Breaking isotropic symmetry: Polarization-dependent enhanced vibrational Raman conversion to the first Stokes in hollow-core gas-filled fibers

Stimulated Raman scattering (SRS) in gases is commonly described in terms of molecular symmetry: vibrational transitions in isotropic molecules, such as SF6 and CF4, are expected to be polarization-independent because of their spherically symmetric Raman tensors. Here, we demonstrate that this assumption potentially breaks down in structured hollow-core fibers, where the waveguide geometry dominates the polarization response. By systematically rotating the pump angle of linear polarization, pronounced oscillations of the vibrational SRS conversion efficiency are observed, with normalized modulation depths of ∼70% in SF6 and ∼85% in CF4, even though both gases support only isotropic vibrational modes. An output-referenced quantum conversion efficiency exceeding 72% is achieved in SF6 at coupled peak powers of ∼2 kW in a 6 m HC-PBG fiber, corresponding to Stokes output powers above 0.4 W and setting a new benchmark for this fiber-gas class. Direct near-field imaging identifies the mechanism as polarization-dependent coupling of the Stokes light to less tightly confined higher-order and surface modes near the photonic bandgap edge, which distorts the mode profile and reduces the spatial overlap with the fundamental pump mode. We show that in the high-gain regime, even minute variations in the overlap integral are amplified into significant fluctuations in conversion efficiency. This fiber-geometry-driven polarization sensitivity defines a new design principle for optimizing nonlinear sources.