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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. Hybrid electro-optic modulator combining silicon photonic slot waveguides with high-k radio-frequency slotlines

   https://doi.org/10.1364/OPTICA.411161  

   Ummethala, Sandeep ; Kemal, Juned N. ; Alam, Ahmed S. ; Lauermann, Matthias ; Kuzmin, Artem ; Kutuvantavida, Yasar ; Nandam, Sree H. ; Hahn, Lothar ; Elder, Delwin L. ; Dalton, Larry R. ; et al ( April 2021 , Optica)

   Electro-optic (EO) modulators rely on the interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced using dielectric slot–waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherentRClimitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot–waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies on$\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm long Mach–Zehnder modulator that offers a 3 dB bandwidth of 76 GHz and a 6 dB bandwidth of 110 GHz along with a small$\mathrm{\pi <\#comment/>}$voltage of 1.3 V ($U_{\mathrm{\pi <\#comment/>}}L=1.3\mathrm{V}\mathrm{m}\mathrm{m}$). We further demonstrate the viability of the device in a data-transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz.

    

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3. In-vacuum measurements of optical scatter versus annealing temperature for amorphous Ta 2 O 5 and TiO 2 :Ta 2 O 5 thin films

   https://doi.org/10.1364/JOSAA.415665  

   Capote, Elenna M. ; Gleckl, Amy ; Guerrero, Jazlyn ; Rezac, Michael ; Wright, Robert ; Smith, Joshua R. ( March 2021 , Journal of the Optical Society of America A)

   Optical coatings formed from amorphous oxide thin films have many applications in precision measurements. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo use coatings of$\mathrm{S}\mathrm{i}{\mathrm{O}}_2$(silica) and$\mathrm{T}\mathrm{i}{\mathrm{O}}_2:\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$(titania-doped tantala) and post-deposition annealing to 500°C to achieve low thermal noise and low optical absorption. Optical scattering by these coatings is a key limit to the sensitivity of the detectors. This paper describes optical scattering measurements for single-layer, ion-beam-sputtered thin films on fused silica substrates: two samples of$\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$and two of$\mathrm{T}\mathrm{i}{\mathrm{O}}_2:\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$. Using an imaging scatterometer at a fixed scattering angle of 12.8°, in-situ changes in the optical scatter of each sample were assessed during post-deposition annealing to 500°C in vacuum. The scatter of three of the four coated optics was observed to decrease during the annealing process, by 25–30% for tantala and up to 74% for titania-doped tantala, while the scatter from the fourth sample held constant. Angle-resolved scatter measurements performed before and after vacuum annealing suggest some improvement in three of the four samples. These results demonstrate that post-deposition, high-temperature annealing of single-layer tantala and titania-doped tantala thin films in vacuum does not lead to an increase in scatter, and may actually improve their scatter.

    

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4. Growth and characterization of Sc 2 O 3 doped Ta 2 O 5 thin films

   https://doi.org/10.1364/AO.59.00A106  

   Fazio, Mariana ; Yang, Le ; Markosyan, Ashot ; Bassiri, Riccardo ; Fejer, Martin M. ; Menoni, Carmen S. ( December 2019 , Applied Optics)

   We present the optical and structural characterization of films of${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$,${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$, and${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$doped${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$with a cation ratio around 0.1 grown by reactive sputtering. The addition of${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$as a dopant induces the formation of tantalum suboxide due to the “oxygen getter” property of scandium. The presence of tantalum suboxide greatly affects the optical properties of the coating, resulting in higher absorption loss at$\mathrm{\lambda <\#comment/>}=1064\mathrm{n}\mathrm{m}$. The refractive index and optical band gap of the mixed film do not correspond to those of a mixture of${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$and${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$, given the profound structural modifications induced by the dopant.

    

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5. Emissions in short-gated ns/ps/fs-LIBS for fuel-to-air ratio measurements in methane-air flames

   https://doi.org/10.1364/AO.418453  

   Gragston, Mark ; Hsu, Paul ; Jiang, Naibo ; Roy, Sukesh ; Zhang, Zhili ( May 2021 , Applied Optics)

   A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550–800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the${\mathrm{N}}_{\mathrm{I}\mathrm{I}}(565\mathrm{n}\mathrm{m})$and${\mathrm{N}}_{\mathrm{I}\mathrm{I}}(594\mathrm{n}\mathrm{m})$emissions is presented. In studying line emission ratios for FAR calculation, it is found that${\mathrm{H}}_{\mathrm{\alpha <\#comment/>}}(656\mathrm{n}\mathrm{m})/{\mathrm{N}}_{\mathrm{I}\mathrm{I}}(568\mathrm{n}\mathrm{m})$is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while${\mathrm{H}}_{\mathrm{\alpha <\#comment/>}}(656\mathrm{n}\mathrm{m})/{\mathrm{O}}_{\mathrm{I}}(777\mathrm{n}\mathrm{m})$is optimal for the ps-LIBS and fs-LIBS cases. Due to low continuum background and short time delay for spectra collection, fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS.

    

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