Strong-field terahertz optical mixing in excitons

Driving a double-quantum-well excitonic intersubband resonance with a terahertz (THz) electric field of frequency ${\omega }_{\mathrm{THz}}$ generated terahertz optical sidebands $\omega ={\omega }_{\mathrm{THz}}+{\omega }_{\mathrm{NIR}}$ on a weak near-infrared probe. At high THz intensities, the intersubband dipole energy which coupled two excitons was comparable to the THz photon energy. In this strong-field regime, the sideband intensity displayed a nonmonotonic dependence on the THz field strength. The oscillating refractive index which gives rise to the sidebands may be understood by the formation of dressed states that oscillate with the same periodicity as the driving THz field.