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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. Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces

   https://doi.org/10.1364/OL.413764  

   Kang, Lei ; Bao, Huaguang ; Werner, Douglas H. ( January 2021 , Optics Letters)

   Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high$Q$-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative$Q$-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter$\mathrm{\alpha <\#comment/>}$of the system. Furthermore, high$Q$-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a$Q$-factor of$\sim <\#comment/>8\times <\#comment/>{10}^4$, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of$3.3{\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$. This suggests that such high$Q$-factor LN metasurfaces may be good candidates for practical blue–ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.

    

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3. Demonstration of using two aperture pairs combined with multiple-mode receivers and MIMO signal processing for enhanced tolerance to turbulence and misalignment in a 10  Gbit/s QPSK FSO link

   https://doi.org/10.1364/OL.391120  

   Song, Haoqian ; Li, Long ; Pang, Kai ; Zhang, Runzhou ; Zou, Kaiheng ; Zhao, Zhe ; Du, Jing ; Song, Hao ; Liu, Cong ; Cao, Yinwen ; et al ( May 2020 , Optics Letters)

   We utilize aperture diversity combined with multiple-mode receivers and multiple-input-multiple-output (MIMO) digital signal processing (DSP) to demonstrate enhanced tolerance to atmospheric turbulence and spatial misalignment in a 10 Gbit/s quadrature-phase-shift-keyed (QPSK) free-space optical (FSO) link. Turbulence and misalignment could cause power coupling from the fundamental Gaussian mode into higher-order modes. Therefore, we detect power from multiple modes and use MIMO DSP to enhance the recovery of the original data. In our approach, (a) each of multiple transmitter apertures transmits a single fundamental Gaussian beam carrying the same data stream, (b) each of multiple receiver apertures detects the signals that are coupled from the fundamental Gaussian beams to multiple orbital angular momentum (OAM) modes, and (c) MIMO DSP is used to recover the data over multiple modes and receivers. Our simulation shows that the outage probability could be reduced from$><\#comment/>0.1$to$<<\#comment/>0.01$. Moreover, we experimentally demonstrate the scheme by transmitting two fundamental Gaussian beams carrying the same data stream and recovering the signals on OAM modes 0 and$+1$at each receiver aperture. We measure an up to$\sim <\#comment/>10\mathrm{d}\mathrm{B}$power-penalty reduction for a bit error rate (BER) at the 7% forward error correction limit for a 10 Gbit/s QPSK signal.

    

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4. Strong optomechanical interactions with long-lived fundamental acoustic waves

   https://doi.org/10.1364/OPTICA.476764  

   Xu, Wendao ; Iyer, Arjun ; Jin, Lei ; Set, Sze Y. ; Renninger, William H. ( January 2023 , Optica)

   Traveling-wave optomechanical interactions, known as Brillouin interactions, have now been established as a powerful and versatile resource for photonic sources, sensors, and radio-frequency processors. However, established Brillouin-based interactions with sufficient interaction strengths involve short phonon lifetimes, which critically limit their performance for applications, including radio-frequency filtering and optomechanical storage devices. Here, we investigate a new paradigm of optomechanical interactions with tightly confined fundamental acoustic modes, which enables the unique and desirable combination of high optomechanical coupling, long phonon lifetimes, tunable phonon frequencies, and single-sideband amplification. Using sensitive four-wave mixing spectroscopy controlling for noise and spatial mode coupling, optomechanical interactions with long$><\#comment/>2\text{µ<\#comment/>}\mathrm{s}$phonon lifetimes and strong$><\#comment/>400{\mathrm{W}}^{-<\#comment/>1}{\mathrm{m}}^{-<\#comment/>1}$coupling are observed in a tapered fiber. In addition, we demonstrate novel phonon self-interference effects resulting from the unique combination of an axially varying device geometry with long phonon lifetimes. A generalized theoretical model, in excellent agreement with experiments, is developed with broad applicability to inhomogeneous optomechanical systems.

    

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5. Widely tunable cavity-enhanced frequency combs

   https://doi.org/10.1364/OL.389412  

   Silfies, Myles C. ; Kowzan, Grzegorz ; Chen, Yuning ; Lewis, Neomi ; Hou, Ryan ; Baehre, Robin ; Gross, Tobias ; Allison, Thomas K. ( April 2020 , Optics Letters)

   We describe the cavity enhancement of frequency combs over a wide tuning range of 450–700 nm ($><\#comment/>7900{\mathrm{c}\mathrm{m}}^{-<\#comment/>1}$), covering nearly the entire visible spectrum. Tunable visible frequency combs from a synchronously pumped optical parametric oscillator are coupled into a four-mirror, dispersion-managed cavity with a finesse of 600–1400. An intracavity absorption path length enhancement greater than 190 is obtained over the entire tuning range, while preserving intracavity spectral bandwidths capable of supporting sub-200 fs pulse durations. These tunable cavity-enhanced frequency combs can find many applications in nonlinear optics and spectroscopy.

    

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