Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

We demonstrate a tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional (3D) structures in the far-infrared. This THz-wave computed tomography (T-ray CT) system provides sectional images of objects in a manner analogous to conventional CT techniques such as x-ray CT. The transmitted amplitude and phase of broadband pulses of THz radiation are measured at multiple projection angles. The filtered backprojection algorithm is then used to reconstruct the target object, including both its 3D structure and its frequency-dependent far-infrared optical properties.

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T-ray computed tomography.

Terahertz (THz) science will profoundly impact biotechnology It has tremendous potential for applications in imaging, medical diagnosis, health monitoring, environmental control and chemical and biological identification THz research will become one of the most promising research areas in the 21st century for transformational advances in imaging, as well as in other interdisciplinary fields However, terahertz wave (T-ray) imaging is still in its infancy This paper discusses the uniqueness and limitations of T-ray imaging, identifies the major challenges impeding T-ray imaging and proposes solutions and opportunities in this field It also concentrates on the generation, propagation and detection of T-rays by the use of femtosecond optics

/pdf/terahertz-wave-imaging-horizons-and-hurdles-p9bxcja9ea.pdf

Terahertz wave imaging: horizons and hurdles.

Terahertz time-domain spectroscopy (THz-TDS) is a coherent measurement technology. Using THz-TDS, the phase and amplitude of the THz pulse at each frequency can be determined. Like radar, THz-TDS also provides time information that allows us to develop various three-dimensional THz tomographic imaging modalities. The three-dimensional THz tomographic imagings we investigated are: terahertz diffraction tomography (THz DT), terahertz computed tomography (THz CT), THz binary lens tomography and THz digital holography. THz DT uses the THz wave as a probe beam to interact with a target, and then reconstructs the three-dimensional image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired by x-ray CT. THz binary lens tomography uses the frequency dependent focal length property of binary lenses to obtain tomographic images of an object. THz three-dimensional holography combines radar and conventional holography technology. By separating the multiple scattered THz waves of different scattering orders, we used a digital holography method to reconstruct the sparsely distributed scattering centres. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, three-dimensional THz imaging is a very useful tool to inspect or characterize dielectric and semiconductor objects. For example, three-dimensional THz imaging can be used to detect and identify the defects inside a space shuttle insulation tile.

https://iopscience.iop.org/article/10.1088/0022-3727/37/4/R01/pdf

Pulsed terahertz tomography

Terahertz and millimeter waves penetrate various dielectric materials, including plastics, ceramics, crystals, and concrete, allowing terahertz transmission and reflection images to be considered as a new imaging tool complementary to X-Ray or Infrared. Terahertz imaging is a well-established technique in various laboratory and industrial applications. However, these images are often two-dimensional. Three-dimensional, transmission-mode imaging is limited to thin samples, due to the absorption of the sample accumulated in the propagation direction. A tomographic imaging procedure can be used to acquire and to render three-dimensional images in the terahertz frequency range, as in the optical, infrared or X-ray regions of the electromagnetic spectrum. In this paper, after a brief introduction to two dimensional millimeter waves and terahertz imaging we establish the principles of tomography for Terahertz Computed tomography (CT), tomosynthesis (TS), synthetic aperture radar (SAR) and time-of-flight (TOF) terahertz tomography. For each technique, we present advantages, drawbacks and limitations for imaging the internal structure of an object.

https://hal.archives-ouvertes.fr/hal-00968839/document

Review of Terahertz Tomography Techniques

We demonstrate the reconstruction of one- and two-dimensional objects by numerically backpropagating measured scattered terahertz transients. The spatial resolution determined by the Sparrow criterion is found to correspond to approximately 30% of the peak wavelength and 85% of the mean wavelength of the power spectrum of the single-cycle waveform.

Time reversal and object reconstruction with single-cycle pulses

Time reversal and object reconstruction

We demonstrate for the first time the reconstruction of one-and two-dimensional objects by numerically back-propagating scattered terahertz transients.

Lenaic Le Hors

Papers

Time reversal and object reconstruction with single-cycle pulses

Time reversal and object reconstruction

Terahertz imaging via time reversal using single-cycle pulses