Spintronics is a rapidly developing field of electronics, where it is not the charge but the spin that is used to manipulate electrons. I will describe how to spin-polarize electrons in a two-dimensional semiconductor heterostructure by using a field-effect gate control of the spin-orbit interaction. In this scheme, a beam of electrons splits in two spin-polarized components propagating at different angles. The phenomenon is similar to the birefringence of light in crystal optics. I will outline possible devices based on the spin-dependent refraction of the current carriers, including a spin filter and spin switch. The proposed program aims to meet the goals of spintronics by using only nonmagnetic semiconductors.