Ionization-Induced Multiwave Mixing: Terahertz Generation with Two-Color Laser Pulses of Various Frequency Ratios.

Ultrafast strong-field ionization is shown to be accompanied by atypical multiwave mixing with the number of mixed waves defined by the dependence of the ionization rate on the field strength. For two-color laser pulses of various frequency ratios, this results in the excitation of a free-electron current at laser combination frequencies and possibly in the excitation of the zero-frequency (residual) current responsible for terahertz (THz) generation in a formed plasma. The high-order nature of ionization-induced wave mixing may cause THz generation with uncommon laser frequency ratios (such as 2:3 and 3:4) to be virtually as effective as that with the commonly used frequency ratio of 1:2.