Refractive Index Tuning of Siloxane-Based Electro-Optic Self-Assembled Superlattices (21001)
A “wet-chemical” process affording molecule-based electro-optic (EO) material refractive index tuning in self-assembled superlattice (SAS) organic structures. The process retains essential microstructural acentricity, without electric field poling. Application potential exists in a wide range of EO devices including modulators, waveguides, switches, emitters and detectors.
ADVANTAGES: Robust, adherent, intrinsically acentric microstructures are produced without electric field poling. The self-assembly process is amenable to automated production, modular and flexible. EO modulator design is simple and fabrication is straightforward. SAS structures can be grown on a variety of substrates, allowing efficient integration with other device components.
SUMMARY: Molecule-based EO materials integrated into photonic devices offer the potential for greatly increased information density/bandwidth telecommunications and reduced device design/fabrication complexity. Chemisorptive siloxane-based layered self-assembly can yield robust, structurally precise, acentric chromophore self-assembled superlattices (SASs) directly on silicon or related substrates, allowing facile device integration. This approach offers higher EO responses (r33) and lower dielectric constants (e) than conventional inorganics (e.g. EO figure of merit, n3r33/ e= 50-300 pm/V for SA films vs 8.7 pm/V for LiNbO3). SAS modular construction enables the incorporation of tuned EO modulator active region refractive indices beyond conventional pi-electron EO materials promising (i) more effective light confinement in waveguiding active regions due to enhanced refractive index contrast between active and cladding layers and (ii) better velocity matching of the modulating radio frequency and optical waves for optimum high frequency modulation.
This invention provides organic EO material refractive index tuning by intercalating high-Z optically transparent nanoscopic metal oxide sheets into SAS structures while retaining microstructural acentricity, large EO response and structural regularity. An iterative process including 1) chemisorption of a large molecular hyperpolarizability chromophore, 2) covalent capping protection, 3) deposition of Ga or In oxide layers and 4) covalent capping protection afford stable acentric microstructures. The process yields uniform multilayer SAS thin films that strongly adhere to glass, silicon or indium tin oxide-coated glass substrates.
X-ray characterization of the structures confirms equal quantities of chromophore, polysiloxane and metal oxide in each repeating layer. High-Z oxide intercalation into the SAS structure increases the film index of refraction (n=1.76 (Ga) and n= 1.84 (In) at 650 nm), substantially greater than that of the comparable metal free SAS-multilayers (n=1.52). Second harmonic generation (SHG) measurements at 1064 nm indicate a quadratic dependence of the 532 nm light output intensity with the number of layers, indicating preservation of acentric microstructures per layer-by-layer assembly. Nonlinear susceptibilities and macroscopic EO coefficients for SAS structures gave favorable n3r33/ e ~103 ppm/V (Ga) and n3r33/ e ~76 ppm/V (In) values.
Milko van der Boom, Seong-Sik Chang, Seng-Tiong Ho, Tobin J. Marks
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