Robert A. Norwood

Two-Photon Absorption in CdSe Colloidal Quantum Dots Compared to Organic Molecules

We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models.

High-reflectivity Bragg mirrors for IR applications using novel chalcogenide hybrid inorganic/organic polymers (CHIPs) (Conference Presentation)

Bragg mirrors are 1D photonic crystals made of a periodic stack of high and low refractive thin film materials that reflect only a small bandwidth of the spectrum. Such high-reflective devices are commercially available in the visible spectrum at relatively low costs. Functional Bragg mirrors for IR applications are greatly desired, but there exist challenges due to the limited availability of inexpensive, high refractive index, and transparent IR materials. Here, we present the design of high reflectivity Bragg mirrors working in the near and mid-IR range. Our mirror designs use the refractive index values of novel ultrahigh refractive index IR materials known as chalcogenide hybrid inorganic/organic polymers (CHIPs). CHIPs are synthetized from an inverse vulcanization process for elemental sulfur and selenium as reported by the Pyun group [1]. We integrate the optical properties of these materials and those of different low refractive index materials to generate various Bragg mirror designs. The extinction coefficients derived from absorption plots are also taken into account to increase accuracy. The theoretical values for reflectivity, physical thickness, and optical bandwidth are reported, as well as preliminary experimental results. We also present changes in reflectivity and bandwidth due to layer thickness variability. These IR Bragg mirrors have applications in devices and industries which require the use of specific wavelengths in the near and mid-IR range, such as beam-splitters, filters, anti-reflection coatings, etc.

Polymers with unprecedented NLO response

Using efficient electro-optical polymer poling in hybrid sol-gel EO modulators we have achieved 0.65V VπMach-Zehnder modulators in a new EO polymer, AJ309, which undergoes thermal crosslinking during the poling process.