Photonics-integrated terahertz transmission lines

Modern communication and sensing technologies rely on connecting the optical domain with the microwave domain. Terahertz technologies, spanning higher frequencies from 100 GHz to 10 THz, are critical for providing larger bandwidths and faster switching capabilities. Nonlinear processes, such as optical rectification and electro-optic sampling, offer potential solutions for terahertz generation and detection, but are currently limited to discrete components based on bulk nonlinear crystals, missing out miniaturization opportunities from integration of optical circuits with terahertz ones. We address this challenge by integrating phase-matched terahertz transmission lines with photonic circuits in a single hybrid architecture on the thin-film lithium niobate~(TFLN) platform. This design demonstrates phase-matched, broadband terahertz generation and detection spanning four octaves (200 GHz to >3~THz) through broadband bi-directional conversion between optical and terahertz signals. The micron-sized transmission lines provide terahertz field confinement with minimal radiative loss, enabling compact terahertz cavities embedded in integrated photonic circuits. This integration is crucial for leveraging photonics' advantages in low-noise, low-loss, and high-speed for terahertz generation and detection. Our platform can readily be integrated with other mature photonic components, such as electro-optic modulators, femtosecond lasers, or photodiodes, to pave the way for compact and power-efficient terahertz devices with applications in telecommunications, spectroscopy, quantum electrodynamics and optical computing.