As an important composition of terahertz (THz) technology, THz pulsed focal-plane imaging has been paid widely attention since it was invented. Until now, researchers have introduced all kinds of methods to enhance the performance of this imaging technique. Simultaneously, this imaging technique has been tried to apply into various industrial and fundamental research fields. In this paper, recent technique improvements and application researches for THz pulsed focal-plane imaging are reviewed, including the spatial resolution enhancement, signal-to-noise ratio improvement, information acquiring ability as well as applications of this imaging technique in spectroscopic identification inspections, function demonstrations of meta-surface devices, measurements of THz special beams, observations of THz surface electromagnetic waves, and so on. The aim of this paper is to push the technique innovation and application exploration of THz pulsed focal-plane imaging.

Terahertz (THz) radiation, due to its unique propagation characters of low-energy, high-penetration, water-absorption, provides internal structure of objects and comprehensive biological information in phase contrast imaging. It has been applied in biomedical imaging, non-destructive testing, and other fields. As an important part of THz imaging technology, continuous-wave (CW) THz digital holography (TDH) is qualified as a non-invasive and whole-field phase contrast imaging method. In this paper, we review the development and status of off-axis and in-line TDH, including the recording and reconstruction theory, experimental setup, and reconstruction algorithms. The influence of existing THz sources and the reconstruction algorithms on resolution and fidelity of imaging are analyzed. And the development trend of TDH is prospected in the end.

Terahertz (THz) imaging technology has shown great advantages and potential applications in the fields of biomedicine, security, and aerospace, due to its low energy, high transmittance, wide bandwidth, and unique analysis abilities; while low spatial resolution restricts its further applications. Recently, a high-resolution, high-throughput, and broad-bandwidth THz imaging method has been proposed based on the terajet effect produced by dielectric structures with appropriate refractive index. The terajet beam can break through the restriction of the diffraction limit on the spatial resolution of the microscopic system without losing the energy and spectral bandwidth of the THz field. In this paper, firstly, a white-light nanoscopy based on photonic nanojet produced by microspheres is introduced, then the THz microscopy based on terajet effect produced by mesoscopic dielectric structures is reviewed. Finally, the prospect of THz high resolution imaging technology based on terajet effect is prospected.

Terahertz radiation is an electromagnetic wave whose frequency is in the range of 0.1 THz～10 THz. With its many features such as non-ionizing and resonance to many biomolecules, THz wave has great potential applications in biomedical field, especially in tumor detection. Terahertz imaging technology, as a new imaging technology in biomedical field, is studied by many research groups around the world. In this paper, we listed and analyzed many terahertz imaging methods in tumor detection, including terahertz scanning imaging, terahertz tomography, terahertz holography, and terahertz near-field imaging. We introduced the basic principle of these imaging methods and the works done by different groups worldwide. At last, we presented the prospect of terahertz imaging technology applied in biomedical field.

Holography is a kind of technique enabling 3D imaging which has been applied in many practical fields. With the rapid development of computer science and technology, computer generated holography (CGH) has become a common holography design method due to its high convenience and flexibility. Herein, we present a review of our recent progress in metasurface-based terahertz CGH. In these demonstrations, the metasurfaces acting as the holograms have shown the novel capabilities beyond the conventional counterparts. We first present a meta-hologram with simultaneous and independent phase and amplitude control over each pixel, which enables high-quality holographic imaging. Such new characteristic also predicts new holographic imaging performances including holographic images transforming continuously along the propagation direction realized by dielectric metasurface. Then different responses under different incident polarization states are designed. A linear polarization and frequency multiplexed meta-hologram, a reflective circular polarization multiplexed meta-hologram, and a surface-wave-based polarization multiplexed meta-hologram have been achieved respectively. Furthermore, a thermally dependent dynamic meta-hologram which can change the holographic image actively is also given. The proposed method paves a novel way to the design and realization of CGH functional devices in the future and contributes to the development of metasurfaces towards practical applications.

Terahertz near field MIMO-SAR technology has advantage of reducing the number of arrays while ensuring resolution, thus it has important application in human security inspection. This paper firstly introduces the application of terahertz near field MIMO-SAR technology in human body security inspection, then introduces the system composition, imaging algorithm, and finally makes a prospect of development.

Terahertz wave has non-destructive nature and fingerprint characteristics for a large number of biomolecules, thus has a good application prospect in the field of medical imaging. In this review, we presented a brief introduction on the terahertz medical imaging systems, and the applications of terahertz medical imaging in biological tissues from in vitro to in vivo. Terahertz wave can be strongly absorbed by water, then the terahertz imaging contrast will be severely deteriorated in vivo. So the terahertz medical imaging was mainly used for detecting epidermal tissues or biological tissues with pretreatments, including excision, dehydration and so on. This review also concluded the recent development of nanoparticle contrast agents for improving the contrast of terahertz imaging in vivo. Finally, the future development of terahertz medical imaging was predicted.

Terahertz (THz) waves have a good transmissivity through non-polar materials, and have no ionization effects on biomedical tissues. Therefore, it is ideal for the applications such as non-destructive testing and biomedical imaging. The imaging system based on THz quantum well photodetectors (THz QWPs) has higher imaging resolution, faster imaging speed, higher signal-to-noise ratio, and more compact structure than the imaging systems based on other detectors, as the THz QWPs have fast response, high responsivity, low noise equivalent power, and tiny size. This paper reviews the research progress of the imaging technology based on THz QWPs. And the factors affecting the core indicators of the imaging system are analyzed and summarized. Using more stable fixtures to mount the THz QWPs, improving the device response speed, detection sensitivity, array size, can improve the key performance of imaging systems effectively.

Based on a research hotspot in field of terahertz (THz) wave imaging—THz wave ghost imaging, we first reviewed the development history of ghost imaging: from quantum to classical and then to computational. Second, the mathematical principles of the computational ghost imaging were described in details. Then, we reviewed the development history of computational ghost imaging within THz regime, and its applications including sub-diffraction imaging, photoconductivity mapping of graphene, and hyperspectral THz imaging. At last, we looked towards the prospects of THz wave ghost imaging: as an imaging scheme, ghost imaging avoids the problem that economic and efficient focal-plane-array detectors within THz regime are lacking, whose present frame rates, however, are too slow to meet the requirement for fast imaging. With the improvement of devices’ performance and the optimization of imaging algorithms, we believe that the frame rate of THz wave computational ghost imaging can be significantly enhanced in the future.

In this paper, the theoretical model of ununiform plasma sheath is established based on scattering matrix method and the transmission characteristics of 0.1 THz～10 THz wave are simulated. A kind of plasma jet is produced in laboratory environment according to the principle of dielectric barrier discharge. Then the measurement of transmission spectrum of terahertz time-domain spectroscopy (THz-TDS), broadband terahertz source, and the terahertz wave reflective imaging of target under plasma shelter are carried out, respectively. Both theory and experiment results show that terahertz wave has good penetration in plasma, which provides a new way for communication and radar detection in blackout area.

In recent years, terahertz imaging has attracted great attention due to its advantages including penetrability and nondestructive property. The field imaging technology within the terahertz range is expected to enhance the terahertz image quality and improve its application effect. In this paper, an experiment on the data acquisition and digital refocusing of the terahertz light field was conducted. Firstly, the basic principle, system structure, and the method of reconstructing light field imaging were analyzed. Secondly, the terahertz focal plane array camera was used to collect the data about light field and digital refocusing was used to get the computed imaging. Finally, the reconstructed image was enhanced to obtain higher depth resolution, angle resolution, and object contour resolution. Experimental results showed the feasibility and ability of terahertz light field imaging to improve image quality and enrich retrieval effects.