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Chapter 2

Background

2.1 Terahertz imaging

In the 1960’s and early 1970’s, the field of THz faced many challenges: a lack of
suitable sources, sensitive detectors, and other components for the manipulation of
radiation in this wavelength range. The development of terahertz imaging systems
and techniques started in the 1990’s after the invention of terahertz time-domain
spectroscopy (THz-TDS), in which sub-picosecond optical pulses are used to generate
broadband terahertz radiation, also via a second-order nonlinear optical process [IS-
IS]. Hu and Nuss reported the first images acquired using THz-TDS (along with
coining the term ’Т-rays’) in 1995 [16, 17]. These initial images (see Figure 2.1)
inspired a great deal of excitement and much subsequent development of terahertz
imaging systems and techniques. In the subsequent 15 years, the majority of research
development in terahertz imaging are still based on THz-TDS. Therefore, let us first
discuss this important THz system, its advantages and disadvantages, before going
into the details of its application in THz imaging.

2.1.1 Time-domain terahertz system

The performance characteristics of a THz-TDS system has been reviewed in several
previous publications [18-21], and will not be discussed in great detail here. Here
provides a very brief description of the spectroscopic technique, and then focus on the

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