Since its first discovery in 1928, and particularly after the invention of the laser, the inelastic scattering of light by molecules (i.e., the Raman effect) has revolutionized multiple disciplines ranging from bio-spectroscopy and label-free imaging to frequency conversion and amplification of coherent light. In the latter applications, intense pump light incident on the medium induces synchronous molecular oscillations, which enable the amplification of radiation red-shifted by the Raman frequency.

In this work, Mitrofanov and coauthors report the observation of the inverse scattering effect, causing loss instead of gain and less common than its canonical counterpart, in a novel scenario of strong-field physics. By pumping a nitrogen-filled gas cell with ultrashort pulses of 3.9 μm central wavelength, they observed the emission of high-order harmonics reaching the ultraviolet. Remarkably, careful inspection of the spectral features of the 11^th harmonic lying in the vicinity of a prominent transition of nitrogen at 337 nm revealed signatures of both Raman-induced gain and loss on both sides of the ultraviolet pump. These interesting results and their potential applications will certainly open new horizons in the aforementioned fields in the years to come.

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