Applied Physics

A Faster Turn-Off

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Science  23 Jan 2004:
Vol. 303, Issue 5657, pp. 437
DOI: 10.1126/science.303.5657.437c

The rod-shaped molecules of a nematic liquid crystal tend to align with each other to give a local orientational ordering. The direction of the ordering can be biased by the presence of surfaces and can be altered by the application of an electric field; this forms the basis of a simple liquid crystal display. However, even though these basic electro-optic cells switch on very quickly, the return to the ground state is a much slower process.

By making a number of changes to the design of the cell, Alexe-Ionescu et al. found that they could make the switching times equal. They first made the cell anisotropic by coating one of the indium tin oxide (ITO)-covered glass surfaces with an aromatic polymer film, which creates different anchoring effects at the different surfaces. This well-known trick ensures that the nematic switching is gradual instead of being triggered at a critical switching voltage. The second change they made was to replace the dc voltage with square voltage pulses at frequencies from 50 to 15,000 Hz. When this voltage was applied to the polymer side at frequencies above 100 Hz, and when the cell was switched by toggling the ground state side, fast switching off was observed. Under these conditions, there is a buildup of charge at the polymer-nematic interface, which enhances the anchoring of the nematic and thus the return to the ground state. — MSL

Appl. Phys. Lett. 84, 40 (2004).

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