Terahertz (THz) wave generation process in air plasma was studied using femtosecond laser pulse pairs. The effect of preexisting air plasma was investigated using two optical geometries: collinear and orthogonal pumping geometry. When the air plasma is excited in collinear geometry, the power of the generated THz wave shows asymmetric dependence on the relative delay ahisof these pulses. The asymmetry depends on the relative intensities of these pump pulses, and it disappears when the pulses have equal intensities. In the orthogonal pumping geometry, the power of the THZ wave generated by the main pump pulse decreases in the presence of the preexisting plasma generated by the prepulse, and it recovers as the relative delay between the prepulse and main pulses becomes longer. Possible mechanisms of the results are discussed in terms of the dynamics of free electrons in the air plasma.
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