Fabrication of high efficiency, high quality, large area diffractive waveplates and arrays

Abstract

The objective of the present invention is providing a method for fabricating high quality diffractive waveplates and their arrays that exhibit high diffraction efficiency over large area, the method being capable of inexpensive large volume production. The method uses a polarization converter for converting the polarization of generally non-monochromatic and partially coherent input light beam into a pattern of periodic spatial modulation at the output of said polarization converter. A substrate carrying a photoalignment layer is exposed to said polarization modulation pattern and is coated subsequently with a liquid crystalline material. The high quality diffractive waveplates of the present invention are obtained when the exposure time of said photoalignment layer exceeds by generally an order of magnitude the time period that would be sufficient for producing homogeneous orientation of liquid crystalline materials brought in contact with said photoalignment layer. Compared to holographic techniques, the method is robust with respect to mechanical noises, ambient conditions, and allows inexpensive production via printing while also allowing to double the spatial frequency of optical axis modulation of diffractive waveplates.

1 . A method for producing spatially periodic orientation modulation of an anisotropy axis of a photoresponsive material layer, the method comprising:
 (a) a light source emitting a light beam;   (b) a polarization converter periodically modulating in space the polarization of said light beam;   (c) a photoresponsive material characterized by an anisotropy axis that may be formed or aligned according to polarization of said light beam;   (d) exposing at least a portion of said photoresponsive material layer to the polarization modulation pattern produced by said polarization converter.   
     
     
         2 . The method of  claim 1  further comprising optical means for projecting said polarization modulation pattern of said light beam onto at least a part of the area of said photoresponsive material layer, said projection generally changing the size, shape and topography of said polarization modulation pattern obtained at the output of said polarization converter. 
     
     
         3 . The method of  claim 1  wherein said polarization converter comprises at least one diffractive waveplate that may be achromatic, and may be part of an array. 
     
     
         4 . The method of  claim 1  further comprising at least one substrate for controlling at least one of the following properties of said photoresponsive material layer: mechanical shape and stability, thermal conductivity, thickness homogeneity, radiation resistance, and resistance to adverse ambient conditions. 
     
     
         5 . A method for producing spatially periodic orientation modulation of an anisotropy axis of a photoresponsive material layer, the method comprising:
 (a) a light source emitting a light beam;   (b) a polarization converter periodically modulating the polarization of said light beam along a single axis;   (c) a photoresponsive material characterized by an anisotropy axis that may be formed or aligned according to polarization of said light beam;   (d) means for holding and positioning a layer of said photoresponsive material;   (e) means for positioning and projecting said polarization modulation pattern of said light beam onto a part of the area of said photoresponsive material layer;   (d) means for exposing different areas of said photoresponsive material layer to said polarization modulation pattern.   
     
     
         6 . The method of  claim 5  wherein the means for holding and positioning the layer of said photoresponsive material include at least one of the following: a glass substrate; a polymer substrate, a drum, a translation stage, and a rotation stage. 
     
     
         7 . The method as in  claim 5  wherein the means for exposing different areas of said photoresponsive material layer to said polarization modulation pattern includes at least one of the mechanical motions, translation in the direction perpendicular to the polarization modulation axis, and rotation, said motions performed with the aid of at least one of said positioning means: the positioning means of the holder of said photoresponsive material layer, and the positioning means of said polarization modulation pattern. 
     
     
         8 . A method for producing orientation modulation of an anisotropy axis of a photoresponsive material layer at a predetermined spatial period, the method comprising:
 (a) producing a linear polarized light beam;   (b) propagating said light beam through a diffractive waveplate, the diffractive waveplate having optical axis modulation period twice larger compared to said predetermined spatial period.   (b) exposing a photoresponsive material layer to said light beam propagated through said diffractive waveplate, the photoresponsive material having the ability of producing an anisotropy axis modulated according to the polarization of said light beam.   
     
     
         9 . Any of the methods of  claim 1 ,  5 , or  8  further comprising at least one anisotropic material layer with ability of producing an optical axis modulation according to and under the influence of the anisotropy axis of the photoresponsive material layer. 
     
     
         10 . A method of fabricating high quality diffractive waveplates for providing diffraction efficiency greater than 95% over an area of greater than 1″ in diameter, and scattering losses less than 1% comprising:
 (a) a source of a light beam; 
 (b) means for periodically modulating the polarization of said light beam across the beam profile; 
 (c) a photoresponsive material layer with ability of producing an anisotropy axis modulated according to said polarization pattern; 
 (d) exposing said photoresponsive material layer to said polarization modulation pattern for exposure energy density exceeding at least 5 times the exposure energy density sufficient for producing waveplates with homogeneously orientation of optical axis. 
 (e) bringing said photoresponsive layer in contact with at least one anisotropic material layer, said anisotropic material having the ability of producing an optical axis modulation according to and under the influence of the anisotropy axis of said photoresponsive material layer. 
 
     
     
         11 . The method of  claim 9  or  10  wherein said optical axis modulation of at least one of said anisotropic material layers is twisted in the direction perpendicular to the modulation plane of the anisotropy axis of said photoresponsive material layer.