## 2 Materials and Methods  
The fabrication processing flow of the device is illustrated in Figure 1. The standard 1-mm-thick SOI wafer contains a Si substrate, a SiO\({}_{2}\) layer with a thickness of 500 nm, and a 220-nm-thick Si top with a crystal orientation of <100> direction. After cleaning the surface with acetone, ethanol, and deionized water in turn, the surface was uniformly coated with polymethyl methacrylate (PMMA) as the photoresist and etching mask. Next, the electron-beam-lithography (EBL: JEOL, JBX-8100) and inductive coupled plasma (ICP: Oxford PlasmaPro 100) etching were implemented for the pattern that forms the ring resonator and the waveguide structure; the etching depth was 220 nm (Figure 1b). Next, the above process was repeated, but the pattern used was for the grating structure, and the etching depth was 70 nm (Figure 1c). Therefore, light can couple into the waveguide and travel around the ring resonator, as shown in Figure 1d. The packaged device structure of the on-chip silicon WGM ring resonator thermometer is shown in Figure 2a. The chip is stuck to a fiber terminal with UV-curing optical adhesives. The light from the probe laser travels between the fiber and waveguide in the chip through  
Figure 1: Fabrication process of the device. (**a**) A standard SOI substrate; (**b**) waveguide and resonator etching; (**c**) grating etching; (**d**) optical path diagram. the strip grating (Figure 2b). From the scanning electron microscope (SEM) image of the micro-morphology shown in Figure 2c,d, the sidewall of the ring resonator is vertical and smooth, reducing scattering loss and thus improving the confinement of the optical field. The width and depth of the waveguide and ring resonator are 500 and 220 nm, respectively, ensuring single-mode transmission [29]. The gap between them is 90 nm in order to obtain a relatively high coupling efficiency. The radius of the ring resonator is 10 \(\upmu\)m. The geometric parameters of the grating structure were optimized by a finite-difference-time-domain (FDTD) simulation of coupling efficiency. On the condition that the laser incidence angle is 8\({}^{\circ}\) and the grating fill-factor is 0.5, the effects of various etching depths and grating periods on the coupling efficiency are simulated, as shown in Figure 3. When the grating period and etching depth are 615 and 70 nm, respectively, the coupling efficiency at a wavelength of 1550 nm can achieve 47%, which is enough for detection. A detailed description of the grating structure can be found in [30]. Figure 4 shows the schematic of the experimental measurement setup. The emitted light from the laser was separated into two paths by splitter-1. 90% of the light was  
Figure 3: The coupling efficiency as a function of the wavelength of the probe laser source. The comparison of various (**a**) etching depths and (**b**) grating periods. The inset in figure (**a**) shows the schematic and geometric parameters of the optimized structure. Figure 2: (**a**) An artistically enhanced photo of the packaged device. (**b**) Diagram of the device structure and laser path. SEM images of (**c**) the resonator and (**d**) grating area. modulated by an acousto-optic modulator (AOM) and then passed through an in-line polarizer and splitter-2. Afterwards, 10% of the light was directed into a photodetector (PD), thus converting the optical power into voltage. A proportional-integral (PI) controller was used to maintain the PD voltage at a fixed value, and the feedback signal was input to the radio frequency (RF) source for amplitude modulation. Because there is a positive correlation between the optical power of the light passing through the AOM and the amplitude of the RF signal applied to the AOM, the feedback loop (the part marked by a green background in Figure 4) can stabilize the output laser power. The in-line polarizer in the feedback loop was used to guarantee the uniform polarization of the output light, which is necessary for resonator transmission spectra detection.