Static information storage and retrieval – Radiant energy – Liquid crystal
Patent
1994-12-12
1996-07-02
Nguyen, Viet Q.
Static information storage and retrieval
Radiant energy
Liquid crystal
359102, 359 36, 359 45, 359 55, 359 90, G02F 113, C09K 334
Patent
active
055329527
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention is concerned with a method for changing the local orientation pattern in a selectively alterable data memory as well as with an optical data memory.
Bistable optical data memories, which are based on transitions between different liquid crystalline phases and different optical properties of textures are known (Bleha, Proc. Eurodisplay '90, p. 44). It is also known that such changes of the optical properties of smectic A phases can be brought about by laser irradiation with the aid of an electric field. These data memories have severe disadvantages. Thus, data memories which operate with smectic A phases must be temperature-controlled. In general, the turn-on time and the erasure time of information is too long for broad application. In addition, the contrast is insufficient when used as projection display.
Realization of different stable orientation states, without maintaining external fields, was successful so far with nematic liquid crystals only when a highly dispersed solid is incorporated (EP 91 117 274, DE 40 41 682). In this case, upon application of a voltage for a short time, a homeotropic, transparent layer is produced, in which permanent information, characterized by a randomly arranged light-scattering orientation pattern, is written by a laser beam. In addition, by an electrochemical reaction in the orientation layer of such a "twisted" nematic cell (Barberi et al., Proc. IEEE 1991, p. 186), it was possible to reach two stable orientation states. In addition, the bistability of various textures of the cholesteric phase, which is formed by doping of a nematic phase with optically active compounds, was successful with the aid of a polymeric network (D.-K. Yang et al. IEEE, 1991, p. 49). Switching back and forth between different stable molecular orientations in a nematic phase can be done in such displays by short-time application of voltages of different frequencies or by successive application of laser light and then voltage. Changing of an inscribed information is technically very complicated here. It can be achieved by separate electrical control of each information element in a pixel or by erasing the entire information that was inscribed previously with a laser beam by application of a voltage to the large-area electrodes.
The task of the invention is to find a simple optical device which permits a selective and rapid changing of information consisting of a multiplicity of local orientation patterns written in a nematic medium, with no separate electrical control being necessary for the area that is to be changed in the data memory.
This task is solved according to the invention by an optical data memory.
Surprisingly, it was found that a local orientation pattern can be changed selectively in a nematic medium that is located between two plates with inside electrodes and consists of a nematic phase and a solid dispersed in it, by application of an auxiliary electric voltage and simultaneous irradiation with electromagnetic energy of high intensity, for example, from a laser light source, and that this change is retained after completion of the irradiation and maintenance, turning off or alteration of the auxiliary voltage. This makes it possible to change information rapidly and selectively in a simple data memory that has few electrical contacts, in the simplest case, only two contacts, and one can utilize the high resolution that is to be achieved with a laser beam.
The liquid crystals that can be used for producing the liquid crystal medium according to the invention can be low-molecular or polymeric. Preferably, they are low-molecular. They may consist of individual compounds or of mixtures of nematogenic compounds. Such compounds are generally known (see D. Demus, H. Zaschke, Flussige Kristalle in Tabelle [Liquid Crystals in Tables], Volume I (1974) and Volume II (1984), Leipzig). Preferred are compounds having general formula I --R.sub.2 I,
R.sub.1 and R.sub.2, independently of one another, stand for an alkyl or alkenyl group with 1 to 15 carbon atoms, whic
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Eidenschink Rudolf
Kreuzer Markus
NEMATEL Dr, R. Eidenshink
Nguyen Viet Q.
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