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1. Ultralow-threshold thin-film lithium niobate optical parametric oscillator

   https://doi.org/10.1364/OPTICA.418984  

   Lu, Juanjuan; Al_Sayem, Ayed; Gong, Zheng; Surya, Joshua_B; Zou, Chang-Ling; Tang, Hong_X (April 2021, Optica)

   Materials with strong second-order ( ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chip-scale integration of low-loss ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$materials remains challenging and limits the threshold power of on-chip ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$OPO. Here we report an on-chip lithium niobate optical parametric oscillator at the telecom wavelengths using a quasi-phase-matched, high-quality microring resonator, whose threshold power ( $\sim <\#comment/>30\text{µ<\#comment/>}\mathrm{W}$) is 400 times lower than that in previous ${\mathrm{\chi <\#comment/>}}^{\left(2\right)}$integrated photonics platforms. An on-chip power conversion efficiency of 11% is obtained from pump to signal and idler fields at a pump power of 93 µW. The OPO wavelength tuning is achieved by varying the pump frequency and chip temperature. With the lowest power threshold among all on-chip OPOs demonstrated so far, as well as advantages including high conversion efficiency, flexibility in quasi-phase-matching, and device scalability, the thin-film lithium niobate OPO opens new opportunities for chip-based tunable classical and quantum light sources and provides a potential platform for realizing photonic neural networks. 

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2. Widely separated optical Kerr parametric oscillation in AlN microrings

   https://doi.org/10.1364/OL.384317  

   Tang, Yulong; Gong, Zheng; Liu, Xianwen; Tang, Hong_X (February 2020, Optics Letters)

   Here, we report ${\mathrm{\chi <\#comment/>}}^{\left(3\right)}$-based optical parametric oscillation (OPO) with widely separated signal–idler frequencies from crystalline aluminum nitride microrings pumped at $2\text{µ<\#comment/>}\mathrm{m}$. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by $><\#comment/>9\mathrm{T}\mathrm{H}\mathrm{z}$, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip’s temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources. 

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3. Efficient and highly tunable second-harmonic generation in Z-cut periodically poled lithium niobate nanowaveguides

   https://doi.org/10.1364/OL.393445  

   Chen, Jia-Yang; Tang, Chao; Ma, Zhao-Hui; Li, Zhan; Meng_Sua, Yong; Huang, Yu-Ping (July 2020, Optics Letters)

   Thin-film lithium-niobate-on-insulator (LNOI) has emerged as a superior integrated-photonics platform for linear, nonlinear, and electro-optics. Here we combine quasi-phase-matching, dispersion engineering, and tight mode confinement to realize nonlinear parametric processes with both high efficiency and wide wavelength tunability. On a millimeter-long, Z-cut LNOI waveguide, we demonstrate efficient ( $1900\pm <\#comment/>500\mathrm{\%<\#comment/>}{\mathrm{W}}^{-<\#comment/>1}{\mathrm{c}\mathrm{m}}^{-<\#comment/>2}$) and highly tunable ( $-<\#comment/>1.71\mathrm{n}\mathrm{m}/\mathrm{K}$) second-harmonic generation from 1530 to 1583 nm by type-0 quasi-phase-matching. Our technique is applicable to optical harmonic generation, quantum light sources, frequency conversion, and many other photonic information processes across visible to mid-IR spectral bands. 

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4. Optical-parametric oscillation in photonic-crystal ring resonators

   https://doi.org/10.1364/OPTICA.469210  

   Black, Jennifer A.; Brodnik, Grant; Liu, Haixin; Yu, Su-Peng; Carlson, David R.; Zang, Jizhao; Briles, Travis C.; Papp, Scott B. (January 2022, Optica)

   By-design access to laser wavelength, especially with integrated photonics, is critical to advance quantum sensors, such as optical clocks and quantum-information systems, and open opportunities in optical communication. Semiconductor-laser gain provides exemplary efficiency and integration but merely in developed wavelength bands. Alternatively, nonlinear optics requires control of phase matching, but the principle of nonlinear conversion of a pump laser to a designed wavelength is extensible. We report on laser-wavelength access by versatile customization of optical-parametric oscillation (OPO) with a photonic-crystal ring resonator (PhCR). Leveraging the exquisite control of laser propagation provided by a photonic crystal in a traveling-wave ring resonator, we enable OPO generation across a wavelength range of 1234–2093 nm with a 1550-nm pump and 1016–1110 nm with a 1064-nm pump. Moreover, our platform offers pump-to-sideband conversion efficiency of > 10 % and negligible additive optical-frequency noise across the output range. From laser design to simulation of nonlinear dynamics, we use a Lugiato–Lefever framework that predicts the system characteristics, including bidirectional OPO generation in the PhCR and conversion efficiency in agreement with our observations. Our experiments introduce broadband lasers by design with PhCR OPOs, providing critical functionalities in integrated photonics. 

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5. InGaP quantum nanophotonic integrated circuits with 1.5% nonlinearity-to-loss ratio

   https://doi.org/10.1364/OPTICA.440383  

   Zhao, Mengdi; Fang, Kejie (February 2022, Optica)

   Optical nonlinearity plays a pivotal role in quantum information processing using photons, from heralded single-photon sources and coherent wavelength conversion to long-sought quantum repeaters. Despite the availability of strong dipole coupling to quantum emitters, achieving strong bulk optical nonlinearity is highly desirable. Here, we realize quantum nanophotonic integrated circuits in thin-film InGaP with, to our knowledge, a record-high ratio of $1.5\mathrm{\%<\#comment/>}$between the single-photon nonlinear coupling rate ( $g/2\mathrm{\pi <\#comment/>}=11.2\mathrm{M}\mathrm{H}\mathrm{z}$) and cavity-photon loss rate. We demonstrate second-harmonic generation with an efficiency of $71200\pm <\#comment/>10300\mathrm{\%<\#comment/>}/\mathrm{W}$in the InGaP photonic circuit and photon-pair generation via degenerate spontaneous parametric downconversion with an ultrahigh rate exceeding 27.5 MHz/µW—an order of magnitude improvement of the state of the art—and a large coincidence-to-accidental ratio up to $1.4\times <\#comment/>{10}^4$. Our work shows InGaP as a potentially transcending platform for quantum nonlinear optics and quantum information applications. 

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