Very fast liquid crystal electro-optic switching by optical enhancement

Brief Report on EPSRC research grant no. GR/J66720
Principal Investigator: Professor J Roy Sambles, School of Physics, University of Exeter

This two year, EPSRC funded, research project was specifically to explore the potential of fast switching liquid crystal optical structures. Firstly the optical response of a range of cell designs was to be modeled including both prism and grating coupled structures. Once the best choice of structure to give potential for high contrast and fast switching had been found these were then to be fabricated with a view to obtaining better than 1µs switching times with contrast greater than 10.

Thus the work was broken into three stages: Firstly modeling the optical response of a range of liquid crystal cell structures; secondly building such structures; and thirdly characterizing their optical response.

The modeling explored broadly two types of optical systems, both involving coupling of external radiation to sharp resonant modes in optically guiding structures. One type of structure used prism coupling, the other gratings. Initially we concentrated on prism coupling since this only involves planar interface modeling, which is well established and relatively straightforward. A variety of possible geometries were explored, finally settling on a polarization conversion arrangement in which strong s (Transverse Electric) to p (Transverse Magnetic) resonances are found. These sharp resonances are sensitive to the director profile in the thin liquid crystal cell so that small angular changes (of order 1°) strongly affect the signal. Switching the polarization signal using the electroclinic effect in a homogeneously aligned S_A cell is then quite sensitive and fast. While this prism-coupled geometry proved very helpful in unraveling the physics and the detailed director structure in thin liquid crystal cells it is clearly unlikely that real devices will have large glass prisms attached. Better device potential is provided by the use of diffraction gratings to couple light to sharp resonant guided modes. However the optical response of diffractive structures containing liquid crystals is far from trivial. To facilitate the modeling of such structures we developed and implemented new modeling codes for the combination of anisotropic media and gratings. The very sharp modes found in the p to s conversion reflectivity for a range of possible configurations was explored. The optimized structure comprised a gold coated grating as the bottom plate of a LC cell and ITO coated low index top plate. The gold grating strongly converts the incident radiation into diffracted guided modes with light becoming totally internally reflected from the low index glass interface. From such cells there are very sharp p to s conversion reflectivity modes which are exquisitely sensitive to small changes in director twist.

In order to find the most suitable fast switching material we quantified, using the half-leaky guided mode technique, a range of cells containing different candidate materials. A new material, A180693-2, gave, in the S_A phase, largely temperature independent switching times below 2µs for voltages less than 5 Volts. However it was observed that the optical signal did not show just a simple damped response when the cell was electroclinically switched. The call had a strong resonance at about 220kHz and led to a switching time of order 1µs with a contrast above 10. The physics and control of this resonant effect still have to be resolved.