Studies On Novel Effects In Parity-Time Symmetry Synthetic Metamaterials Based On Silicon Waveguides

As electronic integrated circuits have been in accordance with Moore’s Law for 30 years, the quantum critical effect and power consumption issues have make it gradually approach its physical limits. Exploring new information carriers has become an important topic in the development of information technology where photonic technology has become an important candidate. The past 20 years have witnessed great progress of aritificial microstructures like photonic crystals, plasmonics and metamaterials which provide people new materials, ideas and opportunities to manipulate light in demand. On the other hand, in order to make photonic technology compatible with traditional silicon microelectronics technology, silicon photonics has been proposed from the application point of view, which will give a solid foundation for further optoelectronic integration.Designing these optical structures and systems, nature only provides three basic ingredients to be used:real refractive index, gain and loss. In most studies so far, the design of such structures and devices has been based on delicately control of their real refractive index properties. The ubiquitous loss in these systems is typically considered a problem. Gain, however, is valuable in optoelectronics because it provides a mean to overcome loss and even lasing. Recently, notion of parity-time (PT) symmetry has been developed in optics from previous quantum theory. This emerging field alows a controlled interplay among real refractive index, gain and loss, providing a new way to realize novel optical properties and a new generation of optical devices.Based on the designing concept of artificial microstructures, this thesis is dedicated to study novel PT symmetric optical effects around exceptional points and develop novel photonic devices on the platform of silicon photonics. The contents are as follows:(1) In chapter 2, we propose for the first time that by introducing some PT symmetric potentials along propagation direction can realize power oscillations violating left-right symmetry in the propagation direction while previous PT symmetric coupled waveguides show in the transverse direction. We study the PT phase transition process in our system and find that there exists unidirectional mode conversion at the threshold of PT symmetry breaking, which still maitains even with the passive PT symmetry. Using the concept of metamaterial, we delicately mimic the microscopic function by macroscale profile of optical materals through several steps of transformation which are confirmed by numerical simulations. The device on silicon chip is fabricated by CMOS-compatible process and measured by scanning near field microscope (SNOM). The measured electric amplitude and phase clearly verify the unidirectional mode converion which breaks left-right symmetry of power oscillations in the propagation direction.(2) In chapter 3, we propose one kind of one-dimensional PT symmetric scattering system which can break the Hermitian conservation law of transmission and reflection. By studying the eigenvalues of scattering matrix, we find that the corresponding passive PT symmetric scattering system can share similar phase transition process with balanced PT symmetric system despite some background absorption, both of these systems have unidirectional reflectionless phenomenon at the exceptional point which can be used to realize unidirectional invisibility. The numerical simulations and experimental verification consistently show this asymmetric transport effect. Inspired by the concept of metamaterial in the entire complex dielectric permittivity plane, we design unidirectional reflectionless optical metamaterial on a silicon-on-insulator (SOI) platform by engineering profile and thickness of optical materials which is verified by numerical simulations. The designed PT metamaterial is fabricated by conventional CMOS procedures and spectral measurements show asymmetric reflection with above 7dB contrast in a broadband communication wavelength range which confirms the expected unidirectional characteristics. Our work has clearly desmonstrated the exceptional point as well as its associated spontaneous PT symmetry breaking on silicon chips for the first time. This non-Hermitian silicon photonic platform can be used to simulate and measure the dynamics in non-Hermitian quantum mechanics which may pave the way to novel optical devices.(3) Photonic simulation of quantum mechanics and condensed matter physics has important scientific and practical significance since photonic systems has greater controllability and integration compared to electronic systems. In chapter 4, we first time study Hermitian and non-Hermitian optical Bloch oscillations using silicon waveguide array on the silicon chips. We theoretically investigate band diagrams and PT symmetry of both Hermitian and passive PT symmetric straight non-Hermitian photonic lattices. We study the optical Bloch oscillations of both systems under effective potential with consistent coupled mode theory and numerical simulations. A real-valued periodic potential in a Hermitian photonic lattice in conjunction with a linear gradient potential perturbation acting as a constant acceleration force is created by bending an array of identical SOI waveguides, while a complex PT synthetic potential in a non-Hermitian photonic lattice is realized with the same lattice but with an additional loss introduced by chrome (Cr) layer on top of every other bent silicon waveguide. We realize the Hermitian and non-Hermitian photonic lattices with CMOS compatible fabrication processes on a silicon-on-insulator (SOI) platform. By using scanning near-field optical microscope (SNOM), we are able to directly visualize the wave dynamics responsible for the BO continuously for both the Hermitian and non-Hermitian photonic lattice in spatial domain. Our spatial wave dynamics recorded by the SNOM reveal a prominent secondary emission around the BO recovery point for the non-Hermitian system in contrast to the classical picture of BO in the Hermitian system. We analyze this feature in such a PT synthetic photonic lattice is related to the threshold-less PT symmetry breaking. The fully integrated Hermitian and non-Hermitian photonic lattice provides a major step forward in expanding the application scope of on-chip photonics in quantum simulation and also study on unprecedented non-Hermitian transport phenomena.