DEPT OF ELECTRONIC & ELECTRICAL ENGINEERING
LIQUID CRYSTAL MODELLING
UCL logo

INFORMATION FOR

Prospective Students
Current Students

INFORMATION

Home
Group Members
Modelling
Devices
Downloads
Publications

PROJECTS

Funded
Undergraduate
Postgraduate

QUICK LINKS

Optical Devices
Ghent LC Group
GID

 

 

Post Aligned Bistable Nematic Device (PABN)

The dynamic operation of a post aligned bistable nematic (PABN) display developed by HP-labs has been modelled using our 3D Landau-de Gennes finite element program*. 

The device consists of LC material sandwiched between two surfaces. The top surface is even with homeotropic anchoring while the bottom surface c ontains an array of microscopic posts with planar degenerate anchoring. Planar electrodes for switching between the two states are placed above and below the strucuture. A device extending to infinity has been modelled by considering a single cell with periodic side boundary conditions.

The Two Stable States

Two distinct stable states exist: the tilted and the planar state. 
joasfd

tilted state

Planar State

Tilted State
Fig. 1: A cut through the diagonal of the cell showing the stable planar and the tilted states. The colour indicates the tilt angle of the director.

In the planar state, the director field bends in a continuous fashion along the surface of the posts. In the tilted state, discontinuities or point defects exist at the top and bottom corners of one of the post edges (see Fig 1 and 2).

continuous bend discontiuous bend-splay
Fig. 2: Close-ups of the the continuous and discontinuous director configurations at a post edge. The colour indicates the order parameter.

The Switching Dynamics

Switching between the two stable states is achieved by applying monopolar electric potentials to the electrodes. LC material of negative dielectric anisotropy and non-zero flexoelectric polarisability is needed in order to switch between the two states. Depending on the direction of the electric field and the sign of the flexoelectric coefficient, the director field at the edges of the post adopt either the continuous of the discontinuous configurations. The two director configurations at the post edge are separated by a -1/2 defect line running along the edge.

plane through defect line 3D view of defect line
Fig. 3: A plane cutting through the line defect at the edge of the post during switching. 3D view of the same edge, with an iso-surface of the order parameter showing the line defect.

 

  

More information on the PABN device can be found e.g. in:

[1]. S. Kitson, "Bistable Nematic Liquid Crystal Device", U.S. Patent 6,903,790, 2005

[2]. Kitson S. Geisow A. “Bistable Alignment of Nematic Liquid Crystals Around Microscopic Posts”, Mol. Cryst. Liq. Cryst., vol. 412, pp. 153-161, 2004.

[*]. Eero Willman, F. Anibal Fernadez, Richard James, Sally E. Day, “Switching Dynamics of a Post Aligned Bistable Nematic Liquid Crystal Device” ,IEEE Journal of Display Technology,4(3):276-281, September 2008.



University College London - Gower Street - London - WC1E 6BT - Telephone: +44 (0)20 7679 2000 - Copyright © 1999-2005 UCL


Search by Google