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Title:Brillouin scattering induced transparency: Coherent optomechanical interaction leading to slow light and non-reciprocal light transmission
Author(s):Kim, JunHwan
Director of Research:Bahl, Gaurav
Doctoral Committee Chair(s):Bahl, Gaurav
Doctoral Committee Member(s):Toussaint, Kimani; Sinha, Sanjiv; Dragic, Peter; Fang, Kejie
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Brillouin scattering
Optical isolation
Abstract:An optical isolator is a device that allows only one-way light transmission. It is an essential optical component used for protection of laser sources found in many applications such as undersea optical communication networks, spectroscopy, fluorescence microscopy, and laser cutting. Today, all optical isolators that are commercially available are constructed from magneto-optic materials that non-reciprocally rotate the polarization of counter-propagating light. In free-space and fiber optics, Faraday isolators perform extremely well with the difference in forward and backward transmission (isolation contrast) exceeding 40 dB over a wide range of laser wavelengths. In integrated photonic circuits, however, Faraday isolators are difficult to implement due to the lack of an established fabrication process for permanent magnets on-chip, and due to material limitations such as high absorption coefficient and low magneto-optic coefficient. In this thesis, I investigate a new optical isolation technique that does not rely on the magneto-optic effect and can be implemented using any dielectric material. The technique exploits a common light-sound coupling process called Brillouin scattering to enable one-way light transmission within a simple system composed of a waveguide and a whispering-gallery resonator (WGR). I demonstrate that it is possible to induce a mode split within optical resonances of the WGR through Brillouin coupling with a mechanical mode of the WGR acting as a coherent state. This process is called Brillouin scattering induced transparency (BSIT), and similar effects have previously been observed in both atomic vapors (using electronic coherent states) and optomechanical devices (using standing-wave mechanical resonances). Distinct from past efforts, the BSIT effect is sensitive to propagation direction due to momentum conservation requirements intrinsic to Brillouin scattering, and therefore permits unidirectional optical transparency while optical absorption occurs in the opposite direction. As a result, BSIT can be used to produce an ideal optical isolator. My theoretical analysis of the BSIT system shows that increasing Brillouin coupling leads to a wider transparency window. Using this, I am able to experimentally demonstrate that the light transmits through the system with nearly zero loss in the transparency direction. Such an optical isolator having extremely low forward insertion loss, and which can be implemented in any dielectric, is extremely attractive for integrated applications where magnetic fields are undesirable and the material availability is limited. Lastly, I characterize the BSIT system as a slow light system, an effect that is associated with a rapidly changing optical phase response. I estimate the achievable time-delay and bandwidth of BSIT system and show that they are comparable to that of other slow light systems. Surprisingly, BSIT is shown to surpass other slow light systems in terms of size and power consumption by at least 5 orders of magnitude. Therefore, BSIT may also be used to efficiently generate optical delays in extremely compact photonic integrated circuits.
Issue Date:2019-06-26
Rights Information:Copyright 2019 JunHwan Kim
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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