Fig. 3.

Transient nonlinear polarization and carrier dynamics. (A) Schematics of the nonlinear polarization (P), transient current (J), and emitted electric field (E_emit) for the shift current (Shift) and in-gap photoexcitation (in-gap). The detected terahertz electric fields are further modified (E_obs) due to the diffraction and instrumental factors. (B) Retrieved current dynamics for the shift current and in-gap photoexcitation. Solid lines represent the fitting curves. Shift current accompanies a swing over (relaxation) of the charge with a decay time of ∼0.5 ps, whereas the in-gap photoexcitation appears only within the incident pulse duration. (C and D) Temperature and incident photon energy dependence of the decay time $\tau$. Excitation intensity is 400 nJ, and photon polarization is $E\left({\omega }_{\text{exc}}\right)\parallel E\left({\omega }_{\text{THz}}\right)\parallel c$.