THESIS
2014
ii, 1 leaves, iv-xiv, 80 pages : illustrations ; 30 cm
Abstract
The terahertz (THz) region is defined as frequencies between 0.1 THz and 10 THz. It
lies between the microwave and infrared regions of the electromagnetic spectrum and
has very low photon energy (4.1 meV) such that it will not cause ionization. What’s
more, this radiation is strongly attenuated by water and is very sensitive to water
content. These characteristics mean that terahertz imaging is suitable for uses in fields
like biomedicine, security and telecommunication.
Terahertz 2D imaging has shown potential for diverse applications such as cancer and
concealed weapon detection. Yet most existing terahertz imaging systems use raster
scanning to move samples in front of a single pixel detector and this limits the
acquisition time. Thus single pixel terahertz imaging system wi...[
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The terahertz (THz) region is defined as frequencies between 0.1 THz and 10 THz. It
lies between the microwave and infrared regions of the electromagnetic spectrum and
has very low photon energy (4.1 meV) such that it will not cause ionization. What’s
more, this radiation is strongly attenuated by water and is very sensitive to water
content. These characteristics mean that terahertz imaging is suitable for uses in fields
like biomedicine, security and telecommunication.
Terahertz 2D imaging has shown potential for diverse applications such as cancer and
concealed weapon detection. Yet most existing terahertz imaging systems use raster
scanning to move samples in front of a single pixel detector and this limits the
acquisition time. Thus single pixel terahertz imaging system with compressed sensing
theory is developed. In this system no mechanical scanning or detector-array is
deployed and only a spatial light modulator is inserted behind sample position. This
modulator consists of independent cells which have two states. In one state terahertz
signal will pass through carrying sample information while in another state signal will
be blocked. Each time the modulator generates a random pattern and compressed
sensing technique is applied to the spatial signal received, largely reducing the
number of measurements.
In this thesis, high birefringence liquid crystals and vanadium dioxide are explored as
the switching material for the spatial light modulator (mask). The switching time
between ‘blocking’ and ‘passing’ states of liquid crystals with electrical control,
thermal control and frequency control are simulated and measured. The stability and
complexity of each control method are also evaluated. Electrical control proves to be
the most effective method with our ‘terahertz in-plane and terahertz out-of-plane’
electrode design. Vanadium dioxide is studied because it has an ‘insulating’ state
(high transmission) below a critical temperature value and ‘metal’ state (low
transmission) above that value. A patterned golden layer is added to increase the
contrast between two states. The information we obtained will be used for fabricating
the prototype spatial light modulator.
Keywords --- Terahertz radiation, Liquid crystal, Vanadium Dioxide,
Modulation
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