Giant third-order nonlinearity amplifies second-harmonic transport in Ca3Ru2O7

Nonlinear electrical transport enables the conversion and detection of electromagnetic signals, supporting wireless energy harvesting, sensing and frequency conversion. Achieving both a large response and electrical tunability remains a central materials challenge. Here we harness giant third-order nonlinearity to amplify and electrically control second-harmonic transport in the correlated polar metal Ca3Ru2O7. Superposing a dc bias on an ac excitation enhances the second-harmonic signal by more than an order of magnitude. The bias controls the polarity of the induced response and activates channels forbidden by symmetry at zero bias. Both transverse and longitudinal components scale quadratically with the ac excitation and are odd in the dc bias, with angular dependences consistent with third-order conductivity. The underlying third-harmonic electric-field responsivity reaches 4.5 \times 10-6 m2V-2 at 3 K, providing a quantitative measure of the strong nonlinearity. The induced response emerges below the spin-reorientation transition near 48 K, linking its enhancement to the reconstructed low-temperature electronic state. Theoretical calculations identify quantum-geometric contributions to the nonlinear conductivity and reproduce the observed response symmetry. These results demonstrate how strong higher-order nonlinearity can amplify an effective second-order response, providing a route to electrically tunable nonlinear transport in correlated materials.