Numerical investigation of finite thickness metal-insulator-metal structure for waveguide-based surface plasmon resonance biosensing
Introduction
Surface plasmon resonance (SPR), the resonant excitation of the surface plasmon-polariton (SPP) at a metal-dielectric interface by evanescent optical waves, has been utilized extensively for detecting surface-bound molecules and studying their interactions [1]. So far, the technique has been implemented mostly in laboratory settings because the SPP excitation requires stringently aligned, complex optical setups. To realize compact and rugged SPR sensors, waveguide-based SPP excitation schemes have been proposed and demonstrated. Typical waveguide-based SPR configurations comprise polymer or glass waveguides and a single-layer metallic thin film that supports the SPP, as shown in Fig. 1(a). The guided mode excites the SPP through mode coupling, which causes a drop in the waveguide transmission Ptrans. Researchers have focused on making the mode coupling efficiency (ηcoup) sensitive to changes in the refractive index of the analyte in the superstrate (nsup) so that Δnsup can be detected through ΔPtrans [2], [3], [4], [5], [6], [7], [8].
To practical utilization of the waveguide-based SPR, of higher importance than the sensitivity ∂Ptrans/∂nsup itself is the operation point, i.e., the value of nsup around which the sensitivity becomes maximized. Since biosensing typically involves aqueous solutions (nsup = 1.33) and Δnsup caused by biochemicals is small, the operation point must be close to the index of the aqueous solution itself. The operation point is formed when the effective index of the SPP (nSPP = co/ω·Re{βSPP} where βSPP is the propagation constant of the SPP, as defined in Eq. (3) of Ref. [1]) approaches that of the waveguide mode (ngw = co/ω·Re{βgw}, where βgw is the propagation constant of the waveguide mode). In conventional waveguide-based SPR configurations, however, the waveguide mode mainly occupies the high-index waveguide core region and, hence, exhibits much higher propagation constant than that of the SPP mode which occupies the cladding layer and aqueous solution, both with lower indices as shown in Fig. 1(a). As a result, nSPP is generally lower than ngw. Since the operation point is formed as nSPP approach ngw, the mismatch between nSPP and ngw inevitably places the operation point beyond 1.33. Moreover, in the conventional single-layer metallic film SPR, the mismatch between ngw and nSPP becomes more serious in the near-infrared regime (λo > 780 nm), which is favored for biosensing [9], as shown in Fig. 2(a). Lowering the operation point towards 1.33 requires an additional scheme to enforce nSPP ∼ ngw even when nsup is much lower than nclad and ncore.
Previously proposed schemes include inserting an extremely low-index (n ∼ 1.29) buffer layer between the metal and cladding layers [2], adding a high-index Ta2O5 overlay on the metal layer [3], [4], and utilizing shorter operation wavelengths (λo < 550 nm) at which βSPP becomes higher due to changes in material characteristics [5]. Inserting a buffer layer reduces the SPP intensity in the sensing volume and distorts the mode profile. High-index overlays separate the SPP from the sensing volume and reduce the sensitivity. In short wavelength regime, the absorption due to metal and water also increases and limits the sensing area. Lowering ngw by fabricating the waveguide with special low-index materials (1.4–1.405) has also been demonstrated [6], [7]. Recently, a scheme to reduce the propagation constant of the waveguide mode itself to match that of the SPP has been proposed. Skorobogaity and Kabashin [8] utilized the leaky modes of photonic crystal waveguides whose effective indices can be lower than the indices of waveguide material to enhance coupling to the SPPs at liquid/metal interface. The photonic crystal waveguide, however, requires 27 dielectric layers with alternating refractive indices, which make its implementation challenging, especially in planar format.
In this paper, we present a new scheme to tune the operation point of the waveguide-based SPR without requiring specially fabricated waveguides, short λo operations, or added components that may distort the mode profile or causes excessive reductions in SPR sensitivity. Our scheme exploits the unique characteristics of the SPP modes supported by the finite thickness, asymmetric metal-insulator-metal (f-MIM) structure. Using numerical modeling, we will investigate the SPP mode's propagation characteristics and seek integration with optical waveguides. Based on these, we will establish design rules to tune the operation point and the SPP mode intensity distribution. Finally, we will estimate the sensitivity of waveguide-based SPR integrated with f-MIM.
Section snippets
Principles of operation and structure
The f-MIM and the combined structure of the f-MIM and a waveguide (f-MIM/WG) are shown in Fig. 1(b and c), respectively. It is well-known that MIM structures with semi-infinite metal thickness support SPP modes with increased propagation constant [10]. In our scheme, we made the metal layer thickness finite and asymmetric. Our f-MIM differs from previously reported similar structures [11], [12], [13], [14], [15] by its total asymmetry (nsup ≠ nsub ≠ ni and dm1 ≠ dm2) and by the thinness of the
Method of design and analysis
To obtain the propagation constant and the mode profile of the SPP in 1D, 5-layer f-MIM structure shown in Fig. 1(b), we derived the following characteristic equation by enforcing continuity of electric and magnetic fields’ tangential components:where Ki = 2π/λo·()1/2, ri = (Ki/ɛi − Ki+1/ɛi+1)/(Ki/ɛi + Ki+1/ɛi+1), i is the layer index specified in Fig. 1(b), and ɛi the
Design and characteristics of standalone f-MIM
As the first step, we investigated the characteristics of the SPP mode supported by a standalone f-MIM not yet integrated with a waveguide (f-MIM SPP). To emphasize polymer waveguide-based SPR sensing in aqueous environments, we assumed: nsup = 1.33 and nclad ≡ nsub = 1.48, ncore = 1.54 which are typical for polymer waveguides [16]. For a wide range of wavelength, ngw of the waveguide's fundamental mode is approximately 1.52. We chose Au as the metal and modeled it based on Ref. [17]. Ta2O5 was chosen
Conclusion
In conclusion, we presented a new plasmonic structure, the finite thickness metal-insulator-metal (f-MIM), and demonstrated that it can enable a strong coupling between its surface plasmon-polariton (SPP) mode and guided optical waves. Achieving such opto-plasmonic coupling has been considered to be difficult due to the inherent difference in the two waves’ dispersion characteristics. Using numerical simulations, we showed that the f-MIM provides wide range tunability to the dispersion
Acknowledgement
This work was supported by Iowa Office of Energy Independence through Iowa Power Fund Project 08-02-1073.
Yifen Liu received her bachelor's degree in College of Electronic Information from Wuhan University, China. After that, she has been doing research in optics and plasmonics with Dr. Jaeyoun Kim in Electrical Engineering of Iowa State University as a Ph.D. candidate.
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Yifen Liu received her bachelor's degree in College of Electronic Information from Wuhan University, China. After that, she has been doing research in optics and plasmonics with Dr. Jaeyoun Kim in Electrical Engineering of Iowa State University as a Ph.D. candidate.
Jaeyoun Kim received his Ph.D. degree in Electrical Engineering from University of Michigan at Ann Arbor in 2003. From 2003 to 2006, he has been with Berkeley Sensor and Actuator Center as a postdoctoral researcher. In August 2006, he joined the Department of Electrical and Computer Engineering of Iowa State University where he is currently Harpole-Pentair Assistant Professor. His research encompasses biophotonics, optical sensing, plasmonics, and optical MEMS. Dr. Kim is the recipient of 2010 NSF CAREER Award.