Near-Resonant Raman Amplification in the Rotational Quantum Wave Packets of Nitrogen Molecular Ions Generated by Strong Field Ionization

The generation of laserlike narrow bandwidth emissions from nitrogen molecular ions ($N_2^{+}$) generated in intense near- and mid infrared femtosecond laser fields has aroused much interest because of the mysterious physics underlying such a phenomenon. Here, we perform a pump-probe measurement on the nonlinear interaction of rotational quantum wave packets of $N_2^{+}$ generated in midinfrared (e.g., at a wavelength centered at 1580 nm) femtosecond laser fields with an ultrashort probe pulse whose broad spectrum overlaps both $P$- and $R$-branch rotational transition lines between the electronic states $N_2^{+}(B^2{\mathrm{\Sigma }}_u^{+},v^{\prime }=0)$ and $N_2^{+}(X^2{\mathrm{\Sigma }}_g^{+},v=0)$. The results indicate the occurrence of highly efficient near-resonant stimulated Raman scattering in the quantum wave packets of $N_2^{+}$ ions generated in strong laser fields in the midinfrared region, of which the underlying mechanism is different from that of the air lasers generated in atmospheric environment when pumping with 800 nm intense pulses.

Figure 1

Spectra of the probe pulse exiting from the gas chamber as a function of the time delay between pump and probe pulses.

Figure 2

The spectra of probe pulse measured at time delays $t_0$ (gray dot line), $t_1$ (red dash dot line), and $t_2$ (blue solid line).

Figure 3

(a) Fourier transforms of the $R$-branch absorption spectra in Fig. 1, and the zoomed-in spectra of Fig. 2 in the spectral ranges (b) and (c) fitted with theoretical calculations.

Figure 4

(a) Schematic diagram of resonant and the first near-resonant Raman scattering in nitrogen molecular ions. (b) Schematic diagram of resonant and the second near-resonant Raman scattering in nitrogen molecular ions. (c) Calculated Stokes wavelengths for the near-resonant Raman processes in (a) and (b).