Single-pixel terahertz imaging with enhanced edge detection using angular momentum of structured light

Structured light—electromagnetic waves with spatial inhomogeneity of amplitude, phase, and polarization—because of its fascinating ability to precisely control these properties, has garnered significant attention across various research fields. Particular interest is given to developing imaging systems where the requirement to increase image resolution, contrast, and ability to resolve tiny structures is of utmost importance. This task can be achieved well using single-shot imaging; however, it remains challenging to implement in conventional terahertz (THz) imaging systems using Gaussian modes. A single-pixel imaging scheme is more attractive for implementation in real operational conditions as it employs sophisticated schemes with single-pixel detectors to retrieve images. It was very recently determined that structured light illumination and image collection schemes are beneficial for various metrics in single-pixel THz imaging [S. Orlov et al., Laser Photonics Rev. 18, 2301197 (2024)]. This work introduces angular momentum in structured light illumination for THz image retrieval, demonstrating simultaneous enhancement of object edge detection while maintaining resolution and contrast. We investigated three experimental approaches at 253 GHz: a nonparaxial Fresnel zone plate, a THz vortex with a zone plate, and a THz Bessel beam with a vortex. The diffractive optical elements were fabricated by 3D laser printing, while silicon-based diffractive elements were prepared by laser ablation technology. Through comprehensive experimental studies supported by numerical modeling, we reveal distinct features of structured THz light induced by angular momentum. This approach establishes a new pathway for developing advanced single-pixel THz imaging systems that leverage the angular momentum of structured light, offering the foundation for future improvements in performance via enhanced edge detection and extended material characterization capabilities.