Fig. 1. Cascaded double-ring resonator sensor schematic.

Fig. 2. Mode profiles in the Si waveguide (550nm×220nm).

Fig. 3. Principle of thermo-optical tuning.

Fig. 4. (a) Photograph of the chip. (b) Optical microscope image of the CDRR. (c) SEM image of the sensing ring. (d) Photograph of the microfluidic control system.

Fig. 5. Experimental device diagram.

Fig. 6. Transmission spectra of drop port with 1.0% NaCl solution and 1.4% NaCl solution.

Fig. 7. Wavelength shift versus the RI change.

Fig. 8. Peak-to-valley (P-V) curve of the micro-heater.

Fig. 9. Comparison of thermal tuning coefficients of single ring and double ring.

Fig. 10. (a) Retrieved spectra for 1.2% concentration of NaCl solution. (b) The linear fit curve of the thermo-optical tuning sensing sensitivity.