Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1992-05-18
1993-09-21
Sikes, William L.
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359 76, 359 77, 359 78, 359 91, 359102, 359100, G02F 1133
Patent
active
052473791
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND DESCRIPTION OF PRIOR ART
Conventional liquid crystal devices, as exemplified by twisted nematic or supertwist nematic devices, utilize a quadratic electrooptic effect, i.e. an effect sensitive to the magnitude but not to the sign of an applied electric field. The response of the liquid crystal material to the external field is, in this case, of dielectric nature, and the decisive material parameter is the dielectric anisotropy, that is, the difference between the value of the dielectric constant along the long molecular axis and the value perpendicular to that axis, which is also the optic axis. According to whether this anisotropy is positive or negative, an applied electric field will have the tendency to align the material in such a way that the optic axis is along or perpendicular to the field, respectively. According to its nature, this response increases proportional to E.sup.2, the square of the applied field.
Since about a decade a radically different liquid crystal technology is growing based on ferroelectric liquid crystals, first described by R. B. Meyer et al. in Journal de Physique, volume 36, pages L69 to L71, 1975. The first ferroelectric liquid crystal patent described the so-called surface-stabilized ferroelectric liquid crystal (SSFLC) and was filed by N. A. Clark and S. T. Lagerwall in 1980 and issued in 1983 as U.S. Pat. No. 4,367,924. The corresponding first announcement of high speed liquid crystal devices was made by the same authors in Applied Physics Letters, volume 36, pages 899 to 901, 1980. In these devices which are characterized by microsecond or submicrosecond speed, and by the completely new feature of symmetric bistability, the active electro-optic effect is a linear one, i.e. sensitive to the sign of the field. Whereas in a twisted nematic device the two distinct optical states are characterized by the field being ON or OFF, the SSFLC device can be driven between its two distinct states by changing the sign of the applied electric field.
Later linearly responding devices without bistability were described by Lagerwall et al. in U.S. Pat. No. 4,838,663, filed in 1987. The devices use different liquid crystal phases, being orthogonal smectics rather than tilted smectics used in the ferroelectric devices. These paraelectric phases, similar to the solid crystal case, typically exhibit a so-called soft mode, which is detectable as a pretransitional effect immediately before entering the ferroelectric phase. In the liquid crystal case such an effect was described first by S. Garoff and R. B. Meyer in Physical Review Letters, volume 38, page 848, from 1977 and they coined the word electroclinic for the response. This response means that the molecular axis n, which is also the optic axis, rotates a certain angle .theta. when an electric field is applied perpendicular to n, cf. FIG. 1. When the field direction is inversed, the induced tilt angle is in the opposite sense. The induced tilt .theta. is proportional to E, but in the case of Garoff et al., so small that it requires phase sensitive methods for even being detected. In U.S. Pat. No. 4,838,663, which uses a different geometric configuration and explores a different temperature regime, the induced tilt is orders of magnitude larger and the effect is also distinguished by the fact that the response time is independent of the applied field, which makes the field an excellent control variable for .theta. and thereby for a grey scale in devices up to very high frequencies.
Recently it has been discovered that the electroclinic effect can be detected not only in the orthogonal smectic phase, but also in the higher-laying nematic phase, that is in the very opposite regime to that investigated by Garoff et al. This was first reported by one of the authors (Komitov) at the International Conference on Optics and Interfaces in Liquid Crystals, held in Torino, Oct. 14-20, 1988; cf. FIG. 2. Independently, the same finding was reported by Z. Li, R. Petschek and C. Rosenblatt in the Physical Review Letters, volume 62, pages
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Andersson Gunnar
Flatischler Kurt
Komitov Lachezar
Lagerwall Sven T.
Stebler Bengt
Duong Tai V.
S. T. Lagerwall S.A.R.L.
Sikes William L.
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