Surface Plasmon Optics

J. Dostálek

Surface plasmon (SP) is an optical wave which originates from coupled oscillations of density of electron plasma and electromagnetic field on a metallic surface. The field of SP evanescently decays into both the metal and a dielectric medium. In surface plasmon resonance (SPR) biosensors, the binding of biomolecules to receptors anchored on a metallic surface is probed with an SP. This binding induces a refractive index change which can be observed by the spectroscopy of SPs. In addition, chromophore-labeled molecules captured on a metallic surface can be excited with SPs and detected by SP-enhanced fluorescence spectroscopy (SPFS). Currently, we pursue the research aimed for design of novel optical structures, surface chemistries and actuating of SPR for both refractive index and fluorescence-based SPR biosensors. Developed SPR biosensors are applied for the detection of compounds relevant to medical diagnostics and food control. Various target analytes including hormones, toxins, bacteria are detected. Surface chemistry architectures relying on hydrogel three-dimensional binding matrices are investigated.

Long range surface plasmon-enhanced fluorescence spectroscopy

Long range surface plasmon (LRSP) is a surface plasmon mode supported by a thin metallic film embedded between two dielectrics with similar refractive indices. LRSPs can propagate with an order of magnitude lower losses comparing to those of conventional SPs. Therefore, the excitation of LRSPs is associated with a larger enhancement of intensity of electromagnetic field on the metallic surface. In SPFS, this field enhancement is directly translated to the increase in the fluorescence signal. In addition, the high penetration depth of LRSP evanescent field offers the possibility to excite large amount of chromophore-labeled molecules captured in proximity to the biosensor surface. For instance, a hydrogel binding matrix can be used to immobilize receptors within the whole volume probed with LRSP. By this means, an additional increase in the measured signal induced by the binding of target molecules to its receptors can be achieved. In our research, LRSPs guided on a thin metallic films deposited on materials with low refractive index (such as Teflon or nanoporous silicate) are investigated. Hydrogels (e.g. based on dextrane and NIPAAM polymers) are attached to the surface and optimized to serve as an efficient binding matrix.

Fig. a) Scheme of a biosensor based on long range surface plasmon-enhanced fluorescence spectroscopy; b) distribution of the electric field intensity on a thin gold film upon the ATR excitation of LRSP, c); comparison of the magnetic intensity distribution for the ATR excitation of LRSP and conventional SP (the intensity of incidence wave is equal to 1);

Diffraction grating coupled surface plasmons

Metallic diffraction gratings present an alternative means for the excitation of SPs without the need of using high refractive index prisms and refractive index matching fluid which makes the design of SPR sensors simpler. In addition, employing diffraction gratings to SPFS-based biosensors enables increasing the sensitivity of these devices through e.g. diffraction grating out-coupling of fluorescence light trapped to SP modes. Furthermore, new diffraction coupled surface plasmon modes such as Bragg-scattered LRSPs with interesting properties for SPR sensors are investigated. Both theoretical and experimental research is performed. Numerical simulations are carried out using finite element-based diffraction grating solver. In the experimental part of the work, diffraction gratings are prepared with the holographic technique and reactive ion beam etching (RIBE) through a photoresist mask or directly into a substrate by means of focused ion beam lithography (FIB). The replication of grating masters into low-refractive index polymers is performed.

Fig. a) AFM image of a diffraction grating; b) Simulations of electric intensity distribution of LRSPs excited using the grating coupled at the angle of 0 deg with a plane wave. The electric intensity of the incident plane wave is equal to 1. c) Angular reflectivity spectra for the excitation of SP compared to that LRSP and SRSP, gold sinusoidal diffraction grating with the period of 512 nm and depth 30 nm. The grating was replicated into Cytop polymer, coated with gold and brought in contact with water.

References

J. Dostálek, A. Kasry., W. Knoll, Long range surface plasmons for observation of biomolecular binding events at metallic surfaces, Plasmonics (2007) 2, 97-106.