Even harmonic generation in semiconductors below and above the band gap assisted by an intense terahertz field

Application of an intense THz field copropagating with the main driving pulse breaks the field symmetry and promotes generation of even optical harmonics naturally forbidden in centrosymmetric crystals. We show that the 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-band-gap harmonics generated close to the front surface of the crystal. By using a THz field with external strength of 7 MV/cm, we demonstrate generation of second and fourth harmonics of a near-IR $1.65\text{\ensuremath{-}}\textmu{}\mathrm{m}$ driver with efficiency of ${10}^{\ensuremath{-}4}$ and ${10}^{\ensuremath{-}7}$ separated by 3.2 ps in 1.4-mm-long CdS crystal. Application of the extreme THz field leads to a comparable intensity of the even and odd harmonics showing the potential of bichromatic pumping for creation of a synthesized medium with controllable nonlinear properties.