Matériaux organiques pour l’électro-optique
Until now, high-speed integrated electro-optical (EO) modulators devices used in telecommunications networks are base on inorganic materials. However, the limit of their performances has roughly been reached and as future needs require an increase in the rate of modulation above 50 GHz, the development of new approaches is required. Among these, the use of organic EO materials, having a low dielectric permittivity (ε ≈ 4-7) offers the possibility to reach cutoff frequencies higher than those obtained with inorganic materials with high dielectric constants (ε > 10). In addition, the performance of organic materials in terms of EO coefficients now exceeds those of inorganic materials (rorg > 100 pm.V-1, while r≈30 pm.V-1 for LiNO3). Thus, by using properly functionalized organic materials, it becomes possible not only to reduce the switching voltage of the modulators (Vπ), but also to achieve modulation rates above 100 GHz.
EO organic materials described in the literature are mainly constituted of commercial polymers (PMMA, PC) doped with push-pull chromophores oriented by an applied electric field. It is imperative to find a way to maintain this orientation as the dipolar interaction tends to favor a centrosymmetric configuration and thus to annihilate the EO properties. Over time, different strategies for freezing push-pull molecules have been devised, but one that showed excellent results to disperse the molecules within a polymer matrix and orient them with an external electric field. Here, the materials we used have a high glass transition temperature (> 100 °C), which induces a freeze of molecular motions at room temperature. The orientation of the molecules is then done at a temperature close to that of the glass transition (T ≈ Tg-10°C), for which the molecules are mobile and steerable under the applied electric field (of the order of 10 V μm-1). By lowering the temperature back to the ambient while maintaining the field the orientation of the molecules is frozen, creating a non-centrosymmetric distribution in the material.
EO materials are usually packaged in thin films obtained by spin-coating. To investigate their effects on a guided mode, the active material is packed between two layers of material of lower index of refraction. In order to apply the electric field required for polarization and characterization of the material, the samples were prepared on substrates of glass / ITO and a metal electrode is deposited on top by evaporation. Thus, layers of low index materials provide both the confinement of the optical wave and electrical insulation of the entire sample.
We have the possibility to measure the NLO properties of doped polymers both by measuring the second harmonic generation (SHG) intensity and by ellipsometry. We have a specially adapted SHG setup to be able to measure in real time in situ NLO properties, in a cell allowing the control of the atmosphere, of the temperature and the poling of the sample via corona effect. The measurement can be made at 1.9 μm, were the vast majority of organic materials is transparent. We also measure the electo-optical response by ellipsometry at 1.5 μm, a standard wavelength for telecom applications.
We master the various aspect of the doped polymer preparation and characterization. Today, thanks to our collaboration with the Chemistery for Optics group at ENS Lyon, our best material exhibits an electro-optic coefficient of 70 pm.V-1 @ 1.5 μm. This material, comparable to lithium niobate in terms of performance EO modulation as characterized by the product n3r (refractive index n and r EO coefficient) and does not show significant drop of its efficiency after more than 10 months.
Currently, we are exploring new concepts for the development of EO polymers with improved efficiency and stability, with their implementation in integrated optical telecom devices as a final objective.