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1. Loss-free shaping of few-cycle terawatt laser pulses

   https://doi.org/10.1364/OL.516590  

   Railing, L_M; Le, M_S; Lazzarini, C_M; Milchberg, H_M (March 2024, Optics Letters)

   We demonstrate loss-free generation of 3 mJ, 1 kHz, few-cycle (5 fs at 750 nm central wavelength) double pulses with a pulse peak separation from 10 to 100 fs, using a helium-filled hollow core fiber (HCF) and chirped mirror compressor. Crucial to our scheme are simulation-based modifications to the spectral phase and amplitude of the oscillator seed pulse to eliminate the deleterious effects of self-focusing and nonlinear phase pickup in the chirped pulse amplifier. The shortest pulse separations are enabled by tunable nonlinear pulse splitting in the HCF compressor. 

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2. Ultra-low-power nonvolatile integrated photonic switches and modulators based on nanogap-enhanced phase-change waveguides

   https://doi.org/10.1364/OE.411254  

   Zhang, Jieying; Zheng, Jiajiu; Xu, Peipeng; Wang, Yanqun; Majumdar, Arka (November 2020, Optics Express)

   We propose a nanogap-enhanced phase-change waveguide with silicon PIN heaters. Thanks to the enhanced light-matter interaction in the nanogap, the proposed structure exhibits strong attenuation (Δα = ∼35 dB/µm) and optical phase (Δn_eff = ∼1.2) modulation atλ = 1550 nm when achieving complete phase transitions. We further investigate two active optical devices based on the proposed waveguide, including an electro-absorption modulator and a 1 × 2 directional-coupler optical switch. Finite-difference time-domain simulation of the proposed modulator shows a high extinction ratio of ∼17 dB at 1550 nm with an active segment of volume only ∼0.004λ³. By exploiting a directional coupler design, we present a 1 × 2 optical switch with an insertion loss of < 4 dB and a compact coupling length of ∼ 15 µm while maintaining small crosstalk less than −7.2 dB over an optical bandwidth of 50 nm. Thermal analysis shows that a 10 V pulse of 30 ns (1×1 modulator) and 55 ns (1×2 switch) in duration is required to raise the GST temperature of the phase-change waveguide above the melting temperature to induce the amorphization; however, the complete crystallization occurs by applying a 5 V pulse of 180 ns (1×1 modulator) and a 6 V pulse of 200 ns (1×2 switch), respectively. 

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3. Simultaneous Observations of EIP, TGF, Elve, and Optical Lightning

   https://doi.org/10.1029/2020JD033921  

   Østgaard, N.; Cummer, S. A.; Mezentsev, A.; Luque, A.; Dwyer, J.; Neubert, T.; Reglero, V.; Marisaldi, M.; Kochkin, P.; Sarria, D.; et al (May 2021, Journal of Geophysical Research: Atmospheres)

   Abstract On February 8, 2019, the Atmosphere‐Space Interaction Monitor observed a terrestrial gamma‐ray flash (TGF) and an Elve from a positive intracloud (+IC) lightning during the initial breakdown stage of a lightning flash north east of Puerto Rico. A second Elve produced by the return stroke (RS) of a negative cloud‐to‐ground (−CG) lightning was observed 456 ms later about 300 km south of the first one. Radio measurements show that a short (30 μs) and large (280 kA km) energetic in‐cloud pulse (EIP) produced the electromagnetic (EM) wave for the first Elve while the RS of the −CG was the EM source for the second Elve. Assuming that the EIP and the RS were the sources of the 777 nm emissions, both the delay relative to the ultra‐violet pulse and the shape and duration of the 777 nm emissions can be explained by scattering and absorption inside the clouds. The TGF produced by the +IC lightning had the same duration as the EIP (∼30 μs). Due to the ±80 μs timing uncertainty of the TGF, we can only state that TGF was produced just before or most likely simultaneously with the EIP. The large 777 nm pulse indicates that a large part of the EIP was produced by a current flowing in a hot channel, but it is likely that the TGF current also contributed significantly to the EIP. 

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4. Single-shot observation of nonlinear pulse splitting in a Kerr medium

   https://doi.org/10.1364/OL.503170  

   Chang, Yen-Yu; Gulley, Jeremy_R; Li, Zhengyan; Welch, James; Zgadzaj, Rafal; Bernstein, Aaron; Downer, M_C (December 2023, Optics Letters)

   We report single-shot, time-resolved observation of self-steepening and temporal splitting of near-infrared, 50 fs, micro-joule pulses propagating nonlinearly in flint (SF11) glass. A coherent, smooth-profiled, 60-nm-bandwidth probe pulse that propagated obliquely to the main pulse through the Kerr medium recorded a time sequence of longitudinal projections of the main pulse’s induced refractive index profile in the form of a phase-shift “streak,” in which frequency–domain interferometry recovered with ∼10 fs temporal resolution. A three-dimensional simulation based on a unidirectional pulse propagation equation reproduced observed pulse profiles. 

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5. Ion density-enhanced electrostatic precipitation using high voltage nanosecond pulses

   https://doi.org/10.1039/d3va00148b  

   Zhang, Boxin; Aravind, Indu; Yang, Sisi; Weng, Sizhe; Zhao, Bofan; Johnson, Grace; Brown, Lucas; Olfert, Jason; Jung, Heejung; Cronin, Stephen B. (October 2023, Environmental Science: Advances)

   This study evaluates the beneficial effects of discharging nanosecond pulse transient plasma (NPTP) in a coaxial electrostatic precipitator for capturing nanoscale soot particles (∼50 nm) produced by an ethylene flame. 

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