Terahertz-optical mixing in undoped and doped GaAs quantum wells: From excitonic to electronic intersubband transitions

The mixing of near-infrared and terahertz (THz) beams has been observed in n-doped GaAs quantum wells (QWs) and is shown to be quite different from that in undoped QWs. The resonant behavior of the sidebands demonstrates that mixing in doped QWs is primarily sensitive to electronic intersubband transitions while that in undoped QWs is due to excitonic intersubband transitions. These results demonstrate that THz-optical mixing can be used to probe the collective dynamics of a driven electron gas.