Extremely nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wave packet

We report on the generation of cascaded rotational Raman scattering up to 58th order in coherently excited $\mathrm{C}{\mathrm{O}}_2$ molecules. Raman scattering with more than 600 sidebands is obtained using an intense femtosecond laser to impulsively excite rotational coherence and femtosecond-laser-induced ${{\mathrm{N}}_2}^{+}$ lasing to generate cascaded Raman signals. This configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific structures of ${{\mathrm{N}}_2}^{+}$ lasing in the spectral and temporal domains and the ideal spatial overlap of the femtosecond laser and ${{\mathrm{N}}_2}^{+}$ lasing. A Raman spectrum extending above $2000\mathrm{c}{\mathrm{m}}^{-1}$ naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wave packets.

Figure 1

(a) Schematic of the experimental setup for ${{\mathrm{N}}_2}^{+}$ lasing generation and coherent Raman scattering. (b) The measured spectrum of ${{\mathrm{N}}_2}^{+}$ lasing and its spatial profile (inset). (c) The temporal information of ${{\mathrm{N}}_2}^{+}$ lasing (blue solid line) and the 400-nm laser pulses (black dot line) obtained by the cross-correlation technique. In this measurement, the sum frequency of the ${{\mathrm{N}}_2}^{+}$ lasing signal or the 400-nm pulses and a weak 800-nm pulse in a BBO crystal is recorded as a function of the delay. The zero delay is defined as the moment when the 800-nm pump laser arrives at the interaction region in the second chamber, as indicated with a red arrow.

Figure 2

Measured rotational Raman spectra (blue solid curves) at gas pressures of (a) 20 mbar, (b) 60 mbar, and (c) 120 mbar, respectively. The theoretically calculated Raman shifts are indicated by vertical lines. (d) Schematic diagram of the first-, second-, and third-order rotational Raman scattering. (e) Simulated Raman spectra for the first (left), second (middle) and third (right) order Raman scatterings. Each order Raman scattering is normalized independently.

Figure 3

Measured Raman spectra at the gas pressure of 150 mbar (green line), 300 mbar (purple line), 500 mbar (red line), and 1 atm (blue line), respectively. The inset shows the spatial profile of Raman signals at 1 atm.

Figure 4

Temporal structures of Raman signal at 1 atm (red dotted-dashed line) and ${{\mathrm{N}}_2}^{+}$ lasing (blue solid line) measured with the cross-correlation technique.