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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 E0/2, where E0 denotes the peak position of the lowest exciton energy. In the spectral region where photon energies ℏω≥E0/2, 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 E0/2, multiphoton-excited photoluminescence techniques have to be employed in conjunction with open-aperture Z scan to distinguish between 2PA and 3PA.

Auger recombination, quantized Auger rate, and intraband absorption of two-photon-excited carriers in colloidal CdSe quantum dots have been investigated systematically with femtosecond Z scans and transient absorption measurements. The Auger constant is revealed to be on the order of 10−30cm6s−1, while the intraband absorption cross sections are found to be in the range of 10−18–10−17cm2. The authors’ experimental evidence demonstrates that the Auger recombination or the intraband absorption takes place under the condition that the average electron-hole pair per quantum dot is in excess of the unity.