Ultrafast temporal modulation of terahertz generation at an optically pumped ITO interface

Time-varying effects have opened new possibilities for manipulating electromagnetic waves. However, their applications so far are largely focused on exploiting the linear optical responses of the time-varying media. By contrast, corresponding investigations on the prosperous nonlinear optical responses remain underexplored. In this study, we report an experimental demonstration of time-varying effects in nonlinear terahertz (THz) generation using an optically pumped ultrathin epsilon-near-zero (ENZ) film, i.e., indium tin oxide (ITO) film. We show that, upon optical pumping near the ENZ wavelength, ultrafast carrier dynamics in the ITO film modulate its second-order nonlinear susceptibility [ χ (2) ], thereby establishing a temporal boundary for the THz generation process. This results in significant amplitude and spectral modulations on a sub-picosecond timescale. Additionally, by exploiting the tensor properties of χ (2) , we further achieve ultrafast polarization modulations. Our work employs the optical time-varying effect in modulating nonlinear THz generation, creating a pathway for achieving ultrafast and active THz generation and manipulation, as well as understanding the associated material dynamics.