The physics and chemistry of semiconductor nanocrystals in sol-gel derived optical microcavities

Abstract

The incorporation of semiconductor nanocrystals (NCs) into sol-gel derived matrices presents both novel applications as well as a robust platform in which to investigate the nonlinear optical properties of NCs. This thesis summarizes our present understanding of the chemistry of the incorporation process and the applications and underlying optical physics that was gleaned from studying these NC-doped sol-gel structures. Chapter 2 describes the synthesis of NC-doped core-shell silica microsphere composites of tunable size and emission wavelength, as well as their subsequent use for in-vivo imaging of blood vessels. This chapter illustrates how an appreciation of the kinetics of the Stober process allowed for the achievement of highly monodisperse microsphere composites with a uniform incorporation of NCs. Chapter 3 reviews the requirements for achieving stimulated emission in CdSe NCs and details the development of NC-based lasers through sol-gel derived microcavities, from distributed feedback (DFB) grating structures to spherical microresonators exhibiting whispering-gallery mode lasing. Chapter 4 compares and explicates the differences between silica and titania as host matrices for NCs in terms of their chemical stability in the presence of solvents.

(cont.) This chapter explores the possibility of integrating NC-based microcavity lasers with microfluidic networks, thus providing the potential to dynamically tune the optical properties of the laser through interaction with different solvent environments on a miniaturized scale. Extension of the spectral window of NC-based gain media is discussed in Chapter 5, where extremely fast non-radiative Auger relaxation processes encountered in blue-emitting CdSe NCs may be circumvented by employing an alternative semiconductor NC, CdS/ZnS. Through judicious chemistry CdS/ZnS NCs may be uniformly incorporated into a sol-gel derived microcavity to provide room temperature lasing at blue wavelengths. Finally, our investigation into the optical physics of NCs in sol-gel derived microcavities is summarized in Chapter 6, which describes our observation of stimulated emission from multiexcitonic states in CdSe/ZnS NCs incorporated into a titania matrix. We employ transient photoluminescence to optically characterize the emission from these multiexcitonic states, which we attribute to 1P3/2-1Pc transitions. A DFB structure is introduced onto the CdSe/ZnS-titania composite to facilitate simultaneous lasing at two distinct wavelengths.