New wavelength-tuning method in optical ring resonators with liquid crystal cladding: exploiting the longitudinal E-field

Abstract

We demonstrate tuning of the resonance wavelength of silicon-on-insulator optical ring resonators. The devices are clad with a layer of nematic liquid crystal. The electrooptic effect of the anisotropic liquid crystal allows us to change the effective index of the TE waveguide mode with an externally applied voltage. The electric field will reorient the liquid crystal director which alters the refractive index of the cladding layer. The evanescent tails of the waveguide mode feel this change. The change in effective index has a direct effect on the resonance wavelength. In our setup, the director tilts from an orientation parallel to the waveguides to an orientation perpendicular to the substrate. This way, it is the longitudinal component of the electric field of the light that experiences the largest change in refractive index. Starting from this principle, we show experimental tuning of the resonance wavelength over 0.6nm towards shorter wavelengths. Theoretical considerations and simulations with a finite element modesolver capable of handling full anisotropy confirm the experimental results and provide insights in the tuning mechanism.