Ultrafast imaging of THz subluminal Cherenkov radiation in ZnTe

Summary form only given. In a dispersive medium, Cherenkov radiation (CR) takes on one of two qualitative forms, depending on whether /spl nu/ > c/sub 0/ (superluminal) or /spl nu/ < c/sub 0/ (subluminal), where /spl nu/ is the velocity of the source and c/sub 0/ = c/n/sub 0/ is the index of refraction in the medium at low frequencies. Optical techniques allow the detailed study of this phenomenon. An ultrafast optical pulse moving through a medium with /spl chi//sup (2)/ # 0 behaves like a relativistic distribution of dipoles by inducing a polarization proportional to the pulse´s intensity envelope through optical rectification. When the group velocity /spl nu//sub g/ of the pulse is greater than the phase velocity of any infrared frequency inside the pulse´s rectified bandwidth, CR is emitted. The theory for this dipolar source is the same as for conventional CR. Degenerate pump-probe experiments provide a way to directly measure the Cherenkov field. A pump pulse generates the CR field which modulates the intensity of the probe pulse that follows behind through the electro-optic effect. Experiments were performed on [110] oriented ZnTe at room temperature using a Ti:sapphire oscillator producing 1 nJ pulses at a repetition rate of 82 MHz. We show the theoretical and experimental results The data agrees well with theory. The weak beating effect is due to the finite extent of the pump pulse. We have not taken into account absorption in the far IR, which is known to be important in this frequency range for ZnTe at room temperature. This might explain the apparent damping out of the Cherenkov field laterally seen in the experimental plot.