A single notch plasmonic spectral filter design using evanescently coupled resonant ultrathin metal grating is numerically studied in this article. Due to excitation and coupling of long range surface plasmon between the metal grating nanowires, a deep and narrow reflection spectrum dip can be obtained. Narrower spectral bandwidth is achieved through decreased damping from the existence of large dielectric gaps between the grating nanowires. This physical explanation is confirmed by the field distribution calculation. As an example, a single notch filter design with full width half maximum band width less than 3 nm centered at 808 nm is presented.

Coupled long range surface plasmons for the investigation of thin films and interfaces

We report a new approach for the investigation of thin films and interfaces based on the spectroscopy of coupled long range surface plasmons (cLRSPs). These modes are supported by a symmetrical layer structure with two metallic waveguides along which long range surface plasmons (LRSPs) propagate. For sufficiently small gap between the metallic waveguides, coupling between these LRSPs is established which gives rise to two new cLRSP modes. In surface plasmon resonance (SPR) sensors, cLRSPs can simultaneously probe events occurring on the sensor surface that are accompanied with refractive index changes. As the evanescent field of the two cLRSPs exhibits different penetration depths into the sensed medium on the top of the sensor surface, the spectroscopy of cLRSPst allows for the interrogating of the distribution of refractive index changes perpendicular to the surface. This feature advances the performance of current SPR sensors. We demonstrate the potential of the spectroscopy of cLRSP in an application in which we investigated swelling of thin hydrogel films and the diffusion of protein molecules into these gels.

NEW CONCEPTS WITH SURFACE PLASMONS AND NANO-BIOINTERFACES

In classical surface plasmon-based optical biosensors, a surface plasmon mode is resonantly excited on the metallic sensor surface to probe any analyte-binding-induced refractive index changes. The field of the surface plasmon mode evanescently decays from the metal into an adjacent analyte solution with a typical penetration depth of 200 nm. In order to maximize the sensitivity of SPR biosensors, interfacial polymer architectures with binding site densities that considerably exceed planar arrangements through the use of three-dimensional microstructures were introduced. For biosensors based on surface plasmon fluorescence spectroscopy, this type of matrix offers the additional advantage of preventing fluorescence quenching, which is caused by the proximity of the chromophore label to the acceptor states of the noble metal. By means of probing the binding events with long range surface plasmon modes of which field extend much farther into the analyte solution (up to the micrometer range), substantially thicker sensor matrix layers can be used. Into such matrices larger amounts of ligands can be loaded, which enables one to increase the surface density of binding sites and thus to enhance the sensitivity of the biosensor. We present results which show that functionalized hydrogels are very well suited for meeting the demands of these novel biosensor platforms.

Strongly coupled surface plasmons on thin shallow metallic gratings

The optical response of a thin metallic film with shallow corrugations on both surfaces is explored and the structure is found to support a strongly coupled surface plasmon polariton when transverse magnetic radiation is incident in a plane parallel to the grating grooves. Modeling confirms that this strongly excited mode is the short range surface plasmon polariton and its presence is confirmed experimentally in the visible part of the spectrum.

Short Range Plasmon Resonators Probed by Photoemission Electron Microscopy

Short range surface plasmon resonators are investigated at the nanometer scale. Gold nanorods (30 nm in diameter) were microfabricated and probed by photoemission electron microscopy under direct laser light excitation. Resonances presenting various numbers of lobes occur for specific rod lengths. A simple analytical model shows that the successive resonant lengths differ by a multiple of one-half of the wavelength of the supported short-range surface plasmon polariton.

Coupled surface plasmons on thin silver gratings

A thin metal film with corrugations on both surfaces is shown to couple visible photons to both the long range surface plasmon polariton (LRSPP) and the short range surface plasmon polariton (SRSPP). It is found that the first harmonic component of the grating shape causes a significant band gap (an anti-crossing) where the LRSPP and the SRSPP should cross. Experimental data are compared with model calculations using a multilayer, multishape differential grating theory. In addition, to clarify the nature of the modes, the time averaged magnetic field distributions and instantaneous electric field profiles are explored, specifically at the anti-crossings of the first-order modes.

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20nm and ∼10μm, respectively. The lowest propagation loss of ∼1.4dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55μm. With a single-mode fiber, the lowest coupling loss of ∼0.4dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55μm. The lowest insertion losses are obtained 8–9dB with 1.5cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10Gbps optical signal transmission via the LRSPP waveguide with a 14nm-thick, 2.5μm-wide, and 4cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.

Vertical dielectric-sandwiched thin metal layer for compact, low-loss long range surface plasmon waveguiding

In this letter, the author proposes a long range surface plasmon waveguide structure of a vertically standing thin metal layer sandwiched between finite-width dielectric layers to achieve both compact and low-loss photonic waveguiding. Rounded 90° bends in such waveguides, with radii of curvature on the scale of the incident wavelength, are studied. The author analyzes waveguiding modes of such waveguides at the straight and bend parts, and also the coupling of surface plasmon waves between them. The analyses are verified with numerical simulations. High transmission of surface plasmon waves through the bends is demonstrated.

Long Range Surface Plasmons for Observation of Biomolecular Binding Events at Metallic Surfaces

A long range surface plasmon (LRSP) is an electromagnetic wave propagating along a thin metal film with an order of magnitude lower damping than conventional surface plasmon (SP) waves. Thus, the excitation of LRSP is associated with a narrower resonance and it provides larger enhancement of intensity of the electromagnetic field. In surface plasmon resonance (SPR) biosensors, these features allow a more precise observation of the binding of biomolecules in the proximity to the metal surface by using the (label-free) measurement of refractive index (RI) variations and by SP-enhanced fluorescence spectroscopy. In this contribution, we investigate LRSPs excited on a layer structure consisting of a fluoropolymer buffer layer, a thin gold film, and an aqueous sample. By implementing such structure in an SPR sensor, we achieved a 2.4- and 4.4-fold improvement of the resolution in the label-free and fluorescence-based detection, respectively, of the binding of biomolecules in the close proximity to the surface. Moreover, we demonstrate that the sensor resolution can be improved by a factor of 14 and 12 for the label-free and fluorescence-based detection, respectively, if the biomolecular binding events occur within the whole evanescent field of LRSP.

Hybrid three-arm coupler with long range surface plasmon polariton and dielectric waveguides

The characteristics of hybrid three-arm coupler, which consists of the middle long range surface plasmon polariton waveguide and two outside conventional dielectric waveguides, are analyzed numerically with finite element method. Since the middle arm can only transfer the TM mode from one dielectric arm to the other one, this structure is promising for realizing integrated polarization splitter. Compared with the two-arm hybrid coupler, the loss of three-arm hybrid coupler based device can be reduced. Furthermore, different from conventional three-arm dielectric coupler, the authors find that the three-arm hybrid coupler with asymmetric structure can reach a much higher extinction ratio.

Coupling between long range surface plasmon polariton mode and dielectric waveguide mode

Coupling of TM mode between long range surface plasmon polariton (LRSPP) mode and conventional dielectric waveguide mode is demonstrated numerically with finite element method. The characteristics of a hybrid coupler, which consists of the LRSPP waveguide and conventional single mode dielectric waveguide, are analyzed. The high efficient coupling shows a possible route for integrating the SPP device and conventional dielectric optical devices together and a method to excite LRSPP mode with dielectric waveguide. Because the coupling just occurs on TM mode, this kind of the hybrid coupler can be used as polarization mode splitter and combiner.

Optimal self-referenced sensing using long- and short- range surface plasmons

Dual-mode surface-plasmon resonance (SPR) sensors use both long- and short- range surface plasmon waves to differentiate surface binding interactions from interfering bulk effects. We have optimized the design of these sensors for minimum surface limit of detection (LOD) using a Cramer-Rao lower bound for spectral shift estimation. Despite trade-offs between resonance width, minimum reflectivity, and sensitivity for the two modes, a range of reasonable design parameters provides nearly optimal performance. Experimental verification using biotin-streptavidin binding as a model system reveals that sensitivity and LOD for dual-mode sensors remains competitive with single-mode sensors while compensating for bulk effects.

Characteristics of ultra-long range surface plasmon waves at optical frequencies

It has been reported earlier that ultra-long range surface plasmon waves can be supported if dielectric layers with lower index of refraction than that of the dielectric cladding are placed on either side of the thin metal film. In this paper, we report a further investigation of the ultra-long range surface plasmon modes and the condition to support such ultra-long propagation distances at optical frequencies. We studied the effects of the index of refraction contrast between the inner layer and the cladding dielectrics, the metal film thickness, and the dispersion with wavelength. We present a condition which must be satisfied by the waveguide structure to support the bound ultra-long range surface plasmon mode.

Long range surface plasmon fluorescence spectroscopy

Surface plasmon modes, resonantly excited at the two sides of an ultrathin (noble) metal layer in contact with two (nearly) identical dielectric media interact via the overlap of their electromagnetic fields resulting in two new coupled modes, i.e., a short range and a long range surface plasmon (LRSP), respectively. The authors demonstrate that both the enhanced optical field of the LRSP wave at the metal/dielectric interface as well as its increased (evanescent) penetration depth reaching farther into the analyte solution can be used for significant enhancements when using LRSP optics in a fluorescence spectroscopic mode of operation. They demonstrate this for the detection of fluorescence intensities from chromophore labeled proteins bound to the sensor surface matrix.

Polymeric variable optical attenuator based on long range surface plasmon polaritons

Proposed is a polymeric variable optical attenuator based on long range surface plasmon polaritons (LRSPPs) along a thin metal stripe embedded in polymers. The device is operated by controlling radiation loss of the LRSPP mode resulting from the temperature gradient of the polymer cladding caused by a heater. For guiding LRSPPs and efficient coupling of singlemode fibres, gold stripes 20 nm thick, 4 µm wide and 1 cm long are utilised. To obtain a long physical lifetime, the heater is formed on the top of the polymer cladding with a 200 nm Au film which is about ten times thicker than the thin metal waveguide. The fabricated device is characterised at a wavelength of 1.55 µm, exhibiting high attenuation of less than 30 dB with an operating power of 100 mW. A fibre-to-fibre total insertion loss of 6.1 dB is achieved when using singlemode fibres.

Simultaneous excitation of long and short range surface plasmons in an asymmetric structure

An asymmetric multilayer structure allowing nearly 100% simultaneous excitation of both symmetric and antisymmetric surface plasmons is proposed. It is shown that the two surface plasmons can be excited at a fixed angle of incidence with polychromatic light or with monochromatic light at different angles of incidence, depending on the design. The structure can be of interest in high-performance surface plasmon resonance sensing and in all-optical signal processing.

Energy-Time Entanglement Preservation in Plasmon-Assisted Light Transmission

We report on experimental evidence of the preservation of the energy-time entanglement of a pair of photons after a photon-plasmon-photon conversion. This preservation is observed in two different plasmon conversion experiments, namely, extraordinary optical transmission through subwavelength metallic hole arrays and long range surface plasmon propagation in metallic waveguides. Plasmons are shown to coherently exist at two different times separated by much more than their lifetimes. This kind of entanglement involving light and matter is expected to be useful for future processing and storing of quantum information.

Experimental study of the enhanced Raman scattering in the Sarid-type 4-layered attenuated total reflection configurations

We experimentally explored the enhanced Raman scattering in pyridine (C 5H 5N) due to the excitation of long range surface plasmons (LRSP's) guided by a symmetrically bounded silver slab in the Sarid-type ATR (attenuated total reflection) configuration. We have observed that, with decreasing the metallic thickness, the absolute enhancement of the Raman cross section increases while the width of the polar angular distribution of the signal decreases. This observed dependence of the enhanced Raman intensity on the thickness of the thin Ag slab and the collecting solid angle is well explained by assuming that the Raman scattering is mediated by LRSP's. The observed maximum enhancement factor is 4 × 10 6 for the 140 Å silver slab which agrees well with the theoretically predicted value of 6 × 10 6.

Observation of long range surface plasmon decay length by optical second harmonic generation

We report the observation of the spatial decay of the long range surface plasmon mode using optical second harmonic generation. The surface wave is excited at 1.06 microm with prism coupling in a multilayer geometry consisting of a quartz crystal substrate, a thin silver film, and an index-matched liquid. From the asymmetry in the reflected second harmonic profile a decay length of 0.8 mm is determined. This value is a factor of 3 smaller than that predicted due to the effect of surface roughness scattering on the propagation of the long range surface plasmon mode.

Optical field enhancement comparison between long-range surface plasma waves, and waves induced by resonant cavity

We present a comparison between the enhancement of the optical fields obtainable by two different prism-coupling geometries: the long range surface plasmons and a dielectric stratified medium or resonant cavity. The analysis shows that the resonant cavity can give rise to a field enhancement larger than the other geometry, with the advantage of being without damage to the structure because of dissipation of power in the metal.