Methods and apparatus for liquid crystal photoalignment

Abstract

Liquid crystal photonic devices and microcavities filled with liquid crystal materials are becoming increasingly popular. These devices often present a challenge when it comes to creating a robust alignment layer in pre-assembled cells. Previous research on photo-definable alignment layers has shown that they have limited stability, particularly against subsequent light exposure. A method of infusing a dye into a microcavity to produce an effective photo-definable alignment layer is described, along with a method of utilizing a pre-polymer infused into the microcavity mixed with the liquid crystal to provide photostability. In this method, the polymer layer, formed under optical irradiation of liquid crystal cells, is effectively localized to a thin region near the substrate surface and thus provides a significant improvement in the photostability of the liquid crystal alignment. This versatile alignment layer method, which can be used in microcavities to displays, offers significant promise for new photonics applications.

1 . A liquid crystal (LC) cell, comprising:
 a structure defining a microcavity;   LC material disposed within the microcavity;   a dichroic dye layer disposed on an inner surface of the microcavity; and   a polymerized reactive mesogen layer, disposed on and aligned with the dichroic dye layer, to align the LC material with respect to the dichroic dye layer.   
     
     
         2 . The LC cell of  claim 1 , wherein the structure comprises a substrate. 
     
     
         3 . The LC cell of  claim 1 , wherein the dichroic dye layer has a thickness of up to about 10 nm. 
     
     
         4 . The LC cell of  claim 1 , wherein the dichroic dye layer comprises Brilliant Yellow azo dye. 
     
     
         5 . The LC cell of  claim 1 , wherein the polymerized layer has a thickness of up to about 100 nm. 
     
     
         6 . The LC cell of  claim 1 , wherein the polymerized layer comprises reactive mesogen. 
     
     
         7 . A method of aligning liquid crystal material to an inner surface of a microcavity, the method comprising:
 infusing anisotropic dye into the microcavity so as to coat the interior surface of the microcavity with the anisotropic dye;   illuminating the anisotropic dye with polarized light so as to form an anisotropic dye layer aligned with respect to the inner surface of the microcavity;   infusing reactive mesogen and liquid crystal material into the microcavity; and   illuminating the reactive mesogen at a wavelength selected to cause polymerization of the layer of reactive mesogen material so as to form a polymerized reactive mesogen layer aligning the liquid crystal material with respect to the anisotropic dye layer.   
     
     
         8 . The method of  claim 7 , wherein infusing the anisotropic dye comprises infusing at least one of an azo dye or a dye substantially similar to an azo compound. 
     
     
         9 . The method of  claim 7 , wherein infusing the anisotropic dye comprises:
 disposing the microcavity in a dye solution comprising the anisotropic dye and a solvent; and   heating the microcavity so as to evaporate the solvent.   
     
     
         10 . The method of  claim 7 , wherein infusing the reactive mesogen and the liquid crystal material comprises infusing RM257. 
     
     
         11 . The method of  claim 7 , wherein infusing the reactive mesogen and the liquid crystal material comprises:
 infusing a mixture of the reactive mesogen, the liquid crystal material, and a photoinitiator into the microcavity.   
     
     
         12 . The method of  claim 11 , wherein the mixture of the reactive mesogen, the liquid crystal material, and the photoinitiator has a weight ratio of reactive mesogen to liquid crystal material to photoinitiator of about 1.35 to 98.50 to 0.15. 
     
     
         13 . The method of  claim 11 , further comprising:
 heating and mixing the mixture of the reactive mesogen, the liquid crystal material, and the photoinitiator prior to infusing the mixture into the microcavity.   
     
     
         14 . The method of  claim 13 , wherein infusing the reactive mesogen and the liquid crystal material further comprises:
 allowing the reactive mesogen to separate from the liquid crystal material before illuminating the reactive mesogen.   
     
     
         15 . The method of  claim 7 , wherein illuminating the reactive mesogen further comprises:
 applying at least one voltage across at least a portion of the microcavity while illuminating the reactive mesogen so as to lock in alignment of the polymerized reactive mesogen layer with respect to the anisotropic dye layer.   
     
     
         16 . The method of  claim 7 , wherein applying the at least one voltage comprises:
 applying a first voltage across a first portion of the microcavity and a second voltage across a second portion of the microcavity so as to create spatially varying alignment of the anisotropic dye to the liquid crystal material.   
     
     
         17 . The method of  claim 7 , wherein the polymerized reactive mesogen layer has a thickness of less than approximately 100 nanometers. 
     
     
         18 . The method of  claim 7 , further comprising:
 infusing a photoinitiator into the microcavity before illuminating the reactive mesogen with ultraviolet light.   
     
     
         19 . The method of  claim 18 , wherein infusing the photoinitiator comprises infusing Irgacure 651.