Browsing by Author "Wang, Xin"
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Item Design and Fabrication of Integrated Optical Waveguides and Sidewall Bragg Gratings(2014-11-20) Wang, XinIn this dissertation, a novel design platform with arsenic tri-sulfide (As2S3) on titanium-diffused lithium niobate substrate (Ti:LiNbO3) is introduced to provide physical foundation for integrated optical device applications. LiNbO3 possesses excellent birefringence, electro-optical and acousto-optical effects that enable its high efficiency in nonlinear parametric frequency conversions and flexible tuning capabilities. Secondly, high-quality, low-loss channel waveguide can be made by thin-film metal diffusion or proton exchange with high reproducibility. The mode area size of the channel waveguide is close to single-mode fiber, leading to negligible coupling loss. As2S3 has a large index that provides strong mode confinement and tight bending radii for high integration densities. Both materials exhibit broad transparency: 0.4-5.0 ?m for LiNbO3 and 0.63-11.0 ?m for As2S3, making it possible to extend their applications to mid-infrared (3-20 ?m) regime. On this design platform, a hybrid waveguide structure is optimized for efficient mid-infrared radiation at 4.0-4.9 ?m by phase-matched difference frequency generation (DFG). The hybrid waveguide is designed for single mode operation. A normalized power conversion efficiency of 20.52%W^-1cm^-2 is theoretically predicted on a 1 mm-long waveguide pumped at 50 mW, which is the highest efficiency record for LiNbO3. Using a tunable pump at 1.38-1.47 ?m or signal at 1.95-2.15 ?m, a tuning range at 4.0-4.9 ?m is achieved. Such hybrid optical waveguides are feasible for mid-infrared emission with mW powers and sub-nanometer linewidths. Besides, sidewall Bragg gratings in As2S3-Ti:LiNbO3 waveguides are fabricated by electron beam lithography and metal liftoff process. Spectrum measurements are in good agreement with numerical fittings. The measured rejection bandwidth is at 2.4-6.7 nm. Coupling coefficients ranging from 2.5 mm^-1 to 8.9 mm^-1 are obtained by altering the grating depth. A transmission peak with a 3-dB bandwidth of ~0.25 nm is observed on a 432 ?m -long phase-shifted grating. Such integrated sidewall gratings are useful for various optical devices including optical filters, switches, modulators, lasers, sensors, and wavelength division multiplexing (WDM). In addition, optical refractive index sensors are designed with phase-shifted sidewall gratings in slot waveguide based on silicon-on-insulator (SOI) platform. The designed optical sensors have a minimum detection limit on the order of 10-6, a linear response and a compact device dimension as small as 11.7 ?m offering the capabilities for optical sensor array deployment and lab-on-a-chip integration.Item Dispersion Engineering With Leaky-mode Resonance Structures(Electrical Engineering, 2010-07-19)In the thesis, a numerical method for the analysis of optical pulses propagation through leaky-mode/guided-mode resonance (GMR) structures is implemented by integrating a Fourier decomposition technique and the rigorous coupled-wave analysis (RCWA) method. Dispersion properties of several GMR structures such as single-grating-layer GMR filters, coupled GMR filters, and cascaded GMR filters are studied and their interaction with optical pulses investigated. For device applications, a high-Q transmission filter is formed by coupling GMR reflection filters which can withstand typical attenuation in silicon. Wavelength division multiplexing (WDM) filters are proposed by cascading a number of GMR transmission filters. Generally, a N channel DWDM filter can be realized by cascading N+1 GMR transmission filters. The channel bandwidth in this structure is sensitive to the gap width between the two neighboring GMR filters. Both the channel bandwidth and channel spacing are inversely proportional to the number of cascaded GMR filters for a given gap width. For slow light applications, a conceptual optical delay line device with desired time delay and flat-dispersion is proposed by treating double GMR transmission filters as a cavity and then cascading such cavities.Item Monte Carlo simulation of charge transport in Si-based heterostructure transistors(2002) Wang, Xin; Banerjee, SanjayStrain and bandgap engineering of strained materials has emerged as an important technique for improving the device performance other than conventional scaling method. The purpose of this work is to develop a Monte Carlo simulation tool to investigate properties of these strained materials and carrier transport in deep submicron novel devices with heterostructures and strained materials. A general full-band Monte Carlo simulation tool with high flexibility about device structure and material profiles is developed for the first time. The transport model is based on energy-dependent scattering rates including inelastic acoustic phonon scattering with longitudinal and transverse modes, optical phonon scattering, impact ionization, surface roughness scattering, impurity scattering and alloy scattering. The full-band treatment for strained material model substantially advances the state-of-the-art method relying on simpler valley model for the scattering rates. The multi-material profiles in devices are treated with parameterization of band structure. The tunneling across a potential barrier is treated with Feynman’s effective potential scheme. An orthorhombically-strained silicon (OS-Si) is reported in this work. The six degenerate valleys in OS-Si near X points break into three pairs with different energy minima due to the orthorhombic strain. Thus the drift velocity is enhanced under an electric field transverse to the longitudinal-axis of the lowest valleys. The OS-Si grown on a compressively-strained Si0.6Ge0.4 sidewall has a mobility almost twice that of bulk Si and electron saturation velocity approximately 20% higher. For homogenous strained silicon on Si0.7Ge0.3 (001), in-plane mobility of 2670 cm2 /(Vs) for electrons is obtained, with enhancement by a factor of 1.8 compared to the unstrained case. Electron transport in a strained Si nMOSFET with 50 nm channel length is also investigated by full-band Monte Carlo. Strained silicon devices exhibit around 60% increase of drain current compared to unstrained silicon. Strained SiGe is also studied with full-band Monte Carol tool. A 90% enhancement in hole mobility is obtained for strained Si0.7Ge0.3 compared with bulk Si. The preliminary investigation of hole transport in vertical pMOSFETs with graded SiGe channel is also reported in this work.Item Solar Selective Optical Coating Made With Metal Nanopaticles For Photovoltaic Thermal Applications(Materials Science & Engineering, 2009-09-16T18:20:32Z)The goal of this research is to fabricate solar selective reflector with suitable optical properties and high thermal and chemical stability for using in the photovoltaic thermal window. While the reflector allows some transmittance in visible spectrum, the mid/far infrared light needs to be reflected back into the window minimizing the IR radiation loss. Nickel/alumina composite was proposed as the solar selective reflector. This thesis consists of two parts - one part concerning the fabrication and characterization of solar selective reflector and another dealing with an optical modeling to simulate the optical properties of the solar selective reflector. The alumina serves as a template for the formation of the oblated nickel nanoparticles, ca. 300nm in diameter with its length ranging from 109 to 245 nm. The Maxwell-Garnett theory was used to simulate the transmittance curve. The calculated curve shows fairly good agreement with the experiment data by considering the hole porosity due to some missing Ni particles in the composite and by defining an effective volume fraction from the experiment data.