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1. Efficient and compact thermo-optic phase shifter in silicon-rich silicon nitride

   https://doi.org/10.1364/OL.431757  

   Nejadriahi, Hani ; Pappert, Steve ; Fainman, Yeshaiahu ; Yu, Paul ( September 2021 , Optics Letters)

   The design, fabrication, and characterization of low-loss ultra-compact bends in high-index ($n=3.1$at$\mathrm{\lambda <\#comment/>}=1550\mathrm{n}\mathrm{m}$) plasma-enhanced chemical vapor deposition silicon-rich silicon nitride (SRN) were demonstrated and utilized to realize efficient, small footprint thermo-optic phase shifter. Compact bends were structured into a folded waveguide geometry to form a rectangular spiral within an area of$65\times <\#comment/>65\text{µ<\#comment/>}{\mathrm{m}}^2$, having a total active waveguide length of 1.2 mm. The device featured a phase-shifting efficiency of$8\mathrm{m}\mathrm{W}/\mathrm{\pi <\#comment/>}$and a 3 dB switching bandwidth of 15 KHz. We propose SRN as a promising thermo-optic platform that combines the properties of silicon and stoichiometric silicon nitride.

    

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2. Direct chip-scale optical frequency divider via regenerative harmonic injection locking

   https://doi.org/10.1364/OL.413335  

   Bustos-Ramirez, Ricardo ; Trask, Lawrence R. ; Bhardwaj, Ashish ; Hoefler, Gloria E. ; Kish, Fred A. ; Delfyett, Peter J. ( February 2021 , Optics Letters)

   A novel optical frequency division technique, called regenerative harmonic injection locking, is used to transfer the timing stability of an optical frequency comb with a repetition rate in the millimeter wave range ($\sim <\#comment/>300\mathrm{G}\mathrm{H}\mathrm{z}$) to a chip-scale mode-locked laser with a$\sim <\#comment/>10\mathrm{G}\mathrm{H}\mathrm{z}$repetition rate. By doing so, the 300 GHz optical frequency comb is optically divided by a factor of$30\times <\#comment/>$to 10 GHz. The stability of the mode-locked laser after regenerative harmonic injection locking is$\sim <\#comment/>{10}^{-<\#comment/>12}$at 1 s with a$1/\mathrm{\tau <\#comment/>}$trend. To facilitate optical frequency division, a coupled opto-electronic oscillator is implemented to assist the injection locking process. This technique is exceptionally power efficient, as it uses less than$100\text{µ<\#comment/>}\mathrm{W}$of optical power to achieve stable locking.

    

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3. 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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4. Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre

   https://doi.org/10.1364/AO.394123  

   Henderikx Freitas, Fernanda ; Dugenne, Mathilde ; Ribalet, François ; Hynes, Annette ; Barone, Benedetto ; Karl, David M. ; White, Angelicque E. ( July 2020 , Applied Optics)

   Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation (${\mathit{\text{c}}}_{\mathit{\text{p}}}$) and backscattering ($b_{\text{bp}}$) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in${\mathit{\text{c}}}_{\mathit{\text{p}}}$and${\mathit{\text{b}}}_{\text{bp}}$to daytime particle growth and division of cells, with particles$<<\#comment/>7\text{µ<\#comment/>}\mathrm{m}$driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacteriumCrocosphaera($\sim <\#comment/>4-<\#comment/>7\text{µ<\#comment/>}\mathrm{m}$) to be an important driver of${\mathit{\text{c}}}_{\mathit{\text{p}}}$at the time of sampling, whereasProchlorococcusdynamics ($\sim <\#comment/>0.5\text{µ<\#comment/>}\mathrm{m}$) were essential to reproducing temporal variability in${\mathit{\text{b}}}_{\text{bp}}$. This study is a step towards improved characterization of the particle size range represented byin situbulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.

    

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5. Mid-IR spectroscopy of Er 3+ doped low-phonon CsCdCl 3 and CsPbCl 3 crystals

   https://doi.org/10.1364/JOSAB.470152  

   Brown, Ei Ei ; Fleischman, Zackery D. ; McKay, Jason ; Hommerich, Uwe ; Kabir, Al Amin ; Riggins, Jazmine ; Trivedi, Sudhir ; Dubinskii, Mark ( September 2022 , Journal of the Optical Society of America B)

   The mid-IR spectroscopic properties of${\mathrm{E}\mathrm{r}}^{3+}$doped low-phonon${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$crystals grown by the Bridgman technique have been investigated. Using optical excitations at$\sim <\#comment/>800\mathrm{n}\mathrm{m}$and$\sim <\#comment/>660\mathrm{n}\mathrm{m}$, both crystals exhibited IR emissions at$\sim <\#comment/>1.55$,$\sim <\#comment/>2.75$,$\sim <\#comment/>3.5$, and$\sim <\#comment/>4.5\text{µ<\#comment/>}\mathrm{m}$at room temperature. The mid-IR emission at 4.5 µm, originating from the${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition, showed a long emission lifetime of$\sim <\#comment/>11.6\mathrm{m}\mathrm{s}$for${\mathrm{E}\mathrm{r}}^{3+}$doped${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$, whereas${\mathrm{E}\mathrm{r}}^{3+}$doped${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$exhibited a shorter lifetime of$\sim <\#comment/>1.8\mathrm{m}\mathrm{s}$. The measured emission lifetimes of the${}^4{\mathrm{I}}_{9/2}$state were nearly independent of the temperature, indicating a negligibly small nonradiative decay rate through multiphonon relaxation, as predicted by the energy-gap law for low-maximum-phonon energy hosts. The room temperature stimulated emission cross sections for the${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition in${\mathrm{E}\mathrm{r}}^{3+}$doped${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$were determined to be$\sim <\#comment/>0.14\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$and$\sim <\#comment/>0.41\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$, respectively. The results of Judd–Ofelt analysis are presented and discussed.

    

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