Ultra sensitive Bio-Chemical sensors Based on Optical Resonance Shalini Prasad Department of Electrical Engineering Port
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Ultra sensitive Bio-Chemical sensors Based on Optical Resonance Shalini Prasad Department of Electrical Engineering Portland State University Portland, OR 97201 ABSTRACT With the focus on bio terrorism and environmental sensing, there is need for development of smart integrated bio-chemical sensors. We investigate Whispering Gallery Mode (WGM) based High Q micro cavity as a biosensor. We integrate the labon-a-chip approach with High Q technology to develop highly selective and smart sensors for in-situ detection of aqueous media based agents. We have investigated the ring based micro-cavity structure as a sensor. We identify the detection of agent through the changes observed in the wavelength shift and Q factor as a result of enzyme-substrate interactions. We report an experimental demonstration of a compact micro-cavity based biochemical sensor based on the micro ring structure. The micro-cavity, fabricated on a silica-on-silicon substrate, is designed to have a resonant wavelength (l) near 1.34 µm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n =1.0 to 1.5. Chemical detection was achieved by measuring the shift in resonant wavelength, and variation to the quality factor 1. INTRODUCTION Over the past few years, one of the highly focused areas in biosensor research has involved the development of label-free enzyme-substrate based optical biosensors [1-4]. In general, affinity-based sensors [1] detect the selective binding between target molecules and the capture agents. Label-free biosensors are of particular interest because they allow one to study the binding events without using invasive fluorescent visualization schemes. This not only improves the simplicity of the detection assay but also allows the monitoring of the time based kinetics associated with the binding event [5]. Commercial label-free affinity-based optical biosensors detect selective binding by measuring the change in refractive index at the surface of the sensor with methods such as surface-plasmon resonance [2] interferometry in porous silicon [4] and diffraction in a grating coupler [5]. However, such methods require an optical readout beam with a welldefined direction, which results in relatively large sensing area (>1-2 mm2). The use of micro cavity ring resonators as sensors has been investigated due to the potential for a very small high-Q microcavity [6-7] that makes it an attractive candidate for biochemical sensing applications,. This is due to the fact that such devices have small sensing area (~100 µm2) and hence would require only a very small amount (1 nL) of sample analyte. Such a compact sensor could be especially useful for biosensing applications, in which the amount of analyte is often limited. Moreover, a small sensing area would also allow dense integration of many sensors on the same chip. In addition, sensors that are based on other techniques such as absorption measurement can also benefit from using a high-Q micro cavity to create very high optica
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