Extreme nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wavepacket

TL;DR

This paper demonstrates high-order rotational Raman scattering up to the 58th order in CO₂ molecules using a single femtosecond laser, producing a broad frequency comb through impulsive excitation and N₂⁺ lasing, enabling standoff spectral extension.

Contribution

It introduces a novel method combining impulsive rotational excitation and N₂⁺ lasing with a single femtosecond laser for high-order Raman scattering at free-space locations.

Findings

Achieved up to 58th order Raman scattering in CO₂.

Generated a quasiperiodic frequency comb with over 600 sidebands.

Extended Raman spectrum above 2000 cm⁻¹ corresponding to femtosecond pulse trains.

Abstract

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel 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 spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets.