Fig. 3: Direct observation of the DFG process for THz waves.
From: Giant enhancement of THz-frequency optical nonlinearity by phonon polariton in ionic crystals
a–d are snapshots from Supplementary Movie 1 taken at 11, 22, 33, and 44 ps after the THz waves were initially generated. LF and HF indicate the lower and higher frequency components, and their walk-off effect during propagation is clear.
