Plasmonic propagations distances for interferometric surface plasmon resonance biosensing
© Lepage et al; licensee Springer. 2011
Received: 18 October 2010
Accepted: 17 May 2011
Published: 17 May 2011
A surface plasmon resonance (SPR) scheme is proposed in which the local phase modulations of the coupled plasmons can interfere and yield phase-sensitive intensity modulations in the measured signal. The result is an increased traceability of the SPR shifts for biosensing applications. The main system limitation is the propagation distance of the coupled plasmon modes. This aspect is therefore studied for thin film microstructures operating in the visible and near-infrared spectral regions. The surface roughness of the substrate layer is examined for different dielectrics and deposition methods. The Au layer, on which the plasmonic modes are propagating and the biosensing occurs, is also examined. The surface roughness and dielectric values for various deposition rates of very thin Au films are measured. We also investigate an interferometric SPR setup where, due to the power flux transfer between plasmon modes, the specific choice of grating coupler can either decrease or increase the plasmon propagation length.
Surface plasmon resonance (SPR) is a prominent method widely used for the last two decades  in research of label-free characterization and sensing of biological agents, such as viruses and bacteria . To expand the detection capability of SPR, a novel self-referenced interferometric scheme has been proposed to integrate with the SPR architectures. The proposed approach introduces a phase-based signal measurement that complements the classical intensity-based SPR measurement. Multiplexing of those signals leads to an increase precision in the general SPR tracking and thus results in an increased sensitivity of the device.
One of the main limitations of this technique is its reliance on the propagation distance of the coupled SPs (ΛSP), as the efficiency of SPR interferometry is directly related to ΛSP. For applications in biosensing, this represents an important constraint since SPs are coupled at visible (VIS) or near-infrared (NIR) energies (E SP) on very thin, typically less than 45 nm, metallic films. Moreover, one side of the metal is necessarily exposed to the probed media, making biosensing SPR interfaces asymmetric. Under those conditions, the long range SPs (LRSPs) cannot be employed. Therefore, we address the fundamental variables influencing SP propagations. The primary aspect is the nanofabrication itself, where the thin films surface roughness is examined for different materials and deposition methods. In addition to the geometry, the dielectric values of the metallic layers are examined as a function of their deposition rates. Finally, a specific configuration of gratings for the SPR interferometry is presented, in which the SPs can couple with an additional SP mode to result in increased propagation distances.
LRSPs have already been studied extensively . Though they present advantageous properties for integrated plasmonics, self-coherent LRSPs are by nature incompatible with biosensing applications: they are either entrapped in dielectrics layers, supported by thick bulk substrates, or have low energies in the IR. Given the decreasing slope in ε of biochemical materials versus energy , the sensitivity of SPR is strongly diminished for IR. Therefore, more traditional means have to be considered when trying to increase the propagation lengths of SPs while taking into account practical issues for biosensing, such as an open metallic surface, thin-films and SPR at VIS-NIR energies (higher energies damaging the samples while lower energies present poor sensitivities). The first and most practical aspect to consider is the nanofabrication itself.
Nanofabrication and roughness
The SPs propagation distances are limited by thermal losses in the metal at a given energy E SP(ω). Additional losses will occur through radiations in thin films, illustrated by a larger SPs complex wave vectors due to coupling to the other interface (known as Fano modes) . The surface roughness is also known to play a very important role in the limitation of the SPs propagation distances, as the corrugation will diffract a fraction of the SPs light flux. Indeed, the mean free path of the SPs wave has been found to be inversely proportional to the square of the surface roughness height, for a given SP energy and fixed metal dielectric (the complete formulation is available in ). The fabrication of SPR devices to be employed in the VIS-NIR range of energies has become possible in the last decades due to the improved fabrication methods at the nanoscale. Nonetheless, the surface roughness of the films and nanostructures now has a larger impact on SPs modulated signals, as the geometrical structures have sizes comparable to the inherent roughness of the employed fabrication methods. For example, in Figure 1 the grating has a line height of 20 nm, but the grain size of e-beam evaporated Au is around 6 nm. The most straightforward way of increasing the SPs propagation length for SPR interferometry is to reduce the surface roughness to a minimum during the fabrication process of the architectures. In many SPR experiments, a dielectric layer has to be fabricated on the top of a functional substrate such as a semiconductor. This is the case for transmission-based experiments  or for reflection-based experiments in which active components are involved and where one side of the metal film is bounded by a deposited dielectric [10–12].
To estimate the propagation lengths of the surface plasmon modes, we have factored in our simulations the measured experimental dielectric properties of the metallic substrate as well as the underlying structure. A finite incident beam is employed to excite the SP mode within a specific region; the propagating mode's EM field intensity decay is observed outside of that region and fitted with a decay model using non-linear regression, to extract the mean free path ΛSP. To isolate the effect of the dielectrics values, the thin films are considered to have perfectly flat surfaces on both sides. The SPs propagations for these simulations are therefore limited by losses to radiations coupling (to Fano modes) and by electron dampening (thermal loss), but there is no scattering into free space. The increase in experimentally measured dielectric values of the thin films, real and imaginary, induce an overall increase of the SP propagation lengths. The ΛSP on the flat 20 nm layer can increase from 4.69 ± 0.02 μm for the 0.5 Å/s layer (with ε Au = -29.1032 + 2.5736i) to 5.22 ± 0.02 μm for the 7 Å/s (with ε Au = -31.2071 + 3.5632i), a 10% increase. The film with a larger dielectric constant, despite having greater thermal losses to electron damping, results in a better SP mode confinement. This effect would be comparable to increasing the film thickness, reducing the radiation leaks through coupling to the other interface, lowering the SP wave vector and increasing the propagation lengths.
Surface plasmon mode coupling
The presented SPR interferometry method is a relatively straightforward way of enhancing the sensitivity of classical intensity-based SPR biochemical sensing, by introducing SPs phase modulations in the measurements. The number of traceable SPR peaks is multiplied by the SPs interference and tracking those multiplied SPR peaks enable a better resolution on the absolute value of surficial SPR shift. The main limitation of the method is its dependence on the SPs propagation distance ΛSP.
We have therefore examined the principal factors influencing ΛSP in experimental setup for biosensing, which simply consists of a thin film Au layer atop a dielectric, measured in the VIS or NIR regions. The results can apply to various architectures, including Kretschmann-Raether setups.
The initial focus was on surface roughness, playing an important role in thin film SP propagation. We found that a careful optimization of the fabrication process can reduce the SP loss due to quasi-random diffractions by a factor of 13 ×. The resulting films have dielectric values dependent on their deposition rates, which obviously plays a role in the SP wave confinement, and thus its ΛSP. Finally, it was shown that the periodicity of the selected grating can have important impacts, negative and positive, on ΛSP. Various SP modes (or more precisely Fano modes) can be coupled in parallel and anti-parallel behaviours. The specific parallel coupling between SP1 and SP2 through the first diffraction order of the grating has been found to increase the propagation lengths by a factor of 1.5 in the SPR interferometer, enhancing the sensitivity of the method even further.
By carefully addressing the presented aspects, we conclude that SPR interferometry is experimentally feasible and has the potential to increase SPR sensitivity by a factor proportional to the SPs propagation distances, ΛSP.
atomic force microscopy
finite element method
long range SPs
plasma-enhanced chemical vapour deposition
surface plasmon resonance
The authors acknowledge the financial contribution from the Natural Science and Engineering Research Council of Canada (NSERC Strategic grant STPGP 350501-07) and the Canada Research Chair in Quantum Semiconductors Program. The authors also want to thank Etienne Grondin and the CRN2 nanofabrication team for their helpful participation.
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