Abstract:
An electro-optical device including a biaxial liquid crystal having a primary and secondary director is provided. A change in orientation of at least one of the directors due to an applied electric field produces a change in the optical state of the device. Response times are increased dramatically over uniaxial nematic based liquid crystal devices, offering improved performance and efficiency.

Description:
[0001]     This application claims the priority benefit of U.S. application Ser. No. 60/711,046, filed Aug. 24, 2005, the disclosure of which is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST  
       [0002]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. NSF-DMR-03-12792 awarded by the National Science Foundation. 
     
    
     BACKGROUND  
       [0003]     Conventional liquid crystal devices are based on what may be called the “uniaxial nematic” liquid crystal phase, which has one optic axis. The typical molecules constituting the uniaxial nematic phase are roughly cylindrical molecules. In these devices, the optic axis, also known as the director, is oriented in a predetermined configuration using surface effects. Surface alignment films are prepared on two substrates to persuade the director to be parallel to one of the in-plane directions. The substrates are then put together to fabricate a nematic device in a number of different configurations. If the director orientation at the two surfaces is along orthogonal directions, it results in a twisted nematic (TN) device. If it is at 180° or 2700, it is called a high birefringence effect (HBE) or super twisted nematic (STN) device. Several other modes, e.g. parallel or OCB mode, are known and used.  
         [0004]     The operation of such devices relies on producing different optical path-lengths for a beam of light passing through it. An applied electric field is applied by connecting a battery to the conducting electrodes predeposited at the two opposing substrates. The field exerts a torque on the director and tries to make it perpendicular to the substrates. During the change in director reorientation due to the applied field, the elongated LC molecules reorient about their short axis to change from ‘lying down’ at the substrate to ‘standing up’ position. Since the index of refraction parallel and perpendicular to the director are different. The “field-off” and “field-on” states appear different to a beam of propagating polarized light and its state of polarization is altered as it passes through such a device. An analyzer positioned at the exit side of the device at proper orientation is used to change the director reorientation in to optical contrast. This forms the principle of operation of LC display devices.  
         [0005]     Speed of switching in the nematic devices depends on the elastic constants and the effective viscosity of the liquid crystal used. Generally, the switching speed lies in 10-100&#39;s of ms range. The speed of these devices has been one of the limiting factors and the major reason for needing more complex active matrix substrates. It is becoming obvious that faster devices are necessary to make significant advances and produce next generation of LC displays and electro-optical devices. The use of biaxial nematic liquid crystal lends itself to the fabrication of much faster devices.  
         [0006]     The existence of a biaxial nematic liquid crystal phase possessing two orthogonal optic axes has previously been predicted. The recent discovery of novel mesophases formed by bent-core liquid crystals has encouraged scientific interest in the study of this phase and the possibility of the biaxial nematic phase being used for technological applications including display devices. In particular, computer simulation of such “boomerang” type molecules has suggested that the bent-core shape could exhibit a stable biaxial nematic phase. Recently, several studies have confirmed the existence of a stable biaxial nematic phase in several homologous series of liquid crystal compounds.  
         [0007]     The molecules that are likely to form a biaxial nematic phase are made of ellipsoidal molecules that have, on average, three different dimensions in contrast to the cylindrical molecules that form the uniaxial nematic phase. They have two optic axes or directors, denoted as m and n (bold letter implies vector or directional nature). The advantage of such materials is that, in addition to reorientation of the principal director n, the secondary director m can also reorient. Reorientation of m requires rotation of its shorter axes about the long molecular axis, which can happen very fast. This is equivalent to a person turning on ones side while lying on a flat surface. Since the biaxial phase has three different indices of refraction in the three spatial directions, this turning (or, spinning) of molecules also offers a change in optical path length in the direction perpendicular to their length and thus a means to fabricate an electro-optical device.  
         [0008]     It is now proposed herein that higher switching speeds in an electro-optic device can be realized using a liquid crystal exhibiting such a biaxial nematic phase.  
       BRIEF SUMMARY  
       [0009]     In one embodiment, there is provided an electro-optical device including first and second substrates, each of the first and second substrates coated with an alignment layer, and a liquid crystal material positioned between the substrates, the liquid crystal material including a biaxial liquid crystal having a first principal director n and a secondary director m, wherein a change in the optical properties of the device are effected by reorienting at least one of m and n.  
         [0010]     In a second embodiment, there is provided a biaxial retardation film including first and second transparent electrodes, and a liquid crystal material positioned between the electrodes, the liquid crystal material including a biaxial nematic liquid crystal having a first principal director n and a secondary director m, wherein n and m are perpendicular to each other and parallel to the electrodes at an applied electric field of zero.  
         [0011]     In a third embodiment, there is provided a method of forming an electro-optical device comprising depositing a liquid crystal material between first and second substrates, each of the first and second substrates coated with an alignment layer, the liquid crystal material comprising a biaxial liquid crystal having a first principal director n and a secondary director m, wherein a change in the optical properties of the device are effected by reorienting at least one of m and n. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an illustration of various nematic liquid crystal phases and their directors; (a) uniaxial nematic phase of cylindrical molecules, (b) uniaxial nematic phase of disk-like molecules, (c) biaxial nematic phase of a mixture of rod-like and disk-like molecules, (d) biaxial nematic phase of flat roughly rectangular shaped molecules, and (e) biaxial nematic phase formed by bent-core molecules.  
         [0013]      FIGS. 2A and 2B  are an illustration of two different configurations of the m director of a biaxial nematic in a cell while the n director orientation remains unchanged.  
         [0014]      FIG. 3  is a change in x-ray scattered intensity at the Fredericks transition for m in a biaxial nematic liquid crystal as observed with the help of x-ray diffraction experiment.  
         [0015]      FIG. 4  is a graphic depiction of the response characteristics of the reorientation of the n director in a cell filled with bent-core liquid crystal (a) at 160° C. while the liquid crystal is in the biaxial nematic phase, and (b) at 170° C. while the liquid crystal is in the uniaxial nematic phase. The n switching response is about twice as fast in the biaxial phase as in the uniaxial phase.  
         [0016]      FIG. 5  is a structure of a bent core liquid crystal suitable for use in the present embodiments.  
         [0017]      FIG. 6  is a structure of another bent core liquid crystal suitable for use in the present embodiments.  
         [0018]      FIG. 7  is a liquid crystal cell according to one embodiment wherein the cell is in a transmitting state with no applied voltage.  
         [0019]      FIG. 8  is a liquid crystal cell according to one embodiment wherein the cell is in a non-transmitting state with an applied voltage. 
     
    
     DETAILED DESCRIPTION  
       [0020]     As detailed above, a biaxial nematic phase has recently been discovered in various bent-core or banana-shaped liquid crystal and in tetrapodic liquid crystals of the type shown in  FIG. 6 .  FIG. 1  shows a schematic illustration of various uniaxial nematic phases as well as several recently discovered biaxial nematics with major and minor directors.  FIG. 1 ( a ) shows a uniaxial nematic comprised of cylindrically symmetric (calamitic) mesogens while (b) shows a uniaxial discotic nematic. Biaxial nematic phases are shown in (c), (d) and (e), which consist of a mixture of rods and disks, anisometric parallelpiped platelets, and bent-core mesogens, respectively.  
         [0021]     With respect to bent-core molecules that have chevron-like shape, the direction of n and m lie along the average long axis and the apex of the molecule, respectively. In an electro-optic device, when the biaxial nematic phase of these materials is sealed between two substrates with an accompanying alignment layer, the director n aligns along the rubbing direction of the alignment layer. In a zero voltage state, the plane containing n and m is parallel to the substrates, as seen in  FIG. 2A . When an electric field is applied between the substrates, m reorients and become perpendicular to the substrate and parallel to the field, as seen in  FIG. 2B . This reorientation, known as Frederick&#39;s transition, is observed in x-ray diffraction as a change in scattered intensity from the cell, as seen in  FIG. 3 . Although the reorientation response time in this x-ray experiment has not been measured, it is expected to be 10 to 100 times faster than in the uniaxial nematic phase.  
         [0022]     In one embodiment, the bent core liquid crystal displaying biaxial nematic phase comprises an oxadiazole molecule having the formula:  
                         
 
 where R 1  and R 2  independently comprise a straight chain alkyl or substituted alkyl having from 5 to 20 carbon atoms. Additional bent core liquid crystal molecules suitable for use in the present embodiments include those shown in  FIGS. 5 and 6 . These examples are not meant to be limiting but merely exemplary of suitable liquid crystal molecules. 
 
         [0023]     In addition to the expected speed increase in the reorientation of the m director in devices utilizing biaxial nematics, the speed of reorientation of the n director in the biaxial nematic phase has also been found, in one case, to be approximately 2 times faster than in the uniaxial phase of same material, as can be seen in  FIG. 4 . In  FIG. 4 , the response characteristics of the reorientation of the n director in a cell filled with bent-core liquid crystal is shown. While in the uniaxial nematic phase at 170° C. (right), the reorientation response time is approximately 15 ms, while in the biaxial nematic phase at 160° C. (left), it is found to be less than 7 ms. This discrepancy is thought to be due to the fact that the ratio of viscosity to the elastic constant is smaller in the biaxial phase than in the uniaxial phase. Thus the use of the biaxial nematic phase offers an advantage even when used like a uniaxial nematic phase (i.e., exploiting the reorientation of n) in devices. The devices using biaxial nematic liquid crystal switch faster than the uniaxial nematic devices, irrespective of whether the electro-optical effects are realized through the reorientation of the m or n director.  
         [0024]      FIGS. 7 and 8  show an exemplary electro-optical device in accordance with one embodiment. A biaxial nematic liquid crystal  10  having n and m directors is positioned between a pair of opposing substrates  12 ,  14 , which are preferably coated with an alignment layer (not shown), such as rubbed polyimide. Crossed polarizers  16 ,  18  are positioned on either side of the substrates. The director n aligns along the rubbing direction of the alignment layer. In a zero voltage state, the plane containing n and m is parallel to the substrates, as seen in  FIG. 7 . This state reorients the polarized light  20  and allows it to be transmitted  22  through the second polarizer. When an electric field is applied between the substrates using a voltage source  24 , n reorients vertically, while m remains unchanged, as seen in  FIG. 8 , or takes up random orientation. Polarized light is prevented from passing through the second polarizer, resulting in a darkened state. This reorientation of n is used to create optical contrast and thus a display device. Such a device switches faster then a device based on uniaxial nematic liquid crystal as shown in  FIG. 4 ( b ).  
         [0025]     A second significant use of the biaxial nematic, which may also be extended to the biaxial smectic phase that has been confirmed in bent-core materials, is in the fabrication of tunable biaxial retardation films. Such films have been employed to reduce light leakage at 45° to the polarizer and analyzer directions in LC devices. The use of biaxial nematic liquid crystal makes it relatively easy to fabricate retardation films that can be electrically tuned.  
         [0026]     Presently, commercial biaxial retardation films are prepared by constructing multi-layered stacks of uniaxial liquid crystal layers including arrangements of distinct planar and vertically aligned liquid crystal layers with precise control of the thickness and director n orientation. In a fraction of the films in the multi-stack, the uniaxial optical axis is oriented in plane (planar) while in others it is perpendicular to the film&#39;s plane (vertical alignment). Composite films of this type behave as biaxial films of constant biaxiality but are cumbersome to produce at high cost. Moreover, once formed, the degree of biaxial retardation that it offers can not be altered.  
         [0027]     A uniformly aligned film, of the type shown in  FIG. 2 , of biaxial nematic phase of bent-core liquid crystal functions, by its very nature, as biaxial retardation film. The fabrication of such a film is essentially one step process of preparing one liquid crystal layer, and thus quite easy. In addition to this ease of construction, an enormous advantage is that the retardation of such a biaxial nematic retardation film is tunable with the application of an electric field.  
         [0028]     Specifically, in devices made with biaxial nematic phase liquid crystal, one can exploit both directors. One possibility is that in a single device, one could manipulate n with, say low-frequency electric field to control the optical path length to create optical contrast while controlling m with the help of a high-frequency field to ‘compensate’. In this manner, no additional component will need to be incorporated in the device. Elimination of additional components also removes many optical boundaries in the device and reduces spurious light scattering from various interfaces, thereby increasing its efficiency and light throughput. If both directors were exploited for dynamic performance in a device made of the biaxial N phase, it will offer four extreme optical states corresponding to the possibilities of their orientation with respect to the transmitted beam of light.  
         [0029]     An exemplary biaxial retardation film comprises two transparent substrates with optically transparent but electrically conducting electrodes deposited on them. The electrodes may be films of indium-tin-oxide (ITO) or conducting polymer. The conductor layer is covered with a polymer which is either treated with polarized UV or mechanically rubbed to produce liquid crystal alignment. Most common method used in the LC industry is a film of polyimide (PI) which is mechanically rubbed after processing it through a prescribed thermal cycle. The methods of deposition, thermal processing, and rubbing are known to those skilled in the art of making LC display devices.  
         [0030]     The substrates are then assembled in to a cell with a cell gap, the magnitude of which can be varied depending on the two birefringences of the biaxial nematic phase and desired retardation. The cell gap is typically fixed with the use of glass or plastic spherical or rod-shaped beads as spacers. Normally, the spacers are sprayed on to one of both surfaces before the substrates are assembled in to a cell.  
         [0031]     The cell is then filled with the selected liquid crystal, for example with A103 shown in  FIG. 5 . This compound aligns with n parallel to the rubbing direction and m in the plane of the cell but perpendicular to n. In this state, the cell offers fixed retardations to the two polarization of incident light. The devices architecture resembles the schematic drawing in  FIG. 2 . The retardation offered by such devices can be controlled by applying a dc or ac electric field of different frequencies to control the orientation of m and/or n. These films act as tunable biaxial retardation films. The special feature of tenability and easy fabrication are possible only with the use of biaxial nematic liquid crystal.  
         [0032]     There are many other possibilities for use of biaxial LC devices such as in beam steering, holographic, and optical communications to define and control beam polarization, wave front shaping, and phase and amplitude of electric field associated with propagating light beams. These materials can also be used in photonic crystals.  
         [0033]     Much of the above described method of making a device applies to the biaxial smectic-A phase also, which have been recently reported to exist. The difference lies in how the smectic planes and the director m can be aligned for device fabrication. Being a smectic-A phase, some of the problems plaguing the ferroelectric SmC* technology may be absent, rendering this phase a preferred one.  
         [0034]     The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.