Frontiers in Terahertz Imaging Applications beyond Absorption Cross-Section and Diffraction Limits

Label-free imaging technology is highly desirable for various bioengineering, medicine, and chemistry applications. Most existing optical imaging techniques require labeling of the biospecimens and have limitations that may affect the intrinsic properties of the target species. The electromagnetic waves in the terahertz (THz) spectral range can be an excellent alternative to visible light, which provides plentiful vibrational signatures of many molecules with very low photon energy (1 THz is equivalent to 4 meV), completely free from damage to the biomaterials. For this reason, THz waves possessing a broadband spectrum have emerged as a critical technology for fundamental research in bio/chemical detection and medical imaging, as well as in solid-state physics, chemistry, material science, and highly anticipated 6G next-generation telecommunications. The limited performance of THz waves as a sensing or imaging tool has been a considerable drawback resulting from low sensitivity or diffraction-limited low spatial resolution. Nevertheless, many successes in obtaining increased sensitivity with additional nanostructures and improving spatial resolution with geometric beam shaping opened up a way for highly efficient real-time THz imaging. From this Perspective, recent trends in innovative THz sensing and imaging research deserve an introduction regarding the level of reliability and sensitivity that can evolve into an actual medical device and other applications. It can also be expected to enable progress in analysis algorithms (compressive phase retrieval, reconstruction, or machine/deep learning), enabling better data sampling, denoising, deblurring, and efficient computing cost, thus finally providing a leap forward in the THz imaging area beyond the absorption cross-section and diffraction limits.