Electrically Gated, Ultra-Broadband THz Amplitude Modulation via Solid-State Organic Electrochemical Devices

Wireless systems for imaging, sensing, and communications continue to march onward into higher frequency bands for improved bandwidth, resolution, directionality, and other benefits. In this context, the THz regime (from 0.1 to 10 THz) has recently gained attention. However, THz radiation presents unique challenges regarding modulation and reconfigurability that are needed for many device applications. The so-called “THz gap” refers to the nascent state of development for the generation and manipulation of THz radiation. The necessary picosecond timescales place stringent requirements on the operation of THz devices that renders conventional materials and fabrication approaches less effective at higher frequencies. Consequently, most research on devices for the manipulation of THz radiation, such as 2D systems or periodic structures/metamaterials, rely on novel materials and complex fabrication techniques. Such approaches present additional challenges to solving the THz gap in scalability, reproducibility, and integration with existing technologies, hindering their widespread adoption.<br/><br/>Organic semiconductors have shown great promise in the context of facile processing and fabrication for scalable, large area devices. While organic semiconductor-based displays are well established, areas of active device research include organic electrochemical transistors (OECTs) and electrochromic devices (ECDs). Such devices utilize organic mixed ionic-electronic conductors (OMIECS) such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as channel layers in contact with an adjacent electrolyte that provides mobile ions. When a bias is applied between the electrolyte and channel, ion migration is induced into the channel material, leading to charge compensation that can modulate conductivity. These reversible interactions are accompanied by significant color changes, leading to their use in ECDs. Such devices show unique properties, including low-voltage operation, chemical tunability, ability to interface with soft and flexible substrates, and ease of fabrication. Additionally, the conductivity of such polythiophenes can often be processed to near-metallic levels, allowing for high frequency operation.<br/><br/>In this talk, we demonstrate the use of electrochemical doping and de-doping processes in a thin channel of an OMIEC to electrically manipulate THz radiation across a 1 THz bandwidth. We demonstrate a fully solid-state device based on a gel electrolyte laminated onto a polythiophene thin film, achieving up to 75% modulation depth with a single layer. We also explore the electrochemical stability limits of such devices, as well as the response times and long-time biasing capabilities. Further, we demonstrate the simultaneous DC, THz, and optical (i.e., electrochromic) switching behaviors of the films. These results show the promise of OMIEC-based devices for THz operation, opening new avenues for advanced applications in high frequency bioelectronics, sensing, and communication technologies.