Towards terahertz microscopy

Abstract

Terahertz (=1012 Hz) radiation is a form of electromagnetic radiation that is at this moment used rarely for imaging purposes. However, there are indeed reasons to assume that imaging with terahertz radiation could be very useful. First, many materials, such as paper, plastics and clothing are transparent for terahertz radiation, while they block visible light. This opens the opportunity to look through objects. One can, for instance, consider checking the storage life of milk without opening the milk carton, or the security checks on concealed weapons at airports. Secondly, many materials have characteristic properties in the terahertz region that make a clear contrast between these materials possible. For instance, it is possible to specifically measure the concentration of gasses in a gas mixture. Also, different forms of DNA can be distinguished. By the development of stronger sources and more sensitive detection methods, imaging with terahertz radiation becomes ever more attractive. However, for some applications, such as imaging biological cells, the resolutions of many terahertz imaging techniques are not good enough. This is caused by a fundamental physical limit, the diffraction limit, which dictates that the resolution of ordinary imaging techniques is limited to about half the wavelength of the radiation used. For terahertz radiation, the diffraction limit on the resolution is about 0.1 mm. To use terahertz radiation for imaging microscopic objects, such as cells, the diffraction limit will thus have to be circumvented. This thesis explores different aspects of terahertz imaging with the ultimate goal of the development of a terahertz microscopy technique. Two of these aspects are the generation and detection of terahertz pulses. We also describe the different noise sources in our measurements and discuss how the influence of these sources can be minimized. Terahertz waves that propagate over metal surfaces are studied, because of the possibly large influence of these waves in our microscopy setup. The last part of this thesis presents a new terahertz microscopy technique.