Electrostatic gating provides a powerful approach to tune the conductivity of the two-dimensionalelectron liquid between two insulating oxides. For the LaAlO3/SrTiO3 (LAO/STO) interface, suchgating effect could be further enhanced by a strong lattice polarization of STO caused by simultaneousapplication of gate field and illumination light. Herein, by monitoring the discharging process uponremoving the gate field, we give firm evidence for the occurrence of this lattice polarization at theamorphous-LaAlO3/SrTiO3 interface. Moreover, we find that the lattice polarization is accompaniedwith a large expansion of the out-of-plane lattice of STO. Photo excitation affects the polarizationprocess by accelerating the field-induced lattice expansion. The present work demonstrates the greatpotential of combined stimuli in exploring emergent phenomenon at complex oxide interfaces.