Plasmon-sensor design can enhance sensing capability

Achieving high sensitivity is one of the most crucial requirements in developing chemical and biological sensing systems. Waveguide-based surface-plasmon-resonance (SPR) sensors (surface plasmons are optical excitations of free-electron oscillations on a metal surface), including fiber-optic devices, are excellent candidates for a wide range of sensing applications. Their competitiveness follows from their small size, remote-sensing capability, high sensitivity, and rapid detection speed.¹ However, the strong dependence of sensor characteristics on device structure renders designing optimal configurations difficult. We have developed a novel approach to SPR design, which we used to fabricate a new family of sensors.

Determination of the sensor's operational wavelength range as well as its structural parameters (such as the thickness of the metallic layer and the period of the grating-coupled structure in this layer) is necessary because even small parameter variations can change important sensor characteristics. This analysis has conventionally been done on the basis of numerical calculations,² such as through mode expansion and propagation,³ matrix formalism for multilayer systems, and the rigorous coupled-wave analysis method (RCWA).⁴ We analyzed our sensors' spectral characteristics using RCWA, combined with a genetic algorithm (GA: an optimization-search technique) to obtain optimized structural parameters.

Figure 1. Fiber-optic surface-plasmon-resonance sensors. The right-hand side shows detailed structural examples for optimization consisting of chromium/silver/gold (Cr/Ag/Au) or Cr/Au/Au gratings. Surface-plasmon wave vector. ε: Permittivity. n_core, n_f: Refractive index of fiber core and sensing medium, respectively. d: Layer thickness. w₁: Grating period. w₂: Width of engraved section. θ_inc: Angle of incident light.

Usually, a single-mode fiber (SMF) is polished, etched, or tapered and deposited with metallic layers such as gold or silver. This has become a common structure in many fiber-optic SPR sensors. Consider, for example, a symmetrically etched SMF SPR sensor with metallic layers composed of chromium, silver, and gold. RCWA considers only a small portion of the fiber near the metal film. Hence, we assume that the incident light from the half-infinite fiber region is reflected by the metallic layers (see Figure 1). We thus obtain the sensor's transmission spectrum and can calculate the objective function for evaluation. The latter must be minimized during GA optimization.⁵ Adjusting the weights and references of the objective function enables us to obtain optimized transmission spectra for sensors with different characteristics.

Considering factors such as the resonance wavelength (500nm), narrow bandwidth, dip depth, and the sensor's high sensitivity, we can obtain optimized transmission spectra (see Figure 2). To acquire these at visual wavelengths, we must control the thickness of the sensor's metallic layers. If we apply the technique to grating-coupled structures (see Figure 1), the grating period and width of the engraved section are additional control parameters. Spectral analysis of the SPR sensor for IR sensing can be obtained as shown in Figure 3. We clearly confirm the effect of perturbations of the metallic layer.

Figure 2. Simulated transmission spectra for Cr, Ag, and Au films in the initial and final states (black and red lines, respectively) based on genetic-algorithm iterations, for a refractive index in the sensing region of 1.33 and 1.34 (solid and dotted lines, respectively). The numerical values are for the final state. RIU: Refractive-index unit.

Figure 3. Sensor transmission spectrum for a metallic grating on the surface of the Au layer based on genetic-algorithm iteration from (1) to (4) for a refractive index in the sensing region of 1.33. The numerical values are for the final state.

Using this optimization method, we can now obtain the configuration parameters required for developing fiber-optic SPR-sensor applications. We are planning to fabricate a sensor-array system adopting this technique, which might enable monitoring of refractive-index changes for multiple resonance wavelengths and high sensitivities. We will also apply this GA technique to optimize various plasmonic devices, including light shaping and hot-spot generation.

This work was supported by the National Research Foundation and the Ministry of Education, Science, and Technology of Korea through the Creative Research Initiatives program (Active Plasmonics Application Systems).