## 4 Applications in Photonic Devices
### Passive Components  
In the domain of passive photonic components, polymer materials have demonstrated remarkable versatility and functionality. Polymer waveguides serve as fundamental building blocks for integrated photonic circuits, offering capabilities beyond simple light guiding. For instance, PMMA waveguides doped with chromophores enable temperature sensing through thermo-optic effects [109], while SU-8 waveguides functionalized with specific receptor molecules have shown excellent capabilities for bio-sensing applications [110]. The incorporation of stimuli-responsive polymers like poly(N-isopropylacrylamide) (PNIPAm)  
creates adaptive waveguides that respond to environmental changes, enabling switchable routing and sensing applications [111]. Polymer-based optical filters utilize various mechanisms for spectral selection, with Bragg filters fabricated from alternating layers of polymers with different refractive indices, such as PS/PMMA multilayers, providing efficient wavelength filtering [112, 113]. More sophisticated approaches include holographic polymer-dispersed liquid crystals (H-PDLCs), where the periodic variation in refractive index creates selective reflection bands [114, 115, 116]. The incorporation of azobenzene-containing polymers enables photoswitchable filters whose spectral response can be modified with light, adding an active component to traditionally passive structures [117]. The field of polymer photonic crystals offers unique opportunities for creating responsive optical structures. Block copolymers like PS-b-PMMA can self-assemble into periodic structures with photonic band gaps, becoming particularly interesting when one component is responsive to external stimuli [118, 119]. Hydrogel-based photonic crystals incorporating poly(acrylic acid) demonstrate color changes in response to pH or ionic strength, useful for sensing applications [120, 121]. Similarly, photonic crystals based on cholesteric liquid crystalline polymers provide a tunable selective reflection that can be modified through mechanical strain or temperature [122, 123, 124, 125]. More complex passive structures combine multiple functions or incorporate responsive elements. Multimode interference (MMI) devices using epoxy-based polymers enable optical power splitting [126], while ring resonators fabricated from fluorinated polymides provide narrow-band filtering capabilities [127]. Photonic crystal fibers using PMMA or COC (cyclic olefin copolymer) offer specialty light guidance properties, and Mach-Zehnder interferometers incorporating electro-optic polymers demonstrate advanced sensing capabilities, as illustrated in Figure 3[128, 129, 130]. In each case, the polymer material not only provides basic optical functionality but also enables additional features through mechanical flexibility, easy surface modification, response to environmental conditions, cost-effective fabrication methods, and integration possibilities with other materials and devices. Figure 3: Architecture and working principle of the Mach–Zehnder interferometer (MZI) blood glucose sensor: (**a**) Working principle of the MZI blood glucose sensor. Glucose oxidase (GOD) catalyzes the production of gluconic acid and \(\mathrm{H_{2}O_{2}}\) from blood glucose. (**b**) Layout of the MZI, which consists of a \(1\times 2\) beam splitter, a \(2\times 1\) beam combiner, and two branch waveguides (sensing arm and reference arm) of the same length. The red line and the green line indicate the phases of the sensing arm and reference arm, respectively. The phase shift between the reference arm and the sensing arm is \(\pi\). (**c**) TE electric field mode distribution of the single-mode waveguide at 1550 nm. (**d**) Schematic demonstration of the overall fabrication process for preparing the MZI blood glucose  
sensor along with magnified detailed SEM images. Step-by-step illustration showing the overall fabrication process of the polymer-based MZI blood glucose sensor.