Polarization control is a crucial functionality for photonic integrated circuits (PIC) used for telecommunication, sensing and quantum key distribution, to name just a few applications [1]. Several approaches have been proposed and demonstrated in silicon and thin-film lithium niobate (TFLN) PICs, entailing birefringence control via stress engineering or waveguide form-birefringence, most often mediated by coupling to higher-order guided modes and combined with electro or thermooptic phase-shifting circuitry [2-3]. Here we present a novel device concept relying on fundamental TE-TM mode coupling in integrated Bragg gratings via the longitudinal fields arising in nanophotonic wires. The device principle can be realized in any PIC platform that provides high light confinement. For its proof-of principle we used our TFLN PICs, achieving perfect agreement between theory and experiments.