Description
Correlated oxide interfaces host quantum-confined phenomena, such as the two-dimensional electron gas (2DEG) and superconductivity, holding great promise for next-generation electronics. However, dynamically manipulating these states and directly probing gate-driven inversion symmetry breaking at buried interfaces remain experimentally challenging. Here, we utilize interface-sensitive sum-frequency generation (SFG) spectroscopy to investigate gate-tunable dynamics in LaAlO3/SrTiO3 and LaAlO3/KTaO3 heterostructures. We observe a giant, unidirectional nonlinear optical modulation: a negative back-gate bias drives a massive ~93.3-fold enhancement of the Ti-O6 anti-phase vibrational mode (103 meV) at -200 V and 35 K, whereas positive bias leaves the signal nearly unchanged. Strikingly, optical and transport responses decouple; the SFG signal exhibits negligible hysteresis and a fully recoverable fatigue decay under continuous sweeping, contrasting sharply with the robust, hysteretic transport resistance. Mechanistically, the negative back-gate narrows and steepens the interfacial quantum well, depleting the 2DEG and suppressing electronic screening. This allows the DC electric field to penetrate deeply into the bulk STO, breaking the macroscopic inversion symmetry and inducing robust polarization. Our findings demonstrate that the giant optical enhancement originates primarily from this electric-field-induced (EFI) bulk response, providing a new paradigm for understanding electro-optic modulation at correlated interfaces.
