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THz tomography
2002 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

In THz tomography the information found in the reflection and transmission of a pulse is interpreted to determine the structure of the refractive index of a sample. The pulses considered are short, time durations in the order of [fs], with the result that the pulses will have a large bandwidths. The frequency distributions of the pulses are centered around a frequency is in the THz region. The reflection from a sample can give information about change in refractive index,n. A step like profile of n can be recognized by the number of reflections, where the time delay between the reflections reveals the thickness of a region if n is known or vice versa. The reflections from such a structure also contain information about the difference of n at either side of an interface. A reflection against a region with larger n than the region in which the pulse is traveling will cause a phase shift 180 degrees, while the opposite relation of refractive indexes at an interface conserves the phase. The fraction of reflected power at an interface is also a measure of the difference in magnitude of n at either side of the interface. The complex part, or the absorption frequencies, of the refraction index can also be recognized by THz tomography. The decay in amplitude of the transmission of a pulse through a sample indicates that there is absorption in the frequency range of the pulse. A broadening in time of the transmitted pulse indicates a frequency dependence of n. A pulse broadening of this kind is either caused by absorption of certain frequencies or the simple fact that different frequency components travel with different speed in a medium where the refractive index is a function of frequency. In order to show that these properties are found in the reflection and transmission of a pulse different functions for the refractive indices have to be analyzed. This analysis is done by assuming a value of n and solving the wave equation for that particular situation where the initial conditions are given by the initial pulse. Before solving the wave equation it has to be verified that the expression of the wave equation used is valid for the chosen form of n. Once the wave equation is found an appropriate numerical method can be used to obtain a solution. The solution gives the reflection and transmission of the pulse and the shapes of these for the chosen n are found and the above mentioned properties are verified.

Place, publisher, year, edition, pages
Keyword [en]
Technology, Tera-hertz tomography, time domain spectroscopy, refractive, index, dielectric function
Keyword [sv]
URN: urn:nbn:se:ltu:diva-58190ISRN: LTU-EX--02/088--SELocal ID: ec6443f5-16c4-43ba-8640-ef678bc4a5faOAI: diva2:1031578
Subject / course
Student thesis, at least 30 credits
Educational program
Engineering Physics, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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