Enhanced nonlinear THz detection using a GaP photonic crystal waveguide and plasmonic antenna coupler

We present a terahertz (THz) detection platform that integrates two key components: a microstructured photonic crystal waveguide fabricated in 110-oriented gallium phosphide (GaP) and a gold plasmonic coupling antenna. In this architecture, a collimated near infrared pulse co-propagates with a THz signal inside a nonlinear GaP crystal to increase detection efficiency. The engineered THz waveguide enables satisfaction of phase matching conditions within the 1.7–3.5 THz spectral window and supports a coherence length greater than 2 mm across a 1 THz bandwidth. Measurements obtained with five distinct waveguide geometries consistently reveal, in comparison to bulk GaP, improved THz detection over a range of frequencies dictated by the waveguide core diameter. The metallic antenna with a bull’s-eye design is mounted on the input waveguide facet, efficiently collecting freely propagating THz waves and coupling them into a guided mode. The hybrid waveguide–plasmonic platform achieves enhanced detection sensitivity across a broad spectral range (1.9–3.9 THz), with a maximum 17-fold improvement, attributed to improved spatial mode overlap over an extended nonlinear interaction length. This scheme addresses key limitations in THz detection, including phase mismatch and beam divergence, and offers a compact, scalable, and robust solution with promising applications in high-speed THz communications and single-pixel imaging.