Terahertz diffractive optics
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Terahertz radiation research is an emerging field with significant work only being possible in the last decade with the development of new high speed laser equipment. Sources of terahertz are a continuing problem however several methods now available are making this technology more reliable and less expensive. With these developments much research is being conducted into the potential applications of this window in the electromagnetic spectrum. An integral part of any such application is the ability to focus and manipulate beams. Traditional optics systems achieve this with reflective, refractive or diffractive elements the later of which has not been utilised in the terahertz frequencies and is the subject of this thesis.
The wavelength range of terahertz waves of the order of hundreds of micron make fabrication difficult with milling or machining. Planar fabrication techniques using microfabrication technology have been used to create micro-optics for much shorter wavelengths. This project has involved developing a microfabrication process that can be applied to the large structure sizes and depths required at terahertz frequencies. High diffraction efficiency demands the structuring of multiple level structures. This was achieved using a repeated binary fabrication process in silicon. A set of Fresnel lenses were produced and some anti-reflection structures to demonstrate the application of this technology to the formation of high quality terahertz diffractive optics.
Different applications may demand the use of either a continuous wave or pulsed emission and detection. An appreciation of the devices' performance in a single frequency and pulsed broadband system is given. These systems were set up for the measurements of anti-reflection gratings and both single and pairs of Fresnel lenses of varying complexity. These results demonstrate potential uses as frequency and or spatial filters and focusing elements for tomography applications.
Using Fresnel and Fraunhofer diffraction theory lenses have been simulated to further explain the results measured and to give an appreciation of how design modifications can be used to improve the efficiency. Because the fabrication process described is tailored for the construction of a broad range of optics in the terahertz frequency range there are a wide range of potential feature sizes and etch depths. A general guide to how processing defects inherent in the microfabrication process can effect lens performance is investigated to give a designer tolerances to remain within during production.