Date

2017

Author

Akkaya, Merve

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Electrical and optical characterization of thin films has been studied for almost the past thirty years in the terahertz region. In the last few years promising thin film candidates such as vanadium dioxide (VO2) and various superconductors have opened up a new era of possibilities in terahertz optical electronics. Vanadium dioxide’s and superconductors conductivity is of particular interest since vanadium dioxide’s conductivity changes when temperature is above the critical temperature and also conductivity of superconductors at room temperature differ from those at low temperature. In this study, the conductivity of thin films made of vanadium dioxide, superconductors on dielectric (sapphire, fused silica) substrates was investigated using experimental methods based on in-house developed spectroscopy systems aided by analytical and numerical simulations of the layered media. Furthermore, such conductivity tunable materials can be especially important for developing devices that work in the terahertz region which historically has been a challenge. Metamaterials and two dimensional type of metamaterials called metasurfaces have an important role to fill this so called THz Gap. After analyzing the conductive properties of films based on yttrium barium copper oxide (YBCO), vi gold and VO2 in the THz region various metasurface designs were simulated for THz transmission using commercially available software, Computer Simulation Technology (CST), Microwave Studio. The simulations were carried out at the various temperatures where the materials showed unique conductive properties and compared to measurements done for single layer YBCO, quadcross shape patterned gold and YBCO film, as well as unpatterned VO2 at different temperatures by using terahertz (THz) time domain spectroscopy (TDS) system. A closed cycle cryostat is used for controlling the temperature. Therefore, it is found that critical temperature of YBCO film is nearly at 90K and below the critical temperature YBCO film behaves as metal. When temperature is decreased, conductivity of YBCO film increases. Therefore at low temperatures YBCO film has high conductivity. Moreover, in these measurements the conductivity of gold film did not change with temperature. In addition to these, critical temperature of VO2 film is found nearly at 340 K. Below the critical temperature VO2 film is in the insulating state and it is transparent to THz radiation. However, above the critical temperature it blocked THz radiation because it is in the metallic state. This change in conductivity is also evident when examining the transmission using different substrates with the same patterned VO2 film whereby an etalon effect was more pronounced for sapphire than fused silica allowing for the determination of the refractive index of these substrates. 

Subject Keywords

Thin films., Superconductors., Radioisotopes., Particle accelerators.

URI

http://etd.lib.metu.edu.tr/upload/12621215/index.pdf
https://hdl.handle.net/11511/26621

Collections

Graduate School of Natural and Applied Sciences, Thesis