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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Spontaneous Parametric Down-Conversion Beaming from a Lithium Niobate Nanostructured Resonator

Attilio Zilli, Vitaliy Sultanov, Michael Poloczek, Marzia Ferrera, Yigong Luan, E. T. Kokkinakis, Tomás Santiago‐Cruz, Luca Carletti, Andréa Toma, Marco Finazzi, Maria V. Chekhova, Michele Celebrano

Published Jun 26, 2023• 0 citations
Publisher

Abstract

Lithium niobate (LiNbO3) is a prime material for photonics thanks to a unique mix of properties, including strong electro-optical and nonlinear optical responses as well as a broad transparency window spanning the visible and near-infrared (NIR) range (0.35 to 4.5 μm). Despite the inherent challenges in the nanofabrication of LiNbO3, caused by its hardness and resistance to chemical etching, recent technical advances eventually enabled the realization of deeply sub-μm features in LiNbO3,thereby enabling a series of breakthroughs in the field of nano-and meta-photonics [1]. Nonlinear nano-optics has especially benefitted from such improvements thanks to the large second-order diagonal nonlinearity of LiNbO3. Several LiNbO3metasurfaces have been designed to enhance and steer second-harmonic generation (SHG) [1] emitted up to the visible range [2]. Similar designs were also exploited for generating photon pairs via spontaneous parametric down-conversion (SPDC) [3], a quantum process where a single photon is converted into two photons of the same total energy, see Fig. 1c (right).

DOI
10.1109/cleo/europe-eqec57999.2023.10232211

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Lithium niobate (LiNbO3) is a prime material for photonics thanks to a unique mix of properties, including strong electro-optical and nonlinear optical responses as well as a broad transparency window spanning the visible and near-infrared (NIR) range (0.35 to 4.5 μm). Despite the inherent challenges in the nanofabrication of LiNbO3, caused by its hardness and resistance to chemical etching, recent technical advances eventually enabled the realization of deeply sub-μm features in LiNbO3,thereby enabling a series of breakthroughs in the field of nano-and meta-photonics [1]. Nonlinear nano-optics has especially benefitted from such improvements thanks to the large second-order diagonal nonlinearity of LiNbO3. Several LiNbO3metasurfaces have been designed to enhance and steer second-harmonic generation (SHG) [1] emitted up to the visible range [2]. Similar designs were also exploited for generating photon pairs via spontaneous parametric down-conversion (SPDC) [3], a quantum process where a single photon is converted into two photons of the same total energy, see Fig. 1c (right).
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.
Nonlinear wavelength conversion is essential for many classical and quantum pho-tonic applications. The underlying second-order nonlinear optical processes, however, generally exhibit limited spectral bandwidths that impact their application potential. Here we use a high-Q X-cut lithium niobate microdisk resonator to demonstrate both second-harmonic generation and spontaneous parametric down-conversion on chip. In particular, our lithium niobate microresonator, with its wide-range cyclic phase matching and rich optical mode structures, is able to achieve ultra-broadband spontaneous parametric down-conversion, with a bandwidth over 400 nm, inferred from recorded spectra of the down-converted photons. The produced biphoton pairs exhibit strong temporal correlation, with a coincidence-to-accidental ratio measured to be 43.1. Our device is promising for integrated quantum photonics where optical frequency could be used as a degree of freedom for signal processing.
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.
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.
Lithium niobate on insulator is a promising platform for integrated quantum photonics. Its strong nonlinear coefficient and electro-optic effect allow the integration of photon pair sources and fast reconfigurable interferometers for many applications like boson sampling, QKD and quantum state generation. The photons are generated via Spontaneous Parametric Down-Conversion (SPDC). Two key aspects of the photon pairs are their bandwidth and the possibility to split them deterministically. Here we present a new source type where the generated photons travel in opposite directions and feature a narrow bandwidth of 5 nm in the forward and 20nm in the backward propagation. The reduction of bandwidth and the separability are big advantages over the type-0-SPDC sources. Compared to the well-known type-2-SPDC sources we achieve a 3.6 times higher efficiency with similar bandwidths.
We design and fabricate shallow-etched periodically poled lithium niobate waveguides to realize highly efficient broadband spontaneous parametric down-conversion (SPDC) on nanophotonic chips. The shallow-etched waveguide can tolerate the non-uniformities of waveguide width induced by fabrication imperfections, enabling the generation of photon pairs with high count rate and bandwidth. We demonstrate photon-pair generation with a high brightness of 11.7 GHz/mW and bandwidth of 22 THz in a 5.7-mm-long PPLN waveguide. The generated photon pairs exhibit a strong temporal correlation with a coincidence-to-accidental ratio of up to 16262±850. Our results confirm the feasibility of shallow etching in the fabrication of an efficient SPDC device on the platform of lithium niobate on an insulator, and benefit quantum information processing with a broadband photon source.