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Nanodomain poling unlocking backward nonlinear light generation in thin film lithium niobate

Alessandra Sabatti, Jost Kellner, Robert J. Chapman, Rachel Grange

Published Dec 1, 2025• 0 citations
Publisher

Abstract

Nonlinear frequency conversion offers powerful capabilities for applications in telecommunications, signal processing, and computing. Thin-film lithium niobate (TFLN) has emerged as a promising integrated photonics platform due to its strong electro-optic effect and second-order nonlinearity, which can be exploited through periodic poling. However, conventional poling techniques in x-cut TFLN are constrained to minimum period sizes on the order of microns, restricting access to highly phase-mismatched interactions such as counter- and backward-propagating frequency conversion. In this work, we demonstrate scalable periodic poling of x-cut TFLN with domains periods as short as 215 nm and realize devices that support both counter- and back-propagating phase matching. We estimate conversion efficiencies of 1,474 %/W/cm2 and 45 %/W/cm2 for the two interaction types, respectively. Sum frequency generation measurements confirm that the nonlinear generation takes place in the desired direction. Furthermore, we report spontaneous parametric down conversion for the counter-propagating configuration and, for the first time, for a backward propagating device. This breakthrough provides unprecedented control over engineering of ferroelectric domain geometries in TFLN, leading into the generation of photon pairs with precisely tailored spatial and spectral characteristics. Such capabilities hold strong potential for advancing quantum signal processing, scalable quantum computing architectures, and precision quantum metrology.

DOI
10.3929/ethz-c-000791243

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Nonlinear frequency conversion offers powerful capabilities for applications in telecommunications, signal processing, and computing. Thin-film lithium niobate (TFLN) has emerged as a promising integrated photonics platform due to its strong electro-optic effect and second-order nonlinearity, which can be exploited through periodic poling. However, conventional poling techniques in x-cut TFLN are constrained to minimum period sizes on the order of microns, restricting access to highly phase-mismatched interactions such as counter- and backward-propagating frequency conversion. In this work, we demonstrate scalable periodic poling of x-cut TFLN with domains periods as short as 215 nm and realize devices that support both counter- and back-propagating phase matching. We estimate conversion efficiencies of 1,474 %/W/cm2 and 45 %/W/cm2 for the two interaction types, respectively. Sum frequency generation measurements confirm that the nonlinear generation takes place in the desired direction. Furthermore, we report spontaneous parametric down conversion for the counter-propagating configuration and, for the first time, for a backward propagating device. This breakthrough provides unprecedented control over engineering of ferroelectric domain geometries in TFLN, leading into the generation of photon pairs with precisely tailored spatial and spectral characteristics. Such capabilities hold strong potential for advancing quantum signal processing, scalable quantum computing architectures, and precision quantum metrology.
Abstract Nonlinear frequency conversion offers powerful capabilities for applications in telecommunications, signal processing, and computing. Thin‐film lithium niobate (TFLN) has emerged as a promising integrated photonics platform due to its strong electro‐optic effect and second‐order nonlinearity, which can be exploited through periodic poling. However, conventional poling techniques in x‐cut TFLN are constrained to minimum period sizes on the order of microns, restricting access to highly phase‐mismatched interactions such as counter‐ and backward‐propagating frequency conversion. In this work, we demonstrate scalable periodic poling of x‐cut TFLN with domains periods as short as 215 nm and realize devices that support both counter‐ and back‐propagating phase matching. We estimate conversion efficiencies of 1,474 %/W/cm 2 and 45 %/W/cm 2 for the two interaction types, respectively. Sum frequency generation measurements confirm that the nonlinear generation takes place in the desired direction. Furthermore, we report spontaneous parametric down conversion for the counter‐propagating configuration and, for the first time, for a backward propagating device. This breakthrough provides unprecedented control over engineering of ferroelectric domain geometries in TFLN, leading into the generation of photon pairs with precisely tailored spatial and spectral characteristics. Such capabilities hold strong potential for advancing quantum signal processing, scalable quantum computing architectures, and precision quantum metrology.
High-fidelity periodic poling of thin film lithium niobate (TFLN) waveguides is critical for robust, quasi-phase-matched three-wave mixing process such as second-harmonic generation and spontaneous parametric down conversion. Over the past decade, extensive research has been performed on design and fabrication of TFLN based optical waveguides, demonstrating high-performance electro-optic modulators, efficient wavelength converters and revealing their great potentials in heterogeneous integration with the conventional silicon photonic material platform. However, studies on poling of TFLN are still lacking, and uniformity of TFLN poling needs to be further improved, to achieve a comparable conversion efficiency as in its bulk counterparts. This dissertation discusses evaluation and optimization of TFLN poling, waveguide design, and device demonstration through characterization of efficient second-harmonic generation and high-quality entangled photon-pair generation. The results presented here indicate promising applications for robust and efficient TFLN devices in nonlinear and quantum optics in the telecommunications regime.
Within the last decade, thin-film lithium niobate (TFLN) has emerged as a leading integrated photonics platform with immediate applications in classical communications. Concurrently, TFLN components tailored to the requirements of photonic quantum information processing have also garnered considerable interest. Chief among these is the quasi-phase matched (QPM) nonlinear frequency mixer—implemented via periodic domain inversion in ferroelectric lithium niobate—which can be used to generate photon pairs, squeezed states of light, and to perform single photon frequency conversion. Here, we demonstrate progress towards realizing a spectrally separable photon pair source in lithium niobate via waveguide dispersion engineering—a technique uniquely enabled by sub-wavelength optical mode confinement in the thin-film platform. Subsequently, we design optimize a scalable fabrication process to produce QPM TFLN devices for applications that require strict adherence to a specified operating wavelength, such as quantum frequency conversion in a quantum communications network. Finally, we explore how high-performance electro-optic devices can be combined with these nonlinear optical devices to realize a multi-functional platform in which quantum states of light can be generated and manipulated within a single, compact photonic integrated circuit.
Spontaneous parametric down conversion in nonlinear material is widely exploited to generate entangled photon pairs in quantum optics experiments and applications, including quantum computing and communication. Periodically poled thin film lithium niobate (PPTFLN) has emerged as a promising platform for efficient entangled photon pair generation, offering enhanced nonlinear interaction through quasi-phase matching (QPM) and tight confinement of light. However, achieving optimal performance requires careful control of the QPM condition since the waveguide in TFLN is highly dispersive to changes in the geometric parameter. In this study, we fabricate PPTFLN rib waveguides to generate entangled photon pairs at telecommunication wavelengths, varying geometric parameters. QPM condition is confirmed with the second harmonic generation experiments and Pair generation rate and coincidence-to-accidental count ratio are also estimated by temporal coincidence measurement. Digital etching process is introduced to control the QPM condition, resulting in incremental peak wavelength shift by discrete etching step. This is expected to contribute to synchronizing wavelength of quantum nodes.
Spontaneous parametric down-conversion (SPDC) has become a key method for generating entangled photon pairs. Periodically poled thin-film lithium niobate (TFLN) waveguides induce strong SPDC but require complex fabrication processes. In this work, we experimentally demonstrate efficient SPDC and second harmonic generation using modal phase matching methods. This is achieved with inverse-designed optical mode converters and low-loss optical waveguides in a single nanofabrication process. Inverse design methods provide enhanced functionalities and compact footprints for the converter. Despite the extensive achievements in inverse-designed photonic integrated circuits, the potential of inverse-designed TFLN quantum photonic devices has been seldom explored. The device shows an on-chip conversion efficiency of 3.95% W−1 cm−2 in second harmonic generation measurements and a coincidence count rate up to 21.2 kHz in SPDC experiments. This work highlights the potential of the inverse-designed TFLN photonic devices and paves the way for their applications in on-chip nonlinear or quantum optics.
Spontaneous parametric down-conversion (SPDC) has been utilized in generating non-classical light, such as correlated photon pairs and squeezed light, significantly influencing optical quantum information research. Advances in thinfilm lithium niobate (TFLN) technologies have brought the benefits of χ(2)nonlinearity from bulk optics to integrated photonics. Targeting fully exploitation of high Electro-optic (EO) controllability and the periodic poled lithium niobate (PPLN), enabling quasi-phase matching (QPM), offer the realization especially under the X-cut based platform [1]–[3]. Moreover, the small footprint is also key figure of merit for efficient processing of SPDC spectrum. In this work, we demonstrate SPDC photon pair generation in a small footprint size with a 0.614 nm FSR at the IR wavelength, based on X-cut PPLN racetrack resonator.