Spectral and dynamic properties of excitons and biexcitons in type-II semiconductor nanocrystals.

Using time-resolved photoluminescence spectroscopy, we analyze single and multiexciton emission energies and decay dynamics of CdS/ZnSe core/shell nanocrystals (NCs). The NCs exhibit a characteristic type-II band alignment that leads to spatially separated charges; this effect is at the origin of long radiative exciton lifetimes, repulsive biexciton interaction energies, and reduced Auger recombination efficiencies. We determine these properties as a function of ZnSe shell thickness and find that the exciton emission energies and the decay times depend little on this parameter. In contrast, the spectral and dynamic properties of biexcitons vary strongly with the shell thickness. The considerable shell dependence of the biexciton decay lets us conclude that the associated Auger process involves the excitation of holes localized in the ZnSe shell. In NCs with thick shells, the large blue shift of the biexciton emission energy is mainly caused by Coulomb repulsion between electrons localized in the CdS core. The different sensitivity of exciton and multiexciton characteristics on the ZnSe shell thickness provides a unique opportunity to tune them independently.

Near-field spectroscopy of surface plasmons in flat gold nanoparticles.

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.

Femtosecond near-field scanning optical microscopy.

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with approximately 150 nm lateral resolution, approximately 250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump-probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump-probe studies of carrier dynamics in GaAs/AIGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within approximately 1 microm of the implanted stripes, an effect which could not have been resolved with a far-field system.