## 4 Organic Field-Effect Waveguides (OFEWs)
In this sense, it emerges an organic integrated optoelectronic device known as the Organic Field-effect Optical Waveguide (OFEW), which seamlessly integrates a field-effect transistor and an optical waveguide. In this device, the propagation of the optical waveguide within the active organic semiconductor can be precisely controlled by the third terminal--the gate electrode of the transistor--enabling controllable modulation depths of up to 70% and 50% in parallel and perpendicular directions of charge transport relative to the optical waveguide, respectively. Furthermore, the optical waveguide, oriented in different directions, can effectively modulate the field-effect of the device, with a photo-dependence ratio of up to 14,800. The successful integration of an active field-effect transistor with a semiconductor waveguide modulator significantly broadens the possibilities for achieving scalable integration of electronics and photonics on a single chip. In an investigation performed by [PERSON] and col. [54], the first \"OFEWs\" were developed, demonstrating the potential utilization of organic field-effect transistors for tuning organic optical waveguides. An organic semiconductor, 2,8-dichloro-5,11-dihexyl-indolo(3,2-b)carbazole (5) (Figure 10), was employed to construct OFEWs, which were assembled based on individual organic semiconductor crystals with gold stripes serving as source and drain electrodes. Figure 11 illustrates the optoelectronic device structure, device performance, and proposed operational principles of the constructed OFEW (Figure 11a). In both operational modes, the output intensities of the waveguide exhibited a significant dependence on the gate and source-drain voltages (Figure 11b,c). Subsequent investigations revealed that OFEWs function similarly to phototransistors (Figure 11d,e). These findings suggest that photons can be envisioned as self-propagating transverse oscillating waves of electric and magnetic fields within the organic field-effect optical waveguides, consistent with [PERSON]'s equations. Furthermore, the authors proposed a working mechanism, as indicated in Figure 11f. Moreover, the OFEWs reported by [PERSON] and col's. team can indeed be implemented on flexible substrates, facilitating the development of stretchable electronics with significant potential for biochips. It is anticipated that this discovery will inspire other research groups to develop flexible optoelectronics for bio/medical and space applications. Figure 10: Structure of 2,8-dichloro-5,11-dihexyl-indolo(3,2-b)carbazole (**5**) used for the organic field-effect waveguide (OFEW) described by [PERSON] and col [54]. However, the search for new organic materials with application in these hybrid OFEWs is not easy because we have to achieve organic materials with application as optical waveguides and as semiconductors in OFETs at the same time and then explore their possible implementation in the hybrid devices. Many times, compounds with optical waveguiding behavior that are usually small molecules do not work as semiconductors in OFETs. On the other hand, compounds with architectures appropriate for OFETs are too big to form the necessary fibers or crystals with optical waveguiding behavior. Bearing in mind this complicated challenge, [PERSON] and col. are looking for this kind of organic material with both applications for the designing of these new hybrid devices. In this sense, they have already developed multifunctional materials with application simultaneously as optical waveguides and as semiconductors in OFETs, with potential  
Figure 11: (**a**) Diagram illustrating the architecture of organic field-effect optical waveguides (OFEWs) utilizing individual organic semiconductor crystals with gold stripes serving as source and drain electrodes. Two optical waveguide direction models are depicted: along the conducting channel (model I) and perpendicular to the conducting channel (model II). (**b**,**c**) Characteristics of modulation in organic molecule-based OFEW devices.