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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 (${\mathrm{\chi <\#comment/>}}^{(2)}$) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chip-scale integration of low-loss${\mathrm{\chi <\#comment/>}}^{(2)}$materials remains challenging and limits the threshold power of on-chip${\mathrm{\chi <\#comment/>}}^{(2)}$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${\mathrm{\chi <\#comment/>}}^{(2)}$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. Toward 1% single-photon anharmonicity with periodically poled lithium niobate microring resonators

   https://doi.org/10.1364/OPTICA.403931  

   Lu, Juanjuan ; Li, Ming ; Zou, Chang-Ling ; Al_Sayem, Ayed ; Tang, Hong_X ( November 2020 , Optica)

   The absence of the single-photon nonlinearity has been a major roadblock in developing quantum photonic circuits at optical frequencies. In this paper, we demonstrate a periodically poled thin film lithium niobate microring resonator (PPLNMR) that reaches 5,000,000%/W second-harmonic conversion efficiency—almost 20-fold enhancement over the state-of-the-art—by accessing its largest${\mathrm{\chi <\#comment/>}}^{(2)}$tensor component$d_{33}$via quasi-phase matching. The corresponding single-photon coupling rate$g/2\mathrm{\pi <\#comment/>}$is estimated to be 1.2 MHz, which is an important milestone as it approaches the dissipation rate$\mathrm{\kappa <\#comment/>}/2\mathrm{\pi <\#comment/>}$of best-available lithium niobate microresonators developed in the community. Using a figure of merit defined as$g/\mathrm{\kappa <\#comment/>}$, our device reaches a single-photon nonlinear anharmonicity approaching 1%. We show that, by further scaling of the device, it is possible to improve the single-photon anharmonicity to a regime where photon blockade effect can be manifested.

    

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3. Bell state analyzer for spectrally distinct photons

   https://doi.org/10.1364/OPTICA.443302  

   Lingaraju, Navin_B ; Lu, Hsuan-Hao ; Leaird, Daniel_E ; Estrella, Steven ; Lukens, Joseph_M ; Weiner, Andrew_M ( March 2022 , Optica)

   We demonstrate a Bell state analyzer that operates directly on frequency mismatch. Based on electro-optic modulators and Fourier-transform pulse shapers, our quantum frequency processor design implements interleaved Hadamard gates in discrete frequency modes. Experimental tests on entangled-photon inputs reveal fidelities of$\sim <\#comment/>98\mathrm{\%<\#comment/>}$for discriminating between the$|{\mathrm{\Psi <\#comment/>}}^{+}⟩<\#comment/>$and$|{\mathrm{\Psi <\#comment/>}}^{-<\#comment/>}⟩<\#comment/>$frequency-bin Bell states. Our approach resolves the tension between wavelength-multiplexed state transport and high-fidelity Bell state measurements, which typically require spectral indistinguishability.

    

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4. Utilizing phase delays of an integrated pixel-array structure to generate orbital-angular-momentum beams with tunable orders and a broad bandwidth

   https://doi.org/10.1364/OL.396447  

   Song, Hao ; Zhao, Zhe ; Zhang, Runzhou ; Song, Haoqian ; Zhou, Huibin ; Pang, Kai ; Du, Jing ; Li, Long ; Liu, Cong ; Su, Xinzhou ; et al ( July 2020 , Optics Letters)

   We study the relationship between the input phase delays and the output mode orders when using a pixel-array structure fed by multiple single-mode waveguides for tunable orbital-angular-momentum (OAM) beam generation. As an emitter of a free-space OAM beam, the designed structure introduces a transformation function that shapes and coherently combines multiple (e.g., four) equal-amplitude inputs, with the$k$th input carrying a phase delay of$(k-<\#comment/>1)\mathrm{\Delta <\#comment/>}\mathrm{\phi <\#comment/>}$. The simulation results show that (1) the generated OAM order ℓ is dependent on the relative phase delay$\mathrm{\Delta <\#comment/>}\mathrm{\phi <\#comment/>}$; (2) the transformation function can be tailored by engineering the structure to support different tunable ranges (e.g., $l=\{-<\#comment/>1\},\{-<\#comment/>1,+1\},\{-<\#comment/>1,0,+1\}$, or$\{-<\#comment/>2,-<\#comment/>1,+1,+2\}$); and (3) multiple independent coaxial OAM beams can be generated by simultaneously feeding the structure with multiple independent beams, such that each beam has its own$\mathrm{\Delta <\#comment/>}\mathrm{\phi <\#comment/>}$value for the four inputs. Moreover, there is a trade-off between the tunable range and the mode purity, bandwidth, and crosstalk, such that the increase of the tunable range leads to (a) decreased mode purity (from 91% to 75% for$l=-<\#comment/>1$), (b) decreased 3 dB bandwidth of emission efficiency (from 285 nm for$l=\{-<\#comment/>1\}$to 122 nm for$l=\{-<\#comment/>2,-<\#comment/>1,+1,+2\}$), and (c) increased crosstalk within the C-band (from$-<\#comment/>23.7$to$-<\#comment/>13.2\mathrm{d}\mathrm{B}$when the tunable range increases from 2 to 4).

    

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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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