We studied the optical transient bleaching of ∼40 Å, ammonia-passivated CdS clusters in a polymer with nanosecond and picosecond pump-probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30-ps pump laser pulse width, we tentatively attribute the bleaching to the exciton-exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron-hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron-hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron-hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron-hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron and hole are present, the lowest exciton absorption is red-shifted from the original exciton absorption, and this transition has a weak oscillator strength, which explains the observed efficient bleaching. The model also predicts that a trapped electron is more efficient than a trapped hole for bleaching the excitonic absorption of CdS clusters in the size regime considered here. This is confirmed by pulse radiolysis results. Finally, we discuss the possible effects of charged surface defects on the linear absorption spectra of semiconductor clusters.

We report a picosecond pump–probe study of 55 Å cadmium sulfide microcrystallites embedded in polymer films. Large negative absorbance changes at wavelengths corresponding to energies near the band gap are observed. This absorption bleaching and the associated changes in refractive index are mainly responsible for the large nonlinearity observed in degenerate four-wave mixing experiments. Based on photoluminescence data, the known electron-trapping cross section of defects, and these pump–probe experiments, we show that the conventional carrier density-dependent band-filling mechanism cannot account for the data, and the absorption bleaching is due to the saturation of the excitonic transition. We further show that the phase-space filling and exchange effects from exciton–exciton and exciton-free carrier interactions fail to account for the observed data. Instead, we propose that the exciton-trapped carrier interaction is mainly responsible for the observed bleaching of the excitonic absorption. This interaction is unique for small semiconductor clusters since the presence of a high density of defects (most likely on the surfaces) causes the extremely rapid trapping of free carriers. According to this model, the recovery time of the absorption bleaching is determined by the trapped-carrier relaxation time, which is sensitive to the fabrication methods and can be controlled by surface chemistry. Our study also demonstrates that one needs to understand the effects of surfaces and control the surface chemistry before the important question of size effects on the nonlinear optical properties can be addressed.

Low temperature absorption spectra are reported for NpCl3 and NpBr3 in the range 3300–40 000 cm−1. Term assignments are made to a number of excited levels not previously identified, and the data are interpreted in terms of a refined free-ion model. Some of the parameters associated with this model are found to have similar values for both 3+ actinide and lanthanide chlorides. The values determined for other of the parameters are in good agreement with those calculated independently using Hartree-Fock methods. The similar crystal-field quantum number ordering of levels in the ground states of actinide trichlorides and analogous lanthanides doped into LaCl3 is noted and discussed.

A polymer (Nafion) film doped with 55 Å CdS microcrystallites exhibits a large resonant third order nonlinearity.

Polymers and glasses doped with small semiconductor clusters represent a new class of materials. These materials are interesting for two reasons. First, the size of the semicondutor clusters can be controlled to vary from a few to hundreds of Å. This provides a vehicle to study the transition of a semiconductor from molecular to bulk. Secondly, by doping polymers or glasses with these small semiconductor clusters, utilizing their large resonant third order nonlinearity, new optically nonlinear composite materials can be prepared. In this paper we will discuss the linear and nonlinear optical properties of PbS and CdS clusters in polymer films.