Fig. 2.

THz photon echoes and NR signals in the time domain. (A) Experimental time domain traces $E_{NL}$ at delays $\tau <T_{\text{rev}}/2$ between THz pulses A and B. THz pulse B is fixed at time 0. The rephasing (R) signals appear at $t=\tau +n{T}_{\text{rev}}$ (n = 0 and 1 shown), and the nonrephasing (NR) signals appear at $t=-\tau +n{T}_{\text{rev}}$ (n = 1 and 2 shown). For $\tau <T_{\text{rev}}/2$, each R signal appears earlier than its counterpart NR signal. The vertical dashed lines mark the position of pulse B (t = 0) and its first two revivals ($t=T_{\text{rev}}$ and $2T_{\text{rev}}$), where the PP signals appear. (B) Experimental time domain traces $E_{NL}$ at delays $\tau >T_{\text{rev}}/2$. Each R signal appears later than the counterpart NR signal. (C and D) Simulated time domain response of the derivative of the orientation factor, $d\cos {\langle \theta \rangle }_{\text{NL}}/dt$, at delays (C) $\tau <T_{\text{rev}}/2$ and (D) $\tau >T_{\text{rev}}/2$. Except for the effects of dephasing that are not accounted for, the simulations show good agreement with the experimental results.