Enhancement of Refractive Index Sensitivity Using Small Footprint S-Shaped Double-Spiral Resonators for Biosensing

Abstract

We demonstrate an S-shaped double-spiral microresonator (DSR) for detecting small volumes of analytes, such as liquids or gases, penetrating a microfluidic channel. Optical-ring resonators have been applied as label-free and high-sensitivity biosensors by using an evanescent field for sensing the refractive index of analytes. Enlarging the ring resonator size is a solution for amplifying the interactions between the evanescent field and biomolecules to obtain a higher refractive index sensitivity of the attached analytes. However, it requires a large platform of a hundred square millimeters, and 99% of the cavity area would not involve evanescent field sensing. In this report, we demonstrate the novel design of a Si-based S-shaped double-spiral resonator on a silicon-on-insulator substrate for which the cavity size was 41.6 µm × 88.4 µm. The proposed resonator footprint was reduced by 680 times compared to a microring resonator with the same cavity area. The fabricated resonator exposed more sensitive optical characteristics for refractive index biosensing thanks to the enhanced contact interface by a long cavity length of DSR structures. High quality factors of 1.8 × 10⁴ were demonstrated for 1.2 mm length DSR structures, which were more than two times higher than the quality factors of microring resonators. A bulk sensitivity of 1410 nm/RIU was calculated for detecting 1 µL IPA solutions inside a 200 µm wide microchannel by using the DSR cavity, which had more than a 10-fold higher sensitivity than the sensitivity of the microring resonators. A DSR device was also used for the detection of 100 ppm acetone gas inside a closed bottle.

Keywords: cavity length; evanescent field sensing; refractive index; ring resonator.

Figure 1

(a) Schematic of the double-spiral resonator (DSR) biosensor with a compact cavity size. The DSR is able to detect a small-volume analyte inside the thin-width microchannel. (b) Refractive index distribution of a waveguide cross-section. (c) Bending loss caused by the curvature radius. (d) Coupling ratio when the spacing distance (D_s) changes. D_s is defined as the inset figure. (e) Power loss caused by the bending angles at S-shaped channel (θ_s) as defined in the inset figure.

Figure 2

(a) Cross-section of the E field of a DSR waveguide with a width of 450 nm and a height of 240 nm in the TE (E_x). (b) Simulated transmission spectrum for the DSR cavity with a spacing distance of 5 µm. (c) The images, Q factors, FSRs, and cavity lengths of different DSR designs with different spacing distances.

Figure 3

(a) Images of two different types of C-turns. The A-type C-turn had an arc diameter (R_c) as large as the spacing distance D_s. The B-type C-turn had two arcs with R_c = 5 µm linked to each other at an angle of θ_C. (b) The column chart of Q factors (the left axis) for different cavities of conventional microring resonators (light blue) with R = 20 µm and 189 µm, and DSR cavities (light green) with different C-turn types. The orange square markers (the right axis) are the cavity lengths of the plotted cavities.

Figure 4

Schematic diagram of custom measurement setup. The PDMS microfluid channel adhered to the Si chip to make the microchannel. A micropipette was used to pump 1 µL of solution into the microchannel for detecting solution bulk sensitivity. In the VOC gas detection, acetone-infused cotton was placed inside a closed PFA bottle connected to the microfluidic inlet by a silicone tube, and the outlet was blocked.

Figure 5

(a) SEM images of the 20 µm radius MRR, 1 mm length DSR, 1.2 mm length DSR (A-type C-turn), and 1.2 mm length DSR (B-type C-turn), starting from the left. The DSR cavity size was 41.6 µm × 88.4 µm. (b) SEM images of two types of C-turns. (c) SEM images of taper-edged coupler. (d) Transmission spectra in DI water for fabricated cavities with different cavity lengths.

Figure 6

(a) Resonant wavelength shift for 1.2 mm length DSR cavity (B-type C-turn) over different refractive indices by using IPA solutions with concentrations of 0.1%, 0.2%, 0.3%, and 0.4%. (b) Measured transmission spectrum of IPA solution (0.1%) using DSR cavity at 0 h, 1 h, 2 h, and 3 h.

Figure 7

(a) Resonant wavelength shift for 1.2 mm length DSR cavity (B-type C-turn) over different acetone concentrations of 100 ppm, 200 pp, 300 ppm, and 400 ppm. (b) Resonant wavelength shifts as the environment inside the bottle changed in the sequence of air, acetone, and air. Acetone gas had a concentration of 100 ppm.