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We visualize the ultrafast dynamics caused by intense femtosecond laser pulses in both thin flexible glass as well as gaseous atoms and molecules using single-shot Frequency Domain Holography (FDH) [1-3]. FDH is a robust, single-shot, time-resolved visualization technique that employs chirped pulses. Femtosecond laser micromachining of glass materials relies critically on the Kerr effect and ionization, thus direct observation of their dynamics can help produce optical devices such as waveguides. For gases, single-shot visualization of laser-matter interactions will allow for a better understanding of nonlinear optical phenomena such as filamentation [4] and Raman-induced extreme spectral broadening [5]. Using FDH, we have previously observed the ionization dynamics of thin, flexible glass and measured its nonlinear index [3], and are currently investigating the ultrafast dynamics of gases under intense laser fields. [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] S. Huang et. al., OFC 1-3 (2014). [4] A. Couairon et al., Phys. Rep. 441, 47 (2007). [5] D. Dempsey et al. Opt. Lett. 45, 1252-1255 (2020). 

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.

 

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. 

We present an experimental and theoretical study of wavelength-dependent electron collision times in plasma and its striking effects on laser-matter interactions during laser filamentation in a solid. 

Abstract We present experimental and numerical investigations of high-energy mid-infrared filamentation with multi-octave-spanning supercontinuum generation (SCG), pumped by a 2.4 μm, 250 fs Cr:ZnSe chirped-pulse laser amplifier. The SCG is demonstrated in both anomalous and normal dispersion regimes with YAG and polycrystalline ZnSe, respectively. The formation of stable and robust single filaments along with the visible-to-mid-infrared SCG is obtained with a pump energy of up to 100 μJ in a 6-mm-long YAG medium. To the best of the authors’ knowledge, this is the highest-energy multi-octave-spanning SCG from a laser filament in a solid. On the other hand, the SCG and even-harmonic generation based on random quasi-phase matching (RQPM) are simultaneously observed from the single filaments in a 6-mm-long polycrystalline ZnSe medium with a pump energy of up to 15 μJ. The numerical simulations based on unidirectional pulse propagation equation and RQPM show excellent agreement with the measured multi-octave-spanning SCG and even-harmonic generation. They also reveal the temporal structure of mid-infrared filaments, such as soliton-like self-compression in YAG and pulse broadening in ZnSe. 

We use frequency domain holography (FDH) to spatio-temporally visualize the laser-matter interaction caused by the optical Kerr effect and plasma in flexible Corning® Willow® Glass in a single-shot. 

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. 

We perform single-shot frequency domain holography to measure the ultrafast spatio-temporal phase change induced by the optical Kerr effect and plasma in flexible Corning Willow Glass during femtosecond laser–matter interactions. We measure the nonlinear index of refraction ($n_2$) to be$(3.6\pm <\#comment/>0.1)\times <\#comment/>{10}^{-<\#comment/>16}{\mathrm{c}\mathrm{m}}^2/\mathrm{W}$and visualize the plasma formation and recombination on femtosecond time scales in a single shot. To compare with the experiment, we carry out numerical simulations by solving the nonlinear envelope equation.

 

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.