Abstract

Creating a Switchable Diffraction Grating in a Liquid Crystal Cell

Ekaterina Sergan, University of California Davis

John Noe, Laser Teaching Center, Stony Brook University

This project focused on the study of polymer stabilized liquid crystals (LCs).

Nematic LCs are birefringent rod-shaped molecules that respond to an electric field. In our case, the applied field polarizes the molecules along their long molecular axis and re-orients them along the field direction. Creating a polymer network at the boundaries of an LC cell disables the ability of the LC molecules to respond in this way. If the polymer network is periodic, then a voltage-controlled grating can be created. The polymer network is formed by irradiating the cell with a wavelength of light that actives the photoinitator. At the same time a relatively high voltage is applied across the cell to not only re-orient the LC molecultes but also drive the polymer precursor (monomer) to the two substrates of the cell. After the voltage is removed, the areas that were not exposed to the light relax back to their original planar orientation and a variable phase shift between the ordinary and extraordinary waves of polarized light incident normally to the cell results in a spatially varying head-on retardation. Applying a switching voltage switches the planar areas between high and low retardation.

We used LC cells (gap ~20 microns) that were filled with a mixture of an LC material, about 1% of a photo-curable monomer, and about 0.01% of a UV (~300 nm) sensitive photoinitiator. The high voltage used was 120 VAC (rms) and the light source was a convential RadioShack blacklight. The cell was irradiated for about 30 minutes though a mask with an approximately 50-50 pattern and a period of slightly greater than 1.0 line/mm.

The ability of the pattern to diffract light was studied by illuminating it with red light (633 nm) from an unpolarized HeNe laser. We observed a diffraction pattern that consisted of several fringes and a bright central spot. The fringe pattern changed as an AC control voltage was applied to the cell. The largest pattern (about 20 visible fringes) was observed at a peak to peak voltage of 3.00 V. The spacing in the pattern was consistent with the spacing in the mask to better than 10%. We are currently analyzing this spacing more carefully, in part to see if it is affected by the applied switching voltage. The diffraction pattern could be turned off by applying > 40 V peak-to-peak. From this we conclude that we did indeed create an electrically controlled diffraction grating. Some very interesting polarization effects were also observed. In one orientation of a linear polarizer only fringes could be seen, while in the perpendicular orientation only the undiffracted beam spot was visible. The polarizer had the same effect whether it was placed before or after the grating.

We thank the Optics Lab at California State University Sacramento for providing the prepared LC cells. We would also like to thank T. Sergan, V. Sergan and H. Metcalf for their invaluable contributions. This work was supported by the National Science Foundation (PHY-0851594).

References:

1. V. Sergan, T. Sergan, and P. Bos, Chem. Phys. Lett, 486, 123 (2010)

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