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1. 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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2. Ultrafast 1  MHz vacuum-ultraviolet source via highly cascaded harmonic generation in negative-curvature hollow-core fibers

   https://doi.org/10.1364/OPTICA.395688  

   Couch, David_E; Hickstein, Daniel_D; Winters, David_G; Backus, Sterling_J; Kirchner, Matthew_S; Domingue, Scott_R; Ramirez, Jessica_J; Durfee, Charles_G; Murnane, Margaret_M; Kapteyn, Henry_C (July 2020, Optica)

   Vacuum-ultraviolet (VUV) light is critical for the study of molecules and materials, but the generation of femtosecond pulses in the VUV region at high repetition rates has proven difficult. Here we demonstrate the efficient generation of VUV light at megahertz repetition rates using highly cascaded four-wave mixing processes in a negative-curvature hollow-core fiber. Both even- and odd-order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with high energy resolution of $\sim <\#comment/>40\mathrm{m}\mathrm{e}\mathrm{V}$. In contrast to direct high harmonic generation, this highly cascaded harmonic generation process requires lower peak intensity and therefore can operate at higher repetition rates, driven by a robust $\sim <\#comment/>10\mathrm{W}$fiber-laser system in a compact setup. Additionally, we present numerical simulations that explore the fundamental capabilities and spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source can enhance the capabilities of spectroscopies of molecular and quantum materials, such as photoionization mass spectrometry and time-, angle-, and spin-resolved photoemission. 

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3. Cascaded metamaterial polarizers for the visible region

   https://doi.org/10.1364/OL.411234  

   Bootpakdeetam, Pawarat; Hemmati, Hafez; Magnusson, Robert (December 2020, Optics Letters)

   Polarizers serve many application fields such as imaging, display technology, and telecommunications. Focusing on the visible spectral region, we provide the design and fabrication of compact high-efficiency resonant polarizers in the crystalline silicon-on-quartz material system. We experimentally verify the improved efficiency attained by a cascaded dual-module polarizer assembled with building blocks of elemental subwavelength grating structures. We obtain a measured extinction ratio (ER) of $\sim <\#comment/>3000$in a 2 mm thick stacked prototype device across a bandwidth of $\sim <\#comment/>110\mathrm{n}\mathrm{m}$in the 570–680 nm spectral domain. The ridge width of the constituent nanograting is $\sim <\#comment/>84\mathrm{n}\mathrm{m}$. Computed results show a high ER in spite of the lossy nature of crystalline silicon in the visible region, enabling cascaded metasurfaces while preserving high transmission. 

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4. Ultrabroadband nonlinear optics in nanophotonic periodically poled lithium niobate waveguides

   https://doi.org/10.1364/OPTICA.7.000040  

   Jankowski, Marc; Langrock, Carsten; Desiatov, Boris; Marandi, Alireza; Wang, Cheng; Zhang, Mian; Phillips, Christopher_R; Lončar, Marko; Fejer, M_M (January 2020, Optica)

   Quasi-phase-matched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this paper, we demonstrate the first generation of devices that combine the dispersion engineering available in nanophotonic waveguides with quasi-phase-matched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasi-static interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude compared to conventional devices, from picojoules to femtojoules. We experimentally demonstrate two effects associated with second harmonic generation (SHG). First, we observe efficient quasi-phase-matched SHG with $<<\#comment/>100\mathrm{f}\mathrm{J}$of pulse energy. Second, in the limit of strong phase-mismatch, we observe spectral broadening of both harmonics with as little as 2 pJ of pulse energy. These results lay a foundation for a new class of nonlinear devices, in which coengineering of dispersion with quasi-phase-matching enables efficient nonlinear optics at the femtojoule level. 

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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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