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1. Wavelength scaling of electron collision times in filament-produced plasma in solids

   Nagar, G C ; Dempsey, D ; Shim, B ( June 2021 , Bulletin of the American Physical Society)

   null (Ed.)

   We report an anomalous regime of laser-matter interactions, which is created by the wavelength dependence of electron collision time during filamentation in solids. Experiments are performed using femtosecond-time-resolved interferometry by varying the filament driver wavelength from 1.2 to 2.3 μm and using a 0.8-μm probe. Information on the phase and absorption via interferometry enables simultaneous measurements of plasma densities and electron collision times during filamentation. Although it is expected that the plasma density decreases with increasing wavelength due to larger plasma-defocusing at longer wavelengths [1-4], our measured plasma densities are nearly constant for all the pump wavelengths. This observation is successfully explained by the measured wavelength-dependence of electron collision time: electron collision times in filament-produced plasma decrease with increasing wavelength, which creates an anomalous regime of plasma-defocusing where longer wavelengths experience smaller plasma defocusing. In addition, simulations with the measured electron collision times successfully reproduce the observed plasma density scaling with wavelength [5]. [1] L. Bergé et al., Phys. Rev. A 88, 023816 (2013). [2] Y. E. Geints et al., Appl. Opt. 56, 1397 (2017). [3] S. Tochitsky et al., Nat. Photonics 13, 41 (2019). [4] R. I. Grynko et al., Phys. Rev. A 98, 023844 (2018). [5] Nagar et al., submitted. 

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2. Wavelength scaling of electron collision time in plasma for strong field laser-matter interactions in solids

   https://doi.org/10.1038/s42005-021-00600-9  

   Nagar, Garima C. ; Dempsey, Dennis ; Shim, Bonggu ( May 2021 , Communications Physics)

   Although the dielectric constant of plasma depends on electron collision time as well as wavelength and plasma density, experimental studies on the electron collision time and its effects on laser-matter interactions are lacking. Here, we report an anomalous regime of laser-matter interactions generated by wavelength dependence (1.2–2.3 µm) of the electron collision time in plasma for laser filamentation in solids. Our experiments using time-resolved interferometry reveal that electron collision times are small (<1 femtosecond) and decrease as the driver wavelength increases, which creates a previously-unobserved regime of light defocusing in plasma: longer wavelengths have less plasma defocusing. This anomalous plasma defocusing is counterbalanced by light diffraction which is greater at longer wavelengths, resulting in almost constant plasma densities with wavelength. Our wavelength-scaled study suggests that both the plasma density and electron collision time should be systematically investigated for a better understanding of strong field laser-matter interactions in solids.

    

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3. Simultaneous measurements of plasma densities and electron collision times in plasma via time-resolved interferometry

   Nagar, Garima ; Dempsey, Dennis ; Shim, Bonggu ( November 2020 , APS Division of Plasma Physics Meeting 2020)

   We present time-resolved interferometry to simultaneously measure plasma densities and electron collision times for strong field laser-matter interactions. First, an intense femtosecond pump pulse generates plasma in a solid and second, a weak 800-nm femtosecond probe traverses the pump-induced plasma and is sent to an interferometer with controlled time delay between pump and probe. By analyzing the interferograms using Fourie methods, we can extract plasma densities and electron collision times in plasma simultaneously with micrometer spatial and femtosecond temporal resolutions. Using the technique, we study the plasma dynamics when a wavelength-varied (λ= 1.2-2.3 μm) pump pulse undergoes laser filamentation in solid materials. 

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4. Measurements of plasma densities in laser filamentation in solids at various wavelengths spanning from near and mid infrared

   https://doi.org/10.1364/CLEO_AT.2019.JTh2A.7  

   Nagar, Garima C. ; Dempsey, Dennis ; Shim, Bonggu ( July 2019 , Conference on Lasers and Electro-Optics (CLEO) 2019)

   We measure plasma densities in laser filamentation in fused silica using single-shot time-resolved interferometry when the filament driver wavelength is varied between 1.2 and 2.3 µm. The experimental results are compared with numerical simulations. 

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5. Plasma-assisted light bullets and wavelength scaling of laser filamentation in the long-wavelength infrared

   Grynko, Rostislav ; Nagar, Garima ; Shim, Bonggu ( October 2018 , APS Division of Plasma Physics Meeting 2018)

   We model laser filamentation in ZnSe in the mid-infrared (Mid-IR, wavelengths λ = 4 and 6 μm) and the long-wavelength infrared (LWIR, λ = 8 and 10 μm) using carrier-resolved unidirectional pulse propagation equations (UPPE). We predict an unprecedented propagation regime at λ = 8 μm that supports light bullets, which are spatio-temporally non-spreading electromagnetic pulses. Furthermore, in contrast to the previous report in air in the mid-IR, we predict that LWIR light bullets in solids critically rely on plasma-mediated dispersion, which dynamically evolves during multiphoton and tunneling ionization as peak plasma densities reach ρ 6.6 ×10^18 cm-3 . Finally, the plasma-assisted light bullets propagate with sub-cycle pulse durations and peak intensities I = 1.1 ×10^12 W /cm^2 , making them useful for high-harmonic generation and attosecond pulse generation. 

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