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1. Non-resonant picosecond three-wave mixing in the gas phase

   https://doi.org/10.1364/OL.542597  

   LaCombe, Grayson; Wang, Jianan; Rouxel, Jérémy_R; Simeni_Simeni, Marien (November 2024, Optics Letters)

   We report on the experimental observation of non-resonant, second-order optical sum-frequency generation (SFG) in five different atomic and molecular gases. The measured signal is attributed to a SFG process by characterizing its intensity scaling and its polarization behavior. We show that the electric quadrupole mechanism cannot explain the observed trends and suggest a mechanism based on symmetry breaking along the incident beam path arising from laser-induced species ground state number density gradients. Our results demonstrate that the SFG is about four orders of magnitude stronger than the third-harmonic generation (THG) and independent from any externally applied electric fields. These features make this method suitable for gas number density measurements at the picosecond time scale in reactive flows and plasmas. 

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2. Single-pulse, reference-free, spatiospectral measurement of ultrashort pulse-beams

   https://doi.org/10.1364/OPTICA.462586  

   Goldberger, David; Barolak, Jonathan; Bevis, Charles_S; Ivanic, Bojana; Schmidt, David; Lei, Yuhao; Kazansky, Peter; Mancini, Giulia_F; Durfee, Charles_G; Adams, Daniel_E (August 2022, Optica)

   High-intensity pulse-beams are ubiquitous in scientific investigations and industrial applications ranging from the generation of secondary radiation sources (e.g., high harmonic generation, electrons) to material processing (e.g., micromachining, laser-eye surgery). Crucially, pulse-beams can only be controlled to the degree to which they are characterized, necessitating sophisticated measurement techniques. We present a reference-free, full-field, single-shot spatiospectral measurement technique called broadband single-shot ptychography (BBSSP). BBSSP provides the complex wavefront for each spectral and polarization component in an ultrafast pulse-beam and should be applicable across the electromagnetic spectrum. BBSSP will dramatically improve the application and mitigation of spatiospectral pulse-beam structure. 

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3. Spatial Beam Self-Cleaning in Second-Harmonic Generation

   https://doi.org/10.1038/s41598-020-64080-7  

   Krupa, K.; Fona, R.; Tonello, A.; Labruyère, A.; Shalaby, B. M.; Wabnitz, S.; Baronio, F.; Aceves, A. B.; Millot, G.; Couderc, V. (December 2020, Scientific Reports)

   null (Ed.)

   Abstract We experimentally demonstrate the spatial self-cleaning of a highly multimode optical beam, in the process of second-harmonic generation in a quadratic nonlinear potassium titanyl phosphate crystal. As the beam energy grows larger, the output beam from the crystal evolves from a highly speckled intensity pattern into a single, bell-shaped spot, sitting on a low energy background. We demonstrate that quadratic beam cleanup is accompanied by significant self-focusing of the fundamental beam, for both positive and negative signs of the linear phase mismatch close to the phase-matching condition. 

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4. Ultrafast 1  MHz vacuum-ultraviolet source via highly cascaded harmonic generation in negative-curvature hollow-core fibers

   https://doi.org/10.1364/OPTICA.395688  

   Couch, David_E; Hickstein, Daniel_D; Winters, David_G; Backus, Sterling_J; Kirchner, Matthew_S; Domingue, Scott_R; Ramirez, Jessica_J; Durfee, Charles_G; Murnane, Margaret_M; Kapteyn, Henry_C (July 2020, Optica)

   Vacuum-ultraviolet (VUV) light is critical for the study of molecules and materials, but the generation of femtosecond pulses in the VUV region at high repetition rates has proven difficult. Here we demonstrate the efficient generation of VUV light at megahertz repetition rates using highly cascaded four-wave mixing processes in a negative-curvature hollow-core fiber. Both even- and odd-order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with high energy resolution of $\sim <\#comment/>40\mathrm{m}\mathrm{e}\mathrm{V}$. In contrast to direct high harmonic generation, this highly cascaded harmonic generation process requires lower peak intensity and therefore can operate at higher repetition rates, driven by a robust $\sim <\#comment/>10\mathrm{W}$fiber-laser system in a compact setup. Additionally, we present numerical simulations that explore the fundamental capabilities and spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source can enhance the capabilities of spectroscopies of molecular and quantum materials, such as photoionization mass spectrometry and time-, angle-, and spin-resolved photoemission. 

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5. Stable 2 W continuous-wave 261.5 nm laser for cooling and trapping aluminum monochloride

   https://doi.org/10.1364/OE.441741  

   Shaw, J_C; Hannig, S.; McCarron, D_J (October 2021, Optics Express)

   We present a high-power tunable deep-ultraviolet (DUV) laser that uses two consecutive cavity enhanced doubling stages with LBO and CLBO crystals to produce the fourth harmonic of an amplified homebuilt external cavity diode laser. The system generates up to 2.75 W of 261.5 nm laser light with a ∼2 W stable steady-state output power and performs second harmonic generation in a largely unexplored high intensity regime in CLBO for continuous wave DUV light. We use this laser to perform fluorescence spectroscopy on theA¹Π ← X¹Σ⁺transition in a cold, slow beam of AlCl molecules and probe the A¹Π|v′ = 0,J′ = 1〉 state hyperfine structure for future laser cooling and trapping experiments. This work demonstrates that the production of tunable, watt-level DUV lasers is becoming routine for a variety of wavelength-specific applications in atomic, molecular and optical physics. 

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