Investigation of optical properties of polymethines for potential application in all-optical signal processing

Abstract

Demonstration of ultrafast all-optical signal processing (AOSP) using silicon as the active material has been limited by large two-photon absorption loss and long lifetimes of the resulting free carriers. For AOSP at speeds in the terahertz, an order of magnitude faster than that the fastest current electronic counterpart, a class of π-conjugated organic molecules called polymethines provides a promising alternative to silicon as they possess large third-order nonlinearities, and ultrafast polarization response to an incident field. The challenge in the application of polymethines as active nonlinear optical materials for AOSP is in translating their promising molecular properties into bulk material properties. The large linear polarizability and charged nature of the polymethines molecules strongly promote aggregation and phase-separation in solid blends, offsetting their advantageous molecular optical properties. In this work, polymethines’ resistance to deleterious spontaneous symmetry breaking and aggregation was enhanced by substitutions of metal- and chalcogen- containing terminal groups, and rigid steric groups above and below the π-conjugated plane of polymethine chain. The resulting polymethines/amorphous polycarbonate (APC) blend films demonstrated an unprecedentedly high two-photon figure-of-merit, |Re(χ(3))/Im(χ(3))| and low linear loss. The optical quality of the polymethines/APC films was also improved by replacing the commonly-used alkyl ammonium counterions with more polarizable aryl phosphonium counterions with moderate ground state dipole moment. The resulting dye-polymer blend films showed an enhanced near-infrared transparency while its magnitude of the third-order susceptibility, |χ(3)|, showed a good agreement with that extrapolated from the molecular third-order polarizability, γ. For facile integration of these promising organic materials into SOH, the substrate surface was functionalized using silane coupling chemistry for the reduction of surface energy mismatch between the polymer films and the waveguide containing substrates. The optical and SEM micrographs showed vastly improved coverage and infiltration of the microfeatures. Furthermore, to enable the precise engineering of waveguide cross-sectional dimensions for single-mode propagation in the organic cladding, the dispersion curves of the polymethines/polymer blends were generated using prism coupling and ellipsometry. The combined efforts in the development of molecules and materials discussed in the thesis have culminated into a successful identification and optimization of the polymethines dyes and their polymer blends for imminent demonstrations of on-chip AOSP at terahertz speed.