# Highly Sensitive Liquid M-Z Waveguide Sensor Based on Polymer Suspended Slot Waveguide Structure
###### Abstract  
The slot structure has great advantages in improving the sensitivity of integrated waveguide optical sensors and reducing the detection limit. We propose a polymer Mach-Zehnder interferometer (MZI) optical sensor based on the slot structure and adopted the suspended structure to improve optical field interaction with the analyte, hence boosting the sensor's sensing accuracy. In this paper, the effects of the single waveguide width, slot width, and coupling structure of the slot waveguide on the performance of the sensor operating at a 1550 nm wavelength were analyzed. Under the premise of satisfying single-mode transmission, we designed an MZI with a branch spacing of 10 \(\upmu\)m, arm length of 2045 \(\upmu\)m, branch span of 700 \(\upmu\)m, and slot region of 500 \(\upmu\)m. The sensor's average sensitivity was 972.1 dB/RIU, and its average detection resolution was 1.6 \(\times\) 10\({}^{-6}\) RIU, which is approximately 1.5 times higher than that of the suspended strip waveguide, 1.6 times higher than that of the non-suspended slot structure, and 2.1 times higher than that of the non-suspended strip waveguide. integrated optics; slot waveguide; polymeric Mach-Zehnder interferometer; optical sensing +
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fabrication materials [11]. In the field of inorganic materials, the high refractive index (HI) group IV semiconductor materials, such as silicon on insulators, play a major role in the development of integrated optical sensors. This is mainly due to the possibility of having strong optical field confinement and then extremely small footprint devices [12]. In 2021, [PERSON] et al. proposed a compact optical gas sensor based on the widespread silicon-on-insulator (SOI) technology with a device sensitivity of 1070 nm/RIU and a figure-of-merit (FOM) as high as 280.8 RIU\(-\)1 at the 1.55 \(\upmu\)m wavelength [4]. In 2022, [PERSON] et al. proposed a novel all-pass slot microring resonator (SMRR). The corresponding refractive index sensitivity is 403 nm/RIU, which is approximately six times greater than that of traditional microring resonator sensors [13]. Regarding the organic materials, polymers have been proven to be potential complementary candidates for inorganic PICs [14; 15], they have found a wide utilization in the detection field due to their modifiability and structural designability [16]. The tunable properties of the polymer give the design and fabrication of the photonic devices much more flexibility [13]. Moreover, biocompatibility is another important and unique feature of polymers. Furthermore, polymers are compatible with three-dimensional (3D) printing, dissolution, and embossing techniques, which can reduce the preparation cost. In 2018, [PERSON] et al. fabricated polymer photonic integrated devices by using the UV imprinting technique [17]. In 2020, [PERSON] et al. demonstrated the fabrication of submicron optical waveguides in polydimethylsiloxane (PDMS) using divinylbenzene (DVB) as the photopolymerizable monomer through two-photon polymerization (2 PP) [18]. Imprinting and 3D printing also can implement three-dimensional devices to improve integration and device performance. In this paper, we focused on the study of material flexibility and 3D structure, so a polymer was employed as the material to fabricate a slot waveguide. In the field of polymer sensing, strip waveguides and slot waveguides on substrates are widely employed. In 2020, [PERSON] et al. proposed and demonstrated a cost-effective liquid refractive index (RI) sensor in polymer materials by employing a Mach-Zehnder interferometer (MZI) formed with the strip waveguide [19]. [PERSON] et al. proposed a polymer horizontal slot waveguide as a light-analyte interactive region to implement a low-cost and highly sensitive liquid refractive index sensor [20].