Light modulation and photon pair generation 
based on lithium niobate micro-resonators

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Min-Kyo Seo

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Department of Physics, KAIST

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The strong confinement of photons in a high-quality (Q) factor optical micro-resonator enables the observation and exploitation of strong light-matter interactions and even light-light interactions mediated by matter in emerging research fields, including low-threshold lasing, single photon generation, non-classical light generation, small-footprint spectral frequency combs, optomechanics, and non-Hermitian physics. Meanwhile, numerous key advancements in quantum information techniques, including quantum entanglement, teleportation, quantum key distribution, and boson sampling, have been achieved using optical platforms. However, many of these early demonstrations relied on nonlinear optical bulk crystals and high-power lasers, which are limited by low scalability and high energy costs. It is timely and relevant to develop on-chip quantum photonic integrated circuits based on nonlinear optical micro-resonators to build practical systems for large-scale quantum information processing. In the recent decade, nonlinear optical micro-resonators made of LiNbO3, known for its strong X(2) optical nonlinearities, a broadband transparent spectral window, and high-performance electro-optic and piezoelectric properties, have been intensively investigated and have offered a pathway to address these challenges.

In this talk, we will present innovative methods of light modulation and photon pair generation using lithium niobate micro-resonators. We demonstrate a two-point coupled lithium niobate racetrack micro-resonator that enables electro-optical traversal through a complete set of zero-, under-, critical-, and over-coupling regimes with minimal disturbance to the intrinsic properties of the resonant mode. The resonant frequency shift between the zero- and critical-coupling conditions was only ~344.2 MHz, which is promising for on-chip coherent photon storage and retrieval. Also, we implement a hyperband electro-optic (EO) modulator that operates over an extensive spectral range from 775 nm to 1550 nm on a single device with a half-wave voltage-length product less than 1 V∙cm. Lastly, spontaneous parametric down-conversion photon pair generation at the telecommunication wavelength by small footprint X-cut periodically poled lithium niobate (PPLN) micro-resonator will be presented. Bright correlated photon pair generation with internal and loaded rates of 4.525 MHz/μW and 62.73 kHz/μW, respectively, was successfully achieved.

[1] Optics Letters, 47 (23), 6149-6152 (2022)
[2] Optics Letters, 49 (3), 658-661 (2024) 
[3] Optica Open, preprint 115140 (2024)