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Thin film lithium niobate (TFLN) features a small photonic device footprint and excellent optical properties. Despite the recent progress on plasma-etched TFLN waveguides, scattering loss induced by the sidewall roughness still limits the performance of the integrated photonic devices. Here, a shallow-etched structure that can mitigate the scatteri...
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... Also, similar to silicon-on-insulator (Kakihara et al, 2009;Nguyen et al, 2019;Webster et al, 2007), rib waveguides in LNOI also suffer from lateral mode leakage which is a prominent source of loss in such waveguides, especially when shallow etched (Yu et al, 2022;Kang, 2023). Lateral leakage occurs when the guided mode of the waveguide gets phase-matched with the orthogonally polarized slab mode and leaks into the slab structure. ...
... This leakage is inherent in shallow etched rib waveguides and the only way to avoid them is to choose appropriate waveguide parameters. Lateral leakage in LNOI waveguides has been reported earlier (Yu et al, 2022;Saitoh et al, 2011;Kang, 2023) and investigated for its effects on non-linear optical processes such as second harmonic generation (Boes et al, 2021a) and for designing high extinction filters (Boes et al, 2021b). However, a detailed analysis on the morphology of LNOI waveguides to avoid lateral leakage loss and producing photon pairs with the desired properties using type 0, type I and type II SPDC processes is not available. ...
... However, ∆n ef f varies from negative to positive values for signal and idler modes for different waveguide geometries (Fig.2). For the regions, where ∆n ef f ≤ 0, there is high probability of mode leakage and thus high leakage losses (Kang, 2023;Boes et al, 2021b). For signal modes (Fig. 2c(I-III)), we discuss the case of width 700 nm ( Fig. 2c(I)) and width 800 nm (Fig. 2c(II)) . ...
Photonic integrated optical components, notably straight waveguides, serve as pivotal elements for on-chip generation and manipulation of quantum states of light. In this work, we focus on optimizing waveguides based on lithium niobate on insulator (LNOI) to generate photon pairs at telecom wavelength using spontaneous parametric down-conversion (SPDC). Specifically, we investigate lateral leakage for all possible SPDC processes involving type 0, type I and type II phase matching conditions in an X-cut lithium niobate waveguide and provide a recipe to avoid leakage loss for the interacting photons. Furthermore, focusing on type II phase matching, we engineer the waveguide in the single mode regime such that it also satisfies group index matching for generating spectrally pure single photons with high purity (99.33\%). We also address fabrication imperfections of the optimized design and found that the spectral purity of the generated photons is robust to fabrication errors. This work serves as a tutorial for the appropriate selection of morphological parameters to obtain lossless, single mode LNOI waveguides for building linear optical circuits and photon pair generation at telecom wavelengths using desired phase-matching conditions.
