## 4 Organic Field-Effect Waveguides (OFEWs)  
An optical waveguide is a physical structure that directs electromagnetic waves within the optical spectrum and serves as a crucial component in photonic devices responsible for guiding, coupling, switching, splitting, multiplexing, and demultiplexing optical signals [52]. Typically, an optical waveguide is controlled by an electrical field via a two-terminal diode. While such integration has been extensively utilized in current photonic devices, its application is limited to compact optical chips for bio/medical and space applications [53]. The principle of light propagation in optical waveguides is based on the fundamental physics of waveguide theory, which relies on the concepts of total internal reflection and mode confinement. An optical waveguide is a structure designed to confine and guide light along a specific path, typically consisting of a core material with a higher refractive index surrounded by cladding material with a lower refractive index. This configuration creates a refractive index contrast that is crucial for effective light guidance. The core principle behind light propagation in a waveguide is total internal reflection. When light travels from a medium with a higher refractive index (the core) to a medium with a lower refractive index (the cladding), it is reflected back into the core if the angle of incidence is greater than the critical angle. This critical angle is determined by Snell's Law, which relates the refractive indices of the core and cladding materials to the angle of incidence. Total internal reflection ensures that the light is effectively confined within the core, allowing it to travel along the waveguide with minimal loss. In addition to total internal reflection, the concept of mode propagation is essential in waveguide theory. Light propagates in discrete modes, each with a specific spatial distribution of the electric field. The fundamental mode, known as the guided mode, is the lowest-order mode that is confined within the core and has a distribution that decays exponentially in the cladding. Higher-order modes can also exist, but they are often less stable and more sensitive to waveguide imperfections. The mode confinement is influenced by the waveguide's geometry and the refractive index contrast between the core and cladding. The physical basis of light propagation in waveguides also involves the principles of wave optics and electromagnetic theory. The waveguide supports the propagation of electromagnetic waves by ensuring that the waves are confined to the core region through constructive interference within the core and destructive interference in the cladding. This confinement allows for efficient light transmission and minimizes losses due to scattering or absorption. In practical applications, waveguides are used in various optical devices, including optical fibers, integrated optics, and photonic circuits. The design and optimization of waveguides involve careful consideration of material properties, waveguide dimensions, and operating wavelengths to achieve desired performance characteristics, such as low loss, high coupling efficiency, and specific mode propagation properties. Over the past few decades, flexible organic electronics have garnered significant research attention. Organic field-effect transistors, owing to their ease of fabrication and manufacturing simplicity, have been extensively studied for producing flexible and stretchable electronics for diverse applications. However, realizing all organic optoelectronics via integration of organic field-effect transistors for tuning organic photonics has remained a significant fundamental challenge. Integrating electronics and photonics is of paramount importance for achieving high-density and high-speed optoelectronic circuits. However, the realization of this goal remains challenging due to the complexities involved in merging diverse areas of science and technology.