Zero-bias spin separation

Zero-bias spin separation generated by homogeneous optical excitation with terahertz radiation in quantum wells is reviewed. In gyrotropic semiconductor structures spin-dependent asymmetry of electron scattering induces a pure spin current which results in a spin separation. We consider the relaxation mechanism yielding the spin current due to the energy relaxation of a heated electron gas and the excitation mechanism caused by the scattering assisted free carrier absorption. An experimental access to these phenomena provides the application of an external magnetic field converting the spin current into a measurable net electric current. We discuss microscopic and phenomenological theory of these effects, give an overview of experimental data, and address several applications.