Efficient nearly-circularly-polarized terahertz generation from an air plasma pumped by collinear and circularly polarized two-color laser fields

Efficient circularly polarized terahertz (THz) emission is important for THz imaging technology and basic scientific research. In order to achieve this, we have made a simple, all-passive design to achieve collinear and corotating circularly polarized two-color laser fields. The collinearity allows us to manipulate accurately the relative phase between two-color lasers. For a given relative phase, our experiments show that efficient nearly-circularly-polarized THz emission can be generated from an air plasma pumped by the two-color fields. The emitted THz power is \ensuremath{\sim}2.5 times that pumped by linear-polarized two-color laser fields. Our results also indicate that nearly circularly polarization is stable against different pump power and plasma distance.