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1. 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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2. 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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3. Large-area, high-numerical-aperture multi-level diffractive lens via inverse design

   https://doi.org/10.1364/OPTICA.388697  

   Meem, Monjurul; Banerji, Sourangsu; Pies, Christian; Oberbiermann, Timo; Majumder, Apratim; Sensale-Rodriguez, Berardi; Menon, Rajesh (March 2020, Optica)

   Flat lenses enable thinner, lighter, and simpler imaging systems. However, large-area and high-NA flat lenses have been elusive due to computational and fabrication challenges. Here we applied inverse design to create a multi-level diffractive lens (MDL) with thickness $<<\#comment/>1.35\text{µ<\#comment/>}\mathrm{m}$, diameter of 4.13 mm, and $\mathrm{N}\mathrm{A}=0.9$at wavelength of 850 nm (bandwidth $\sim <\#comment/>35\mathrm{n}\mathrm{m}$). Since the MDL is created in polymer, it can be cost-effectively replicated via imprint lithography. 

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4. Self-adaptive amplified spontaneous emission suppression with a photorefractive two-beam coupling filter

   https://doi.org/10.1364/OL.410695  

   Pettine, Jacob; Zhu, Miao; Anderson, Dana_Z (November 2020, Optics Letters)

   Optical bandpass filters can be utilized to suppress parasitic broadband spectral power prior to laser amplification but are typically designed around specific frequencies or require manual adjustment, thus limiting their compatibility with highly tunable or integrated laser systems. In this Letter, we introduce a self-adaptive volume holographic filter using the dynamic two-beam coupling interaction in photorefractive ${\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_3$, demonstrating $-<\#comment/>10\mathrm{d}\mathrm{B}$suppression of amplified spontaneous emission noise surrounding a tunable 780 nm diode laser peak, with $<<\#comment/>2\mathrm{n}\mathrm{m}$filter bandwidth and 50% power throughput. The spectral filtering is automatically centered on the lasing mode, with an estimated auto-tuning rate of 100 GHz/s under typical conditions. Furthermore, the filter suppression and bandwidth can be optimized via the two-beam coupling intensity ratio and angle, respectively, for versatile control over the self-adaptive filter characteristics. 

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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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