Field-resolved four-wave mixing with multi-THz transients in InSb

Two-dimensional-spectroscopy with near-infrared (NIR) or mid-infrared pulses has given insight into ultrafast dynamics and spectral correlations in many systems. Terahertz (THz) four-wave-mixing (FWM) has attracted growing interest as a potential way to access important low-energy excitations such as inter and intra molecular vibrations or energy gaps in superconductors. However, lead-off experiments have been limited to model systems due to moderate THz fields. Here we employ a recently developed ultraintense multi-THz source to pursue THz four-wave mixing in a bulk semiconductor even under two-photon resonant conditions. The temporal profile of the strong FWM signal shows first indications of a THz photon echo. A non-collinear geometry allows for spatial selection of nonlinear signals and background-free measurements.