The information industry has profoundly affected people’s lives due to advancements in technologies such as 5G, optical computing, the internet, sensing technologies, artificial intelligence, and multimedia/data/signal processing. These innovations have spurred major changes and opportunities within the optoelectronic device industry. One critical challenge is the demand for high-speed communication, which requires the rapid transformation of electronic signals into optical signals.

The heart of this transformation lies the development of high-performance electro-optical (EO) modulators. These devices translate electrical signals into the optical realm, which facilitates the transmission of high-bandwidth information while minimizing electrical interference. EO modulators are essential in optical communication systems, where they regulate optical signals. As the demand for faster EO signal conversion grows, the requirements for EO modulators become increasingly stringent. Key criteria include: 1) low drive voltage; 2) minimal optical loss; 3) low energy consumption; 4) high bandwidth, among others. EO polymers offer distinct advantages over other materials for modulator fabrication. They can achieve an EO coefficient (r₃₃) exceeding 300 pm/V in neat-film and over 100 pm/V in device. In contrast, commercial lithium niobate, a common modulator material, typically shows lower EO coefficients. The high r₃₃ value of EO polymers indicates their ability to achieve significant modulation with lower voltages, which makes them highly efficient for high-speed applications. Additionally, EO polymers exhibit a low microwave/optical velocity mismatch, which simplifies the design of modulator electrodes for achieving rapid modulation. These characteristics enable EO polymer modulators to operate at frequencies exceeding 100 GHz, ideal for applications requiring rapid data transmission and processing. Moreover, EO polymers can be processed and integrated with various materials and components, including semiconductor light sources, detectors, low-voltage CMOS drivers, and both inorganic and polymeric waveguides. This integration capability enhances the versatility of EO polymer modulators, thus allowing for customized optimization to meet specific application and device configurations. In 2002, Mark Lee and his colleagues demonstrated ultra-high bandwidth modulation ranging from 25 to 145 GHz in EO polymer MZI modulators (Fig. 12). With advancements in materials science, the EO coefficient of EO polymers can exceed 100 pm/V, resulting in a V_πL of around 1 V·cm (Fig. 13). Integrating EO polymers with silicon slot waveguides helps foster the development of more compact modulators. Modulation speed of up to 112 Gb/s has been realized using a 1.5 mm long slot waveguide. EO polymers have also been combined with metal plasmonic structures. By filling the polymer into the metal slots, both electric and optical fields can be concentrated within the metal slots, thereby enhancing the EO interaction. Modulators with an effective phase shift length of only 6 μm correspond to V_π=10 V and a modulation bandwidth of 70 GHz. Additionally, ring resonator modulators with high bandwidth and high EO tunability have been developed.

Based on advancements in organic molecular science, guided optics, and microwave theory, EO polymer materials, and their modulator structures have made enormous progress over the past decade. In terms of materials, scientists have systematically addressed several challenging issues through innovations in polymer compositions, chromophores, and host materials. These advancements provide a robust material foundation for the practical implementation of related devices and chips. In terms of device development, researchers from Japan, Germany, Switzerland, and China have successively pioneered “cladding-free structures”, “ultra-thin silicon structures”, “silicon-based slot structures”, “metal plasmonic structures”, and “silicon nitride micro-ring structures”. These innovations fully leverage the unique advantages of EO polymers, including high EO coefficients, broad intrinsic bandwidths, and good compatibility with multiple material systems. These breakthroughs have effectively overcome the limitations of traditional modulation techniques in terms of energy consumption and bandwidth.