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Electron–positron pair-production process is observed in polychromatic light oscillating electric fields. A quantum kinetic theory is presented to obtain the pair-production rate and the momentum information. Oscillating structures of the particle yield and the odd–even structure of the momentum–frequency spectrum are depicted. The roles of the phase factor and the frequency multiplication are analyzed. We find the odd–even structures are remained in the triple-frequency field.

We review our recent progress on power scaling of THz pulses generated from several nonlinear-optical crystals. By using a high-resistivity GaP crystal, we have significally increased the output peak power to as high as 722 W. By stacking three GaP wafers, we have further increased the highest output peak power to 2.36 kW. On the other hand, by using CO₂ laser pulses, we have obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO₃ crystals based on a backward configuration. The highest output power measured by us so far is 10.7 μW.