# Highly Sensitive Liquid M-Z Waveguide Sensor Based on Polymer Suspended Slot Waveguide Structure
###### Abstract
To further exert the compatibility of the polymer with three-dimensional (3D) printing, dissolution, and embossing techniques, we proposed a polymer suspended slot, a three-dimensional structure that adds a test surface in contact with the test fluid, which leads to a higher sensing accuracy. The average sensitivity of the sensor we designed was 972.1 dB/RIU, and its average detection resolution is 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. Regarding the device structure, the device structures of existing optical waveguide sensors are mainly microdisk resonators [21], microring resonators [22], MZI [23; 24; 25; 26], and [PERSON]'s interferometer [27]. Ring resonators are compact and easy to multiplex, but an expensive and bulky high-resolution wavelength-tunable laser is often required to operate the silicon microring resonator and its detection limit is ultimately restricted by the laser resolution [28]. Additionally, in the polymer field, the bending loss of the microring is relatively large, especially for the ridge waveguide, and the optical field will leak in the slab which is not conducive to the sensing of the device. Compared with microring resonator sensors, MZI-based sensors are easier to implement and simpler regarding the design, fabrication, and measurement [28]. MZI can be divided into the symmetric structure of micro-fluidic channel integration and asymmetric without disposition according to the contact manner. In 2008, the MZI waveguide chemical sensor with an inverted ridge structure was presented by [PERSON] et al. [29]. In 2012, [PERSON] et al. presented an improved sensing window with three sensing surfaces, whose sensitivity could be enhanced by a factor of 2.8 in theory [30]. In 2013, [PERSON] et al. proposed a highly sensitive label-free Mach-Zehnder interferometer (MZI) biosensor based on a silicon nitride slot waveguide, and the bulk refractive index sensitivity of it was found to be 1864 \(\uppi\)/RIU (refractiveindex unit) with a 7 mm-long slot waveguide sensing arm [28]. In 2021, [PERSON], [PERSON] et al. investigated a Compact Gas Sensor Using Silicon-on-Insulator Loop-Terminated Mach-Zehnder Interferometer, which uses a slot waveguide for the sensing arm and achieves a device sensitivity of 1070 nm/RIU [4]. In summary, MZI utilizes a sensing scheme based on optical phase shift without spectral detection, primarily measuring the output light intensity, making it more cost-effective [1]. Therefore, we adopted the MZI with a single-arm integrated slot. In the slot integration process, the coupling structure is a significant indicator affecting the coupling loss. The device needs a transition structure from the strip waveguide to slot waveguide, but the transition will cause certain energy loss. To reduce coupling loss, the design of an effective coupled structure is of great necessity. In 2011, [PERSON] et al. proposed a cooled structure with a coupling efficiency of 91.2% [31]. In this paper, to couple the polymer suspended slot, we proposed the tapers in two latitudes, which can reduce the coupling loss. In this paper, the potential of using a suspended slot waveguide as a light-analyte interactive region to implement a highly sensitive liquid M-Z sensor is explored. To enhance the sensing accuracy, we designed the suspended slot waveguide to expand the contact area between the sensor and the test liquid, as well as proposed the coupled structure to reduce the coupled loss. Numerical analysis reveals that with the optimized geometrical parameters, the polymer slot waveguide of a 3D structure can achieve high waveguide sensitivity in comparison with the sensors employing polymer strip waveguides and slot waveguides on substrates.