Picosecond ionization dynamics in femtosecond filaments at high pressures

We investigate the plasma dynamics inside a femtosecond-pulse-induced filament generated in an argon gas for a wide range of pressures up to 60 bar. At higher pressures, we observe ionization immediately following a pulse, with up to a threefold increase in the electron density within 30 ps after the filamentary propagation of a femtosecond pulse. Our study suggests that this picosecond evolution can be attributed to collisional ionization including Penning and associative ionizations and electron-impact ionization of excited atoms generated during the pulse. The dominance of excited atoms over ionized atoms at the end of the pulse also indicates an intrapulse inhibition of avalanche ionization. This delayed ionization dynamics provides evidence for diagnosing atomic and molecular excitation and ionization in intense laser interaction with high-pressure gases.

Figure 1

Experimental setup. BS, beamsplitter; RM, rooftop mirror; MO, microscope objective. Inset (i): Shadowgraph recording plasma-induced absorption imprinted on a probe. Inset (ii): Interferogram recording plasma-induced phase alterations imprinted on a probe.

Figure 2

(a) Single-shot shadowgraphs for two delay times. (b) Lineout measurements for several delay times (from top to bottom: 0- to 51-ps delay). Each lineout is averaged over 10 shots. Here the width of the shadow broadens due to multishot averaging. (c) Transmission extracted from the shadowgraph versus delay at 60 bar. Each data point represents a 10-shot average.

Figure 3

(a) Single-shot phase images extracted from interferograms at $p=40$ bar—(i) $\mathrm{\Delta }t=0$ and (ii) $\mathrm{\Delta }t=46$ ps—and at $p=60$ bar—(iii) $\mathrm{\Delta }t=1$ ps and (iv) $\mathrm{\Delta }t=43$ ps. (b) Measured phase shifts vs delay at several pressures. Each data point is the average of 25 single-shot measurements. Error bars represent the standard deviation. The standard deviation at 60 bar is large because the position is near the end of the filament.

Figure 4

(a) Time evolution of the normalized density of electrons and excited atoms. (b) Time evolution of the normalized electron density $N_e$ with (solid blue line) and without (dashed red line) the energy gain from Penning ionization and when recombination and Penning ionization are not included (dash-dotted green line). (c) Time evolution of $T_e$ for the same condition as in (b).