Phase switching of ordered arrays of liquid crystal emulsions

Polymer dispersed liquid crystals (PDLC) are a well established technology for electrooptic devices used in displays and light switches. They rely on dispersions of nematic liquid crystal material in a polymer matrix. In the absence of an electric field, the director field n(r), which characterizes the local average orientation of the rodlike nematic mesogens, is randomly oriented, leading to a mismatch between the average index of refraction of the liquid crystal and that of the polymer; this results in strong scattering making the film opaque. An external electric field aligns the nematic director parallel to the direction of light propagation, and, if the average index of refraction of the aligned liquid crystal matches that of the polymer, scattering is eliminated making the film transparent [1]. To control the direction of the light, a holographic version of this same technology (H-PDLC) incorporates a periodic modulation of the liquid crystal; switching then induces a phase grating, leading to controllable diffraction of the light [2]. Formation of PDLCs usually involves the phase separation of liquid crystal during the curing of the polymer matrix; H-PDLCs are formed when intensity gratings from crossed laser beams are imposed on the film during curing. Because of the strong pinning of the liquid crystal to the polymer matrix, PDLCs require relatively large switching fields. An alternate fabrication technique, which can significantly reduce the switching fields, relies on the formation of well defined liquid crystal droplets suspended in a polymer matrix. However, these liquid crystal emulsions are typically comprised of a highly polydisperse mixture of droplets limiting their use to controlling the transmission of light, and precluding their use in any H-PDLC application. In this letter we report a unique technique for producing phase gratings from liquid crystal emulsions. We accomplish this by using monodisperse liquid crystal droplets formed into a precisely ordered two-dimensional hexagonal close-packed monolayer. The spacing of this grating is controlled by the droplet size. This allows us to switch both the intensity and the direction of the light, producing a very simple new material equivalent to a H-PDLC. Moreover, very low switching voltages (0.1 V/μm) and relatively fast switching speeds can be achieved by exploiting the interference effects of these phase gratings. Monodisperse droplets of nematic liquid crystal where FIG. 1: Photomicrographs of the crystalline monolayer before (a) and after drying (b). The bipolar character of the droplets having two surface disclinations and parallel boundary conditions on the director is indicated by the texture when viewed between crossed polarizers shown in (c) with the droplet axis along the polarizer P and (d) rotated by 45◦. The insets in (c) and (d) indicate the director pattern inside each droplet. The gray shaded regions indicate approximate regions of extinction.