In the past few decades, the applications of terahertz (THz) spectroscopy and imaging technology have seen significant developments in the fields of biology, medical diagnosis, food safety, and nondestructive testing. Label-free diagnosis of malignant tumours has been obtained and also achieved significant development in THz biomedical imaging. This review mainly presents the research status and prospects of several common continuous-wave (CW) THz medical imaging systems and applications of THz medical imaging in biological tissues. Here, we first introduce the properties of THz waves and how these properties play a role in biomedical imaging. Then, we analyse both the advantages and disadvantages of the CW THz imaging methods and the progress of these methods in THz biomedical imaging in recent ten years. Finally, we summarise the obstacles in the way of the application of THz bio-imaging application technology in clinical detection, which need to be investigated and overcome in the future.

Continuous-Wave THz Imaging for Biomedical Samples

High stable common-path digital holographic interferometers can be widely applied in long-term time-lapsing interferometric measurements, threedimensional imaging, and quantitative phase imaging. Scientists in China reviewed the commonpath off-axis digital holography and categorized the common-path models into lateral shearing, point diffraction and other types, and summarized the progress of this topic in detail. Benefiting from compact features, common-path digital holography is very promising for the manufacture of highlystable optical measurement and imaging instruments in the future.

A review of common-path off-axis digital holography: towards high stable optical instrument manufacturing

Spatial light modulators (SLM), capable of dynamically and spatially manipulating electromagnetic waves, have reshaped modern life in projection display and remote sensing. The progress of SLM will expedite next-generation communication and biomedical imaging in the terahertz (THz) range. However, most current THz SLMs are adapted from optical alternatives that still need improvement in terms of uniformity, speed, and bandwidth. Here, we designed, fabricated, and characterized an 8 × 8 THz SLM based on tunable liquid crystal metamaterial absorbers for THz single-pixel compressive imaging. We demonstrated dual-color compressive sensing (CS) imaging for dispersive objects utilizing the large frequency shift controlled by an external electric field. We developed auto-calibrated compressive sensing (ACS) algorithm to mitigate the impact of the spatially nonuniform THz incident beam and pixel modulation, which significantly improves the fidelity of reconstructed images. Furthermore, the complementary modulation at two absorption frequencies enables Hadamard masks with negative element values to be realized by frequency-switching, thereby halving the imaging time. The demonstrated imaging system paves a new route for THz single-pixel multispectral imaging with high reliability and low cost. 

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Dual-color terahertz spatial light modulator for single-pixel imaging

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High-resolution terahertz ptychography using divergent illumination and extrapolation algorithm

Continuous-wave terahertz digital holography (TDH) is a booming full-field phase-contrast imaging method validated in both in-line and Mach–Zehnder off-axis geometries. In this Letter, a self-referencing TDH approach is proposed based on the Fresnel's mirrors, by which the object wavefront is partitioned and reflected. Two beams interfere with each other to form an off-axis hologram. The proposed recording configuration is immune from a superposed twin image and has higher temporal stability than Mach–Zehnder interferometers. To evaluate the phase-contrast imaging performance, different types of samples are measured.

Continuous-wave terahertz self-referencing digital holography based on Fresnel's mirrors

Terahertz (THz) phase imaging is widely spreading in various scenarios, among which full-field phase distributions are commonly retrieved by digital holography or ptychography. In this Letter, the transport of the intensity equation reconstruction method is applied into the THz band. An algorithm named the lensless US-transport of intensity equation (TIE) is proposed to accommodate to an in-line configuration. The object phase is retrieved by primarily conducting iterations between the axial intensity derivative and the phase distribution at the recording plane and subsequent backward diffraction propagation. This method is applicable to both isolated and extended weakly absorbing samples with higher reconstruction quality and remarkably less time cost than holographic phase retrieval algorithms. It can also be attempted in other non-interferometric geometries or using low-cost partially coherent THz sources, which significantly broaden the application scope of THz phase imaging.

Transport of intensity equation-based terahertz lensless full-field phase imaging.

The invention discloses a continuous terahertz wave self-reference digital holographic imaging method based on a Fresnel double-sided mirror. According to the method, high-time-stability common-optical-path off-axis interference holography can be achieved only through few optical elements. The method has the advantages that the number of required optical elements is small, the optical path structure is compact, the period of interference fringes can be flexibly adjusted, and the sampling condition of the detector can be better met. Because the intensities of the object light and the referencelight on are cording plane are almost equal, interference fringes with consistent contrast can be obtained, and the reproduction of an amplitude image and a phase image and the improvement of the precision are facilitated. Under the conditions of the same recording duration and unchanged experimental conditions, experiments prove that the recording device provided by the invention has higher timestability than the traditional optical path system based on a Mach-Zehnder interferometer structure.

Yaya Zhang

Papers

Continuous-Wave THz Imaging for Biomedical Samples

High-resolution terahertz ptychography using divergent illumination and extrapolation algorithm

Continuous-wave terahertz self-referencing digital holography based on Fresnel's mirrors

Transport of intensity equation-based terahertz lensless full-field phase imaging.

Continuous terahertz wave self-reference digital holographic imaging method based on Fresnel double-sided mirror