# Continuous-Wave THz Imaging for Biomedical Samples
## 3 CW THz Full-Field Imaging
### CW THz Digital Holography  
Continuous-wave THz digital holography (CW TDH) is an essential full-field, real-time two-dimensional quantitative phase contrast imaging approach, which has the advantage of high spatial resolution. It can obtain the distributions of amplitude and phase without scanning. By recording the diffraction field information of the object or the interferometric pattern formed by the object and the reference beam, the amplitude and phase information of the wavefront of the object are then reconstructed by the digital numerical calculation [59]. The TDH scheme can be divided into in-line and off-axis TDH according to the angle between the reference and object beam in the detecting plane [60; 61]. In 2014, [PERSON] et al. investigated an in-line CW TDH based on a 2.52 THz FIRL 100 and pyroelectric (PY-III) array detector, as shown in Figure 8 [62], which eliminated the twin images through the iterative phase retrieval and reconstructed the amplitude and phase images of a dragonfly's rear wing by recording a single hologram. In addition, the high-frequency information was obtained by the extrapolation of holograms, and the cross veins of width 35 \(\upmu\)m could be observed in the reconstructed THz image. In 2016, [PERSON] et al. used a QCL source and a microcalorimeter detector to form an in-line CW TDH arrangement [63]. The synthetic aperture method was adopted to expand the size of the hologram from 320 \(\times\) 240 pixels to 660 \(\times\) 580 pixels, corresponding to improving the resolution from \(\approx\)253 to 125 \(\upmu\)m, and the recording distance and the wavelength respectively were 14.75 mm and 97 \(\upmu\)m. The subpixel image registration algorithms could reduce the subpixel displacement error of the sub-holograms before the synthetic process. Finally, the phase retrieval method was used to reconstruct the distribution of the amplitude and phase of the sample. The basic outline of the dehydrated mosquito could be observed in the reconstructed images. In 2019, [PERSON] et al. further modified the in-line CW THD system based on a 4.3 THz QCL and a microbolometer detector [64; 65]. Both the synthetic aperture algorithm and shortening the distance together can improve the resolution. The autofocus algorithm was used to find the most accurate recording distance, and a sparsity-based phase retrieval algorithm could solve the twin-imaging problem. The lateral resolution of the reconstructed intensity image was up to 70 \(\upmu\)m at the shortest recording distance of \(\approx\)13.5 mm. The phase contrast images of a cicada's wing, a beetle's leg, and a spider were all high-quality reconstructions. Although the geometry of the in-line CW TDH is simple, shortening the recording distance can enhance the resolution, but the problem of twin-image caused by the propagation of the object and reference beams in the common path can be eliminated only by a phase retrieval algorithm or the phase-shifting method [66], which waste more time during the numerical reconstruction. The size of the object must be smaller than the incident beam to adopt such a scheme, or it should present a weak absorption distribution of THz. To overcome the above problems, an independent reference beam is introduced by the THz beam splitter, where the interference between it and the object beam arm an off-axis hologram. The off-axis TDH allows relatively easy reconstruction of a complexamplitude wavefront distribution of the sample in real time. The off-axis scheme not only has no limitation on the size of the tested object, but it can also carry out reflection mode imaging and obtain the complex amplitude wavefront distribution. In 2015, [PERSON] et al. determined an off-axis CW TDH imaging device based on a Mach-Zender interferometer [67]. The imaging system is shown in Figure 9. The system adopted a 2.8 THz QCL (power: 4 mW, operating temperature: \(\approx\) 20 K), and recorded holograms by micro-bolometer focal plane arrays of 640 \(\times\) 480 pixels.