Liquid crystalline materials for optical phase modulation

Details Liquid crystalline materials for optical phase modulation Liquid crystalline materials for optical phase modulation

2001-08-24

2003-11-18

Huff, Mark F. (Department: 1756)

Compositions

Liquid crystal compositions

C428S001100

Reexamination Certificate

active

06649087

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a liquid crystalline material having a lamellar layered or stratified structure, as well as to a device for modulating light which has a liquid crystalline material having a lamellar layered or stratified structure.
BACKGROUND INFORMATION
It is believed that current trends of transmitting and processing information optically have significantly increased the need for components for controlling the properties of light beams. In this context, varying the phase of light is also of fundamental importance for many adaptive optical systems, for example, astronomical or terrestrial imaging systems, optical communications systems, or for systems used to simulate turbulence. Available dielectric, electrooptic crystals, for example, LiNbO
3
, which can be used, for example, for wavefront correction, may exhibit long switching times, small apertures, and minimal variations in refractive indices, the latter necessitating high switching voltages to set a predefined phase modulation.
Recently, interest has intensified in liquid crystals as promising materials for use in wavefront control. In this context, the optical properties are electrically controlled, for example by orienting and deforming the local index ellipsoid of the material. It is believed that some important electrooptic effects in liquid crystals manifest themselves in a simultaneous change in the phase and polarization of the light transmitted through the liquid crystal, the change in polarization, however, being an undesirable secondary effect, for example, in adaptive optical systems and in most other phase modulation applications.
It is believed that the electrically induced transition from a homogeneous, planar orientation to a homeotropic orientation in nematic liquid crystals, described as the S-effect, may bring about a pure phase modulation when linearly polarized light is used, whose polarization plane is situated in parallel to the director of the liquid crystal in the homogeneous, planar orientation. Materials of this kind may be employed in pixelated, electrically controlled, spatially resolved phase modulators, but also in optically addressable variants, which additionally have a photoconductive layer. In both cases, several ten milliseconds may be needed to achieve a phase modulation of 2&pgr;, higher phase delays requiring relatively large layer thicknesses of about 5 &mgr;m and an electrically controllable refractive index variation of about &Dgr;n=0.15. Since switching times increase with cell thickness, the most often used wavefront correctors, which work on the basis of nematic liquid crystals, have a pulse frequency within the range of a few Hz.
In comparison, chiral-smectic, ferroelectric liquid crystals render possible switching times in the microsecond range. Available methods heretofore essentially consider two effects in the use of ferroelectric liquid crystals of this kind. U.S. Pat. Nos. 4,838,663 and 4,563,059 discuss a bistable switching between two surface-stabilized orientation states, and Swiss Patent No. CH-3722/87 discusses the deformation of the helical superstructure. While in the first case, virtually only the optical axis is rotated and no change in the refractive index occurs, in the second case, a rotation of the optical axis and a variation in the refractive index occur simultaneously. Due to the rotation of axes, the effects in the available chiral-smectic, ferroelectric liquid crystals always result in a polarization change in the light transmitted through the liquid crystal, which makes the available ferroelectric crystals appear to be unsuited for pure wavefront corrections.
Polyphilic liquid crystals, whose molecules may be composed of a rigid, readily polarizable central group including two or three phenyl rings, and of two, more flexible wing groups, in particular aliphatic chains, are also available. When an aliphatic wing group is replaced by a perfluorinated chain, a polyphilic separation of the molecular components may be achieved, the molecules thereby ordering themselves in the liquid crystalline state such that the perfluorinated groups predominately face in the same direction. It is believed that this behavior was verified in the reference, “Ferroelectrics”, Tournilhac et al., volume 114, pages 283-287, 1991 and in “Liquid Crystals”, volume 14, pages 405-414, 1993. In the investigated material F(CF
2
)
8
(CH
2
)
11
—O—Ph—Ph—CN, one ascertained an optical tilt angle &thgr; of 48° and an inclination angle &thgr;
F
of the fluorinated chain of 28°; in the material F(CF
2
)
8
(CH
2
)
11
—O—Ph—
N
Ph—CN, an optical tilt angle &thgr; of 49° and an inclination angle &thgr;
F
of the fluorinated chain of 30°; and in the substance F(CF
2
)
8
(CH
2
)
11
—O—Ph—Ph—CO—O—CH
2
CF
3
, an optical tilt angle &thgr; of 51° and an inclination angle &thgr;
F
of the fluorinated chain of 33°; Ph being an abbreviation for a phenyl ring and
N
Ph for a pyridine ring. For the mentioned polyphilic liquid crystals having perfluorinated chains, the references discusses that the optical tilt angle &thgr; specific to the rigid central group and the inclination angle &thgr;
F
of the perfluorinated chain differed from the layer normal z. Further, when perfluorinated chains are used, there may be a reduction in the molecular rotational viscosity of ferroelectric liquid crystals, so that low switching times within the range of less than 15 sec may be attainable.
However, the discussed liquid crystals may have some generally occurring polarization modulation, which may not be acceptable for most phase modulation applications.
SUMMARY OF THE INVENTION
An exemplary embodiment of the present invention is directed to providing a liquid crystalline material having a lamellar structure which can be used exclusively for phase modulation.
Another exemplary embodiment of the present invention is directed to a new liquid-crystal material class having a lamellar layered structure, the liquid crystal has arcuated or angular dimer molecules, which each include two central units, the longitudinal axes of the two central units having at least approximately opposite inclination angles, for example, opposite with respect to the layer normal. With the arrangement or system in accordance with the present invention of the two central units of the dimer molecule, the molecular index ellipsoid, which is substantially composed of the components of the two readily polarizable central units, may be positioned such that the optical axis is always parallel to the layer normal, so that a polarization modulation of light may be prevented, which is transmitted in a defined direction, perpendicularly to the layer normal through the liquid crystalline material.
To maximize the desired phase modulation, depending on the specific embodiment of the present invention, the amount of the two inclination angles may be within the range of about 10 to 90°. In order to construct the dimer molecule and thereby form a vertex or peak, the two central units may be bonded or joined together, it being possible for the bond to be formed by at least one neutral, molecular chain. Depending on the specific embodiment of the present invention, the vertex between the two central units of the dimer molecule can be an angle apex or, in the case that an arc may be formed between the central units, it may also be an arc midpoint.
The formation (or configuration or geometry) of the liquid crystalline material in accordance with the present invention can be applied to a multiplicity of thermotropic and lyotropic liquid crystals. In this context, the vertices (or peaks) of the dimer molecules of adjacent layers can be nearly unidirectional or, in another specific embodiment, approximately directed in opposite directions, thereby forming a double-layer structure.
To produce a helical structure having a predefined pitch, which can be canceled by applying an external electric field, so that a phase modulation is adjustable, the azimuth angles of the vertices of successive layers may change uniformly by a p

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