Alignment-Dependent Terahertz Radiation in Two-Color Photoionization of Molecules

The generation of terahertz radiation from an ensemble of aligned air molecules in intense two-color laser fields has been investigated. Our experiments show that terahertz radiation yields are strongly modulated by the alignment of nitrogen and oxygen molecules. We explain this phenomenon in the context of the plasma current model combined with alignment-dependent ionization, in which nitrogen molecules aligned parallel to the laser field yield more ionization and plasma currents, resulting in enhanced terahertz radiation. Our measurements also provide the ratio of the ensemble-averaged ionization rates of nitrogen molecules aligned parallel versus orthogonal to the laser field.