Colossal Cryogenic Electro‐Optic Response Through Metastability in Strained BaTiO ₃ Thin Films

Abstract

Abstract The search for thin film electro‐optic materials that can retain superior performance under cryogenic conditions has become critical for quantum computing. Barium titanate thin films show large linear electro‐optic coefficients in the tetragonal phase at room temperature, which is severely degraded down to ≈200 pm V −1 in the rhombohedral phase at cryogenic temperatures. There is immense interest in manipulating these phase transformations and retaining superior electro‐optic properties down to liquid helium temperature. Utilizing the thermodynamic theory of optical properties, a large low‐temperature electro‐optic response is designed by engineering the energetic competition between different ferroelectric phases, leading to a low‐symmetry monoclinic phase with a massive electro‐optic response. The existence of this phase is demonstrated in a strain‐tuned BaTiO 3 thin film that exhibits a linear electro‐optic coefficient of 2516 ± 100 pm V −1 at 5 K, which is an order of magnitude higher than the best reported performance thus far. Importantly, the electro‐optic coefficient increases by 100 × during cooling, unlike the conventional films, where it degrades. Further, at the lowest temperature, significant higher order electro‐optic responses also emerge. These results represent a new framework for designing materials with property enhancements by stabilizing highly tunable metastable phases with strain.