## 5 Material Platforms for Integrated Photonics  
Several material platforms are utilized in the development of integrated photonics, each offering exceptional advantages tailored to specific applications [105]. Si photonics, leveraging the mature semiconductor industry, employs Si as the substrate due to its compatibility with complementary metal oxide semiconductor (CMOS) fabrication processes, enabling cost-effective large-scale integration [106, 107]. Si\({}_{3}\)N\({}_{4}\) provides low optical loss and compatibility with visible and near-infrared wavelengths, making it suitable for applications requiring high performance and broadband operation [14]. III-V compound semiconductors, such as InP and gallium arsenide (GaAs), offer superior optical properties and are frequently used for light sources, detectors, and modulators in high-speed communication systems. Meanwhile, hybrid integration techniques combine multiple materials, such as Si and III-V materials, to harness the benefits of each, enabling the realization of complex photonic circuits with enhanced functionality and performance [108]. Germanium-Tin (GeSn) alloys have garnered significant attention due to their potential for monolithic integration with CMOS technology, offering promising avenues for electronic and optoelectronic applications [109, 110]. Previous research has focused on optimizing growth techniques, such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), to achieve high-quality GeSn films with uniform composition and minimized defects [111]. These efforts have encountered several challenges, including managing the high lattice mismatch between GeSn and Si substrates, controlling Sn segregation, and mitigating strain-induced defects. Additionally, maintaining the thermal stability of GeSn during post-growth processing steps is crucial for device performance [112]. Researchers have explored various approaches to overcome these obstacles, such as using buffer layers, optimizing growth parameters, and implementing advanced annealing techniques [113]. Despite these challenges, the continued advancements in GeSn growth techniques underscore its potential as a CMOS-compatible material, paving the way for its integration into next-generation semiconductor devices. Organic materials and polymers are also explored for their flexibility, low-cost fabrication, and compatibility with flexible substrates, enabling applications in wearable devices and biochemical sensing [114]. Flexible integrated photonics represents photonic devices manufactured on pliable polymer substrates, resilient to mechanical deformations such as bending, folding, rolling, twisting, stretching, or compression, all while upholding their optical competency [115]. The hallmark of these devices lies in their mechanical adaptability, unlocking a realm of pioneering applications. The applications of this technology include flexible imaging and display arrays, short-reach optical links, wearable photonic textiles, solar cells, broadband tunable photonic devices, strain gauges, and optical systems that are capable of seamless incorporation with curved surfaces or biological tissues. Moreover, the incorporation of flexible substrates potentially revolutionizes manufacturing, paving the way for cost-effective, large-scale device production through roll-to-roll (R2R) processing [116]. Overall, the diverse range of material platforms in integrated photonics permits tailored solutions to meet the demands of various applications, spanning telecommunications, computing, sensing, and beyond. The information presented in Table 2 offers a general comparison of typical characteristics and applications of different material platforms. However, actual performance may vary depending on specific implementations, fabrication techniques, and device designs.