High-sensitivity, miniaturization, and integration sensors are still in great demand in medical detection, environmental monitoring, and other fields. One of the hot issues in the current research on sensors is to improve the sensitivity and reduce the size of sensors. Mach-Zehnder interferometer (MZI) is an important part of integrated optics, which is widely used in optical filters, optical lasers, optical sensors, and other fields. The refractive index sensor based on MZI has been widely utilized in sensing according to its advantages of simple structure, convenient manufacture, and large process tolerance. In general, MZI-based sensors implement sensing by detecting the variety of interference signals between the reference arm and the sensing arm. The proposed sensor schemes based on MZI usually have large sizes and complex structures. Therefore, the study of small-size and high-sensitivity MZI sensors is very suitable for the current development needs.

To improve the sensitivity of the sensor without increasing the complexity, we choose to add a new type of waveguide to the MZI-based sensor. The sensitivity of the MZI-based sensor is mainly determined by the difference in the length of the two MZI arms and the group refractive index of the optical signal transmitted in the waveguide. In traditional MZI-based sensor schemes, the light transmits in the same mode in the sensing arm and the reference arm, so the group refractive index difference is limited. To solve this problem, we transmit different modes on different MZI arms to obtain ultra-sensitive refractive index sensors. It is found that the sensitivity of a sensor can be improved effectively by increasing the intensity and the contact area between the waveguide and analyte. Therefore, we emit suspended slot mode into the sensing arm, launch TE₀ mode into the reference arm, and adjust the length of both arms to maximize the sensitivity of the sensor.

According to formula (13), the sensitivity of the sensor is mainly determined by the waveguide sensitivity (S_W) and the device sensitivity (S₁). The magnitudes of S₁ and S_W are dependent on both the length of the two arms of the MZI and the waveguide structure. The slot gap (g)_{is assumed as 0.1 μm, the height of silicon (H) as 0.22 μm, and the hanging height (d) as 1 μm. According to the calculations, it is shown that within the range of 0.4-0.5 μm, the SW of the strip waveguide decreases as the width (Ws) of the strip waveguide increases. The SW of the slot waveguide increases as the waveguide width (Wslot) increases within the range of 0.25-0.26 μm, and the SW of the slot waveguide decreases as the Wslot increases within the range of 0.26-0.5 μm. SWof SSlot waveguide increases with the increase in SSlot waveguide width (WSSlot) in the range of 0.2-0.23 μm and it decreases with the increase in WSSlot in the range of 0.25-0.5 μm. The sensitivity of the suspended slot waveguide is superior to that of the slot waveguide and strip waveguide, reaching a maximum of 1.313 (Fig. 5). After optimizing the length of the tapered waveguide (Ltaper) and the width of the tapered waveguide (Wtaper), the conversion efficiency between TE0 mode and slot mode reaches 97.3% (Fig.7). The refractive index n of the analytical component region is set from 1.00029 to 1.00049, and the main bandwidth of the sensor spectral line decreases with the increase in refractive index. Through calculation, the sensitivity of the MZI sensor reaches 9.824×10⁴ nm/RIU (Fig. 11).}

We propose a refractive index sensor with high sensitivity, utilizing an MZI based on silicon-on-insulator (SOI). The transmission and sensitivity formulas of the sensor are derived and analyzed. The structures of different waveguides are compared and analyzed, and it is found that the suspended slot waveguide outperforms the other two waveguides. Thus, the strip waveguide is selected as the reference arm and the suspended slot waveguide is selected as the sensing arm in the MZI sensor. Then, we conduct tests on the conversion efficiency between the TE₀ mode and the slot mode in the sensor. After optimizing the length and width of the tapered waveguide, we can achieve a conversion efficiency of 97.3%. Finally, we optimize the arm length of the MZI and change the refractive index of the analyte region to obtain different transmission spectra. By detecting the wavelength shift between different transmission spectra, the sensitivity of the sensor is calculated by the formula to reach 9.824×10⁴ nm/RIU. Our scheme also has the advantages of small size and simple manufacture and can be widely applied in biomedicine, environmental monitoring, and other fields.