Theses Doctoral

Efficient Optical Modulation and Complete Wavefront Manipulation Using Integrated Photonics

Huang, Heqing

Creating compact, efficient, highly-controllable optical systems has been one of the central goals of optics and photonics research. Integrated photonics provides a powerful platform for manipulating light efficiently and flexibly by guiding light in waveguide circuits on chip. Among the rich family of integrated photonic devices, integrated optical modulators and wavefront generators are two types of components for a great many applications such as optical communications, VR/AR displays, and LiDAR. Current approaches to creating these two types of devices – integrated optical modulators based on waveguides, active wavefront transceivers based on phased arrays, and passive wavefront transceivers based on grating couplers or integrated metasurfaces – suffer from large footprint, high power consumption, low beam quality, and limited controllability. It is desirable to improve the performance of such devices by exploring new device physics and architectures.In this thesis, we propose and investigate several novel approaches for efficient optical modulation and wavefront manipulation using integrated photonics.

First, we show that efficient optical phase modulation can be achieved using a micro-resonator operating in the strongly over-coupled regime. Theoretical analysis, simulations, and experimental demonstrations of thermally tuned silicon nitride adiabatic micro-ring resonators operating at the visible and near-infrared wavelengths are conducted. Compared with traditional waveguide-based devices, our resonator-based phase modulators operating at the visible wavelengths showed order-of-magnitude reductions in both device footprint and power consumption. Through a statistical study of the device performance, our adiabatic micro-ring device architecture also showed significantly improved robustness against fabrication variations when compared with the regular micro-ring architecture.

Second, we invent a new category of integrated wavefront-shaping devices – leaky-wave metasurfaces – that possess the simple form factor of a grating coupler and the capability of complete wavefront manipulation over all the four optical degrees of freedom: amplitude, phase, polarization ellipticity, and polarization orientation. The working principle of the leaky-wave metasurfaces is based on symmetry-broken photonic crystal slabs supporting quasi-bound states in the continuum (q-BICs). We extended the mechanism of q-BICs excited by free-space planewaves into q-BICs excited by guided waves, and developed a semi-analytical model describing the mapping between the four structural parameters and the four optical parameters of a meta-unit. We experimentally demonstrated multiple leaky-wave metasurface devices that convert light confined in an optical waveguide to an arbitrary optical pattern in free space, realizing custom polarization control, phase-amplitude control, and complete wavefront control, and validating the theory and capability of this platform.

Lastly, we explore strategies to optimize the beam quality and efficiency of integrated optical phased arrays. We show that a two-dimensional disordered hyperuniform array layout is promising for generating a radiation pattern with high directionality with performance surpassing uniform arrays, constrained random arrays, and non-redundant arrays. We experimentally demonstrated a passive 32-channel phased array operating at the blue wavelength that showed a high percentage of power in the main beam and suppressed side lobes. We further propose and discuss the use of efficient, resonator-based modulators in phased arrays to improve the system compactness, power efficiency, and scalability.

The approaches we investigated in this thesis provide a concrete set of solutions for interfacing free-space optics and integrated photonics. These two platforms have traditionally been studied by investigators from different subfields of optics and have led to commercial products addressing different needs. Our work suggests new ways to create “hybrid” systems consisting of partly integrated photonics and partly free-space optics and utilize the best of both worlds to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.

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More About This Work

Academic Units
Applied Physics and Applied Mathematics
Thesis Advisors
Yu, Nanfang
Degree
Ph.D., Columbia University
Published Here
June 7, 2023