TiO2 nanoparticles loading on the microwave and optical properties of the electro-optic polymer PMMA-DR1 for optimization of microwave photonic components

Thanks to a very good velocity matching between the microwave signals and optical waves in polymeric materials and progress in the synthesis of high hyperpolarizability chromophores, electro-optic (EO) polymers offer a very attractive possibility to realize low cost photonic microwave components with a very wide bandwidth and a very low driving signal. Indeed, on one hand, the EO coefficient of EO polymers achieves up to 350 pm/V [1], which is much higher than 31 pm/V, that of the popular inorganic EO crystal Lithium Niobate (LiNbO
₃
). On the other hand, RF bandwidth of 110 GHz is demonstrated for stabile E-O polymer based optical Mach-Zehnder (MZ) modulators [2]. We study the influence of dielectric nanoparticles on the microwave and optical properties of the polymers. It consists of a fine tuning of the microwave and optical properties of the EO polymers by loading with dielectric nanoparticles, aiming at improving the bandwidth by reducing the difference between the effective refractive index and dielectric constant [3]. For our investigations, a classical polymer such as poly (methyl methacrylate) (PMMA) filled or not with disperse red 1 (DR1) EO dyes is used as host polymer. Titanium dioxide (TiO
₂
) nanoparticles are chosen due to the transparency of TiO
₂
at the telecommunication wavelength of 1.55 μm [4]. The 1,1,2-trichloroethane (TCA) is the solvent used for the PMMA polymer matrix, the DR1 chromophore and the polymer-nanoparticles suspensions. The TiO
₂
nanoparticles are homogeneously incorporated in the EO polymer by an ultrasonic stirring. Different TiO
₂
nanoparticles contents ranging from 1 to 3wt.% with respect to PMMA are used. A rigorous control of the agglomeration is performed by dynamic light scattering. TEM observations and Raman spectra are used for the classification of the nanoparticles. The thin film necessary for this study is produced by spin coating from the parent solution (PMMA + DR1 + TiO
₂
+ solvent). The first step of our investigation is to study the influence of TiO
₂
on the microwave dielectric properties (permittivity and loss tangent) up to 10 GHz and the refractive index at telecommunication wavelength of the PMMA as a function of the TiO
₂
loading. The obtained results show that both the dielectric constant and the refractive index of PMMA and PMMA-DR1 films may be continuously tuned by incorporating TiO
₂
nanoparticles. This results show a continuous increase of the dielectric constant in the frequency range from 100 MHz to 10 GHz with TiO
₂
nanoparticles loading, by 22% at 8 GHz from 1 to 3%wt TiO
₂
nanoparticles with respect to PMMA. Interestingly, the dielectric losses are not increased to the same extent by TiO
₂
filling (x% at 8 GHz). The refractive index increases with the TiO
₂
nanoparticles loading by less than 1%. The obtained results will be used to optimize the bandwidth of EO modulator based in polymer by using HFSS software.