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

Abstract

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.

Terahertz Imaging and Sensing for Healthcare: Current Status and Future Perspectives

“…Molecular biomarkers have fingerprint spectral features in the broad THz regime, however the spectral information of the spectral fingerprints can be easily lost when they are dissolved in water or superposed in a mixed form. It is therefore a challenging task to expect sharp fingerprinting for larger biological species such as cells, proteins, or body organs [214].…”

Recent Progress of Terahertz Spatial Light Modulators: Materials, Principles and Applications

“…With the predefined modulation patterns and correlated detection signals, the object can be reconstructed through matrix inversion. Detailed theory can be found in related reviews [ 31 , 108 , 109 , 110 , 111 ]. Since the intensity and phase patterns are both reconstructable [ 112 ], the imaging system is applicable to various materials, such as metallic resolution chart [ 71 ], knifes [ 65 ], plastic [ 14 ], semitransparent materials [ 113 ], leaves [ 67 ] and circuit boards [ 70 ].…”

Broadband THz Edge-Enhanced Imaging Technology Based on Electro-Optic Sampling

“…In the frequency domain, this important information corresponds to high-frequency components, which will decay exponentially with the increase of propagation distance. Therefore, such a typical imaging method cannot obtain terahertz images with ideal contrast, which makes edge detection and feature recognition [8][9][10][11][12] in subsequent image processing complex.…”