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.

The development of a reliable technology for domain engineering in thin film lithium niobate is crucial to leveraging its disruptive potential for integrated nonlinear optics. However, thin film formats present outstanding challenges for traditional poling techniques with specific concern to non-polar cuts and short periods. Here, a novel approach is developed for the periodic poling of x-cut ≈500nm-thick lithium niobate on insulator (LNOI), relying on electron beams. High-quality ferroelectric gratings with periods in the 3.5–0.37 µm range are successfully fabricated, and a comprehensive analysis of their properties by piezoresponse force microscopy is presented, providing evidence for poling in highly non-equilibrium regimes, yielding regular domain gratings that remain stable over several years. Moreover, seamless integration with undoped and 5 mol% MgO-doped LNOI photonic nanowires is demonstrated, together with their nonlinear optical functionality in both co- and counter-propagating waveguide experiments. This novel poling technology appears ideally suited for submicrometric domain patterning of the most widely used cut for LNOI photonic integrated circuits and holds promise for unlocking their full potential for the realization of ultralow-footprint all-optical signal processing chips exploiting engineerable nonlinearities for classical and quantum applications.

Polarization control in photonic integrated circuit (PIC) waveguides is receiving broad attention for application in quantum systems and telecommunication [1]. Thin film lithium niobate (TFLN) is an ideal platform for polarization control applications due to its birefringence and electro-optic properties [2]. We observe polarization coupling between fundamental TE and TM modes in TFLN waveguides.

Fano resonances in nanophotonic structures are attracting significant attention for the engineering possibilities they may offer in applications such as lasers, sensing and optical signal processing [1]. Most widely explored device architectures in the quest for fully integrated implementation scenarios involve the side-coupling of different waveguide-cavity systems providing the narrowband and broad resonances (discrete and continuum states, respectively) whose interference gives rise to the signature asymmetric Fano lineshapes. Here we present a brand-new approach to achieve Fano resonances in ultracompact 1D waveguide formats through a polarization diversity scheme, exploiting the longitudinal field components of guided modes in high confinement photonic wires in combination with integrated Bragg-resonant structures achieved by sidewall modulation of the same.