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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.

 

The multimodal carrier-resolved unidirectional pulse propagation equation is solved to study the wavelength-dependent (λ = 1, 2, 3 and 4 μm) spatio-temporal dynamics, particularly pulse self-compression during high-intensity laser pulse propagation in gas-filled capillaries. We find that pulse self-compression in gas-filled capillaries due to plasma is more efficient for short wavelengths in contrast to wavelength-dependent pulse self-compression in laser filamentation [1]. To explain our finding, a detailed analysis is performed by quantifying the contributions of higher-order modes and calculating the temporal delay among modes, which reveals that pulse self-compression at longer wavelengths does not occur due to larger group velocity mismatch between the fundamental and higher-order modes for longer wavelengths [2]. Our study has important implications for the various fields of high-intensity nonlinear optics in gas-filled capillaries such as supercontinuum generation and high-order harmonic generation [3]. [1] L. Bergé et al., Phys. Rev. A 88, 023816 (2013). [2] G. Nagar and B. Shim, submitted. [3] T. Popmitchev et al. Science 336, 1287 (2012). 

We fabricate waveguides in Corning® flexible glass using Femtosecond Laser Micromachining (FLM) and visualize the ultrafast plasma dynamics which lead to waveguide formation via time-resolved interferometry. Due to minimal thermal effects and highly-nonlinear optical processes [1], FLM is an ideal tool to fabricate waveguides in glass with high precision and without post processing. We optimize laser fabrication of waveguides by varying scanning speed and pulse energy and, in particular, achieve waveguides with circular cross-sections using slit beam shaping [2]. Further optimization requires investigation of the underlying dynamics of how structural changes in glass are made during and after laser-glass interactions. Thus, we visualize the creation and recombination of plasma in glass which leads to the formation of waveguides using time- resolved interferometry [3]. [1] Rafael R. Gattass and Eric Mazur, Nature Photonics 2, 219–225 (2008)); [2] M. Ams et al. Opt. Express 13, 5676-5681 (2005); [3] G. C. Nagar, D. Dempsey, and B. Shim, Communications Physics 4, 96 (2021). 

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). 

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.