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Lithium Niobate on Insulator Integrated Optics for Low Propagation and Coupling Losses

Prabhakar, Karan
http://orcid.org/0000-0002-4716-2387
http://rave.ohiolink.edu/etdc/view?acc_num=osu1678890337245306

Abstract Details

2023, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
Lithium niobate (LN) has attracted significant interest over the past few decades as a potential platform for next generation nonlinear optical devices, high speed optical interconnects and modulators, and quantum light sources. Sub-micrometer thick lithium niobate on insulator (LNOI) is a promising integrated photonic platform that provides optical field confinement and high optical nonlinearity useful for state-of-the-art electro-optic modulators, wavelength converters, and acousto-optical devices. With fabrication foundry technologies enabling realization of low loss LNOI waveguides, devices fabricated using LNOI substrates and have been able to achieve record high harmonic generation efficiencies, and low insertion and propagation losses.Fabrication of LNOI on a silicon substrate through ion-slicing is advantageous for enabling velocity matching between microwave and optical copropagating fields in electro-optic modulators and for electronic-photonic integration but is challenging because of debonding and cracking due to thermal expansion coefficient mismatch between silicon and LN. Moreover, current techniques to pattern low loss waveguides with smooth sidewalls in LNOI rely on chemical mechanical polishing and electron beam lithography. Chemical mechanical polishing can result in film etching thickness variations, while electron beam lithography is not suited for high throughput production.Lastly, current schemes for fiber to chip edge coupling rely on the use of specialty optical fibers, such as lensed fibers, and there is a requirement for an efficient packaging solution that can utilize standard single mode cleaved optical fibers. Fabrication of thin film lithium niobate on insulator on a silicon handle wafer is achieved via ion-slicing, informed by structural modeling, and facilitated by accommodating for dissimilar wafer bows using a bonding apparatus. Structural finite element analysis of strain energy and stress, due to thermal expansion coefficient mismatch at elevated temperatures, is conducted. High strain energies and stresses which result in debonding and cracking, respectively, are studied through modeling and reduced by selecting optimized substrate thicknesses followed by an experimental technique to bond substrates with dissimilar bows. A lithium niobate thin film with a thickness of 800 nm thermally exfoliated and transferred to an oxidized silicon wafer with a root mean square surface roughness of 5.6 nm. Next, we present lithography and argon plasma etching of lithium niobate on insulator (LNOI) rib waveguides using reflowed photoresist etch masks and 405 nm photolithography. Melting the photoresist at temperatures greatly exceeding its glass transition temperature while minimizing feature distortion through photoresist adhesion control reduces sidewall surface roughness and allows the photoresist to be used both as the pattern mask and the hard etch mask. Waveguide sidewall surfaces exhibiting sub-nm root mean square roughness are fabricated. Dependence of sidewall roughness and angle on feature width, and propagation loss on thermal annealing of the fabricated devices is characterized. The fabricated microresonators exhibit quality factors of two million. LNOI rib waveguides and resonators with low propagation loss increase nonlinear optical conversion efficiencies and are useful for efficient electro-optic modulation. Photolithography compatible fabrication of low loss LNOI photonic integrated circuits facilitates scalable commercialization. We also present the design of cantilever couplers for fiber-to-chip coupling between standard cleaved single mode optical fiber and LNOI rib waveguides. Modeling shows 0.5 dB losses over 440 nm of bandwidth at infrared wavelengths for a coupler length of 155 μm. The design is tolerant to fiber to chip misalignment errors and lithography layer alignment errors. The design provides a path to achieve low overall fiber-to-fiber losses in packaged LNOI devices with input and output optical fiber ports for classical and quantum applications.
Ronald Reano (Advisor)
Patrick Roblin (Committee Member)
Robert Lee (Committee Member)
Fernando Teixeira (Committee Member)
211 p.
Electrical Engineering; Electromagnetics; Engineering; Nanotechnology; Optics
lithium niobate; LiNbO3; lithium niobate on insulator; LN; LNOI; integrated optics; photonics; waveguides; microresonators; fiber to chip; coupler; fabrication; nanophotonics; thin film lithium niobate; nonlinear optics; low loss; reflow; wafer bonding; wafer bow; periodic poling; ion implantation; exfoliation; quality factor; waveguide design

Recommended Citations

Citations

  • Prabhakar, K. (2023). Lithium Niobate on Insulator Integrated Optics for Low Propagation and Coupling Losses [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1678890337245306

    APA Style (7th edition)

  • Prabhakar, Karan. Lithium Niobate on Insulator Integrated Optics for Low Propagation and Coupling Losses. 2023. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1678890337245306.

    MLA Style (8th edition)

  • Prabhakar, Karan. "Lithium Niobate on Insulator Integrated Optics for Low Propagation and Coupling Losses." Doctoral dissertation, Ohio State University, 2023. http://rave.ohiolink.edu/etdc/view?acc_num=osu1678890337245306

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.