Time-varying Nonlinear Effects in Terahertz Generation

TL;DR

This paper experimentally investigates how ultrafast carrier dynamics in ITO films cause time-varying nonlinear effects, leading to amplitude, frequency, and polarization modulations in terahertz generation, advancing the control of electromagnetic waves in the time domain.

Contribution

It demonstrates the transfer of strong time-varying material responses from near-infrared to terahertz frequencies via nonlinear frequency down-conversion.

Findings

Significant amplitude and frequency modulations in THz generation.

Observation of polarization modulations due to tensor properties of (2).

Establishment of a temporal boundary for THz waves based on ultrafast carrier dynamics.

Abstract

Time-varying effects have unveiled new possibilities for manipulating electromagnetic waves through the temporal dimension. In this study, we experimentally explore these effects in the nonlinear optical process of terahertz (THz) generation using optically pumped indium tin oxide (ITO) films. The ultrafast carrier dynamics in the ITO film endow the second-order nonlinear susceptibility (\c{hi}(2)) with sub-picosecond temporal evolution, establishing a temporal boundary for the generated THz waves. We observe significant amplitude and frequency modulations in the THz generation at various transients, attributed to the time-varying complex amplitude of the \c{hi}(2). Moreover, we also observed polarization modulations when further exploiting the tensor properties of \c{hi}(2). This work advances the exploration of time-varying effects into the nonlinear regime through frequency down-conversion, effectively transferring the strong time-varying material response from the near-infrared (NIR) band to the THz band. These findings open up new opportunities for realizing time-varying phenomena that specifically require single-cycle modulation.