# Colossal Cryogenic Electro‐Optic Response Through Metastability in Strained BaTiO3 Thin Films

## TL;DR

Researchers created a barium titanate thin film with a massive electro-optic response at cryogenic temperatures by stabilizing a unique phase using strain.

## Contribution

A new method for enhancing electro-optic properties at low temperatures by stabilizing a metastable monoclinic phase in BaTiO3 thin films.

## Key findings

- A BaTiO3 thin film achieved a linear electro-optic coefficient of 2516 ± 100 pm V⁻¹ at 5 K.
- The electro-optic coefficient increased 100× during cooling, unlike conventional films.
- Significant higher-order electro-optic responses emerged at the lowest temperatures.

## 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 BaTiO3 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.

Utilizing the thermodynamic theory of optical properties, a colossal cryogenic electro‐optic response in BaTiO3 thin films is designed and demonstrated by stabilizing a low symmetry metastable monoclinic phase via epitaxial strain tuning with an electro‐optic response reaching ≈2516 pm V−1 at 5 K. This approach represents a new paradigm for engineering large property enhancements in materials applied toward cryogenic photonics.