## 5 Results and Discussion  
CO\({}_{2}\), a colorless, odorless, and non-flammable greenhouse gas, is primarily generated through the combustion of carbonaceous materials or animal metabolism. Its escalating concentration and growth rate pose severe threats to the environment, including global warming and unsustainable environmental challenges [25]. Consequently, there is a pressing need for highly sensitive CO\({}_{2}\) gas sensors to monitor greenhouse gas concentrations effectively. With increasing CO\({}_{2}\) levels in the surrounding medium, the refractive index of functionalization layers undergoes discernible changes as shown in Table 2.  
After fabricating the primary sensing structure using electron-beam lithography (EBL) and reactive ion etching (RIE), a second lithography process is required. In this process, a photoresist layer is spin-coated over the entire sample. Subsequently, a small sensing window is left open on the sensing arm of the device. PHMB is then spin-coated within this window, and the thickness of the PHMB layer is precisely controlled by adjusting the spin-coating speed. For effective CO\({}_{2}\) gas sensing applications, a PHMB thickness of approximately 240-250 nm is optimal [26]. Polyhexamethylene biguanide (PHMB), featuring basic amide-bearing functional groups and belonging to the guanidine polymer family, emerges as a prime candidate for functionalization polymer layers due to its sensitivity and selectivity to low CO\({}_{2}\) concentrations at room temperature and atmospheric pressure [27]. Notably, unlike previous functional materials, PHMB does not necessitate the presence of water vapor for CO\({}_{2}\) absorption and release. Gas sensors employing PHMB exhibit a linear correlation between the resonance wavelength blueshift and CO\({}_{2}\) concentration [22; 29]. Remarkably, there are no reported instances of gases other than CO\({}_{2}\) interfering with the sensitivity of the PHMB polymer layer, rendering it highly suitable for CO\({}_{2}\) gas sensing applications. In a study analyzing PHMB's selectivity as a functional layer for CO\({}_{2}\) gas against hydrogen gas, it was confirmed that the sensor displayed no response to hydrogen or nitrogen gas, ensuring accurate detection of CO\({}_{2}\) without false positives [26]. The concentration of CO\({}_{2}\) gas deemed dangerous for humans typically begins to pose health risks when it exceeds certain thresholds. While CO\({}_{2}\) is naturally present in the atmosphere at concentrations around 400 ppm, indoor environments can experience elevated levels due to poor ventilation or specific activities such as combustion. Generally, concentrations above 1000 ppm are considered to cause discomfort and may lead to symptoms such as headaches, dizziness, and drowsiness. At higher concentrations, exceeding 5000 ppm, CO\({}_{2}\) can induce more severe symptoms, including difficulty breathing, increased heart rate, and impaired cognitive function. Extremely high concentrations, reaching tens of thousands of ppm, are life-threatening and can result in unconscious or even death. Therefore, maintaining CO\({}_{2}\) levels below 1000 ppm in indoor environments is advisable to ensure the safety and well-being of occupants. Initially, the MZI structure without an end loop is investigated for its CO\({}_{2}\) gas sensing capabilities. The schematic diagram of this structure is depicted in Figure 5a. The intensity interrogation method in an MZI structure involves monitoring changes in the intensity of light at the output_1 port to detect variations in external conditions, such as gas concentration. In this method, the interference pattern of light, which results from the \begin{table}
\begin{tabular}{c c} \hline \hline
**PHMB Refractive Index** & **Gas Concentration (ppm)** \\ \hline
1.55 & 0 \\
1.54 & 215 \\
1.53 & 262 \\
1.52 & 328 \\
1.51 & 366 \\
1.49 & 434 \\
1.48 & 524 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Refractive index of PHMB versus CO\({}_{2}\) gas concentration [27; 28].