Light polarization and electron’s spin angular momentum are very intimately connected. One can easily create left circularly polarized (LCP) or right circularly polarized (RCP) using a special kind of filter called a circular polarizer (the same all photographers use to eliminate reflections from the surface of the water for example). When RCP light (or an RCP laser) is incident on a semiconductor, only electrons of one spin interact with that light. When the light interacts with the electrons it can give the electrons enough energy to free them from the valence shells of their parent atom. Free from the valence shells, but still in the material, these electrons are called free because they have basically complete mobility if exposed to an electric field, and can form a spin -polarized electric current. Conversely, we can use light to detect spin populations in semi conductors. Light composed of half LCP and half RCP is said to be linearly polarized and we can assign a specific spatial orientation to this polarization. When lightbounces off a material with a net spin population, the polarization of the light changes by an angle directly proportional to the spin population in the material, in physics jargon this is called the Kerr Effect. This effect allows us to detect and map out the regions of different spin population. These Pictures took advantage of both of these Phenomena to collect data. What you see are the results of a laser beam focused down to a 10 micron beam diameter onto a Gallium Arsenide thin film. The bands of color represent different amounts of spin polarization (detected by the Kerr effect) in the excited electrons as they drift under the bias of an electric field.
(Each image above is 90 by 180 microns in size)