Optical gain in silicon nanocrystals

Adding optical functionality to a silicon microelectronic chip is one of th e most challenging problems of materials research. Silicon is an indirect-b andgap semiconductor and so is an inefficient emitter of light. For this re ason, integration of optically functional elements with silicon microelectr onic circuitry has largely been achieved through the use of direct-bandgap compound semiconductors. For optoelectronic applications, the key device is the light source-a laser. Compound semiconductor lasers exploit low-dimens ional electronic systems, such as quantum wells and quantum dots, as the ac tive optical amplifying medium. Here we demonstrate that light amplificatio n is possible using silicon itself, in the form of quantum dots dispersed i n a silicon dioxide matrix. Net optical gain is seen in both waveguide and transmission configurations, with the material gain being of the same order as that of direct-bandgap quantum dots. We explain the observations using a model based on population inversion of radiative states associated with t he Si/SiO2 interface. These findings open a route to the fabrication of a s ilicon laser.