The effect of light on the properties of ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ superlattices is studied using first-principles calculations in their paraelectric phase $p$ as well as in the, $c, r$, and $aa$ phases that are defined by polarization lying along the [001], $\left[11v\right]$, and [110] directions, respectively. While the $r$ phase is the ground state in dark conditions, illumination triggers a rotation of the polarization towards the out-of-plane direction, which results in the $r$ phase transforming into a $c$ phase at a large concentration of photoexcited carriers. We further reveal that a space charge region of free carriers builds up at the interface under illumination, implying that these superlattices can be thought as acting as $p-i-n$ junctions under light. These free charges screen the polarization discontinuity at the interface, effectively reducing the electrostatic depolarizing field.

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• Ferroelectricity
• First-principles calculations
• Photoinduced effect