Complex diffraction device

Details Complex diffraction device Complex diffraction device

2002-03-22

2004-06-15

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Liquid crystal optical element

Liquid crystal diffraction element

Reexamination Certificate

active

06750941

ABSTRACT:

FILED OF THE INVENTION
This invention relates to a complex diffraction device, which is applicable to various fields, such as optics, optoelectronics, optical information recording, and liquid crystal display devices and to various uses such as those of security and design, and can be produced so as to have a large area and handled easily.
BACKGROUND OF THE INVENTION
Diffraction devices or holograms, a type thereof, utilizing a photo-diffraction phenomenon have various functions such as a lens function, a spectrum function, a branching/multiplexing function, and an optical intensity distribution conversion. Due to these functions, they have been widely used in spectroscopic devices, hologram scanners of bar-cord readers and optical pickups of compact disks. Moreover, they have also been used for the purpose of preventing credit cards or various notes from being counterfeited, by utilizing the difficulty in counterfeit and designability of the holograms as well.
The diffraction devices are classified according to the shapes thereof into amplitude type diffraction devices and phase type diffraction devices. The amplitude type diffraction devices are those in which a light is allowed to pass through non-light-transmitting parts with a uniform thickness, such as long thin wires, periodically arranged so as to obtain diffracted light. The phase type diffraction devices are further classified into those in which periodic grooves are formed on a surface of a substrate which does not absorb light and refractive index modulation type devices in which regions where the refractive index is periodically varied are formed in a layer with a uniform thickness. Unlike the amplitude type diffraction devices, the phase type diffraction devices can be enhanced in diffraction efficiency because of the absence of a region which does not transmit light. Examples of the phase type diffraction devices each having grooves on its surface are those obtained by forming grooves on a surface of a glass, metal, or plastic. Examples of the refractive index type diffraction device are holograms made using gelatin dichromate or photo polymers.
The helical structure of a smectic liquid crystal is also known to function as one of the refractive index type diffraction devices as described in, for example, Jpn. J. Appl. Phys., Vol. 21, page 224, (1982).
In the above-mentioned use of preventing credit cards or notes from being counterfeited, a hologram is produced by embossing a thermoplastic film so as to form grooves. However, in the case of seeking more enhanced designability and anti-counterfeit properties, there is a limit for such a hologram. If a light made incident on a diffraction device can be diffracted in a plurality of directions or angles therefrom, the device can be expected to find more extensive uses.
The object of the present invention is to provide a complex diffraction device which has a refractive index modulation type diffraction function using the helical structure of a liquid crystalline phase in combination with a diffraction function using grooves formed on its surface, resulting in enhanced designability, and is easy to set diffraction angles and to handle, and adaptable to size increase.
DISCLOSURE OF THE INVENTION
The diffraction device according to the present invention is characterized in that a diffraction function originating from an uneven pattern is imparted to a diffraction device comprising a liquid crystal layer where the helical orientation of a smectic liquid crystalline phase having a helical structure is maintained.
The diffraction device according to the present invention has two types of diffraction functions one of which is obtained from the fixed helical orientation of the smectic liquid crystalline phase and the other of which is obtained from an uneven pattern formed on the surface thereof.
The smectic liquid crystalline phase of the liquid crystal layer used in the present invention denotes a liquid crystalline phase wherein the liquid crystalline molecules form a smectic layer structure which is one-dimensionally crystal and two-dimensionally liquid.
Examples of the smectic liquid crystalline phase are smectic A phase, smectic B phase, smectic C phase, smectic E phase, smectic F phase, smectic G phase, smectic H phase, smectic I phase, smectic J phase, smectic K phase, and smectic L phase. Among these, preferred are those wherein liquid crystalline molecules are aligned in tilting relation to a normal direction of the smectic liquid crystal layer, such as smectic C phase, smectic I phase, smectic F phase, smectic J phase, smectic G phase, smectic K phase, and smectic H phase.
Alternatively, in the present invention, there may be suitably used liquid crystalline phases exhibiting optical activity and ferroelectricity such as chiral smectic C phase (SmC*) chiral smectic I phase (SmI*), and chiral smectic F phase (SmF*), liquid crystalline phases exhibiting optical activity and antiferroelectricity such as chiral smectic C
A
phase (SmC
A
*), chiral smectic I
A
phase (SmI
A
*), and chiral smectic F
A
phase (SmF
A
*), and liquid crystalline phases exhibiting optical activity and ferrielectricity such as chiral smectic C
&ggr;
phase (Sm C
&ggr;
*), chiral smectic I
&ggr;
phase (Sm I
&ggr;
*), and chiral smectic F
&ggr;
phase (Sm F
&ggr;
*).
Further alternatively, there may be suitably used those which are chiral and exhibit a smectic phase having a helical structure as described in J. Matter. Chem. Vol. 6, page 1231 published in 1996 or J. Matter. Chem. Vol. 7, page 1307 published in 1997.
However, with the objective of easy synthesis of liquid crystalline materials, easy orientation of the helical structure in the smectic liquid crystalline phase, easy variation of the helical pitches, and stability of the helical structure, the most preferred is chiral smectic C phase or chiral smectic C
A
phase.
The term “helical structure in the smectic liquid crystalline phase” used herein denotes a structure wherein the longitudinal axes of the liquid crystalline molecules tilt at a certain angle from the vertical direction of each smectic layer, and the tilt directions twist little by little from one layer to another. The center axis of the helix in this helical structure is referred to as “helical axis”, while the length in the helical axis direction for one helical turn is referred to as “helical pitch”.
When a light is allowed to pass through a liquid crystal layer comprising a helical-structured smectic liquid crystalline phase, the diffraction direction of the light depends on the helical axes direction of the liquid crystalline phase. For example, in the case where the helical axes are parallel to the liquid crystal layer, a light made incident vertically thereto is diffracted to the helical axes direction. No particular limitation is imposed on the helical axes direction of the liquid crystal layer forming the complex diffraction device of the present invention. Therefore, the helical axes direction may be properly selected such that the desired characteristics can be exhibited. For example, the helical axes direction may be parallel to or tilted with respect to the liquid crystal layer surface. Furthermore, the tilt angle may be varied discretely or continuously. Moreover, the helical axes direction may be microscopically determined by the domain and may be macroscopically directed to various directions or to the same direction. The helical structure is not necessarily formed entirely in the liquid crystal layer and thus may be formed on the surface area or interior of the liquid crystal layer, or a part thereof.
The helical pitch in the liquid crystal layer is usually from 0.1 to 20 &mgr;m, preferably 0.2 to 15 &mgr;m, and more preferably 0.3 to 10 &mgr;m. The helical pitches may be constant in the liquid crystal layer, but may be varied depending on positions therein. The variation may be continuously or discretely. The helical pitches can be properly adjusted in a conventional manner, for example, by adjusting the orientation conditions such as temperature, the o

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