Despite significant achievements in characterizing the properties of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ over the last three decades, the precise nature of its electronic ground state is still unresolved. In this work, we provide a missing piece of the puzzle by uncovering evidence of electronic nematic order in the normal state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, revealed by ultrafast time-resolved optical dichroism measurements of uniaxially strained thin films. This nematic order, whose domains are aligned by the strain, spontaneously breaks the fourfold rotational symmetry of the crystal. The temperature dependence of the dichroism resembles an Ising-like order parameter, and optical pumping induces a coherent oscillation of its amplitude mode. The existence of electronic nematic order in the normal state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ may have consequences for the form and mechanism of superconductivity in this material.

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• Nematic order
• Ruthenates
• Reflectivity
• Ultrafast pump-probe spectroscopy