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1. 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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2. Temperature dependence of collisional broadening and shift for the Kr 4 p 6 S 01→5 p [3/2] 2 electronic transition

   https://doi.org/10.1364/AO.380102  

   Sahoo, Abinash ; Zelenak, Dominic ; Narayanaswamy, Venkateswaran ( February 2020 , Applied Optics)

   Temperature scaling of collisional broadening parameters for krypton (absorber)$4p^6{S}_0^1\to <\#comment/>5p[3/2{]}_2$electronic transition centered at 107.3 nm in the presence of major combustion species (perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the fluorescence due to the two-photon excitation scan of krypton. Krypton was added in small amounts to major combustion species such as${\mathrm{C}\mathrm{H}}_4$,${\mathrm{C}\mathrm{O}}_2$,${\mathrm{N}}_2$, and air, which then heated to elevated temperatures when flowed through a set of heated coils. In a separate experimental campaign, laminar premixed flat flame product mixtures of methane combustion were employed to extend the investigations to higher temperature ranges relevant to combustion. Collisional full width half maximum (FWHM) ($w_C$) and shift (${\mathrm{\delta <\#comment/>}}_C$) were computed from the absorption spectrum by synthetically fitting Voigt profiles to the excitation scans, and their corresponding temperature scaling was determined by fitting power-law temperature dependencies to the$w_C$and${\mathrm{\delta <\#comment/>}}_C$data for each perturber species. The temperature exponents of$w_C$and${\mathrm{\delta <\#comment/>}}_C$for all considered combustion species (perturbers) were$-<\#comment/>0.73$and$-<\#comment/>0.6$, respectively. Whereas the temperature exponents of$w_C$are closer to the value ($-<\#comment/>0.7$) predicted by the dispersive interaction collision theory, the corresponding exponents of${\mathrm{\delta <\#comment/>}}_C$are in between the dispersive interaction theory and the kinetic theory of hard-sphere collisions. Comparison with existing literature on broadening parameters of NO, OH, and CO laser-induced fluorescence spectra reveal interesting contributions from non-dispersive interactions on the temperature exponent.

    

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3. 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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4. Widely separated optical Kerr parametric oscillation in AlN microrings

   https://doi.org/10.1364/OL.384317  

   Tang, Yulong ; Gong, Zheng ; Liu, Xianwen ; Tang, Hong X. ( February 2020 , Optics Letters)

   Here, we report${\mathrm{\chi <\#comment/>}}^{(3)}$-based optical parametric oscillation (OPO) with widely separated signal–idler frequencies from crystalline aluminum nitride microrings pumped at$2\text{µ<\#comment/>}\mathrm{m}$. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by$><\#comment/>9\mathrm{T}\mathrm{H}\mathrm{z}$, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip’s temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources.

    

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5. 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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