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1. 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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2. Long-wavelength-infrared laser filamentation in solids in the near-single-cycle regime

   https://doi.org/10.1364/OL.389456  

   Qu, Shizhen; Chaudhary_Nagar, Garima; Li, Wenkai; Liu, Kun; Zou, Xiao; Hon_Luen, Seck; Dempsey, Dennis; Hong, Kyung-Han; Jie_Wang, Qi; Zhang, Ying; et al (April 2020, Optics Letters)

   We experimentally demonstrate long-wavelength-infrared (LWIR) femtosecond filamentation in solids. Systematic investigations of supercontinuum (SC) generation and self-compression of the LWIR pulses assisted by laser filamentation are performed in bulk KrS-5 and ZnSe, pumped by $\sim <\#comment/>145\mathrm{f}\mathrm{s}$, 9 µm, 10 µJ pulses from an optical parametric chirped-pulse amplifier operating at 10 kHz of repetition rate. Multi-octave SC spectra are demonstrated in both materials. While forming stable single filament, 1.5 cycle LWIR pulses with 4.5 µJ output pulse energy are produced via soliton-like self-compression in a 5 mm thick KrS-5. The experimental results quantitatively agree well with the numerical simulation based on the unidirectional pulse propagation equation. This work shows the experimental feasibility of high-energy, near-single-cycle LWIR light bullet generation in solids. 

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3. 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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4. 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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5. Femtosecond timing synchronization at megahertz repetition rates for an x-ray free-electron laser

   https://doi.org/10.1364/OPTICA.396728  

   Sato, Tokushi; Letrun, Romain; Kirkwood, Henry_J; Liu, Jia; Vagovič, Patrik; Mills, Grant; Kim, Yoonhee; Takem, Cedric_M_S; Planas, Marc; Emons, Moritz; et al (June 2020, Optica)

   A critical challenge of pump-probe experiments with x-ray free-electron lasers (XFELs) is accurate synchronization of x-ray and optical pulses. At the European XFEL we observed megahertz rate timing jitter of $24.0\pm <\#comment/>12.4\mathrm{f}\mathrm{s}$. 

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