## 3 Material Systems  
The diversity of polymer materials suitable for photonic applications is extensive and still continuously expanding. The ability to tailor molecular structures through synthetic chemistry enables precise control over optical, electronic, and physical properties. Rather than providing an exhaustive list of all available materials, this review focuses on fundamental polymer families that have demonstrated significant impact in photonic applications and serve as platforms for understanding structure-property relationships in these systems. At the core of polymer-based photonic materials are conjugated polymers, which feature alternating single and double bonds along their backbone. This conjugation creates delocalized \(\pi\)-electron systems that extend over multiple monomer units, enabling unique optical and electronic properties not found in conventional polymers. Unlike saturated polymers with localized \(\sigma\)-bonds, conjugated polymers possess significantly reduced bandgaps (typically 1.5-3 eV), placing them in the semiconductor regime and enabling strong interactions with visible light. This semiconductor behavior, combined with solution processability, has positioned conjugated polymers as versatile materials for a wide range of photonic applications, from simple light guiding to complex nonlinear optical phenomena. The development of conjugated polymers has evolved from early systems like polyacetylene to increasingly sophisticated architectures designed to enhance specific optical properties. Contemporary materials often incorporate various functional moieties that influence light absorption, emission, charge transport, and processing characteristics. This molecular engineering approach has yielded a rich landscape of materials with properties tailored for specific photonic applications. Among these, several key conjugated polymer families have emerged as particularly important platforms, which we discuss in detail below. Conjugated polymers can be divided into several classes. Polyfluorens (PFs) feature a rigid biphenyl unit with a bridging carbon atom at the 9-position, allowing for side-chain substitution that influences solubility and processing [30]. Their rigid backbone structure promotes extended conjugation and high photoluminescence quantum yields. The prototypical poly(9,9-dioctylfluorene) (PFO) exhibits excellent blue emission and can form different conformational phases affecting its optical properties [31]. More complex systems like F8 BT (poly(9,9-dioctylfluorene-alt-benzothiadiazole)) incorporate electron-accepting units to red-shift emission and enhance charge transport [32; 33]. Polyfluorens find applications in light-emitting devices, optical amplifiers, and lasers, though their susceptibility to oxidative degradation requires careful encapsulation. Polyphenylene vinylenes (PPVs) comprise phenylene units connected by vinylene bridges, creating a highly conjugated system with strong visible light absorption and emission [34; 35]. The archetypal poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) shows efficient yellow-orange emission and good solution processability [36; 37]. Derivatives like \"Super Yellow\" PPV offer improved stability and quantum yield [38]. These materials excel in electroluminescent devices and have shown promise in optical switching applications. Their relatively large Stokes shift helps minimize self-absorption in waveguide applications, though photo-oxidation of the vinylene bonds can limit long-term stability. Polythiophenes (PTs) feature five-membered sulfur-containing heterocycles in their backbone, providing good environmental stability and strong \(\pi\)-\(\pi\) interactions [39; 40]. Poly(3-hexylthiophene) (P3 HT) represents the most studied system, showing good charge transport and optical properties that depend strongly on molecular weight and regioregularity [41; 42; 43].