A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Measurement of radiation pressure and tailored momentum transfer through switchable photonic devices(2016) Ma, Dakang; Munday, Jeremy N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light carries momentum, which can be transferred to an object upon reflection or absorption. The resulting force per unit area from light, so called radiation pressure, is rather weak but can have macroscopic consequences. For example, sunlight imparts momentum on dust particles causing a comet's tail to be directed away from the sun. In a microscopic world, micro/nano-mechanical transducers have become sensitive enough that radiation pressure can influence them greatly. However, photothermal effects often accompany and overwhelm the radiation pressure, complicating its measurement. In this thesis, we first show a quantitative measurement of the radiation force on an uncoated silicon nitride microcantilever in an ambient condition. We identify and separate the radiation pressure and photothermal effects through an analysis of the cantilever's frequency response. Further, we demonstrate the first measurement of a wavelength-dependent radiation pressure due to optical interference in a silicon microcantilever. We utilize an in-situ optical transmission measurement at the excitation position to determine the local optical properties. Another interesting application of radiation pressure is a solar sail. Solar sails use solar radiation pressure for propulsion and offer an opportunity for propellant-free space travel, enabling long-term and long-distance missions that are impossible with traditional methods. Although solar sail propulsion alleviates the need to carry chemical fuel, attitude control and steering are still performed using traditional methods involving reaction wheels and propellant ejection. In the second part of the thesis, we present a steerable solar sail concept based on a polymer dispersed liquid crystal (PDLC) device that switches between transparent and scattering states, enabling attitude control without mechanically moving parts or chemical propellant. Devices are fabricated and characterized (transmission, reflection, absorption and scattering) over the visible and near infrared range of the solar spectrum (400 nm - 1100 nm) and are found to outperform previous designs by more than a factor of four in terms of over-all weighted momentum switchablility between on and off states. Devices require no power in the diffusely reflective state and dissipate less than 0.5 mW/cm^2 while in the on state, showing great potential as a low-power switching mechanism for solar sail attitude control.Item Design, Fabrication, and Testing of a Microsystem for Monitoring Bacterial Quorum Sensing(2009) Koev, Stephan Todorov; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Most pathogenic bacteria communicate with each other using signaling molecules. Their coordinated behavior, known as quorum sensing (QS), enables them to infect host organisms collectively and form drug-resistant biofilms. The study of bacterial signaling pathways may lead to discovery of new antimicrobials. Lab-on-a-chip technology can significantly accelerate the screening of candidate drugs that inhibit QS. This dissertation develops for the first time miniaturized sensors embedded in microfluidic channels to monitor the activity of an enzymatic pathway that produces signaling molecules. These devices can be used as building blocks of future high-throughput systems for drug discovery. The sensors presented here are gold-coated microcantilevers, and they detect the aminoacid homocysteine, a byproduct of the bacterial signaling pathway. It binds to the gold surface, causing stress and cantilever displacement that is measured optically. Samples are synthesized using bacterial enzymes and tested with the sensors. The minimal detected concentration of homocysteine is 1uM. It is demonstrated that deactivation of the enzymes causes a change in the sensor response; this effect can be used for finding drugs that inhibit the enzyme. The traditional method for measuring cantilever displacement requires an elaborate optical setup, and it can only test one device at a time. Two new methods are developed here to overcome these limitations. The first one uses a transparent cantilever which is also an optical waveguide. Light is coupled from the cantilever to a fixed output waveguide and measured with a photodetector. The cantilever displacement is determined from the change in output power. The change is approximately 0.7% per nanometer displacement. The minimal detectable displacement and surface stress are 6nm and 1.3 mN/m respectively. The second measurement method uses a transparent cantilever that is close to a reflective substrate. When the device is imaged with an optical microscope, an interference pattern forms. The cantilever displacement is calculated from the lateral shift of the interference fringes. This shift is determined from images of the device using custom software. The response of multiple cantilevers is captured by translating the microscope stage. The minimal detectable displacement and surface stress are 1nm and 340 uN/m respectively.Item Microcantilever Biosensors with Chitosan for the Detection of Nucleic Acids and Dopamine(2007-05-07) Koev, Stephan; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microcantilever biosensors allow label-free detection of analytes within small sample volumes. They are, however, often limited in sensitivity or specificity due to the lack of proper bio-interface layers. This thesis presents the use of the biopolymer chitosan as a bio-interface material for microcantilevers with unique advantages. Sensors coated with chitosan were designed, fabricated, and functionalized to demonstrate two distinct applications: detection of DNA hybridization and detection of the neurotransmitter dopamine. The first demonstration resulted in signals from DNA hybridization that exceed by two orders of magnitude values previously published for sensors coated with SAM (self assembled monolayer) interface. The second application is the first reported demonstration of using microcantilevers for detection of the neurotransmitter dopamine, and it is enabled by chitosan's response to dopamine electrochemical oxidation. It was shown that this method can selectively detect dopamine from ascorbic acid, a chemical that interferes with dopamine detection in biological samples.