Organic/silica hybrid films which possess very large second-order nonlinear optic effect were prepared and their nonlinear optical properties, temporal stability, thermal stability and photobleaching effect were investigated. Using the films, passive and active waveguide devices were fabricated and their performances were characterized.
The hybrid film where organic chromophore molecules with large hyperpolarizability were incorporated into a silica matrix by physical blending (host-guest) or chemical bonding (side-chain) was prepared by sol-gel process. The side-chain films of a good optical quality were fabricated and the doping rate of the chromophore was well above 50 wt.%. The atomic force microscopy (AFM) measurements revealed that the surface roughness of the film was smaller than 1.5 nm.
The macroscopic second-order nonlinearity in the hybrid film was established by both the corona poling and the DC contact poling methods. No nonlinearity was observed in the poled host-guest film, which may be due to low chromophore concentrations (below 5 wt.%) and a fast relaxation. However, depending on the poling conditions, the values of nonlinear optic coefficient were distributed over a wide range for the side-chain films and the films retained the initial $r_{33}$ value over 800 hours at room temperature. The measured value of nonlinear optic coefficient strongly depends on poling conditions such as a poling temperature, a poling time, a heating rate and a soft baking time. The largest value obtained for the electro-optic coefficient,$r_{33}$ was 16 pm/V at a wavelength of $1.3\mu m$, which is larger than the values reported so far for the various sol-gel systems to the best of my knowledge.
During the poling above the glass transition temperature $T_g$, a transient behavior of second harmonic (SH) signal was observed for the first time. The SH intensity grew very fast at the initial stage of the poling process and gradually decayed to an equilibrium value.
Relaxation phenomena above and below $T_g$ were investigated by measuring dielectric constants, a second harmonic intensity and electro-optic coefficients as a function of time and temperature. The distribution function of the relaxation time and the glass transition temperature could be estimated from the measured values.
It was found that the thermal stability of the film was greatly enhanced as a poling time increased and a poling temperature became higher.
The refractive index change of the film by photobleaching effect was measured and the index distribution inside the film was calculated by the inverse WKB approximation. The photobleaching time for reducing the index by 0.01 was shorter than 200 minutes and thus the photobleaching effect can be utilized to fabricate a low loss optical channel waveguide.
Passive and active waveguide devices such as a channel waveguide, a Y-branch, a directional coupler and a Mach-Zehnder modulator were successfully fabricated using the hybrid film as a core layer. The single mode operation of passive devices was confirmed by observation of guiding mode profiles. For a Mach-Zehnder modulator, the fabricated channel waveguide did not satisfy the single mode condition in such a way that a mode-mode coupling complicated the resulting mode profile. However a periodic variation of a mode pattern was observed with a modulation voltage period of 48V.
