Biomedical Applications of Terahertz Spectroscopy and Imaging.

Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.

Toward real-time terahertz imaging

Traditional terahertz (THz) equipment faces major obstacles in providing the system cost and compactness necessary for widespread deployment of THz applications. Because of this, the field of THz integrated circuit (THz IC) design in CMOS and SiGe HBT technologies has surged in the last decade. An interplay of advances in silicon process technology, design technique, and microelectronic packaging promises to narrow the gap between the requirements and the reality of system cost and performance of THz components. Furthermore, the scalability, reconfigurability, and signal processing features of silicon technology have initiated research in complex THz ICs that expand the functionality of THz systems; this has enabled new applications, methods, and algorithms. This paper reviews the progress in THz IC research and investigates several realizations of THz imaging and sensing applications with silicon-based components regarding their motivation, system performance, and challenges. THz computed tomography, broadband multicolor imaging, high-resolution FMCW radar imaging, subwavelength resolution near-field imaging, and compressed sensing are presented.

Terahertz Imaging and Sensing Applications With Silicon-Based Technologies

Thermal nondestructive testing (TNDT) may be considered to be a more widely applicable method than many traditional techniques, such as X ray, ultrasonic, eddy current, liquid penetrant, etc. It can be applied to both metals and non-metals containing subsurface defects such as cracks, foreign inclusions, disbonds, delaminations, variations in thermal properties, etc. This is especially true for composite materials, and TNDT is very appropriate for screening purposes. TNDT test results may be analyzed by advanced image processing algorithms. This paper provides a concise review of composite NDT using TNDT in combination with other inspection techniques, providing an opportunity for data fusion.

Thermal (IR) and Other NDT Techniques for Improved Material Inspection

3D terahertz computed tomography has been performed using a monochromatic millimeter wave imaging system coupled with an infrared temperature sensor. Three different reconstruction methods (standard back-projection algorithm and two iterative analysis) have been compared in order to reconstruct large size 3D objects. The quality (intensity, contrast and geometric preservation) of reconstructed cross-sectional images has been discussed together with the optimization of the number of projections. Final demonstration to real-life 3D objects has been processed to illustrate the potential of the reconstruction methods for applied terahertz tomography.

https://hal.archives-ouvertes.fr/hal-00577605/document

Investigation on reconstruction methods applied to 3D terahertz computed tomography

Three-dimensional terahertz computed tomography has been used to investigate dried human bones such as a lumbar vertebra, a coxal bone, and a skull, with a direct comparison with standard radiography. In spite of lower spatial resolution compared with x-ray, terahertz imaging clearly discerns a compact bone from a spongy one, with strong terahertz absorption as shown by additional terahertz time-domain transmission spectroscopy.

/pdf/three-dimensional-terahertz-computed-tomography-of-human-93nbjgpft2.pdf

Three-dimensional terahertz computed tomography of human bones

A monochromatic millimeter-wave imaging system coupled with an infrared temperature sensor has been used to investigate historic objects preserved at the Museum of Aquitaine (France). In particular, two-dimensional and three-dimensional analyses have been performed in order to reveal the internal structure of nearly 3500-year-old sealed Egyptian jars.

/pdf/terahertz-tomographic-imaging-of-xviiith-dynasty-egyptian-2nn6f85etz.pdf

Terahertz tomographic imaging of XVIIIth Dynasty Egyptian sealed pottery

Recent advances for the measurement of TeraHertz (THz) radiation by using original IR temperature flux sensors are presented. The bolometer principle is used for designing simple thermal converters for THz radiations (measurement of the temperature increase of a sensitive absorber). Most of these sensors are efficient, sensitive and fast enough for quantitative measurement of THz source power as well as for 2D and 3D THz imaging. By combining optical and thermal technologies, we extend and adapt the use of thermal sensors to large THz wavelength till 3 mm (0.1 THz). A large variety of mono- or arrayed- thermal sensors is used and optimized for real-time room temperature THz imaging using adapted IR focal-plane microbolometers array (FPMA) camera. Optimisation and adaptation of such FPMA is discussed and a new arrayed prototype device of THz-Thermal Converter, “TTC”, for full-field real-time THz imaging is presented. This small size, low cost and efficient prototype design is discussed from the thermal poi...

Photothermal converters for quantitative 2D and 3D real-time TeraHertz imaging

https://hal.archives-ouvertes.fr/hal-00731822/document

Advantage of terahertz radiation versus X-ray to detect hidden organic materials in sealed vessels

GigaHertz (GHz) thermoreflectance technique is developed to measure the transient temperature of metal and semiconductor materials located behind an opaque surface. The principle is based on the synchronous detection, using a commercial THz pyrometer, of a modulated millimeter wave (at 110 GHz) reflected by the sample hidden behind a shield layer. Measurements were performed on aluminum, copper, and silicon bulks hidden by a 5 cm thick Teflon plate. We report the first measurement of the thermoreflectance coefficient which exhibits a value 100 times higher at 2.8 mm radiation than those measured at visible wavelengths for both metallic and semiconductor materials. This giant thermoreflectance coefficient κ, close to 10−3 K−1 versus 10−5 K−1 for the visible domain, is very promising for future thermoreflectance applications.

/pdf/thermoreflectance-temperature-measurement-with-millimeter-31rnb4jd3u.pdf

Jean-Pascal Caumes

Papers

Three-dimensional terahertz computed tomography of human bones

Terahertz tomographic imaging of XVIIIth Dynasty Egyptian sealed pottery

Photothermal converters for quantitative 2D and 3D real-time TeraHertz imaging

Advantage of terahertz radiation versus X-ray to detect hidden organic materials in sealed vessels

Thermoreflectance temperature measurement with millimeter wave.