Multi-photon excitation and relaxation in colloidal semiconductor quantum dots

Abstract

Colloidal semiconductor quantum dots (QDs) have received increasing attention as promising two-photon absorbers for optical applications such as bio-imaging, optical limiting, stabilization, optical communication, optical information. As far as these applications are concerned, two-photon absorption (TPA) cross-sections as well as subsequent recombination processes following interband excitation are important aspects. In this thesis, we report the systematic experimental study on the TPA excitation and relaxation in colloidal CdSe QDs and CdTe QDs. Theoretical work has also been carried out to investigate the TPA spectra in strong confinement CdTe QDs.

For the experimental study, various techniques have been applied to investigate the characteristics of the above nanomaterials such as high-resolution transmission electron microscopy (HRTEM), UV-visible absorption spectroscopy, photoluminescence (PL) spectroscopy, etc. For the study of TPA in QDs, open-aperture Z-scans have been performed at different wavelengths with femtosecond laser pulses. The relaxation processes have been determined by time-resolved, frequency-degenerate pump-probe technique.

For the theoretical calculation, a TPA theory for QDs based on eight-band Pidgen and Brown (PB) model has been developed. Numerical calculations based on the theory have been performed to investigate the spectra of TPA in strong confinement CdTe QDs.

For colloidal CdSe QDs with 2 nm in radius, the TPA spectra have been measured with Z-scan from 750 nm to 950 nm and compared with published calculation results. The Auger process as well as intraband absorption, after TPA excitation, have been analyzed with frequency-degenerate, pump-probe technique and open-aperture Z-scan technique, respectively. For TPA spectra, the measured cross section is in the range from 10E-47 to 10E-46 cm4s 1/photon, depending on the wavelength. These values are in the same range as the published computation result based on a simple four-band parabolic model. The Auger constant is revealed to be of the order of 10E-30 cm6/s , while the intraband absorption cross-sections are found to be in the range from 10E-18 to 10E-17 cm2 from 680 to 780 nm. Our 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 greater than unity.

For the study on colloidal CdTe QDs, TPA spectra of three-different-sized QDs in very strong confinement regime have been investigated both experimentally and theoretically. The size-dependent TPA cross-section is unambiguously measured from 720 to 950 nm with Z-scan technique. the TPA cross-sections are measured to be on the order from 10-47 to 10-46 cm4¿s¿photon-1, depending on the wavelength and the size of CdTe QDs. Based on the eight band PB model, calculation on the spectra of TPA in CdTe QDs has also been carried out. By taking into account of the conduction-valence band mixing and the complex structures of the valence bands, the theory can give more accurate prediction for TPA of CdTe QDs in the strong confinement regime. Both the experiment and theory show that at a certain wavelength, the TPA in QDs rises un-monotonously with size. The increase of TPA for larger size is due to two factors: the increasing number of transitions for larger size and the red shift of the transitions of larger size. Another findings from the theory work is that, though the maximum peaks increases for larger size, the normalized maximum values of TPA by the QDs volume does not show size dependence.

The studies presented in this thesis will provide first-hand information for many applications based on two-photon absorption of QDs in strong confinement.