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This chapter is divided into mainly three sections. The first section will give a brief introduction about terahertz radiation, the distinct properties of the terahertz radiations, the terahertz sources and the detection techniques. This will be followed by the vast applications of terahertz radiations in different fields. In the second section, a background on the imaging techniques and methods those are associated with the non destructive method that finally leads to the aims and the motivations of this research.
Section 1: Introduction
Terahertz Region: A part of the Electromagnetic Spectrum
Terahertz radiation was discovered in 1896 and it was first isolated in 1897 by Heinrich Rubens . The term THz was first coined by Fleming in 1974 which at that time was used to describe the spectral line frequency coverage of Michelson interferometer. Terahertz radiations form a part of the broad electromagnetic spectrum that includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Terahertz radiations lie between microwaves, whose wavelengths measure from centimetres to millimetres, and infrared, with wavelengths measured in nanometres. The gap in between the boundary of infrared and microwave regions is called as the terahertz gap âˆ’ contains wavelengths from 3.0 mm and 30 Âµm, and a frequency band lying between 100 GHz to 10 THz. [2,3,4]
1.2 Features of Terahertz radiations
Terahertz radiations have several interesting features which has triggered the research for developing the frequency domain techniques for imaging applications. Terahertz waves are non destructive, non contact, have low photon energy  and also non ionizing.
The wavelength of terahertz radiations is shorter than wavelengths of microwave radiations, with the associated improvement in spatial resolution, but long enough to be free from Rayleigh scattering suffered by infrared radiations.
Terahertz radiation is highly sensitive to polar substances, such as water. Water molecules absorb terahertz waves and this limits the penetration of the terahertz waves in the moist substances. This makes terahertz radiations more suitable to detect tumours and cancers than X-Rays. Terahertz radiation is however transparent to non-polar substances, such as plastics, wood, fibres, cloth, ceramic etc and hence can penetrate through them with reasonable attenuation.
The particle properties of terahertz radiations demonstrate that the molecules have strong emission and absorption lines in this band for translational, rotational and vibrational excitations that is generally absent in optical, X-Ray and nuclear magnetic images.
The universe has terahertz radiations in abundance and it has been nearly untapped because of the failure of the optical properties to operate below a few hundred of Terahertz and failure of the electronic methods to work above few hundred gigahertzes.
Below the terahertz range, electric field of propagation is detected using an antenna whereas at higher frequencies the intensity proportional to flux gets detected. Terahertz radiation is in between the quantum mechanical and classical descriptions of electromagnetic waves and their interaction with materials.
1.3 Description of Terahertz Sources and Detectors
It is because of the disadvantages of the Terahertz sources and detectors it is the most significant limitation for the development of the most efficient Terahertz systems. The thesis initially describes the problems of using the sources and detectors to record data in the Terahertz range and then it briefly describes each of these potential sources and detectors without going into deep details.
1.3.1 Problems Faced
Acquisition Times: Till date the fastest terahertz system still requires about 6 minutes to acquire a one hundred by one hundred pixel image
High dimensionality of data is required: A terahertz `pixel' consists of 29 - 212 time samples, and some form of parametric extraction needs to be carried out before an image can be formed.
Large data sets: Because of the high dimensionality of the terahertz data, a large storage space and larger bandwidth is needed for transmission. A single terahertz image, 50*30 pixels, would usually be of the size of 25mb.
System instability: Most terahertz systems are still built on large optical benches using delicate optical components that are prone to failure and drift, as well as being extremely sensitive to misalignment and even variation in atmospheric conditions.
Resolution: Both the spatial and temporal resolution of the terahertz scanners has room for improvement.
Noise: There is noise present in almost all recorded terahertz signals. The coherent generation and detection techniques however give a better signal to noise ratio.
1.3.2 THz Sources
Terahertz radiations are emitted by all objects, animate or inanimate which is at temperatures greater than 12K. Before the advent of these bright sources, terahertz radiation was generated using sources similar to those used in infrared Fourier Transform Spectroscopy, which generated weak and incoherent radiation, or by bulky complex equipment like free electron lasers or by optically pumped gas lasers . Since the breakthrough in the 1980s, terahertz imaging technology has spread rapidly, and a terahertz scanner was built at the University of Leeds as part of the European Union .Teravision. project that ran from 2000-2003, alongside five other academic and commercial entities across Europe..
Presently, Terahertz sources can be broadly classified into incoherent thermal sources, broadband sources and narrowband sources. Each of these sources has different output power, efficiency, bandwidth, dynamic range and implementations. Narrowband systems are generally continuous wave sources whereas broadband sources are pulse driven
Continuous-wave (CW) systems generally have higher peak power, better image signal to noise, have faster acquisition times for simple and large scale measurements and can be more cost-effective than pulsed systems. However, pulsed terahertz imaging is superior in contexts where multivariate information is desired, or when pulse time-of-flight measurements are used or information about the layered surfaces are required.
Laser driven sources utilise short laser pulses to excite or probe different materials. Photoconductive antennas and electro-optic crystals are the commonly used techniques for producing broadband terahertz sources.
In photoconductive method there is an electrically biased antenna deposited on an electro optic substrate like Gallium Arsenide .A femtosecond....................Emission of upto 15Thz has been reported..Another alternative method.....................The optical rectification method has provided very high bandwidths usually upto 50Thz. Another kind of broadband source include semiconductor surfaces being used for generating THz radiations with ultra short laser pulses focused onto ambient air. A crystal such as beta barium borate is inserted at the focal point to create a second harmonic generation that acts as an AC bias to polarize the plasma by drifting the electrons away from the nuclei and this process creates a transient photocurrent and a strong THz field. This process is called surface emission.
1.3.3 THz Detectors
Terahertz spectroscopy can be performed in the frequency- domain (FDS) or time-domain (TDS). While FDS can be performed using tunable, narrow bandwidth sources. TDS techniques are based on broadband pulsed terahertz sources with coherent detection. In this case, the transient terahertz electric field is sampled via an optical delay line and Fourier transformed to obtain amplitude and phase spectra.....in detectors
1.4 Potential Applications of Terahertz Imaging and Spectroscopy
Terahertz technology are applicable in different domains for many research related fields including medical science, biology, material science, security, astronomy, information and communication technology, environmental science etc.
Some of the applications are described briefly here:
Spectroscopy in molecule fingerprinting: A molecule is stimulated with a broadband Terahertz radiation and the frequencies that get absorbed by the molecule are observed. Each molecule has its own characteristic vibrational resonances and measurement of these vibrations help to distinguish one molecule from another through the technique of molecular fingerprinting. It was possible to notice a difference between salt and anthrax . Broadband THz analysis can also trace complete gas compositions in a device. Applications for gas sensing like monitoring of combustions processes, or plasma sensing e.g. to control plasma etching processes are used. Gas sensing has found interest because it can be used in both industrial and homeland monitoring. Guo et.al demonstrated that high resolution spectra of polar gases like CO, NO can be measured in a wide frequency range.
Screening: Terahertz radiations have enormous potential for security screening - complementing rather than replacing X-ray technology. Terahertz radiations can effectively see through packaging - as paper, plastic, clothing, even wood appear transparent under this. A great interest in the detection of drugs and explosives by THz techniques has emerged. This has also helped in the detection of weapons and suicide bombers.
Astronomy: Astronomy and remote sensing of the Earth's atmosphere have provided a powerful incentive for the development of THz technology. In Earth Observation vertical profiles of temperature and humidity of the atmosphere are determined from meteorological satellites using passive millimetre wave sounders operating at specific spectral lines of high atmospheric attenuation from low orbits. Astronomers use the terahertz frequency region to look at stars because a lot of space radiation is in the terahertz region. Other areas where terahertz radiations could be used include measurement of precipitation, cloud motion vectors, cloud water profiles, imaging of sea ice and snow, determination of concentrations of ozone and other trace gases such as CFCs that are affecting its concentration .
Using orbiting observatories and high altitude balloon measurements has led to the development of a significant range of missions, entailing the observation of both interstellar and extragalactic regions of space. Past missions such as the NASA Cosmic Background Explorer (COBE) and the development of Planck and Herschel have given greater amount of information of the universe. Another space application for terahertz sensors is in planetary and small-body (asteroids, moons, and comets) observations.
Medical Imaging: Unlike X-rays, Terahertz radiations are safe for human application because they are non-ionizing radiations. For detecting cancer in the surface, X-rays are not that good whereas terahertz is gentler and will give more contrast in detecting cancers. Cancerous tissues with Terahertz radiations have been reported to exhibit different hydration levels from normal tissues. Researchers have found that THz waves can go much deeper in breast tissues because of their fat content .
Art and Heritage Conservation: Terahertz research entered the field of cultural heritage conservation in 1998. Recent advances in generating and detecting THz radiation have made investigation of art works quite possible. It is the combination of material characterization, time of flight imaging and the preparation in optically opaque materials that gives rise to applications for subsurface imaging. The more powerful non-destructive forms of radiation for imaging, like x-ray, gamma-ray, proton and neutron, are ionizing and they destroy the DNA in artifacts that can and will even more so in the future yield important information; they also affect dating techniques which means that irradiated samples cannot be subsequently dated. Therefore, in terms of conservation, it is essential to employ non-destructive and non-invasive techniques to investigate art objects including paintings, murals, coloured sculptures or furniture.
Section 2: Non Destructive Techniques of Imaging
Non-destructive evaluation is very important especially while studying objects because they help to maintain the structural components and aids in its preservation, conservation and replacement. Non-destructive evaluation is used for various purposes like locating defects within materials, studying inside human bodies, imaging of the art works and also studying the physical characteristics of materials.
2.2. Common Non-Destructive Techniques
For non-destructive inspection, ultrasonic, X-ray, infrared or visible radiations are employed for many years in the field of imaging. .
In ultrasonic testing, high frequency sound waves to determine the presence of flaws and sub-surface defects. This method can provide good penetration depth for flaw detection and provides distance information to characterize the properties of material. But however this method is non contactless. In radiographic testing, electromagnetic radiation of very short wavelength like X-rays, gamma rays, etc is differentially absorbed by the materials to inspect complex shapes and multilayered structures, confirming the location of hidden parts and measuring the thickness of layered object. However these methods have possible human hazards. Infrared thermal imaging remotely senses surface temperatures and temperature gradients to detect imperfections or damage is detected but however this technique is sensitive to ambient room temperatures and surface emissivity variations. In medical field, magnetic resonance imaging uses powerful magnets to polarize and excite hydrogen nuclei in human tissue, and detect signal to produce images of the body. Positron emission tomography is a nuclear imaging technique that produces a 3D image of functional processes in the body .Nuclear magnetic resonance is a technique of current research that exploits the magnetic properties of atomic nuclei for the determination of the physical and chemical properties of atoms and molecules of different objects.
Similarly in the field of conservation of art objects visible range of the electromagnetic spectrum was used since ancient times. In the early 1990's non invasive spectroscopy called Fibre Optic Reflectance Spectroscopy  was introduced for identification of the pigments but the visible range was not sufficient enough to identify all the pigments unless they were properly diluted with white pigments. When this was extended to the near infrared region, the identification of these pigments was much more confident. Hence imaging with near infrared region is now widespread together with X-Ray radiography  for revealing underdrawings and paintings. All of the above mentioned techniques have received much more attention from many years and they currently offer enhanced resolution, greater penetration and higher acquisition speeds. Recently X-Ray imaging technique called as X-Ray florescence is also used. This was able to detect a woman's head hidden under the work "Patch of Grass" by van Gogh . X-Ray computed tomography has also been applied to investigate the works of art and maintain their conservation state also . However, all these methods suffer from some drawbacks and thus non-ionizing and non contact THz imaging technique can find a niche. Unlike the mid-infrared region-which gives intra-molecular information, terahertz spectral features depend on molecular behaviour, weak bonds, as well as phonon absorptions. In fact, the spectra of several mineral and inorganic materials historically used as pigments in artwork- including cinnabar, haematite pigments were observed using a frequency domain terahertz system in 1969 for the purpose of mining research . In the last 20 years, the number of terahertz imaging applications has dramatically increased with the development of electrical, optical and hybrid-based terahertz technology. As a result, there is a wide variety of 2D and 3D terahertz imaging techniques that have been applied to both the analysis of cultural heritage objects and materials and also supply information about the non metal objects such as paintings.
Section 3: Objectives and the Motivations of this Research
Thus analytical techniques, such as micromorphology and spectroscopy, enabled examination of individual microstratigraphic layers of plaster, paint etc. Terahertz systems impose less long term risk to the molecular stability of the historical artifacts and humans. The advantages of the application of terahertz technology to cultural heritage conservation over the other techniques for the preservation and sustainability of the objects and traditions that have defined the human species is the main motivation towards this research work.Terahertz Time domain Spectroscopic Imaging is used to demonstrate the presence of subsurface paint layers under the layers of wall plaster at Çatalhöyük. Çatalhöyük is one of the most important archaeological sites in Turkey . It was discovered in the 1950s and was excavated by James Mellaart from 1961 to 1965. Since 1993 a team of international archaeologists led by Ian Hodder has undertaken new excavations . The site is of particular interest because of its dense occupation, spectacular wall paintings, and its role in the development of early agriculture. Therefore, THz radiation has been used to reveal paintings hidden beneath coats of plaster.
Paintings were practised in á¸ˆatalhöyÅ±k throughout the life of its settlement. At á¸ˆatalhöyÅ±k, a full range of pigments like red, brown, yellow, blue, azurite, green malachite, cinnabar were used in these paintings derived wholly from minerals such as iron oxides, copper ores, mercury oxides etc. Black was obtained from soot and dead white from Pleistocene lake beds. Red and yellow ochres at Catalhöyük were frequently used in ritual contexts as pigments in wall paintings, or on selected human bones and skulls buried in graves below building floors. .The paints were applied on the walls and after the painting served its purpose the wall was covered with white plaster and was repainted later with a new painting. Thus the walls contained obscured paintings embedded within several layers of plaster for years. These paintings were of much significance as it was evident that the pattern inside were repeated throughout the layers of the walls.
The objective of the research is to use novel signal and image processing methodologies for spectral analysis of the reflected beam that would provide information to identify and locate the obscured pigments and visualise the obscured paintings of the archaeological site at á¸ˆatalhöyÅ±k. The research was focused on denoising and removing the artefacts due to non parallel plaster surfaces and uses several image enhancement and segmentation techniques. It aimed to move beyond the conventional terahertz processing techniques and use the combination of various methodologies to analyse the interaction of THz radiation with natural pigments used in these wall paintings and the penetration effect of the radiation through the stratographic layers of plaster.
This research demonstrates that pulsed terahertz imaging can be used to identify the location of obscured paintings and shows the potential for imaging in full using various imaging parameters to the obscured neolithic paintings at Çatalhöyük. It is expected to obtain a brick patterned pigmented area under the layers of plaster.