Distinguishing optical rectification, four-wave mixing, and surface depletion effects in coherent control of terahertz generation in ZnSe under normal and oblique incidence

Coherent control under two-color femtosecond laser excitation plays a significant role in atomic, molecular, and semiconductor materials. Recently, terahertz (THz) emission spectroscopy has been employed to characterize the coherent ultrafast photocurrent from semiconductor materials under two-color light excitation. However, distinguishing the contributions of various optical effects to THz radiation under normal- and oblique-incident excitations with two-color light remains a challenge. Herein, we choose the zinc selenide (ZnSe) crystal as a model sample to study the THz radiation at normal and oblique incidences with two-color light excitation. Based on the dependence of the THz signal on the relative phase difference between the fundamental wave (800 nm, ω) and the second harmonic wave (400 nm, 2ω), the contribution ratio of optical rectification (OR) to four-wave mixing (FWM) for THz radiation is calculated as 1:2.3 at normal incidence. Under oblique incidence, the contribution ratios of FWM, OR, and surface depletion field (SDF) for THz radiation are calculated as 1:1.5:6.7. In particular, we have observed the THz time-domain signals from net FWM contribution with polarity reversal at both normal and oblique incidences. This work not only elucidates the THz radiation properties of ZnSe crystal but also reveals the interplay among OR, SDF, and FWM in the coherent control of ultrafast photocurrent under femtosecond laser excitations.