Optimized sensitivity of Silicon-on-Insulator (SOI) strip waveguide resonator sensor

Abstract

Evanescent field sensors have shown promise for biological sensing applications. In particular, Silicon-on-Insulator (SOI)-nano-photonic based resonator sensors have many advantages for lab-on-chip diagnostics, including high sensitivity for molecular detection and compatibility with CMOS foundries for high volume manufacturing. We have investigated the optimum design parameters within the fabrication constraints of Multi-Project Wafer (MPW) foundries that result in the highest sensitivity for a resonator sensor. We have demonstrated the optimum waveguide thickness needed to achieve the maximum bulk sensitivity with SOI-based resonator sensors to be 165 nm using the quasi-TM guided mode. The closest thickness offered by MPW foundry services is 150 nm. Therefore, resonators with 150 nm thick silicon waveguides were fabricated resulting in sensitivities as high as 270 nm/RIU, whereas a similar resonator sensor with a 220 nm thick waveguide demonstrated sensitivities of approximately 200 nm/RIU.

Keywords: (140.4780) Optical resonators; (280.1415) Biological sensing and sensors.

Fig. 1

Group indices of slab waveguides as functions of slab thicknesses.

Fig. 2

(a) Calculated resonator’s sensitivities, based on simulations, as functions of silicon core thicknesses. The hallowed markers are the simulated sensitivities for the case of a slab waveguide, the black filled markers are the simulated sensitivities for the case of rectangular waveguides with waveguide widths of 750 nm and 900 nm, and the red markers are averages of our experimental results for TM ring resonators with 150 nm and 220 nm thick silicon cores. (b) Contour plot of sensitivity in nm/RIU as functions of waveguide widths and thicknesses. The cross-section corresponding to the dashed line representing the slab is plotted in Fig. 2(a), and the other two cross-sections representing the thicknesses of 150 nm and 220 nm are plotted in Fig. 3(d). The red markers show the fabricated TM mode resonator devices (Star: Width = 900 nm and Thickness = 150 nm, Triangle: Width = 750 nm and Thickness = 220 nm).

Fig. 3

(a, b) Spectra showing that the resonant wavelength shifts as the refractive index of the aqueous cladding medium changes, for the TM resonator sensors with (a) 220 nm thick waveguide cores, and (b) 150 nm thick waveguide cores. (c) Sensitivities of TM waveguide resonators to the aqueous cladding. (d) Sensitivities of TM waveguide resonators to the aqueous cladding, for the optimum silicon core thickness of 150 nm and for the conventional thickness of 220 nm, as functions of the waveguide width. The filled markers show corresponding experimental results. The labels “TM1 is mode 2” and “TM1 is mode 3”, means the first guided TM mode is the second and third guided mode in the waveguide, respectively. The first guided mode is the fundamental TE mode. For the strip waveguides wider than ∼ 650 nm, the first two TE modes are guided before the first TM mode.

Fig. 4

Experimental results includes the measurement of the wavelength shift over time as the various layers of electrostatically charged polymers were introduced to the surface of our conventional TM resonator sensor. Polymers corresponding to the labels A–D are: A = Tris Buffer (0.5 mM, 100 mM NaCl, pH 7.1); B = PEI (solution of positively charged polyethylene imine, 5 mg/ml); C = PSS (solution of negatively charged polystyrene sulfonate, 5 mg/ml); D = PAH (solution of positively charged polyallylamine hydrochloride, 5 mg/ml).

Fig. 5

(a) Schematic representation of model biological sandwich bioassay. Reagent sequencing corresponding to regions [A–E] include: Region A= Protein A adsorption, B= anti-streptavidin (Anti-SA) functionalization, C= Bovine Serum Albumin (BSA) challenge and block, D= streptavidin (SA) target analyte binding, E= Biotin-BSA amplification step. Introduction of each reagent was followed by a PBS wash. (b) Experimental results of biosensing assay following the physisorption of Protein A offline (Region A not shown). The blue-dashed vertical line shortly before Region C denotes the unintended introduction of an air bubble. While the air bubble desorbs some of the immobilized antibody, it does not impact the viability of the subsequent binding steps.