Abstract:
Cycloidal boundary conditions for aligning liquid crystalline materials are obtained by mechanical rubbing of a polymer coating. The rubbing is performed by a rubbing head rotating around an axis perpendicular to the rubbing plane while the alignment polymer film is being translated across the rubbing film such as only a linear portion of the alignment film touches the rubbing film at any given time.

Description:
CROSS REFERENCES 
     U.S. Patent Documents 
       [0001]    U.S. Pat. No. 7,048,619 May 2006 Park et al. 
         [0002]    U.S. Pat. No. 8,045,130 October 2011 Son, et al. 
       RELATED APPLICATIONS 
       [0003]    This application claims priority to Provisional Application No. 61/771,895 filed Mar. 3, 2013, the contents of which are relied upon and incorporated herein. 
     
    
     FIELD OF THE INVENTION 
       [0004]    This invention relates to fabrication of liquid crystal, including liquid crystal polymer diffractive waveplates (DWs) with the aid of mechanical rubbing. DWs are used in imaging, sensor, communication, photonics, laser and display technologies. 
       BACKGROUND OF THE INVENTION 
       [0005]    The structure of one of the optical components of interest is schematically shown in  FIG. 1 . Essentially, it is an optically anisotropic film  100  with the optical axis orientation  101  rotating in the plane of the film, the x,y plane in  FIG. 1 . In simplest realization, the rotation angle α of the optical axis orientation is a linear function of a single coordinate, α=πx/Λ with Λ characterizing the period of the pattern. The thickness L of the film is defined by half-wave phase retardation condition L=λ/n ∥ −n ⊥ ), where n ∥  and n ⊥  are the principal values of the refractive indices of the material; and λ is the radiation wavelength. Such a structure imposes a phase shift 
         [0000]      Φ=±2 α( x,y )
 
         [0000]    on circular polarized beams propagating through it with the sign depending on the handedness of polarization. With account of α=2πx/Λ=qx, where q=2π/Λ, an unpolarized beam is thus diffracted into +/−1 st  diffraction orders with the magnitude of the diffraction angle equal to λ/Λ. The phase Φ in the equation above, known as geometrical or Pancharatnam phase, does not depend on wavelength, hence the broadband nature of the diffraction. Due to its half-wave plate nature, there are well developed techniques for making the component essentially achromatic in a wide range of wavelengths. 
         [0006]    Obtaining large diffraction angles requires that the optical axis modulation period Λ be comparable to the wavelength λ. Liquid crystals (LCs) are the only materials that allow obtaining continuous optical axis modulation patterns at micrometer scale and in a technologically efficient manner. Moreover, due to record high optical anisotropy, Δn=n ∥ −n ⊥ ˜0.1, the thickness of the film providing 100% diffraction efficiency is also comparable to the wavelength. 
         [0007]    The molecules of a LC material are easily aligned along an anisotropy axis of a substrate. There are two major techniques for inducing structural anisotropy on a substrate. In the photoalignment technique demonstrated in  FIG. 2 , in a first step, the substrate  200  is coated with a material that creates a thin layer  202  (˜10-100 nm) of random distribution of molecules  203 . Due to absorption dichroism, the molecules are aligning according to the polarization of typically UV light beam  210 , parallel or perpendicular, depending on the so-called photoalignment material,  FIG. 2B . Perpendicular aligned molecules  204  is shown in  FIG. 2B  for certainty. Lastly, the substrate is coated with LC layer  220  the molecules wherein  221  align along the anisotropy axis produced in the photoalignment material  202 ,  FIG. 2C . The LC can be polymerizable for some applications. 
         [0008]    The cycloidal polarization modulation pattern is typically obtained holographically in the overlap region of right- and left-circular polarized beams. Holographic technique requires expensive lasers providing coherent beams, optics and opto-mechanical stabilization systems. Radiation power and beam size limitations limit the use of the technique to small components only. The materials used for photoalignment are also expensive, not widely available, and often do not provide strong enough orientation conditions for LC molecules. 
         [0009]    Thus, there is a need for a technique that would allow fabricating DWs with the aid of mechanical rubbing of inexpensive polymer films well-developed and commonly used for liquid crystal display technologies. There is a wide prior art related to mechanical rubbing, as for example evident from the U.S. Pat. Nos. 7,048,619 to Park et al. or 8,045,130 to Son, et al. However, to the best of our knowledge none addressed the opportunity for producing general patterns with high spatial resolution. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The objective of the present invention is providing a method for producing boundary conditions for cycloidal alignment of liquid crystalline materials by mechanically rubbing a substrate coated by an alignment polymer. 
         [0011]    The second objective of the present invention is using the mechanical rubbing technique for fabrication of cycloidal diffractive waveplates, particularly, large area waveplates. 
         [0012]    The third objective of the invention is providing a method for producing boundary conditions for nonlinearly patterned alignment of liquid crystalline materials by mechanically rubbing a substrate coated by an alignment polymer. 
         [0013]    The fourth objective of the present invention is using the mechanical rubbing technique for fabrication of diffractive waveplates with nonlinear alignment patterns. 
         [0014]    Still another objective of the present invention is producing patterned boundary conditions with high spatial resolution using mechanical rubbing. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]      FIG. 1  schematically shows the structure of a cycloidal diffractive waveplate (CDW): The angle α made by the optical axis of an anisotropic material (liquid crystal) with a coordinate axis in x,y plane varies along a single coordinate (x), α=πx/Λ, where Λ is the period of the optical axis rotation in space. 
           [0016]      FIG. 2  schematically shows the method of producing photoaligned liquid crystal films: (a) coating a substrate with photoaligning material layer; (b) exposing to polarized UV light creates an anisotropy axis on the substrate; (c) depositing liquid crystal layer (including a polymerizable liquid crystal). 
           [0017]      FIG. 3  schematically shows the method for creating orienting conditions for LCs by mechanical rubbing: the substrate carrying a polymer film (typically, polyimides) is translated under a rotating wheel covered by a rubbing surface (typically, a textile). 
           [0018]      FIG. 4  schematically shows the concept for producing cycloidal orienting conditions due to mechanical rubbing: the rotating wheel in this case carries the alignment polymer while rubbing is performed by an oscillating substrate carrying the rubbing surface. The rubbing direction is rotated due to phase shift between oscillations along x and y axes. 
           [0019]      FIG. 5  schematically shows a system wherein a one-dimensional translation stage is rotated by a rotation stage around the axis perpendicular to the translation plane. 
           [0020]      FIG. 6 . schematically shows a rubbing drum simultaneously rotated around two axes. 
           [0021]      FIG. 7  schematically shows an alignment film spool-wound and pulled across to a take up spool such as only a line of the film touches the rubbing film at any given time. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not limitation. 
         [0023]    The method of aligning LCs due to mechanical rubbing, still the main technique used in fabrication of LCDs, is shown in  FIG. 3 . A wheel  300  with a rubbing film  310  wrapped around it, typically a textile, is rotating around an axis  301  such as the rubbing film  310  touches the surface of a substrate  320  carrying the alignment polymer layer  313 , typically, a polyimide, poly-vinyl alcohol, etc. The substrate is translated exposing fresh areas of the polymer to the rubbing action. The process creates anisotropy due to aligning the polymer fragments  312  as well as scratching the surface of the polymer at nanoscale. This anisotropy is not only sufficient for aligning the LC layer that is coated on top of the film, but it often provides the strongest anchoring strength known in the LC technology. 
         [0024]    A circular rubbing process can be used for obtaining axially symmetric alignment conditions for LCs. Microrubbing with a tip of an atomic force microscope can be used for creating cycloidally patterned alignment conditions. However, the latter is slow and applicable to rather small areas only. 
         [0025]    In the preferred embodiment of the current invention shown in  FIG. 4  the film  411  providing the rubbing is attached to a mechanical stage  401  providing oscillation with an amplitude A and at a frequency (ω) optimized for the rubbing. Said stage is capable of 2D oscillations in the rubbing plane (x,y plane, for certainty). The oscillations along the x and y axes are programmed to provide effective rotation of the rubbing direction at a frequency (Ω) determined by the period of the orientation structure to be produced, 
         [0000]      Δ x=A  cos(ω t )cos(Ω t )
 
         [0000]      Δ y=A  cos(ω t )sin(Ω t )
 
         [0000]    Thus the rubbing angle α is changing as Ωt with time t. 
         [0026]    The alignment polymer  421  is coated on a cylinder  402  that is brought in touch with the oscillating rubbing surface  411 . The cylinder  402  is rotating around axis  403  exposing different linear areas of the alignment film to the rubbing at different angles.  FIG. 4B  presents a view of the alignment polymer with oriented parts  423  and a part still not rubbed  422 . 
         [0027]    Conventional commercially available textile materials well-known in the prior art can be used for providing the rubbing surface  411 . For example, these include velvets. 
         [0028]    Conventional commercially available materials such as polyimides and poly-vinyl alcohol (PVA) can be used as alignment layer. The alignment layer, typically of submicron sizes, can be deposited on a substrate in a number of techniques, including spin-coating, dip coating, printing, etc. For example, 0.5 wt. % solution of PVA in distilled water can be spin coated on a glass substrate by spinning at 3000 rpm for a 60 s. 
         [0029]    To produce strong rubbing, the oscillation frequency ω shall be chosen higher than the rotation frequency a For example, one can chose Ω=1 Hz while ω=10Ω=10 Hz. The specific values for frequencies as well as the oscillation amplitude should be experimentally optimized for best conditions for specific rubbing cloth, velour, rayon, etc. 
         [0030]    A desired effective rubbing length L, by that, is obtained for N=L/4 A full oscillation. For example, to obtain 100 cm effective rubbing length with a 1 cm oscillation amplitude, the number of full oscillations N shall be equal to 25. At 10 Hz oscillation frequency, it will require 2.5 s effective rubbing time. 
         [0031]    In another embodiment shown in  FIG. 5 , the stage  401  with the oscillation capability in two dimensions is replaced by a single-axis translation stage  501  mounted on a rotation stage  502 . 
         [0032]    In another preferred embodiment shown in  FIG. 6 , the stage  401  providing 2D oscillations is replaced by a rubbing drum continuously rotating around two axes. In an example of realization of such 2D rotation shown in  FIG. 6 , the drum  605  with the attached rubbing layer (rubbing cloth)  606  is continuously rotated with the motor  604 . The rotation axle  607  is mounted onto a frame  603 . The frame  603  is mounted on a platform  602  that is brought into rotation by the motor  601 . The arrows in  FIG. 6  indicate the rotation directions around the two orthogonal axes. The diameter of the drum with rubbing layer and the frequency of its rotation can be chosen according to the required rubbing conditions. For example, the drum  605  can have a diameter of 50 mm and be rotated around the axis  607  at 100 rpm. The rotation frequency around the axis  602  can be chosen according to the required pattern. 
         [0033]    The oscillation and rotation can be at constant frequencies or be modulated in time to produce nonlinear alignment patterns. For example, the rotation frequency around the axis  602  can be accelerated to produce parabolic variation of alignment axis on the rubbed surface. 
         [0034]    The cylinder  402  carrying the alignment material in  FIG. 4  and  FIG. 5  is shown for illustration of the concept. In another preferred embodiment shown in  FIG. 7  the rubbing is performed in a roll-to-roll type of process. For example, the band of a polymeric substrate  722  carrying the alignment layer  721  and rolled around the first drum  703  is translated to the second drum  704  while being stretched around a support rotating disk  705 . It is rubbed then with the 2D rubbing system, for example, comprising a 1D oscillation stage  701  rotated by the motor  702 . Instead, the system shown in  FIG. 6  can be used indeed. The rubbed film  723  then is rolled around the axis of the drum  704 . During the translation, the film may be subject to additional processes such as deposition of additional coatings. These coatings, for example, can be polymerizable liquid crystals such as, for example, RMS series materials from Merck. 
         [0035]    Although the present invention has been described above by way of a preferred embodiment, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.