The role of asymmetry in few-cycle, mid-IR pulses during THz pulse generation

The efficiency of terahertz (THz) pulse generation improves at longer driving wavelengths. For this reason, the use of mid-infrared (MIR) sources is more advantageous compared to visible or near-infrared systems. In this work, we investigate how single-color and two-color schemes of MIR pulses with few-cycle pulse durations compare in producing THz pulses. The results reveal that as the duration of the driving pulses decreases, the second harmonic generation crystal can be omitted from the system. Our numerical study pinpointed three regions where the optimal pulse parameters are fundamentally different for the most efficient THz pulse generation. The first is the two-color approach, where the two-color scheme is dominant at 3.2 optical cycles and over. The second is the single-color approach, where the single-color scheme becomes dominant at 1.7 optical cycles and below. Therefore, it simplifies the traditional two-color scheme for THz pulse generation. There is also a third transitional region where the two-color scheme still prevails, but the sign of the relative phase between the input pulses becomes important. Considering the effect of the relative phase and the carrier to envelope phase (CEP) effect on the THz pulse generation, the results have shown that as the pulse duration become shorter, the role of the CEP becomes important for efficient THz generation. By measuring the efficiency of the THz generation in this optical arrangement, quantifying the CEP becomes possible, which could become an important experimental tool for few-cycle, MIR laser technology.