The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10(5) GM (1 GM = 10(-50) cm(4) s phot(-1)), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by k·p calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption.

We investigate the spectral dependence of the linear and two-photon absorption of wurtzite CdS nanoparticles (dots and rods) by means of quantitative one-and two-photon photoluminescence excitation spectroscopy and effective mass theory modeling. Absolute two-photon absorption cross sections free from spectrally varying beam related uncertainties are obtained by means of a new reference dyebased method. The two-photon spectrum features of rods strongly differ from those of dots, due to the distinct energy structure of quasi-one-dimensional systems. The transversal confinement is found to dominate the energy of the absorption maxima while the longitudinal one dominates their absorption intensity. This suggests two-photon transition energy and intensity can be controlled independently in nanorods. For both geometries we observe a sizable spectral shift between the first one-and two-photon absorption maxima, which we conclude is inherent to the small rates of near-bandgap two-photon transitions rather than to the particular geometry of the absorber. The provided understanding of the spectral dependence of the two-photon absorption of CdS dots and rods is of strong interest for the design of nanocrystals with optimized two-photon absorption properties for bioimaging and phototherapy applications.

We present a theoretical and numerical investigation of correlated multi-electron states of hexagonal semiconductor rings. Both single-particle and correlated states show localization patterns in the six corners and energy spectra degeneracies corresponding to a hexagonal benzene ring. Thus, our results can aid the interpretation of energy-loss or near-field experiments that, in turn, shed light on the nature of molecular few-particle orbitals of artificial benzene. Surprisingly, we find that charges get more localized in the corners as the number of electrons increases, up to six, this indicating the deficiency of a picture based on orbitals delocalized on the whole ring. We also expose the presence of several spin-correlated states and the effect of an asymmetry of the system. V C 2012 American Institute of Physics. [http://dx

The transition bridge between zero-dimensional quantum dots and one-dimensional quantum wires is explored theoretically by means of the construction of the addition energy spectra of nanorods with different lengths. Spin density-functional theory supplemented with full configuration interaction calculations are carried out. The addition energy spectra are qualitatively related to the single particle correlation diagram. The transition from charge-density waves to spin-density waves, characterizing the Wigner crystallization in the low density limit is shown.

Few-electron states of AlAs-GaAs-AlAs hexagonal quantum rings pierced by an axial magnetic field are computed through full configuration interaction calculations. The quantum ring is in the low-density regime, populated with N = 1 up to N = 7 electrons. Similar to circular rings, the energy spectra of the hexagonal ones reflect an integer and fractional Aharonov-Bohm regular oscillation pattern for N = 1 and N = 2, 3, respectively. Deviations from the regular fractional period with increasing electron density become apparent for larger N. Remarkably, for N = 6 the Aharonov-Bohm effect is completely suppressed. This is a unique symmetry-related feature of hexagonal rings that only can emerge in the low-density regime.