Аннотация:Application of intense THz field co-propagating with the main driving pulse breaks the field symmetry and promotes generation of even optical harmonics naturally forbidden in centrosymmetric crystals. We show that transient nature of the symmetry switching confines efficient even harmonic generation to the regions close to the crystal surfaces in the case when the length of the crystal significantly exceeds the coherence length of the process. In such a case efficient generation of the absorption limited and the absorption free even harmonics appears to be separated in time by the walk-off time of THz and near-IR driving pulse due to the strong absorption of the above bandgap harmonics generated close to the front surface of the crystal. Using THz field with the external strength of 7 MV/cm we demonstrate generation of second and fourth harmonics of near-IR 1.65-𝜇m driver with efficiency of 10^−4 and 10^−7 separated by 3.2 ps in 1.4-mm-long CdS crystal. Application of the extreme THz field leads to the comparable intensity of the even and odd harmonics showing the potential of bi-chromatic pumping for creation of synthesized medium with controllable nonlinear properties.
