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Periodic poling of thin-film lithium niobate for second-harmonic generation and entangled photon-pair generation

Jie Zhao

Published Jan 1, 2020• 0 citations
OpenAlex

Abstract

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.

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arXiv:1910.03202•Physical Review Letters•Apr 24, 2020•362 citations
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.
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 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.
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.
Thanks to its high quadratic nonlinear susceptibilty and low propagation losses, thin film lithium niobate (TFLN) on insulator is an ideal platform for laser frequency conversion and generation of quantum states of light. Frequency conversion is usually achieved by quasi-phase matching (QPM) via electric-field poling. However, this scheme shows very high sensitivity to the dimensions of the waveguide, poling period and duty cycle, resulting in a lack of repeatability of the phase matched wavelength and efficiency, which in turn limits the spread of TFLN frequency converters in complex circuits and hinders wafer-scale production. Here we propose a layer-poled modal phase matching (MPM) that is 5 to 10 times more robust towards fabrication uncertainties and theoretically more efficient than conventional QPM. By selectively poling the bottom part of the waveguide all along its length, second harmonic is efficiently generated on a higher order waveguide's mode. We validate this approach by poling TFLN waveguides as a post-process after the fabrication in a foundry process. We perform a tolerance analysis and compare the experimental results with conventional QPM second harmonic generation process on the same waveguides. Then, we show how MPM can be exploited to obtain efficient intraband frequency conversion processes at telecom wavelengths by leveraging simultaneous second harmonic and difference frequency generation in the same waveguide.
Abstract Integrated photon-pair sources are crucial for scalable photonic quantum systems. Thin-film lithium niobate is a promising platform for on-chip photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the device implementation faces practical challenges. Periodically poled lithium niobate (PPLN), despite enabling flexible quasi-phase matching, suffers from poor fabrication reliability and device repeatability, while conventional modal phase matching (MPM) methods yield limited efficiencies due to inadequate mode overlaps. Here, we introduce a layer-poled lithium niobate (LPLN) nanophotonic waveguide for efficient photon-pair generation. It leverages layer-wise polarity inversion through electrical poling to break spatial symmetry and significantly enhance nonlinear interactions for MPM, achieving a notable normalized second-harmonic generation (SHG) conversion efficiency of 4615% W-1 cm-2. Through a cascaded SHG and SPDC process, we demonstrate photon-pair generation with a normalized brightness of 3.1×106 Hz nm-1 mW-2 in a 3.3 mm long LPLN waveguide, surpassing existing on-chip sources under similar operating configurations. Crucially, our LPLN waveguides offer enhanced fabrication reliability and reduced sensitivity to geometric variations and temperature fluctuations compared to PPLN devices. We expect LPLN to become a promising solution for on-chip nonlinear wavelength conversion and non-classical light generation, with immediate applications in quantum communication, networking, and on-chip photonic quantum information processing.
A thin-film periodically poled lithium niobate waveguide was designed and fabricated which generates entangled photon pairs at telecommunications wavelengths with high coincidences-to-accidentals counts ratio CAR>67000, two-photon interference visibility V>99%, and heralded single-photon autocorrelation g_{H}^{(2)}(0)<0.025. Nondestructive in situ diagnostics were used to determine the poling quality in 3D. Megahertz rates of photon pairs were generated by less than a milliwatt of pump power, simplifying the pump requirements and dissipation compared to traditional spontaneous parametric down-conversion lithium niobate devices.