Frequency degenerate and nondegenerate two-photon absorption spectra of semiconductor quantum dots

The frequency degenerate and nondegenerate two-photon absorption (2PA) spectra of direct band gap semiconductor quantum dots are studied. Measuring the spectra for both cases in samples of CdSe and CdTe with different quantum dot sizes and size distributions, we observe that the 2PA spectra and the 2PA coefficient are size dependent, so that smaller dots have smaller 2PA even after taking into account the volume fraction. Theory considering the mixing of the hole bands, in a k · p model, explains the data quite well except for the smallest dots. A comparison with the parabolic band approximation is also shown.