Present and future of terahertz integrated photonic devices

Photonic integrated circuits have benefited many fields in the natural sciences. Their nanoscale patterning has led to the discovery of novel sources and detectors from ultraviolet to microwaves. Yet terahertz technologies have so far leveraged surprisingly little of the design and material freedom provided by photonic integrated circuits. Despite photoconduction—the process in which light is absorbed above the bandgap of a semiconductor to generate free carriers—and nonlinear up- and down-conversion being by far the two most widespread approaches to generate and detect terahertz waves, so far, terahertz technologies have been mostly employed in bulk. In this perspective, we discuss the current state-of-the-art, challenges, and perspectives for hybrid optical-terahertz photonic chips. We focus, in particular, on χ(2) and χ(3) nonlinear waveguides and waveguide-integrated photoconductive devices. We highlight opportunities in the micro- and macroscale design of waveguide geometries and printed antennas for the optimization of emission and detection efficiencies of terahertz waves. Realizing complex functionalities for terahertz photonics on a single chip may come into reach by integration and miniaturization compatible with telecom and fiber technologies.