Recollision-Induced Superradiance of Ionized Nitrogen Molecules

We propose a new mechanism to explain the origin of optical gain in the transitions between the excited and ground states of the ionized nitrogen molecule following irradiation of neutral nitrogen molecules with an intense ultrashort laser pulse. An efficient transfer of population to the excited state is achieved via field-induced multiple recollisions. We show that the proposed excitation mechanism must lead to a superradiant emission, a feature that we confirm experimentally.

Figure 1

(a) Schematic diagram of relevant energy levels of nitrogen molecules and its ions. (b) Amplification of the seed pulse inside the plasma filament. The femtosecond pump laser pulse is focused by a convex lens of $f=400\mathrm{mm}$ in pure nitrogen at a pressure of 30 mbar. The seed pulse around 391 nm is generated by second harmonic generation in a thin Beta Barium Borate (BBO) crystal on the second arm of a Mach-Zehnder interferometer. The weak seed pulse is combined with the 800 nm pump pulse with a dichromatic mirror. The seed pulse is also focused by an $f=400\mathrm{mm}$ lens installed before the dichromatic mirror. The spatial overlap and temporal delay of the pump and seed pulses are carefully controlled. The pump pulse energy is 3 mJ. Self-seeded stimulated emission is responsible for the weaker signal observed with pump only. (c) The self-seeded 391 nm signal as a function of pump laser energy in 150 mbar nitrogen.

Figure 2

Temporal evolution of the optical gain at 391 nm measured in 10 mbar nitrogen gas as a function of delay between pump and seed pulse. Pump pulse energy is 3 mJ. Seed pulse characteristics are as in Fig. 1.

Figure 3

(a) 391 and 428 nm lasing emission as a function of the ellipticity of the pump pulses at 800 nm. The gas pressures were 45 and 300 mbar, respectively. (b) Dependence of the high-order harmonic yield in nitrogen gas as a function of the laser ellipticity. The experiments were performed in a 15 mm long gas cell with two $150\mu \mathrm{m}$ holes on the entrance and exit surfaces of the gas cell, which were drilled by the laser pulse itself on the $100\mu \mathrm{m}$ thick aluminum foil windows. Each data point was an average over 10 000 laser shots. The nitrogen gas pressure was about 65 mbar and the pump pulse energy was 3.8 mJ.

Figure 4

(a) Temporal profile of the 391 nm forward radiation measured for different nitrogen pressure in the presence of a constant seed pulse. The narrow peaks at zero delay are the measured seed pulse. These temporal profiles are measured by frequency mixing the 391 nm radiation and a constant 800 nm pulse inside a sum frequency generation (SFG) BBO crystal. The SFG signal at 263 nm is recorded as a function of relative delay between the two pulses. (b) The pulse delay, pulse width, and 391 nm peak intensity are presented for increasing nitrogen pressure.