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PHOTOCONDUCTIVE SWITCH IMPLEMENTED WITH GALLIUM ARSENIDE ON SILICON NITRIDE FOR COPLANAR WAVEGUIDE BASED TERAHERTZ SPECTROMETER
Title
PHOTOCONDUCTIVE SWITCH IMPLEMENTED WITH GALLIUM ARSENIDE ON SILICON NITRIDE FOR COPLANAR WAVEGUIDE BASED TERAHERTZ SPECTROMETER
Creator
Fu, Xiaohuan
Date
2015, 2015-12
Description
Over the years, pulsed terahertz (THz) time domain spectroscopy has attracted a lot of interest since it can not only measure the intensity of...
Show moreOver the years, pulsed terahertz (THz) time domain spectroscopy has attracted a lot of interest since it can not only measure the intensity of the transient electric eld but also determine the amplitude and phase of the spectral components that make up the pulse. This provides a convenient method to obtain the molecular network information of chemical or biological samples. Signi cant e orts have been directed towards free space spectroscopy in which THz pulses travel in free space between photoconductive transceivers. Due to di raction limit, the resolution is insu cient for many applications and it results in extreme power requirements or low power densities of the probe pulse. To overcome this issue, we propose a planar THz spectrometer which uses the same photoconductive excitation and detection mechanism as free space spectroscopy, but the excited THz pulses are directly coupled into lithographically de ned coplanar transmission lines. To reduce the power loss in the substrate, a thin lm with lower permittivity is employed to replace the conventional silicon substrate at the location where the coplanar waveguide is fabricated. To obtain high sensitivity, sample can be directly placed on the coplanar waveguide for tight coupling of the eld. Conceptual design and EM simulations have been completed and a variety of experimental studies have been carried out at Argonne National Laboratory to understand the fundamentals of coplanar waveguide. The fabrication of the coplanar waveguide prototype have been conducted at Northern Illinois University. The challenge of building a photoconductive switch with LTG-GaAs on Si3N4 was encountered and an innovative wafer bonding technique was discovered. Several wafer treatments prior to and after wafer bonding have been demonstrated to improve the bonding strength and reduce the defects. A standard pump-probe optics setup has been built at DePaul University and the photoconductive switch have been tested with femtosecond laser. The results con rmed that the photoconductive switches are functioning as design and the device system is ready for THz spectroscopy application. The historical overview of THz spectroscopy research and fundamentals of coplanar waveguide and photoconductive switches are presented in Chapter 1-3, followed by EM simulation, conceptual design in Chapter 4. The details of microfabrication process and the testing results are discussed in Chapter 5 and 6. The project is summarized in Chapter 7.
Ph.D. in Electrical Engineering, December 2015
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