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1. Chip-based self-referencing using integrated lithium niobate waveguides

   https://doi.org/10.1364/OPTICA.392363  

   Okawachi, Yoshitomo ; Yu, Mengjie ; Desiatov, Boris ; Kim, Bok Young ; Hansson, Tobias ; Lončar, Marko ; Gaeta, Alexander L. ( June 2020 , Optica)

   The measurement and stabilization of the carrier–envelope offset frequency$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$via self-referencing is paramount for optical frequency comb generation, which has revolutionized precision frequency metrology, spectroscopy, and optical clocks. Over the past decade, the development of chip-scale platforms has enabled compact integrated waveguides for supercontinuum generation. However, there is a critical need for an on-chip self-referencing system that is adaptive to different pump wavelengths, requires low pulse energy, and does not require complicated processing. Here, we demonstrate efficient$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$stabilization of a modelocked laser with only 107 pJ of pulse energy via self-referencing in an integrated lithium niobate waveguide. We realize an$f\text{-}2f$interferometer through second-harmonic generation and subsequent supercontinuum generation in a single dispersion-engineered waveguide with a stabilization performance equivalent to a conventional off-chip module. The$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$beatnote is measured over a pump wavelength range of 70 nm. We theoretically investigate our system using a single nonlinear envelope equation with contributions from both second- and third-order nonlinearities. Our modeling reveals rich ultrabroadband nonlinear dynamics and confirms that the initial second-harmonic generation followed by supercontinuum generation with the remaining pump is responsible for the generation of a strong$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$signal as compared to a traditional$f\text{-}2f$interferometer. Our technology provides a highly simplified system that is robust, low in cost, and adaptable for precision metrology for use outside a research laboratory.

    

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2. 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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3. Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate

   https://doi.org/10.1364/OL.439232  

   Higginson, A. ; Wang, Y. ; Chi, H. ; Goffin, A. ; Larkin, I. ; Milchberg, H. M. ; Rocca, J. J. ( October 2021 , Optics Letters)

   We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to$f=1\mathrm{k}\mathrm{H}\mathrm{z}$. Interferograms of the wake generated show that while pulses in a train of repetition rate$f=0.1\mathrm{k}\mathrm{H}\mathrm{z}$encounter a nearly unperturbed environment, at$f=1\mathrm{k}\mathrm{H}\mathrm{z}$, a channel with an axial air density hole of$\sim <\#comment/>20\mathrm{\%<\#comment/>}$is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at$f=1\mathrm{k}\mathrm{H}\mathrm{z}$show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

    

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4. Effects of disorder on two-photon absorption in amorphous semiconductors

   https://doi.org/10.1364/OL.391197  

   Dutta, Nikita S. ; Almeida, Juliana M. P. ; Mendonça, Cleber R. ; Arnold, Craig B. ( June 2020 , Optics Letters)

   Structural disorder inherent to amorphous materials affords them unique, tailorable properties desirable for diverse applications, but our ability to exploit these phenomena is limited by a lack of understanding of complex structure-property relationships. Here we focus on nonlinear optical absorption and derive a relationship between disorder and the two-photon absorption (2PA) coefficient. We employ an open-aperture Z-scan to measure the 2PA spectra of arsenic (III) sulfide (${\mathrm{A}\mathrm{s}}_2{\mathrm{S}}_3$) chalcogenide glass films processed with two solvents that impart different levels of structural disorder. We find that the effect of solvent choice on 2PA depends on the energy of the exciting photons and explain this as a consequence of bonding disorder and electron state localization. Our results demonstrate how optical nonlinearities in${\mathrm{A}\mathrm{s}}_2{\mathrm{S}}_3$can be enhanced through informed processing and present a fundamental relationship between disorder and 2PA for a generalized amorphous solid.

    

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5. Single-shot ultrafast visualization and measurement of laser–matter interactions in flexible glass using frequency domain holography

   https://doi.org/10.1364/OL.382645  

   Dempsey, Dennis ; Nagar, Garima C. ; Renskers, Christopher K. ; Grynko, Rostislav I. ; Sutherland, James S. ; Shim, Bonggu ( February 2020 , Optics Letters)

   We perform single-shot frequency domain holography to measure the ultrafast spatio-temporal phase change induced by the optical Kerr effect and plasma in flexible Corning Willow Glass during femtosecond laser–matter interactions. We measure the nonlinear index of refraction ($n_2$) to be$(3.6\pm <\#comment/>0.1)\times <\#comment/>{10}^{-<\#comment/>16}{\mathrm{c}\mathrm{m}}^2/\mathrm{W}$and visualize the plasma formation and recombination on femtosecond time scales in a single shot. To compare with the experiment, we carry out numerical simulations by solving the nonlinear envelope equation.

    

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