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

Within the last two decades, terahertz systems have become a well-established tool in scientific laboratories and industrial research and development departments. Unlike the methods of pulsed terahertz radiation that mainly rely on ultrafast optical technology, the technology behind continuous-wave terahertz sources and detectors has a long history involving many different types of technical schemes. The intent is to provide a brief review of the continuous-wave terahertz systems that are based on photomixing and current application trends in the field of terahertz technology.

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Review of photomixing continuous-wave terahertz systems and current application trends in terahertz domain

Terahertz time-domain imaging (THz-TDI) has been applied for imaging a hidden portrait and other subsurface composition layers of an 18th century (18C) easel painting by Nicolai Abildgaard, the most important 18C Danish neoclassical painter of historical and mythological subjects. For the first time, a real hidden portrait on an easel painting has been imaged by THz-TDI, with an unexpected richness of detail. THz C- and B-scans have been compared with images obtained by x-ray radiography and invasive cross-sectional imaging, leading to a deeper understanding of the strengths and limitations of this technique for art diagnostic purposes and defining its role among complementary tools for the investigation of art objects. We present a fast and effective method to separate single THz pulse reflections of interest from the entire signal across the image, adapted for uneven surfaces typically encountered in practical applications of the technique. Interfaces between layers of the painting have been successfully imaged, contributing substantially to the understanding of the structure of the painting.

Reflection terahertz time-domain imaging for analysis of an 18th century neoclassical easel painting.

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.

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Philippe Maire

Papers

Three-dimensional terahertz computed tomography of human bones