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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)

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   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. Single-shot ultrafast visualization of plasma dynamics and nonlinear index of refraction in flexible glass using frequency domain holography

   Dempsey, Dennis ; Nagar, Garima ; Renskers, Christopher ; Grynko, Rostislav ; Sutherland, James ; Shim, Bonggu ( October 2019 , APS Division of Plasma Physics Meeting 2019)

   We measure the nonlinear index of refraction (n2) and investigate plasma dynamics in flexible Corning® Willow® Glass using single-shot Frequency Domain Holography (FDH). Flexible glass has received a lot of attention recently due to various applications such as 3-D photonics and wearable devices. Femtosecond laser micromachining (FLM) is a viable tool to fabricate these devices because of minimal thermal effects and thus enables fabrication of small and clean 3-D structures. To control and understand the underlying dynamics of FLM, ultrafast visualization of plasma and optical Kerr effect is important. FDH is a robust femtosecond time-resolved technique in which chirped reference and probe pulses centered at 404 nm are used to measure and visualize the plasma and Kerr effect produced by an intense, ultrashort pump pulse centered at 808 nm. Using FDH, we study laser-matter interactions in Willow Glass and measure its n2 to be 3.41 +/-0.08 ×10-16 cm2/W and visualize the plasma dynamics. 

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4. Experimental and theoretical study of wavelength dependence of plasma dynamics in laser filamentation in solids

   Nagar, Garima ; Dempsey, Dennis ; Shim, Bonggu ( October 2019 , APS Division of Plasma Physics Meeting 2019)

   We experimentally and theoretically investigate plasma dynamics in laser filamentation in fused silica by varying the driver wavelength from 1.2 to 2.3 μm covering the near-zero to the anomalous group-velocity dispersion regimes. First, we perform femtosecond time-resolved interferometry to measure plasma densities in filaments, which unexpectedly reveals that plasma densities are not monotonically decreasing with increasing wavelength. This result is in sharp contrast to recent theoretical work in filamentation in air/gases as well as our own numerical simulations in fused silica in which the electron collision time is assumed to be constant for all the wavelengths. Therefore, to investigate further, we also perform time-resolved shadowgraphy which, combined with interferometry, enables us to determine the electron collision time in plasma. We find out that the electron collision time is not a constant for different wavelengths, which can change the plasma dynamics in filamentation significantly. 

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5. Single-shot spatio-temporal visualization of plasma and optical nonlinearity via single-shot frequency-domain holography

   Dempsey, D. ; Nagar, G. ; Agnes, J. ; Berger, R. ; Shim, B. ( November 2021 , 63rd Annual Meeting of the APS Division of Plasma Physics)

   We observe the ultrafast dynamics of solids and gases under intense femtosecond light in a single shot using Frequency Domain Holography (FDH) [1-3]. FDH is a time-resolved visualization technique that utilizes a pump pulse and two chirped laser pulses (reference and probe) for ultrafast phase measurements. Single-shot visualization of laser-matter interactions will allow for increased understanding of nonlinear optical phenomena such as Raman-induced extreme spectral broadening [4], filamentation [5], and plasma generation and recombination [3]. [1] S. P. Le Blanc et al., Opt. Lett. 56, 764-766 (2000). [2] K. Y. Kim et al., APL, 88 4124-4126 (2002). [3] D. Dempsey et al. Opt. Lett. 45, 1252-1255 (2020) [4] J. Beetar et al., Science Advances 6, eabb5375 (2020) [5] A. Couairon et al., Phys. Rep. 441, 47 (2007). 

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