The researcher first studied different chemical combinations for the ChLC and the alignment layer to observe the re-orientation of the boundary orientation angle. ChLC with planar boundary condition (LC molecules close to the boundary are oriented parallel to the substrate) exhibit strong circular Bragg reflection normal to the substrate. However, our initial study shows that the planar-planar configuration is not adequate to observe the re-orientation of angle due to the strong surface orientational stiffness at the boundary. Consequently, the effect was investigated in hybrid cells (i.e. planar-homeotropic – LC molecules oriented perpendicular - boundary conditions) where the Bragg reflection is expected from the homeotropic side with the negligible stiffness. This option also failed to observe the anticipated effect as it lacks homogeneity in the order of laser beam scale (100s of microns).
Rigorous search in the rich ChLC literature leads us to open an international collaboration with a pioneer in preparing a unique phase of ChLC at Kent University, USA. The key advantage of this ChLC is that an external electric field can change its pitch through the winding/unwinding of the helical structure while preserving its structural periodicity. Hence, one can exploit this extraordinary behaviour to re-configure the orientation angle with the field under the proper boundary conditions. That is, winding or unwinding the helical structure should change the effective boundary orientation angle associated with the homeotropic cell.
Subsequently, a few samples of such ChLC are prepared at Kent university to re-initiate exploration for the BB-Pixel. Unfortunately, the homogeneity of the first generation samples was not adequate (<50microns) to test the anticipated geometric phase modulation of the Bragg reflected beam as a function of an electric field. After the first set of attempts, the second set of samples is prepared at Kent University with an improved protocol for achieving beam scale homogeneous liquid crystal and transferred to us for exploration. Also, we decided to use the liquid crystal in a hybrid boundary condition provides better inhomogeneity while not compromising the efficiency of Bragg reflection. Hence this designed and the realised sample is observed to be suitable for realising the targeted BB-pixel, the building block of the BB-SLM. Electrically controlled phase modulation of Bragg reflected light beam is subsequently observed. These results kick-start the development of a novel generation of optical phase modulation devices and generate both academic and industrial interest.
Overview of the results and their exploitation and dissemination: -
After developing and receiving the unique ChLC sample, we have carefully studied the electric field-induced phase modulation of the Bragg-reflected beam from the ChLC sample in an interferometer with reference to the incident beam. We observed at least three cycles of phase modulation for the reflected beam without changing its reflectance magnitude. Please note that a cycle of phase modulation is sufficient for the progress of the project. These results encourage us to invite additional collaboration with the expertise in electronics and micro-fabrications to realise the prototypic 5X5 pixel SLM. The results are obtained in the last months of the project duration, and the research and development activities will be continued at the host institutes. This will eventually allow us to disseminate the results to the public.
