Exceptional Electro‐Optic Coefficient Unlocked via Polarization Swaying in KNbO ₃ Thin Films

ABSTRACT

The electro‐optic effect powers microwave‐to‐optical quantum transducers, photonic quantum computing switches, and high‐speed optical interconnects. A larger electro‐optic coefficient directly reduces the drive voltage and device footprint of electro‐optic modulators, making the search for materials with large coefficients a central challenge in photonics. Here, guided by phase‐field modeling, we engineer an electro‐optic coefficient of 43 000 pm V ⁻¹ in monoclinic KNbO ₃ thin films, the largest ever measured in any material, exceeding industry‐standard LiNbO ₃ by 1000×. This exceptional response arises from electric field‐driven “lateral swaying” of the ferroelectric polarization vector between symmetry‐equivalent monoclinic variants, which reorients the optical index ellipsoid far beyond the conventional Pockels response that relies on a linear coupling between field and polarization. The ultralow coercive fields of the monoclinic phase (∼1 kV cm ⁻¹ ) place this giant response within reach of standard drive electronics, enabling drive voltages orders of magnitude below the current state‐of‐the‐art. The response is stable to within 1% − 2% over hours, robust under thermal and electrical cycling, and demonstrated up to 1 MHz, establishing low‐symmetry ferroelectric phases as an unexploited design space for next‐generation energy‐efficient electro‐optic modulators for classical and quantum photonic integrated circuits.