A finite element modesolver and beam propagation (BPM) algorithm are applied to the optical analysis of liquid crystal waveguides. Both approaches are used in combination with advanced liquid crystal calculations and include a full dielectric tensor in solving the Helmholtz equation to model the liquid crystal behavior properly. Simulation of the beam propagation in a waveguide with tunable liquid crystal cladding layer illustrates the coupling of a Gaussian beam to the fundamental waveguide mode obtained with the modesolver. Excellent quantitative agreement between both approaches illustrates the potential of these methods for the design of advanced devices. The high accuracy of the BPM algorithm for wide angle propagation, essential in the analysis of high index contrast waveguides, is illustrated for angles up to 40 deg.
A finite element framework is presented to combine advanced three-dimensional liquid crystal director calculations
with a full-vector beam propagation analysis. This approach becomes especially valuable to analyze and design
structures in which disclinations or diffraction effects play an important role. The wide applicability of the
approach is illustrated in our overview from several examples including small pixel LCOS microdisplays with
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.
Nanolithographic fabrication techniques may soon enable electrically-driven LCoS
devices to be manipulated using ultra-nanoscale CMOS transistors. However, questions
as to the switching properties of such LCoS devices arise due to the diminishing dimensions
of their transistors. Thus, experimental investigations into the response times and
the onset-threshold voltages for LCoS devices were embarked upon. Such measurements
were obtained for various electrode dimensions and cell gaps. Furthermore, an interdigitated
(IDT) electrode pattern was used to drive the homeotropically-aligned NLC material
in a direction parallel to the bounding planes of the cell. Experimental findings
revealed that faster response times were achieved when the electrode spacings were decreased.
Such results have shown that a 10μm-thick device with an electrode pitch of 2μm
can achieve a switch-on time of < 5ms. In addition, decreasing the electrode spacing results
in the threshold voltage to drop. The results therefore indicate that improvements in
a LCoS device's switching properties can be realised by using smaller electrode dimensions.
The presence of ion impurities in a liquid crystal cell has a detrimental effect on the performance of the cell in terms of switching times and optical response. Here we present simulated and experimental results showing the effect of these impurities at different states of switching.