Dissertation Title: “Light Guiding and Concentrating using Self-Collimating Spatially-Variant Photonic Crystals”
Advances in integrated photonic devices require low loss, easy-to-integrate solutions for chip-to-chip and chip-to-fiber interfacing. Among the most common solutions are traditional lenses. However, circular lenses require additional mounting mechanisms to ensure proper alignment. Additionally, multi-lens optics are necessary to achieve the numerical aperture needed for efficient light coupling. As the number of optics increases, the tolerances for alignment are further challenged during the assembly process. Further, the beam routing functionality cannot be added to the traditional lenses unless they are combined with mirrors and operate in the reflection mode.
In this dissertation, we investigate lens-embedded photonic crystals (LEPCs) as a solution to flat and multifunctional lenses. PCs contain rich dispersion properties and can be engineered so that the flow of power and the phase of the field are independently controlled. The concept is demonstrated by creating self-collimating lattices containing a gradient refractive index lens (GRIN-LEPC), a binary-shaped lens (B-LEPC), and a Fresnel-type binary-shaped lens (F-B-LEPC). The devices are fabricated in a photopolymer by multi-photon lithography with the lattice spacing chosen for operation around the telecom wavelength of 1550 nm. The lattice is based on a low-symmetry rod-in-wall unit cell that strongly self-collimates light. Light acquires a spatially quadratic phase profile as it propagates through the devices. Although the phase of the field is altered, the light does not focus within the device because self-collimation forces power to flow parallel to the principal axes of the lattice. Upon exiting the device, ordinary propagation resumes in free space and the curved phase profile causes the light to focus. Both the experimentally observed optical behaviors and simulations show that the device behaves like a thin lens, even though the device is considerably thick. The thickness of a B-LEPC was reduced threefold by wrapping phase in the style of a Fresnel lens. Embedding a faster-varying phase profile enables tighter focusing, and NA = 0.59 was demonstrated experimentally. Furthermore, we demonstrate experimentally that a Fresnel lens can also be combined into a bender, so one PC performs both bending and focusing functions, further reducing the footprint of the PC devices.
We also explored a hexagonal lattice and demonstrated wide-angle and broad-band self-collimation. The PCs are fabricated using the same material and method as that of the LEPCs. The PC can be described as a hexagonal array of cylindrical air holes in a block of dielectric material having low refractive index. Optical characterization shows that the device strongly self-collimates light at near-infrared wavelengths that span from 1360 nm to 1610 nm. Self-collimation forces light to flow along the extrusion-direction of the lattice without diffractive spreading, even when light couples into the device at high oblique angles. Numerical simulations corroborate the experimental findings.
Major: Optics and Photonics PhD
BS: 2015, Physics, Harbin Institute of Technology
MS: 2017, Optical Sciences, University of Arizona
Committee in Charge:
Dr. Stephen Kuebler, Chair
Dr. Sasan Fathpour, Co-Chair
Dr. Peter Delfyett
Dr. Xiaoming Yu
Dr. Xun Gong
Approved for distribution by Stephen Kuebler, Committee Chair, on April 18th, 2022.
The public is welcome to attend.