Seeded optically driven avalanche ionization in molecular and noble gases

Abstract

We report experimental and numerical results on the dual laser-pulse plasma excitation in molecular and noble gases at atmospheric pressure. Dilute plasma channels generated through filamentation of ultraintense femtosecond laser pulses in air, argon, and helium are densified through the application of multijoule nanosecond heater pulses. Plasma densification in molecular gases is always accompanied by the fragmentation of the plasma channels into discrete bubbles, while in atomic gases, under certain conditions, the densified channels remain smooth and continuous. The densification effect in atomic gases persists through considerably longer delays between the femtosecond and nanosecond pulses compared to that in molecular gases. Using rate equations we trace this difference in the temporal dynamics of densification to the different cooling mechanisms operative in atomic and molecular cases.

Memory effects in the long-wave infrared avalanche ionization of gases: a review of recent progress

“…Figure 1(a) depicts T kin (solid line) and T pl (dash line) as functions of time, and we see that the temperatures approach equilibrium after the first pulse at about T = 10 ps, in reasonable agreement with T ≈ 10.8 ps from Eq. (22). When the second pulse centered at t = 30 ps arrives, a second boost in the two temperatures is clearly seen.…”

The extreme nonlinear optics of gases and femtosecond optical filamentation

“…Some N 2 schemes are based on two pulses: the filament pulse weakly ionizes the air to $10 16 cm À3 , and then a copropagating "heater" pulse heats these electrons, which then pump the molecular transition, with preliminary experiments using a k ¼ 0.8 lm filament and a k ¼ 1.064 lm heater pulse. 23 Meanwhile, backward N 2 lasing has been demonstrated in a high pressure (6 atm) N 2 /Ar gas cell driven only by a k ¼ 3.9 lm laser pulse, 24 with the laser transition upper state populated via a different mechanism than in Ref. 22.…”

Picosecond laser filamentation in air

“…By contrast, plasma channels produced through fs laser filamentation in air are extended and continuous, but they are dilute and short-lived, with plasma densities of 10 16 -10 17 electrons per cm 3 [20] and plasma lifetime of less than one nanosecond [21]. Attempts to combine the advantages of the fs and ns excitation regimes, through the use of the so-called igniter-heater scheme, have been shown to produce extended and dense plasma channels in air [22,23]. However, plasma channels generated via the hybrid fs-ns excitation are usually fragmented into discrete plasma bubbles.…”