The nonlinear optical phenomena known as "upconversion", or "sum-frequency generation" is used to time-resolve the luminescence emitted from a semiconductor quantum structure. Upconversion involves the "mixing' of two wavelengths of light in a nonlinear crystal to produce photons at an energy equal to the sum of the two incident photon energies. This process only occurs when the incident photons overlap in space, and more importantly, in time.

Thus with the use of ultrafast lasers one can time resolve optical signals by mixing with a short (~100 femtosecond) "interrogation pulse" whose time delay can be controlled. In the schematic shown below, the sample is excited with a short optical pulse (blue) which selectively excites either spin-up or spin-down excitons into the quantum structure. These excitons spin-scatter and recombine, emitting luminescence during their ~20 picosecond lifetime. The luminescence is time resolved with the probe pulse (red) which interrogates the luminescence intensity at controlled 100fs time-slices. Polarization analysis allows for direct measurement of spinflip-scattering rates.