Enhancing Raman redshift via cross-circular polarization in an air-filled multipass cell for frequency down-conversion.

We demonstrate efficient and tunable ultrafast frequency down-conversion by exploiting polarization-dependent Raman dynamics in an air-filled multipass cell. By utilizing circularly or elliptically polarized 1030-nm, 800-fs driving pulses, we excite a cross-circularly polarized Raman process that effectively breaks the phase-matched four-wave mixing coupling. This mechanism allows Raman gain to dominate, driving a discrete energy transfer to Stokes waves that produces a significantly stronger and highly asymmetric spectral redshift compared to linear polarization. Simply varying the input energy allows the redshifted spectral lobe to be continuously tuned from 1040 to 1100 nm. At 1090 nm, the filtered pulses exhibit a near-transform-limited duration of 250 fs with an 18% conversion efficiency. This provides a stable and compact approach for generating high-energy, long-wavelength driving sources.