Optical Properties of Nanoparticles in Composite Materials

Nanoparticles are synthetic structures with dimension from 1 to 100 nanometers and are various in types. Some favorable properties peculiar to the nanoparticles (generally owing to size effects) make them prevailing and beneficial for applications in different scientific and engineering fields. A large portion of these properties find their connection to optics and photonics. In the context of optics, the thesis is devoted to study of two specific categories of nanoparticles, gold nanoparticles and CdSe-CdS core-shell quantum dots, aiming at investigating the influence and potential of the particles in applications of lasing and medical diagnosis/treatment. Gold nanoparticles have been widely exploited in radiative decay engineering to achieve fluorescence enhancement or quenching of fluorophores, with the help of a localized surface plasmon resonance band in visible range. As the technique is recently introduced to lasing applications, the influence of the gold nanoparticles on the photostability of the gain medium needs more attention. In this work, the effect of size and concentration of gold nanoparticles on altering the photostability of aqueous solution of Rhodamine 6G in lasing process is demonstrated and analyzed. Energy transfer and nanoparticle induced heat are found to be responsible for the acceleration of photobleaching. It is shown that coating the gold nanoparticles with a 15 nm thick silica layer can effectively diminish the photostability degradation of the gain medium. Gold nanorods are popular for in vivo diagnostic and therapeutic applications due to their strong absorption of near-infrared light. A novel type of multimodal nanoparticles based on gold nanorods is synthesized here and optically characterized. The coating of silica and gadolinium oxide carbonate hydrate renders the nanoparticles superior performance as MRI/CT contrast agents than commercially available products. Meanwhile, the precise temperature control of bio-tissues using the particles under laser irradiation makes them promising for photothermal treatment of cancer cells. The thesis also addresses several open questions with respect to CdSe-CdS core-shell quantum dots. A numerical model is built to study the spatial separation of electrons and holes in the dots with different core/shell sizes. QDs in different geometrical shapes are investigated. It is found that the spherical core-shell QDs can be flexibly tuned between the type-I and the type-II regime by varying the dimensions of the core and the shell. The feature is confirmed by time-resolved photoluminescence measurements, in which the carrier recombinations from different spatial paths can be distinguished. A sign of amplified spontaneous emission is observed with spherical dots of an appropriate combination of core radius and shell thickness, indicating the potential of the QDs for lasing applications.