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DTSTART;TZID=America/New_York:20240417T120000
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DTSTAMP:20260511T093429
CREATED:20240410T205702Z
LAST-MODIFIED:20240410T210230Z
UID:43332-1713355200-1713360600@coe.northeastern.edu
SUMMARY:Raana Sabri Khiavi PhD Dissertation Defense
DESCRIPTION:Name:\nRaana Sabri Khiavi \nTitle:\nTheory and Design of Spatiotemporal Metasurfaces for Comprehensive Control of Light \nDate:\n4/17/2024 \nTime:\n12:00:00 PM \nLocation:\nIn person: Exp 311 \nCommittee Members:\nProf. Hossein Mosallaei (Advisor)\nProf. Josep Jornet (Co-advisor)\nProf. Charles Dimarzio\nProf. Siddhartha Ghosh \nAbstract:\nPhotonic metasurfaces are key components for manipulating almost all properties of light such as amplitude\, phase\, polarization\, wave vector\, pulse shape and orbital angular momentum at subwavelength scale. They are capable of sculpting the wavefront of the scattered light through imparting spatial or temporal modulation. Recently\, considerable efforts have been devoted to design active metasurfaces that enable real-time tuning and post-fabrication control of the optical response. Toward achieving this goal\, electro-optically tunable materials such as doped semiconductors\, MQWs\, and atomically thin sheets are incorporated into the building blocks of the geometrically-fixed metasurfaces. Despite the significant progress in this field\, there have been several limitations imparted to the optical response of such so-called quasi-static metasurfaces. Remarkably\, the strong resonant dispersion in such metasurfaces leads to narrow spectral and angular bandwidths. In addition\, the co-varying amplitude and phase response as well as the limited phase modulation give rise to undesired artefacts manifested on their output profiles. The slow response time to the external stimuli is another drawback that restricts the performance of the metasurfaces. Introducing time into the external stimulus of the metasurfaces\, as an additional degree of freedom\, offers a way out to surmount the obstacles facing the quasi-static metasurfaces. Modulation in time enables myriad of exotic space-time scattering phenomena\, where possibility to break the reciprocity and generation/manipulation of the sideband scattered signals offer the most appealing functionalities. In a space-time device\, the reciprocity constraint is lifted\, and time-reversal symmetry is broken. This effect can enable optical isolation and circulation\, while allowing for attaining full-duplex communication by rejecting the interference between up and down communication links. In addition\, sideband generation/manipulation provides access to the dispersionless modulation-induced phase shift with full 2pi span as well as a constant amplitude. The objective of this dissertation is to investigate the mechanisms for yielding reconfigurable plasmonic/all-dielectric metasurfaces in both space and time. Several realizations of quasi-static and time-modulated devices integrated with the electro-optical materials such as  ITO and InAs with the potential for high reflection and wide phase modulation are presented. It has been shown that time-modulated metasurfaces are superior to their quasi-static counterparts. Novel and unique applications of space-time photonic metasurfaces by spatiotemporal manipulation of light for all-angle\, broadband beam steering\, suppressing the undesired sidelobes\, high speed continuous beam scanning\, single sideband suppressed carrier modulation\, dispersionless dynamic wavefront engineering\, and magnetless power isolation at free space have been studied.
URL:https://coe.northeastern.edu/event/raana-sabri-khiavi-phd-dissertation-defense/
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DTSTART;TZID=America/New_York:20240417T120000
DTEND;TZID=America/New_York:20240417T140000
DTSTAMP:20260511T093429
CREATED:20240410T205431Z
LAST-MODIFIED:20240410T210347Z
UID:43336-1713355200-1713362400@coe.northeastern.edu
SUMMARY:Bengisu Ozbay PhD Dissertation Defense
DESCRIPTION:Name:\nBengisu Ozbay \nTitle:\nFast Semi-Algebraic Clustering for Efficient System Identification and \nGeometric Scene Understanding \nDate:\n4/17/2024 \nTime:\n12:00:00 PM \nLocation:(EV) 102 \nCommittee Members:\nProf. Mario Sznaier (Advisor)\nProf. Octavia Camps\nProf. Taskin Padir\nProf. Rifat Sipahi \nAbstract:\nAs the demand for data-driven techniques in machine learning and computer vision continues to rise\, the reliance on unsupervised learning methods becomes increasingly prevalent. Piecewise linear or affine models offer versatile solutions across various domains\, including system identification and computer vision tasks. \nThis dissertation introduces an efficient methodology that relies solely on singular value decomposition of matrices\, maintaining a fixed size independent of the total number of data points. Remarkably\, this method only requires execution a number of times equivalent to the number of clusters. Through singular value decomposition (SVD) of the empirical moments matrix containing the data\, we demonstrate the feasibility of identifying the polynomials representing hyperplanes. Central to this approach is the utilization of polynomials and Christoffel functions\, facilitating the partitioning of data into distinct clusters\, each with its own set of parameters extracted using application-specific techniques. \nThe dissertation explores various challenges\, including semi-algebraic clustering\, identification of switching auto-regressive models with exogenous inputs (SARX)\, affine linear subspace clustering\, two-view motion segmentation\, identification of Wiener systems\, and switched nonlinear system identification using block-oriented models. The proposed semi-algebraic clustering framework identifies reliable subsets from data\, sequentially segments data using Christoffel polynomials\, and extends the approach beyond linear affine arrangements to address challenges involving quadratic surfaces in two-view motion segmentation and higher order algebraic varieties in switched-Wiener system identification. \n 
URL:https://coe.northeastern.edu/event/bengisu-ozbay-phd-dissertation-defense/
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