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Nanophotonic devices in thin film lithium niobate

Time: Tue 2024-04-16 09.30

Location: Room Pärlan, Hus 1, Albano Campus

Language: English

Subject area: Physics, Optics and Photonics

Doctoral student: Alessandro Prencipe , Nanostrukturfysik, Nonlinear Quantum Photonics

Opponent: Rachel Grange,

Supervisor: Katia Gallo, ; Val Zwiller, ; Mohammed Amin Baghban,

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QC 2024-04-03


Photonic devices play a fundamental role in today’s society and are a central building blocks for numerous applications, ranging from modern internet to sensing and manufacturing, and photonics is foreseen to be the backbone of future quantum internet and quantum communication systems thanks to the long coherence time of light. At variance with electronic integrated circuits, where silicon has been the material of election for many decades, in the case of photonic integrated circuits (PICs) there are numerous options available for the material substrate. One of the most promising platforms for future PICs is thin film lithium niobate (TFLN). 

Lithium niobate (LN) is a ferroelectric crystal characterized by a wide transparency window and excellent electro- and nonlinear optical properties. Additionally the thin film format allows the implementation of submicrometric devices, characterized by a footprint similar to the one realized on silicon and silicon nitride but with improved capabilities in terms of coherent electrical control of light and more efficient on-chip photon-photon interactions. This thesis demonstrates a few novel nanophotonic devices and contributes to the quickly developing TFLN technology platform, encompassing also hybrid integration processes. In term of monolithic TFLN nanowaveguide components, nanostructuring capabilities for ultrasmall footprint photonic devices have been developed and utilized to implement high quality factor resonators based on waveguide integrated phase shifted Bragg gratings (PSBG). The response of these devices, operating at telecom wavelength and characterized by a transmission bandwidth as narrow as 8.8 pm (corresponding to a quality factor Q in excess of 1.7×105) and by a footprint smaller than 500 μm2, was analyzed with the help of a model based on coupled mode theory (CMT) showing excellent agreement with the experimental data. This model provides insights on fabricated device losses and useful guidelines for the design of optimized PSBG. While spanning the full parameter space for device fabrication and optimization, deviations from such a model were also experimentally observed. Upon further investigation, these effects were explained as the results of a Fano resonance occurring in the PSBG structure involving the interaction of TE00 and TM01 modes in the supporting waveguide. The effect results in a narrowband and asymmetric response which can be tuned upon changing the waveguide design, as confirmed by experiments and simulations. 

The excellent sensitivity to refractive index changes of PSBG devices was leveraged to develop a comprehensive study of the electro- and the thermo- optical properties of X-cut TFLN. The study highlights the advantages and the limits of both approaches to device trimming and reconfigurability. The thesis includes also experimental contributions to dispersion-engineered TFLN waveguides, whose properties were characterized for fundamental TE and TM modes as function of the waveguide geometry in the telecom band by means of dual comb spectroscopy. 

This thesis addresses also hybrid photonic devices for on-chip light detection and emission, specifically demonstrating the integration of superconducting nanowire single-photon detectors (SNSPDs) and erbium emitters in TFLN. SNSPDs based on niobium-titanium-nitride (NbTiN) were integrated onto single mode TFLN nanowaveguides and their spectral sensitivity was leveraged to implement on-chip wavelength meters working in the telecom C- and L- bands, achieving photon counting and spectral sensitivity on a single waveguide- integrated device. Furthermore, a process for the effective incorporation of Er ions in TFLN was successfully developed in collaboration with researches at the university of Manchester. Structural and optical characterization of the Er:TFLN samples indicates essentially no disruption to the intrinsic properties of the LN crystal. The photoluminescence from the implanted films, emitting in the C-band, was studied as a function of temperature. 

The results hold promise for the implementation of a complete platform for on-chip quantum photonic circuits in the 1550 nm band, capable of operating at cryogenic temperature comprising on-demand single photon sources, electro-optic photon routing and manipulation as well as detection in TFLN circuits.