Nematic liquid crystals can switch the orientation of the director under influence of an electric field. Liquid crystals can
be combined with waveguides in many different ways: the liquid crystal can be in the core, in the cladding or in both. In
the recent past liquid crystals have been combined with glass fibers and with silicon-on-insulator waveguides. Important
progress has been achieved in the modeling of liquid crystals near inhomogeneous boundaries and the modeling of
optical waveguides with anisotropic materials. This paper discusses these recent advancements and illustrates how
waveguides with voltage tuned cutoff may be designed.
To come up to the demand for extremely sensitive biosensors for parallel real-time bioanalyses, we present several
configurations of label-free biosensors on Silicon-on-Insulator (SOI) optical chips. We discuss results on microring
resonators with a non-fouling polymer coating, increased sensitivity with slotted wire resonators and the design
and fabrication of an integrated surface plasmon resonance interferometer. The high refractive index contrast
of SOI offers submicron-size features with high quality for dense integration, high sensitivity and detection with
very low analyte volumes. The fabrication method, 193nm deep-UV lithography, allows for mass production of
cheap disposable biochips.
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.
Liquid crystals can switch under influence of an electric field or under influence of incident light. In this paper we
provide a mathematical description including electrical, optical and elastic torques. Depending on the applied voltage
and the incident light, bistability in the director orientation may be possible. Under certain conditions, the sequence of
applying incident TM polarized light and a static voltage allows to access different states.