Short- and long-term gain dynamics in N2+ air lasing

Abstract

Air lasing in the nitrogen molecular ion is not well understood because the complex physics responsible for gain is interwoven with pulse propagation in an extreme environment. Here we use a short gas jet to limit the interaction length, thereby removing the propagation effects. We report on several mechanisms that contribute to the decay of gain in different conditions, and experimentally isolate two decay timescales: the decay of long-term gain due to collisional state mixing, and short-term gain that cannot be explained by population inversion. To test the former, we control the inelastic electron scattering rate by varying the gas concentration while keeping the propagation length fixed, and predict the change of the decay using a model of collisional state mixing. We show that the same mechanism causes the decay of rotational wave packets in the states of the ion. Finally, we simulate the complex modulations of gain due to rotational wave packets and the propagation of the probe pulse through the evolving rotationally excited and inverted medium.

Control of N2+ air lasing

“…For clarity, we refer to the additional pulse as "control." This experiment minimizes the interaction length using a narrow gas jet in vacuum to isolate gain from the effects of propagation [18,19]. By vary-ing both control and probe delays, we also observe modified gain and emission.…”

Rotational Quantum Beat Lasing without Inversion

“…For increasing pump intensity, the oscillating gain starts to occur for virtually all pump-probe time delays as observed in the nitrogen gas jet experiments in Ref. [39].…”

“…Taking into account the effects of gain narrowing and inelastic collisions that destroy the rotational coherences in the ionic states, our results reproduce the typical observations made in pumpprobe "air lasing" experiments (see e.g., Refs. [14,21,38,39]): a rapid increase of the gain as a function of the pump-probe delay during the first few hundreds of femtoseconds, followed by a slow decay of the gain for delays up to tens of picoseconds that is periodically modulated by the molecular alignment dynamics.…”

“…Most experiments studying the amplification effect around the 391 nm line in N + 2 are done in a pump-probe scenario [14,[20][21][22]24,26,27,[30][31][32][34][35][36][37][38][39][41][42][43][44]: an intense pump pulse prepares the gas, and the transient absorption (or gain) of a delayed probe pulse is measured. The theoretical results presented below focus on this scenario, with the pump pulse carried at 800 nm.…”

Understanding the Seeding Pulse-Induced Optical Amplification in N 2 + Pumped by 800 NM Femtosecond Laser Pulses

“…Later, under similar experimental conditions, the optical gain was found to last for less than 10 ps [26,27], in an obvious contradiction with the earlier results. Recently, Britton and co-workers performed the gain dynamics measurement for gas cell and gas jet [28]. They reported that the decay process of the gain depends sensitively on the concentration of the N 2 .…”