The Free space optical (FSO) communication has long been considered a promising solution for high-speed broadband data transfer. A conventional FSO receiver consisting of a focusing lens and a small high-speed detector works efficiently for a limited range of angles of arrival (AoA). This makes the optical link vulnerable to AoA fluctuations jitter. Even with a dedicated Pointing, Acquisition, and Tracking (PAT) system, residual jitter with variance in the range of microradian is unavoidable in FSO communication. The jitter causes BER fluctuations over a long time. Moreover, multi-beam optical communication in which one receiver needs to connect with multiple transmitters dispersed in 3D space at the same time cannot be implemented with the conventional receiver. This thesis presents two receiver designs based on metalenses that can offer solutions for these problems. To reduce the AoA fluctuation, a metalens is introduced at the focal plane of the bulk aperture lens of a conventional receiver. The phase profile of the lens is optimized to allow efficient capture of beams with AoAs as large as 2.5mrad. This is a significant improvement over a conventional receiver without metalens, which can capture only 195μrad with the same efficiency. For multi-beam communication, a two-metalens receiver system with optimized phase profiles is proposed. A proof-of-concept three-beam communication link is demonstrated in which the maximum AoA of 2° can be captured with 67% efficiency. This shows 35X improvement over conventional systems with a single detector in terms of acceptable AoA. The performances of the proposed receivers are analyzed with diffractive optics calculation. Metalens unit cells at two different wavelengths (1μm and 1.55 μm) are presented in this thesis. Full metalens systems are created based on the phase response of unit cells with varying diameters and simulated in Lumerical and COSMSOL. The simulation result validates the efficiency of the proposed receivers. When implemented, the proposed receiver will increase the capacity of communication links as well as maintain the same capacity over a long period of time. The proposed method can further be used to design receiver modules for specific situations such as for CubeSat/ astrophysical observation.

Nerve activity in a biological neural network is mostly characterized by generation and propagation of impulses through the nerves. These impulses, known as action potentials, are generated when the nerves are excited by a stimulus either as an external input or as a means of internal communication between nerves. This project is aimed to develop an optical imaging based minimally invasive technique for neural recording. Existing technologies largely limit the analysis of neuronal processing to a single or small cluster of neurons using different varieties of electrodes or the introduction of exogenous contrast agents and most of these techniques are invasive in some ways. Nerves undergo rapid transient thickness changes during propagation of action potentials. These activity associated structural changes are usually in the order of nanometers which is well beyond the usual limit of resolution of most common imaging technologies, including optical coherence tomography (OCT). However, recent advances in phase-resolved OCT (pr-OCT), a specific modality of OCT, have enabled the measurement of subnanometer changes in optical path length. This study demonstrates pr-OCT's capability to detect and measure rapid transient thickness changes in nerves during activity. Optically detected changes in nerve fiber thickness have timing and duration similar to the propagating electrical signal recorded in electrophysiology. Averaging of 8-10 impulses improves the SNR of detected optical signal. However, most of the results demonstrated here are single shot detections. Since OCT collects data from every depth points within a single A-line simultaneously, the changes in phase at every depth location are examined. Results demonstrate that these transient changes are present at different depths and this allows representing activity as a map of thickness changes. A custom-built cold block system has been used for switchable control of activation and deactivation of action potential propagation through the Limulus nerve. Optical recording has been compared with simultaneous electrical recording at every stages of cold block operation: activated nerve before deactivation, nerve after deactivation and nerve after reactivation. Hopefully, this study will serve as the basic ground work for future experiments and will have an overall significant impact on functional neuroimaging.

Apart from the COVID-19 pandemic, two chronic problems affecting Californians are high housing costs and road congestion. Although high housing costs and the determinants of commuting have separately received a lot of attention from academic researchers, to my knowledge very few papers have analyzed the linkage between them. In this dissertation, I present three essays that will enhance our understanding on the relationship between commuting, land use, housing costs, and the impact of COVID-19 on telecommuting. In all three essays, I use Structural Equation Model (SEM). In my first essay, I propose a framework for understanding the impact of housing costs on commuting time and commuting distance in one worker-households in Los Angeles County, which is the most populous county in the US. After analyzing data from the 2012 California Household Travel Survey (CHTS), I find that households who can afford more expensive neighborhoods have on average a commute 3.1% shorter per additional $100k to their residence median home values. In my second essay, I analyze the commutes of two-worker households to understand some of the trade-offs they need to make, since two-worker households have dual work constraints. My data for this essay come from 2017 National Household Travel Survey (NHTS) respondents who reside in five U.S. MSAs (San Francisco, Los Angeles, Dallas, Houston, and Atlanta). Results show that women do not commute as far as men on average, although their commuting time is not necessarily shorter than men’s, and that the commuting times of men and women are weakly positively correlated. Moreover, households have faster commutes by 14.5% for men and 22.7% for women per additional $1000 to their residence median monthly housing cost. My third essay investigates the impact of the COVID-19 pandemic on telecommuting by analyzing a unique dataset collected at the end of May 2021 by IPSOS via a random survey of California members of KnowledgePanel®. I find that an additional 4.2% of California workers would engage in some level of telecommuting and more educated workers are expecting to telecommute more (0.383* for bachelor’s degree) post-pandemic. Teasing out the impact of high housing costs on commuting is important at a time when concerns about the environmental impacts of transportation have turned reducing vehicle-miles traveled (VMT) into a policy priority. More generally, a better understanding of the determinants of commuting is critical to inform housing and transportation policy, improve the health of commuters, reduce air pollution, and achieve climate goals.