Two- and three-photon absorption of semiconductor quantum dots in the vicinity of half of lowest exciton energy

We present a demonstration of the unambiguous determination of two-photon absorption (2PA) and three-photon absorption (3PA) in semiconductor quantum dots in the vicinity of E(0)/2, where E(0) denotes the peak position of the lowest exciton energy. In the spectral region where photon energies h omega >= E(0)/2, the 2PA cross section is determined by open-aperture Z scans with femtosecond laser pulses at relatively lower irradiances, and then the 3PA cross section is extracted from the Z scans measured at higher irradiances. As photon energies are less than but near E(0)/2, multiphoton-excited photoluminescence techniques have to be employed in conjunction with open-aperture Z scan to distinguish between 2PA and 3PA.