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1. 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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2. 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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3. Ultra-high extinction ratio polarization beam splitter with extreme skin-depth waveguide

   https://doi.org/10.1364/OL.420824  

   Ahmed, Syed_Z; Ahmed, Ishtiaque; Mia, Md_Borhan; Jaidye, Nafiz; Kim, Sangsik (April 2021, Optics Letters)

   In this Letter, we present a high extinction ratio and compact on-chip polarization beam splitter (PBS), based on an extreme skin-depth (eskid) waveguide. Subwavelength-scale gratings form an effectively anisotropic metamaterial cladding and introduce a large birefringence. The anisotropic dielectric perturbation of the metamaterial cladding suppresses the TE polarization extinction via exceptional coupling, while the large birefringence efficiently cross-couples the TM mode, thus reducing the coupling length. We demonstrated the eskid-PBS on a silicon-on-insulator platform and achieved an ultra-high extinction ratio PBS ( $\approx <\#comment/>60\mathrm{d}\mathrm{B}$for TE and $\approx <\#comment/>48\mathrm{d}\mathrm{B}$for TM) with a compact coupling length ( $\approx <\#comment/>14.5\text{µ<\#comment/>}\mathrm{m}$). The insertion loss is also negligible ( $<<\#comment/>0.6\mathrm{d}\mathrm{B}$). The bandwidth is $><\#comment/>80$(30) nm for the TE (TM) extinction ratio $><\#comment/>20\mathrm{d}\mathrm{B}$. Our ultra-high extinction ratio PBS is crucial in implementing efficient polarization diversity circuits, especially where a high degree of polarization distinguishability is necessary, such as photonic quantum information processing. 

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4. Spectroscopic study of the 4 f ⁷ 6 s ^{2 8} S 7/2∘−4 f ⁷ ( ⁸ S ^∘ )6 s 6 p ( ¹ P ^∘ ) ⁸ P _9/2 transition in neutral europium-151 and europium-153: absolute frequency and hyperfine structure

   https://doi.org/10.1364/JOSAB.467968  

   Herd, M_T; Maruko, C.; Herzog, M_M; Brand, A.; Cannon, G.; Duah, B.; Hollin, N.; Karani, T.; Wallace, A.; Whitmore, M.; et al (September 2022, Journal of the Optical Society of America B)

   We report on spectroscopic measurements on the $4f^{7}6s^2{}^8{S}_{7/2}^{\circ <\#comment/>}\to <\#comment/>4f^7{(}^8{S}^{\circ <\#comment/>}\left)6s6p{(}^1{P}^{\circ <\#comment/>}\right){}^8{P}_{9/2}$transition in neutral europium-151 and europium-153 at 459.4 nm. The center of gravity frequencies for the 151 and 153 isotopes, reported for the first time in this paper, to our knowledge, were found to be 652,389,757.16(34) MHz and 652,386,593.2(5) MHz, respectively. The hyperfine coefficients for the $6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$state were found to be $\mathrm{A}\left(151\right)=-<\#comment/>228.84\left(2\right)\mathrm{M}\mathrm{H}\mathrm{z}$, $\mathrm{B}\left(151\right)=226.9\left(5\right)\mathrm{M}\mathrm{H}\mathrm{z}$and $\mathrm{A}\left(153\right)=-<\#comment/>101.87\left(6\right)\mathrm{M}\mathrm{H}\mathrm{z}$, $\mathrm{B}\left(153\right)=575.4\left(1.5\right)\mathrm{M}\mathrm{H}\mathrm{z}$, which all agree with previously published results except for A(153), which shows a small discrepancy. The isotope shift is found to be 3163.8(6) MHz, which also has a discrepancy with previously published results. 

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5. 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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