Near infrared and optical light applied near the band edge of a semiconductor can lead to the formation of bound electron-hole pairs known as excitons. Using growth methods such as molecular beam epitaxy, semiconductor heterostructures can be engineered to have properties beneficial to particular experiments. The formation of thin layers of semiconductor can lead electrons and holes to be confined along a particular direction, and one structure that can be grown is called a quantum well. Confinement of charge in the quantum well increases the Coulomb interaction between the electron and hole, increasing exciton formation. When excitons are driven with an intense THz field, changes to the optical properties of the semiconductor are observed.

These changes to the optical properties can not only provide interesting information about the exciton system, but also may provide insight on modulating optical beams at THz frequencies, information that may be necessary to further improve the speed of our cable and internet connections. By growing InGaAs quantum wells with AlGaAs barriers on a GaAs substrate, we have observed strong changes to the optical spectrum due to intense THz fields. We find that when the strong THz field is applied to an intersubband transition in the quantum well, the applied field can significantly shift the energy of that intersubband transition. This shift is unexpected within the approximations often used to describe this system, and we find that full numerical simulations of the system are necessary to interpret our results.

When the strong THz field is polarized in the plane of the quantum well, we are able to observe optical light at up to 11 frequencies that were not present before application of the THz. The new frequencies are separated from the optical frequency by multiples of the THz frequency and are often referred to as sidebands. To understand the origin of the high-order sidebands observed, which are present up to 8 th order, a recollision model between electrons and holes is adopted. Strong agreement with theoretical predictions indicates that recollisions are responsible for high-order sideband generation providing an intriguing connection between atomic and condensed matter phenomena.