Browsing by Author "Chen, Yu"

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    High quality silicon photonic devices based on heterogeneous integration method
    (2014-08) Chen, Yu
    Having been widely utilized as the foundation material for CMOS industry, with high refractive index and large spectrum transparency window, silicon has also long been considered as a perfect platform for photonics applications. Optical structures such as microcavities, photonic crystals, interferometers and etc. have already been demonstrated as on-chip silicon photonic devices. Such platforms have already been utilized in different applications such as high speed optical signal processing, optomechanical system demonstration, optical nonlinearity research and etc. Moreover, since the optical property of integrated silicon photonic devices are highly susceptible to the change of the refractive index of the surrounding medium, ultrasensitive optical sensor has also been demonstrated in various fields such as chemical and biological sensing, fiber strain analysis, EM (electromagnetic) field sensing, mechanical motion sensing and etc. However, silicon dioxide, as the material for the buried layer of silicon on insulator (SOI) substrate, which is the most widely adopted silicon photonic device platform, has limited both the optical and mechanical potential for silicon based optical sensor since it possesses a very narrow transparency window and is highly rigid. Within the past decades, flexible electronics based on inorganic material has been successfully demonstrated by using stamp-assisted heterogeneous integration method, which could also be applied to the field of silicon photonics. This thesis has been focusing on utilizing various heterogeneous fabrication methods such as integrating high quality silicon photonic devices onto materials other than silicon dioxide, or applying polymer based materials on top of SOI substrate in order to demonstrate devices with novel applications which are inaccessible with traditional silicon photonic devices. Firstly, a highly sensitive strain sensor is demonstrated by transferring silicon ring resonator and Mach-Zehnder Interferometer (MZI) onto stretchable PDMS substrate. Secondly, fully integrated silicon photonic circuit with grating couplers and ring resonators has been successfully transferred onto thin and flexible plastic substrate. Thirdly, by using a photoresist-pedestal assisted transfer method, a microcavity-enhanced mid-infrared optical chemical sensor is successfully demonstrated by using a silicon-on-calcium difluoride platform. Lastly, by applying a thin layer of polymer on one-dimensional photonic crystal cavity, an ultrasensitive infrared optical chemical sensor is realized.
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    Magnetic field tuned nonequilibrium transport in quasi-one dimensional Zn nanowires
    (2009-12) Chen, Yu
    Nonequilibrium transport was studied in superconducting Zn nanowires, connected with wide Zn electrodes. The wire exhibited drastically different behaviors when the electrodes were tuned from the superconducting state to the normal state, by applying magnetic fields. In the regime in which electrodes were superconducting, a suprising enhancement of superconductivity was observed upon the application of a small magnetic field. This enhancement was exhibited as an increase of the current or temperature at which the wire left its zero resistance state. Further experiments were carried out to study the dependence of the effect on various parameters such as magnetic field orientation, wire length and wire width. The results revealed that this enhancement is a result of nonequilibrium effect involving the boundary electrodes. In addition, it is more appropriate to treat it as a recovery of superconductivity, which was suppressed by the applied current. In the regime in which elecrodes were normal, we observed a nonzero residual resistance associated with the proxmity effect between the electrodes and the wire. Comparing the temperature and current dependence of this residual resistance revealed the breakdown of superconductivity at currents well below the depairing current. Further analysis suggests the breakdown of superconductivity in this situation does not originate from the usual supercurrent-depairing mechanism. Instead, it may be associated with the nonequilibrium distribution of quasiparticles. The breakdown is also charaterized by a critical voltage rather than a critical current.