|Nonlinear Optical Waveguides from Dilute Composites Consisting of Polymer with Partially Aligned Nanorods||7.02 MB|
Thesis Type:MSc thesis
The expected permittivity and third order nonlinear susceptibility, of a low filling fraction composite consisting of semiconductor nanorods dispersed in a polymer host are derived, using the Maxwell-Garnett model for anisotropic nonlinear inclusions. The semiconductor nanorods are modeled both as prolate spheroids and more realistic capsule shapes. A new generalized model is presented for various nanorod axis orientation statistics, achieved by an aligning electric field. The angular distribution function of the nanorods is calculated for nanorods with a permanent electrical dipole moment, which assists the alignment of the nanorods. Using the angular distribution function, the composite macroscopic characteristics are found for a composite with random orientation, partially aligned and nematic array nanorods. As the alignment field strength increases, the composite optical properties
asymptotically converge towards the nematic case. Different parameters relate to the nanorods geometry are examined, concluding that the main parameter influencing the alignment is the single NR volume, while for the nematic array the single nanorod axes aspect-ration is the major parameter. Due to the symmetry of the nanorods, the composite characteristics depend on the polarization of the optical electrical field, with a symmetry that resembles a uniaxial crystal.
A nonlinear waveguide with a core made of such a composite is simulated, in order to find the nonlinear parameter of the waveguide. The model takes into account two electrodes for the alignment process, far enough from the waveguide core, in order to avoid losses to the optical mode. Significant optical response can be achieved even for randomly oriented nanorods composite, with a nonlinear parameter of 68(W×m)-1. The alignment process increases the nonlinear parameter significantly even at elevated temperature that are needed for polymerization of the polymer host, typically 150oC. Aligning field strength of 107 V/m results with very high value for the nonlinear parameter – 120(W×m)-1, much higher than ordinary glass based nonlinear optical fibers, that result with nonlinear parameter up to 50(W×m)-1.