We have measured the nonlinear optical response of Cadmium Sulfide quantum dots (CdS QD) in a poly(propyleneimine) dendrimer matrix having diaminobutane (DAB) core. Large refractive nonlinear coefficients and low absorption losses were observed at all wavelengths. Dendrimers are nanosize, highly branched, tree like monodisperse macromolecules that emanate from a central core with a branch occurring at each monomer unit. Dendrimers encapsulations convey stability, control of emission wavelengths by QD size. The branching points in the interior of the dendrimers are occupied by tertiary nitrogen to provide numerous nucleation sites to drive formation of QD clusters of small size. The dendrimer-stabilized CdS QDs were stable at room temperature, both in solution and in solid state for several weeks. Thin films were deposited by spin casting from methanol solutions. The resulting samples consisted of a 1mm thick quartz substrate with a 200-400 Å nonlinear optical film on one side. The Z-scan technique was used to characterize the NLO response. A mode-locked YAG laser provided the laser pulses with 30-ps duration at 355 nm, 532 nm and 1064 nm at a 20-Hz repetition rate with energies per pulse ranging from few microjoules to several millijoules. These results indicate relatively large values for the nonlinear response (> 10<sup>-10</sup> esu) at all three wavelengths. Our calculations indicate that quantum dot-organic systems have large optical nonlinearity due to interactions between excitons in the quantum dots and the organic medium. We calculate that an increase of the QD radius to ~4-8 nm will result in a substantial enhancement of the nonlinearity.
We calculate the imaginary part of the third order optical non-linearity for an array of semiconductor quantum dots in an organic host and show that it leads to large two-photon absorption. The calculated two-photon absorption is greater than currently measured materials. The large non-linearity results from a hybrid exciton formed in the inorganic-organic medium. The band gap of the semiconductor dot determines the spectral region of the resonances that vary from the visible to the near, mid and far infrared regions. We show that relatively small changes in the ratio of the quantum dot size to the quantum dot-to-dot spacing result in significant changes in the non-linearity. We briefly describe applications in communications, optical filters, and bio photonics for thin films comprising these hybrid excitions.
Multiphoton processes are playing a significant role in microfabrication, patterning of nanostructures, formation of photonic band gap materials and ordered micro/nanoarrays. However, many of the chromophores with large two-photon absorption used for these applications are actually hybrid materials in which the two-photon absorption is coupled to an excited state absorption. This coupling makes the detailed analysis of the photophysics significantly more complex. We have developed a numerical technique to investigate hybrid multiphoton processes. Our numerical method compares very well with published results.
We examine the effects of multiple incident pulses and determine that for closely spaced pulses adjacent pulse interactions lead to pulse coalescence resulting in increased peak energy and greater absorption. Furthermore, trailing pulses reach a reduced transmission level.