Terahertz tomography

THz imaging can be used for screening of luggage and postal mails due to the fact that molecular crystals can be viewed clearly, based on particular features demonstrated in the THz spectrum. Explosives and illicit drugs can be located and identified [4][5][6][7][8][9][10][11][12][13][14], for example, several liquid explosives can be distinguished given the dramatic change in dielectric response in THz range versus the alcohol percentage [15]. Although dangerous metalwares, such as knives, can be recognized by their shapes through certain pattern recognition algorithms [16], it’s impossible to see through metallic packages with THz waves. And that’s why THz spectrometers cannot replace X-ray scanners. However, it is still useful in providing additional information for low density materials and chemical separation [17].

THz systems can also be used for production control in paper and polymer industries [18]. It enables differentiations between different thickness and moisture content in paper in paper industry [19], and demonstration of conductive properties, moisture level, fiber orientation and the glass-transition temperature in polymer industry [20][21][22][23].

THz systems are helpful in both metallic and nonmetallic contamination detection in food [24]. For example, THz waves made it possible to detect metallic and nonmetallic components in chocolate bars [25], since food with low water contents, such as chocolates, is almost transparent in THz waves. Moisture quantification and non-destructive analysis of cork substance are also possible with THz waves, which is of great significance for wine or alcohol making companies.

Pharmacy is another filed where potential applications of THz systems are addressed. Polymorphic forms detection can be realized considering crystalline structures formed by different isomers have different spectra fingerprints in the THz range. As chirality being the major concern when it comes to the pharmaceutical performance of the active principle [26], this detection is crucial in pharmacy. Besides, THz is a great tool to achieve quality control of tablet coatings [27].

Non-destructive analysis that can be conducted onsite for valuable artworks can also be realized with THz imaging, which can reveal the hidden layers and transmittance of several pigments of paintings [28][29]. Interests in THz tomography are aroused to analyze samples in a 3-D visualization [30][31].

In diffraction tomography, the detection beam interacts with the target and then USES the target scattered waves to build a 3D image of the sample[32]. The diffraction effect and the diffraction slice theorem are used to shine light on the surface of the scattered object and record the reflected signal to obtain the diffraction field distribution after the sample, so as to explore the surface shape of the target object. For fine samples with more complex surface structure, this technique is effective because it can provide a sample refractive index distribution[33].But there are also drawbacks. The imaging speed of terahertz diffraction tomography is faster, but its imaging quality is poor due to the lack of effective reconstruction algorithm. In 2004, S.waang et al. first used the diffraction chromatography based on thz-tds system to image polyethylene samples[34], and their experimental system was shown in figure 1.

The difference of fault synthesis chromatography is that it is less difficult to reconstruct. The reconstruction can be done by several projection angles, making the image faster. This technique can be regarded as a kind of lost information CT technology, which has low resolution but faster imaging speed [35]. This technique also has a huge advantage over terahertz CT. Terahertz CT is seriously affected by reflection and refraction, especially for the wide and flat plate sample, which has a large incidence Angle at the edge and a serious signal attenuation. Therefore, it is difficult to obtain complete projection data and obtain a lot of noise information at the same time. However, the terahertz fault synthetic tomography is not affected by refraction and reflection because of the small incidence Angle during projection. It is an effective method for local imaging, rapid imaging, or incomplete sample rotation. In 2009, N.unaguchi et al. in Japan used continuous terahertz solid-state frequency multiplier with frequency of 540 GHz to conduct TS imaging on three letters "T", "H" and "Z" at different depths of post-it notes. [36]The back projection method and wiener filter were used to reconstruct the spatial distribution of three letters. Their imaging mechanism is shown in figure 2.

Terahertz fault chromatography can reconstruct the three - dimensional distribution of refractive index.This is because when the terahertz pulse is reflected at different depths in the sample, the depth distribution information of the refractive index can be obtained according to the time delay of the peak value of the reflected pulse.For structured samples, good information can be obtained.The longitudinal resolution of time-of-flight tomography depends on the pulse width of terahertz waves (usually in the tens of microns).Therefore, the vertical resolution of flight time chromatography is very high.In 2009, J.Takayanagi et al. designed an experimental system that successfully tomography a semiconductor sample consisting of three sheets of superimposed paper and a thin layer of GaAs just two microns thick[37].

THz beam can be incorporated into 3D holography if the differentiation of each multiple scattered THz waves of different scattering orders is enabled [38]. With both intensity and phase distribution recorded, the interference pattern generated by object light and reference light encodes much more information than a focused image. Reconstructed according to Fourier optics theory [39], the holograms can provide researchers a 3D visualization of the object of interest. However, it remains a challenge to obtain high quality images with this technique because of the scattering and diffraction effects required for measurement, which is what this technology relies on. The high order scattering measurement usually result in poor signal to noise ratio (SNR) [40].Typical terahertz 3d digital holographic experimental system is shown in figure 4.

Fresnel lenses can serve as a replacement of traditional refractive lenses [41] with the advantages of being small and light-weighted. Given that their focal lengths depend on frequencies, we can image samples at various locations along the propagation path to the imaging plane [42], which can be applied to perform tomographic imaging.In 2003, s. waang et al. conducted terahertz Fresnel lens chromatography for the first time, and used the broadband terahertz pulse transmission sample generated by thz-tds system to image three letters distributed at different positions in space by changing the frequency, as shown in figure 5[43].

Synthetic aperture processing (SA) differs from traditional imaging systems in the way of collecting data. In contrast to point-to-point measurement scheme, SA uses a diverging or unfocused beam [44]. The information of phase collected by SA can be adopted for 3D reconstruction.

Taken pulsed THz radiation into use, time reversal imaging serves as an innovative method of indirect imaging. Exploiting the time-reversal symmetry of Maxwell’s equations to derive an image reconstruction algorithm, it was made possible with this technique to reconstruct 1D, 2D and 3D [45] amplitude and phase contrast objects to the measurement of the diffracted THz field at multiple angles accordingly [46]. A very small resolution of 674 μm has been achieved with time reversal THz imaging, which is greatly smaller than the average THz source.

THz computed tomography record both amplitude and spectral phase information, compared to X-ray imaging. With the implement of THz CT, identification or comparison of different substances and non-destructively locating them have been made possible. But the transfer of this technology from laboratories to the real world is limited by the thickness of the sample due to absorption, complexity, high costs of strong-power THz sources, and sufficient transparency required in the THz bandwidth [47]. New reconstruction algorithms are also needed because of the unique propagation of THz beam.