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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 of$g_{\mathrm{e}\mathrm{o}}=2\mathrm{\pi <\#comment/>}\times <\#comment/>0.043\mathrm{G}\mathrm{H}\mathrm{z}$, 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. Simultaneous turbulence mitigation and channel demultiplexing for two 100  Gbit/s orbital-angular-momentum multiplexed beams by adaptive wavefront shaping and diffusing

   https://doi.org/10.1364/OL.383714  

   Zhang, Runzhou ; Song, Hao ; Zhao, Zhe ; Song, Haoqian ; Du, Jing ; Liu, Cong ; Pang, Kai ; Li, Long ; Zhou, Huibin ; Willner, Ari N. ; et al ( January 2020 , Optics Letters)

   We experimentally demonstrate simultaneous turbulence mitigation and channel demultiplexing in a 200 Gbit/s orbital-angular-momentum (OAM) multiplexed link by adaptive wavefront shaping and diffusing (WSD) the light beams. Different realizations of two emulated turbulence strengths (the Fried parameter$r_0=0.4,1.0\mathrm{m}\mathrm{m}$) are mitigated. The experimental results show the following. (1) Crosstalk between OAM$l=+1$and$l=-<\#comment/>1$modes can be reduced by$><\#comment/>10.0$and$><\#comment/>5.8\mathrm{d}\mathrm{B}$, respectively, under the weaker turbulence ($r_0=1.0\mathrm{m}\mathrm{m}$); crosstalk is further improved by$><\#comment/>17.7$and$><\#comment/>19.4\mathrm{d}\mathrm{B}$, respectively, under most realizations in the stronger turbulence ($r_0=0.4\mathrm{m}\mathrm{m}$). (2) The optical signal-to-noise ratio penalties for the bit error rate performance are measured to be$\sim <\#comment/>0.7$and$\sim <\#comment/>1.6\mathrm{d}\mathrm{B}$under weaker turbulence, while measured to be$\sim <\#comment/>3.2$and$\sim <\#comment/>1.8\mathrm{d}\mathrm{B}$under stronger turbulence for OAM$l=+1$and$l=-<\#comment/>1$mode, respectively.

    

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3. 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.

    

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4. Design of infrared microspectrometer based on phase-modulated axilenses

   https://doi.org/10.1364/AO.390610  

   Chen, Yuyao ; Britton, Wesley A. ; Dal Negro, Luca ( June 2020 , Applied Optics)

   We design and characterize a novel axilens-based diffractive optics platform that flexibly combines efficient point focusing and grating selectivity and is compatible with scalable top-down fabrication based on a four-level phase mask configuration. This is achieved using phase-modulated compact axilens devices that simultaneously focus incident radiation of selected wavelengths at predefined locations with larger focal depths compared with traditional Fresnel lenses. In addition, the proposed devices are polarization-insensitive and maintain a large focusing efficiency over a broad spectral band. Specifically, here we discuss and characterize modulated axilens configurations designed for long-wavelength infrared (LWIR) in the 6 µm–12 µm wavelength range and in the 4 µm–6 µm midwavelength infrared (MWIR) range. These devices are ideally suited for monolithic integration atop the substrate layers of infrared focal plane arrays and for use as compact microspectrometers. We systematically study their focusing efficiency, spectral response, and cross-talk ratio; further, we demonstrate linear control of multiwavelength focusing on a single plane. Our design method leverages Rayleigh–Sommerfeld diffraction theory and is validated numerically using the finite element method. Finally, we demonstrate the application of spatially modulated axilenses to the realization of a compact, single-lens spectrometer. By optimizing our devices, we achieve a minimum distinguishable wavelength interval of$\mathrm{\Delta <\#comment/>}\mathrm{\lambda <\#comment/>}=240\mathrm{n}\mathrm{m}$at${\mathrm{\lambda <\#comment/>}}_c=8\text{µ<\#comment/>}\mathrm{m}$and$\mathrm{\Delta <\#comment/>}\mathrm{\lambda <\#comment/>}=165\mathrm{n}\mathrm{m}$at${\mathrm{\lambda <\#comment/>}}_c=5\text{µ<\#comment/>}\mathrm{m}$. The proposed devices add fundamental spectroscopic capabilities to compact imaging devices for a number of applications ranging from spectral sorting to LWIR and MWIR phase contrast imaging and detection.

    

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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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