Engineering high Pockels coefficients in thin-film strontium titanate for cryogenic quantum electro-optic applications

Materials which exhibit the Pockels effect are notable for their strong electro-optic interaction and rapid response times and are therefore used extensively in classical electro-optic components for data and telecommunication applications. Yet many materials optimized for room-temperature operation see their Pockels coefficients at cryogenic temperatures significantly reduced - a major hurdle for emerging quantum technologies which have even more rigorous demands than their classical counterpart. A noted example is $\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$, which features the strongest effective Pockels coefficient at room temperature, only to see it reduced to a third (i.e. ${\mathrm{r}}_{\mathrm{e}\mathrm{f}\mathrm{f}}\approx$ 170 pm/V) at a few Kelvin. Here, we show that this behaviour is not inherent and can even be reversed: Strontium titanate ($\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$), a material normally not featuring a Pockels coefficient, can be engineered to exhibit an ${\mathrm{r}}_{\mathrm{e}\mathrm{f}\mathrm{f}}$ of 345 pm/V at cryogenic temperatures - a record value in any thin-film electro-optic material. By adjusting the stoichiometry, we can increase the Curie temperature and realise a ferroelectric phase that yields a high Pockels coefficient, yet with limited optical losses - on the order of decibels per centimetre. Our findings position $\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}{\mathrm{O}}_3$ as one of the most promising materials for cryogenic quantum photonics applications.