Kim, Changhyun Bae, Munseong Choi, Minho Lee, Sangbin Lee, Myunghoo Kim, Chihyeon Jung, Hojoong Chung, Haejun Kwon, Hyounghan 2025-03-23T11:00:45Z 2025-03-23T11:00:45Z 2025-03-19 2025-06 2192-8606 https://pubs.kist.re.kr/handle/201004/152066 Thin-film lithium niobate (TFLN) has emerged as a promising platform for integrated photonics due to its exceptional material properties. The application of freeform topology optimization to TFLN devices enables the realization of compact designs with complex functionalities and high efficiency. However, the stringent fabrication constraints of TFLN present significant challenges for optimization, particularly in nonlinear photonic devices. In this work, we propose an inverse design methodology that successfully addresses these challenges and demonstrates the development of an efficient freeform TFLN mode converter. The numerically optimized mode converter achieves a transmission efficiency of 67.60 % and a mode purity of 84.58 %. Experimental validation through nonlinear processes, including second harmonic generation and spontaneous parametric down-conversion, shows that the fabricated devices improve the efficiency of these processes by factors of two and three, respectively, compared to devices without freeform designs. The proposed inverse design framework provides a powerful tool for advancing the development of TFLN-based devices, with broad applicability to nonlinear and quantum photonics. English WALTER DE GRUYTER GMBH Freeform thin-film lithium niobate mode converter for photon-pair generation Article 10.1515/nanoph-2024-0515 1 Nanophotonics, v.14, no.11, pp.1949 - 1960 Nanophotonics 14 11 1949 1960 Y scie scopus 001414366600001 2-s2.0-85217796732 Nanoscience & Nanotechnology Materials Science, Multidisciplinary Optics Physics, Applied Science & Technology - Other Topics Materials Science Optics Physics Article INVERSE DESIGN mode converter inverse design topology optimization spontaneous parametric down conversion lithium niobate