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| Dissertation / PhD Thesis | PreJuSER-9718 |
2009
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/3602
Report No.: Juel-4301
Abstract: Within this thesis, a novel approach for the investigation of aqueous solutions interacting with electromagnetic radiation at micro-to-millimetre wave frequencies was developed and employed with relevance to biological applications. Whispering-gallery modes in cylindrically shaped dielectric disks machined from low-loss single crystalline materials such as sapphire or quartz allow for very high quality factors, because the electromagnetic fields are strongly confined inside the dielectric disk such that radiation losses can be neglected. Based on this resonator type, three different liquid sensing approaches were developed, analysed and optimized with emphasis on the determination of the complex dielectric permittivity of liquids. As a first method, whispering-gallery sapphire resonators at 35-37 gigahertz were developed and optimized for the investigation of small droplets of biochemical liquids with volumes ranging from 100 picolitres up to about a few nanolitres. Employing a micrometer controlled microinjection system, droplets could be spotted at arbitrary positions on the surface of the sapphire disk. At the surface of the disk, an evanescent field emerges, such that the droplet induces a slight change of the resonance frequency and a reduction of the inverse quality factor depending on complex dielectric permittivity of the liquid under test. The optimum spotting position on the resonator surface corresponding to the maximum electric field was determined experimentally. Two types of resonator excitation schemes were investigated and optimized for the Ka frequency band (26.5 – 40 Gigahertz). At first, resonators were embedded in a semi-open metal shielding cavity and excited in a standing wave regime by coaxial loop antennas fixed at the walls of the cavity. Secondly, dielectric disks without shielding cavity were investigated, comprising excitation of whispering-gallery running waves by dielectric image guides. In contrast to standing wave loop excitation, the electric field strength of the running whispering gallery waves is nearly independent of the azimuthal angle, which is of great advantage for the positioning of very small samples. In addition, this scheme can be extended up to the terahertz frequency range. As a first demonstration, whispering gallery resonances were excited in a quartz disk resonator at 170 Gigahertz with quality factors of order of 30000. The results on droplet induced changes of resonant frequency and inverse quality factor were found to be in quantitative agreement with perturbation theory in conjunction with numerical field simulation of the unperturbed resonator. In particular it was found, that the effect of real and imaginary part of permittivity could be separated. Both measured quantities were found to be nearly linear dependent on the droplet volume, as expected by perturbation theory. Due to the very high quality factor, droplets down to 100 picoliters volume could be analysed by this technique. Droplets of distilled water were compared with aqueous solutions of ethanol, sodium chloride, glucose and albumin. For all solutes, the measured results were clearly different from water for concentrations above 5%. Within perturbation theory it was found, that the ratio of droplet induced change of inverse quality factor and droplet induced frequency shift,[...]
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