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1. Modifications of ion beam sputtered tantala thin films by secondary argon and oxygen bombardment

   https://doi.org/10.1364/AO.59.00A150  

   Yang, Le; Randel, Emmett; Vajente, Gabriele; Ananyeva, Alena; Gustafson, Eric; Markosyan, Ashot; Bassiri, Riccardo; Fejer, Martin; Menoni, Carmen (January 2020, Applied Optics)

   Amorphous tantala ( ${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$) thin films were deposited by reactive ion beam sputtering with simultaneous low energy assist ${\mathrm{A}\mathrm{r}}^{+}$or ${\mathrm{A}\mathrm{r}}^{+}/{\mathrm{O}}_2^{+}$bombardment. Under the conditions of the experiment, the as-deposited thin films are amorphous and stoichiometric. The refractive index and optical band gap of thin films remain unchanged by ion bombardment. Around 20% improvement in room temperature mechanical loss and 60% decrease in absorption loss are found in samples bombarded with 100-eV ${\mathrm{A}\mathrm{r}}^{+}$. A detrimental influence from low energy ${\mathrm{O}}_2^{+}$bombardment on absorption loss and mechanical loss is observed. Low energy ${\mathrm{A}\mathrm{r}}^{+}$bombardment removes excess oxygen point defects, while ${\mathrm{O}}_2^{+}$bombardment introduces defects into the tantala films. 

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2. Quasi-monolithic heterodyne laser interferometer for inertial sensing

   https://doi.org/10.1364/OL.473476  

   Zhang, Yanqi; Guzman, Felipe (September 2022, Optics Letters)

   We present a compact heterodyne laser interferometer developed for high-sensitivity displacement sensing applications. This interferometer consists of customized prisms and wave plates assembled as a quasi-monolithic unit to realize a miniaturized system. The interferometer design adopts a common-mode rejection scheme to provide a high rejection ratio to common environmental noise. Experimental tests in vacuum show a displacement sensitivity level of $11\mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$at $100\mathrm{m}\mathrm{H}\mathrm{z}$and as low as $0.6\mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$above $1\mathrm{p}\mathrm{m}$. The prototype unit is $20\mathrm{m}\mathrm{m}\times <\#comment/>20\mathrm{m}\mathrm{m}\times <\#comment/>10\mathrm{m}\mathrm{m}$in size and weighs $4.5\mathrm{g}$, allowing subsequent integration in compact systems. 

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3. 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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4. Thermal noise and mechanical loss of SiO ₂ /Ta ₂ O ₅ optical coatings at cryogenic temperatures

   https://doi.org/10.1364/OL.413758  

   Robinson, John_M; Oelker, Eric; Milner, William_R; Kedar, Dhruv; Zhang, Wei; Legero, Thomas; Matei, Dan_G; Häfner, Sebastian; Riehle, Fritz; Sterr, Uwe; et al (January 2021, Optics Letters)

   Mechanical loss of dielectric mirror coatings sets fundamental limits for both gravitational wave detectors and cavity-stabilized optical local oscillators for atomic clocks. Two approaches are used to determine the mechanical loss: ringdown measurements of the coating quality factor and direct measurement of the coating thermal noise. Here we report a systematic study of the mirror thermal noise at 4, 16, 124, and 300 K by operating reference cavities at these temperatures. The directly measured thermal noise is used to extract the mechanical loss for ${\mathrm{S}\mathrm{i}\mathrm{O}}_2/{\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$coatings, which are compared with previously reported values. 

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5. 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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