When light shines on a semiconductor, it excites the electrons in the semiconductor into higher energy states. This leaves behind the absence of an electron, which is equivalent to a positively charged space called a “hole.” The electron and the hole attract each other through the electrostatic force and form a bound pair called an exciton. Excitons can then interact with other unpaired charges. This interaction alters the typical way in which a beam of light that propagates in a material shifts that material’s positive and negative charges. This response is called nonlinear, and it can make the beam change its shape, direction, and/or frequency. This change allows for optical processing of information.
