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1. Electro-optic interface for ultrasensitive intracavity electric field measurements at microwave and terahertz frequencies

   https://doi.org/10.1364/OPTICA.384160  

   Benea-Chelmus, Ileana-Cristina ; Salamin, Yannick ; Settembrini, Francesca Fabiana ; Fedoryshyn, Yuriy ; Heni, Wolfgang ; Elder, Delwin L. ; Dalton, Larry R. ; Leuthold, Juerg ; Faist, Jérôme ( May 2020 , Optica)

   Electro-optic quantum coherent interfaces map the amplitude and phase of a quantum signal directly to the phase or intensity of a probe beam. At terahertz frequencies, a fundamental challenge is not only to sense such weak signals (due to a weak coupling with a probe in the near-infrared) but also to resolve them in the time domain. Cavity confinement of both light fields can increase the interaction and achieve strong coupling. Using this approach, current realizations are limited to low microwave frequencies. Alternatively, in bulk crystals, electro-optic sampling was shown to reach quantum-level sensitivity of terahertz waves. Yet, the coupling strength was extremely weak. Here, we propose an on-chip architecture that concomitantly provides subcycle temporal resolution and an extreme sensitivity to sense terahertz intracavity fields below 20 V/m. We use guided femtosecond pulses in the near-infrared and a confinement of the terahertz wave to a volume of$V_{\mathrm{T}\mathrm{H}\mathrm{z}}\sim <\#comment/>{10}^{-<\#comment/>9}({\mathrm{\lambda <\#comment/>}}_{\mathrm{T}\mathrm{H}\mathrm{z}}/2{)}^3$in combination with ultraperformant organic molecules ($r_{33}=170\mathrm{p}\mathrm{m}/\mathrm{V}$) and accomplish a record-high single-photon electro-optic coupling rate ofmathvariant='normal'>Hz, 10,000 times higher than in recent reports of sensing vacuum field fluctuations in bulk media. Via homodyne detection implemented directly on chip, the interaction results into an intensity modulation of the femtosecond pulses. The single-photon cooperativity is$C_0=1.6\times <\#comment/>{10}^{-<\#comment/>8}$, and the multiphoton cooperativity is$C=0.002$at room temperature. We show$><\#comment/>70\mathrm{d}\mathrm{B}$dynamic range in intensity at 500 ms integration under irradiation with a weak coherent terahertz field. Similar devices could be employed in future measurements of quantum states in the terahertz at the standard quantum limit, or for entanglement of subsystems on subcycle temporal scales, such as terahertz and near-infrared quantum bits.

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2. 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 stimulatedmore »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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3. Spectrally simplified approach for leveraging legacy geostationary oceanic observations

   https://doi.org/10.1364/AO.465491  

   Houskeeper, Henry F. ; Hooker, Stanford B. ; Cavanaugh, Kyle C. ( September 2022 , Applied Optics)

   The use of multispectral geostationary satellites to study aquatic ecosystems improves the temporal frequency of observations and mitigates cloud obstruction, but no operational capability presently exists for the coastal and inland waters of the United States. The Advanced Baseline Imager (ABI) on the current iteration of the Geostationary Operational Environmental Satellites, termed the$R$Series (GOES-R), however, provides sub-hourly imagery and the opportunity to overcome this deficit and to leverage a large repository of existing GOES-R aquatic observations. The fulfillment of this opportunity is assessed herein using a spectrally simplified, two-channel aquatic algorithm consistent with ABI wave bands to estimate the diffuse attenuation coefficient for photosynthetically available radiation,$K_d(\mathrm{P}\mathrm{A}\mathrm{R})$. First, anin situABI dataset was synthesized using a globally representative dataset of above- and in-water radiometric data products. Values of$K_d(\mathrm{P}\mathrm{A}\mathrm{R})$were estimated by fitting the ratio of the shortest and longest visible wave bands from thein situABI dataset to coincident,in situ$K_d(\mathrm{P}\mathrm{A}\mathrm{R})$data products. The algorithm was evaluated based on an iterative cross-validation analysis in which 80% of the dataset was randomly partitioned for fitting and the remaining 20%more »was used for validation. The iteration producing the median coefficient of determination ($R^2$) value (0.88) resulted in a root mean square difference of$0.319{\mathrm{m}}^{-<\#comment/>1}$, or 8.5% of the range in the validation dataset. Second, coincident mid-day images of central and southern California from ABI and from the Moderate Resolution Imaging Spectroradiometer (MODIS) were compared using Google Earth Engine (GEE). GEE default ABI reflectance values were adjusted based on a near infrared signal. Matchups between the ABI and MODIS imagery indicated similar spatial variability ($R^2=0.60$) between ABI adjusted blue-to-red reflectance ratio values and MODIS default diffuse attenuation coefficient for spectral downward irradiance at 490 nm,$K_d(490)$, values. This work demonstrates that if an operational capability to provide ABI aquatic data products was realized, the spectral configuration of ABI would potentially support a sub-hourly, visible aquatic data product that is applicable to water-mass tracing and physical oceanography research.

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4. Variability of relationship between the volume scattering function at 180° and the backscattering coefficient for aquatic particles

   https://doi.org/10.1364/AO.383229  

   Hu, Lianbo ; Zhang, Xiaodong ; Xiong, Yuanheng ; Gray, Deric J. ; He, Ming-Xia ( February 2020 , Applied Optics)

   Properly interpreting lidar (light detection and ranging) signal for characterizing particle distribution relies on a key parameter,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, which relates the particulate volume scattering function (VSF) at 180° (${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$) that a lidar measures to the particulate backscattering coefficient ($b_{\mathit{\text{bp}}}$). However,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$has been seldom studied due to challenges in accurately measuring${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$and$b_{\mathit{\text{bp}}}$concurrently in the field. In this study,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, as well as its spectral dependence, was re-examined using the VSFs measuredin situat high angular resolution in a wide range of waters.${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, while not measured directly, was inferred using a physically sound, well-validated VSF-inversion method. The effects of particle shape and internal structure on the inversion were tested using three inversion kernels consisting of phase functions computed for particles that are assumed as homogenous sphere, homogenous asymmetric hexahedra, or coated sphere. The reconstructed VSFs using any of the three kernels agreed well with the measured VSFs with a mean percentage difference$<<\#comment/>5\mathrm{\%<\#comment/>}$at scattering angles$<<\#comment/>{170}^{\circ <\#comment/>}$. At angles immediately near or equal to 180°, the reconstructedstretchy='false'>)depends strongly on the inversion kernel.${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$derived with the sphere kernels was smaller than those derived with the hexahedra kernel but consistent with${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$estimated directly from high-spectral-resolution lidar andin situbackscattering sensor. The possible explanation was that the sphere kernels are able to capture the backscattering enhancement feature near 180° that has been observed for marine particles.${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$derived using the coated sphere kernel was generally lower than those derived with the homogenous sphere kernel. Our result suggests that${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$is sensitive to the shape and internal structure of particles and significant error could be induced if a fixed value of${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$is to be used to interpret lidar signal collected in different waters. On the other hand,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$showed little spectral dependence.

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5. 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$4f^{7}6s^2{}^8{S}_{7/2}^{\circ <\#comment/>}\to <\#comment/>4f^7{(}^8{S}^{\circ <\#comment/>})6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$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$\mathrm{A}(151)=-<\#comment/>228.84(2)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(151)=226.9(5)\mathrm{M}\mathrm{H}\mathrm{z}$and$\mathrm{A}(153)=-<\#comment/>101.87(6)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(153)=575.4(1.5)\mathrm{M}\mathrm{H}\mathrm{z}$, 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.