Research Of Adjustable Period Optical Grating Based On Flexoelectric Effect

Bent-core nematic(BCN)liquid crystals have some special electro-fluid effects and appear plentiful structures of flexodomains under a certain amplitude and frequency of external applied voltage.When a planar aligned nematic or hybrid aligned nematic liquid crystal cell with appropriate temperature(the liquid crystal in the cell at nematic state)under a dc voltage or ultra-low frequency ac voltage,some flexodomains will appear and their structures are like optical grating which have diffraction property.The period of this grating is on the micron scale and can be clearly observed by a polarizing optical microscope.The period and diffraction efficiency of this grating can be changed by the temperature as well as the amplitude and frequency of external applied voltage.In addition,the different rubbing treatments on internal surface of substrates and the cell gap of liquid crystal cell can also influence the period and diffraction efficiency.The BCN liquid crystal of 2,5-bis{4-[difluoro(4-heptylphenyl)methoxy] phenyl}-1,3,4-oxadiazole(7P-CF2O-ODBP)was filled in planar aligned nematic liquid crystal cells with different pretilt angles at the boundaries and different cell gaps.The changed structures of flexodomains were observed by a polarizing optical microscope and tested by the light diffraction technique.The period decreased with the increased amplitude of external applied voltage at a certain temperature in a liquid crystal cell.When a certain voltage was applied to liquid crystal cell,the period increased with the increased temperature and had a linear relationship with the temperature.In addition,when the temperature and voltage were certain,the period of the grating also increased with the increase in cell gaps and the decrease in pretilt.The experiment results showed that the period changes with the cell gap were more obvious than the pretilt angle at the boundaries of liquid crystal cell,thus,changing the cell gaps would change the range of the period obviously.The period of this grating based on the flexoelectric of a BCN liquid crystal can be adjusted continuously without a special treatment on the electrodes of a liquid crystal cell or a complicated design of driving circuit.The continuously adjustable period optical grating should be a potential application of optical grating in the future.