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Illumination & Displays

Liquid crystal Bragg gratings for high-brightness spatial light modulators

A new method for recording holograms in electrically switchable liquid-crystal/polymer composites can reduce incoherent scattering while simultaneously boosting diffraction efficiency.
27 November 2007, SPIE Newsroom. DOI: 10.1117/2.1200711.0936

Spatial light modulators (SLMs) deliver images to a screen or printer, but are also utilized in direct-write lithography systems, holographic data storage, and optical computing. Bragg reflection gratings are ideal for SLMs wherever it is possible to take advantage of their strong wavelength selectivity and efficient diffraction into a single beam.1,2 Using liquid-crystal/polymer composite media with these devices, complex holograms can be readily recorded. Unfortunately, they have not achieved the high reflectance (∼99%) and low haze required of SLMs, for example, in high-brightness displays.

Holographic liquid crystal (LC) Bragg gratings consist of periodic LC droplet monolayers sandwiched between polymer layers.3 High diffraction efficiency (DE) requires a large difference in refractive index between the polymer and LC. Efforts to improve DE include using large concentrations of highly birefringent LCs.3–5 Unfortunately, due to the inhomogeneous nature of the LC layers, such attempts have not worked well in reflection gratings.3 Because droplet orientation is random, the droplet-polymer index differences within the monolayer and droplet-droplet birefringence differences produce unacceptably large incoherent scattering. Nor is DE generally improved as larger LC droplets wash out the gratings. Our approach is to control the shape and orientation of LC droplets and hence to more effectively employ the large birefringence to simultaneously improve DE and decrease scattering.

Shape, orientation, and concentration of LC droplets affect both DE and scattering, but in different ways. Elongating the droplets increases their effective birefringence and also enables higher packing density. Orienting them in the same direction at once increases the droplet-polymer differences while decreasing the droplet-droplet differences. At the same time, higher packing density reduces the regions of droplet-polymer inhomogeneity. Shape, orientation, and packing density thus raise the index contrast for higher DE, yet the effects of orientation and higher packing density (made possible by shape) can lead to reduced inhomogeneities for lower haze.

LC condenses from a polymerizing solution when the mixture is irradiated by a light intensity pattern.6 As the molecular weight of the polymer increases, the solution's high viscosity allows a shear to be effectively transmitted from confining windows to incipient droplets throughout the polymer. The resulting lateral stress distorts the forming droplets and orients the LC. Eventually, polymer gelation locks in the stress and droplet distortion.

Figure 1 illustrates the concept. Droplets have a larger effective birefringence because the elongated shape energetically favors a nematic configuration where more (less) of the extraordinary (ordinary) refractive index is utilized (ne > no). Smaller "gaps" between droplets and identical, rather than random, orientation of the droplet optical axes reduce the index inhomogeneity. The grating becomes highly anisotropic, yielding polarization dependent diffraction. If the droplets can be oriented more parallel with the layer, the DE can be improved even further. DE is high (low) for light polarized parallel (perpendicular) to the shear. Plots in Figure 2 illustrate how modification of these parameters can optimize DE while reducing haze (baseline).

Figure 1. Schematic diagram illustrates the concept of applying lateral shear stress to shape and align liquid crystal droplets in the polymer. Arrows indicate directions of droplet optical axes.

Figure 2. The transmittance (T) of a liquid crystal Bragg grating is plotted as a function of wavelength, illustrating high (low) diffraction efficiency for light polarized parallel (perpendicular) to the shear. Diffraction efficiency is 1–T. The inset compares results for an unsheared sample. Lines are calculations based on a theoretical model,3 and symbols are data.

To demonstrate this concept, we used typical mixtures of monomer, LC, and photoinitiator sandwiched between transparent electrodes7 in a modified holographic sample configuration.8 Lateral movement of one window with respect to the other, under computer control, transmits a shear parallel to the plane of the film. Timing and magnitude of the applied shear are critical to the desired grating properties. This process is aided by monitoring the growth of the grating in real time. The data plotted in Figure 2 illustrate how DE is maximized for light polarized along the shear direction, and baseline (i.e., haze) is improved. Electron micrographs corroborate LC droplet shape deformation and orientation (Figure 3). Electrical switching of this grating has also been confirmed.

Figure 3. Scanning electron micrographs of cryogenically prepared samples demonstrate the reshaping and orientation of nearly spherical droplets in unsheared samples (left) into elongated droplets in sheared samples (right).

Highly-efficient switchable Bragg gratings with low haze for SLMs appear to now be within our technological reach. So far, we have achieved DE ∼ 99% with reduced haze in small (∼5 mm) samples. Future work will involve scaling up the process to achieve highly uniform, large area gratings. Numerical models of this system predict that, through further parameter refinement, there is potential for DE improvement to 99.9% or even 99.99% with further reductions in haze.

This work is supported by Air Force Research LaboratoryMaterials and Manufacturing Directorate and Air Force Office of Scientific Research Chemistry and Life Sciences Directorate.

Richard Sutherland, Vincent Tondiglia, Lalgudi Natarajan
Science Applications International Corporation (SAIC)
Dayton, OH Materials and Manufacturing Directorate
Air Force Research Laboratory (AFRL)  
Wright-Patterson Air Force Base, OH

Richard Sutherland, PhD (physics) is a technical fellow with SAIC and principal investigator for a materials research program at AFRL. His current research interests are in the physics of liquid crystals, nonlinear optics of organic molecules, and photonic structures in biological tissues. He is a fellow of the American Physical Society and a member of SPIE and OSA.

Pamela Lloyd
UES, Inc
Dayton, OH Materials and Manufacturing Directorate
Air Force Research Laboratory
Wright-Patterson Air Force Base, OH
Timothy Bunning
Dayton, OH Materials and Manufacturing Directorate
Air Force Research Laboratory
Wright-Patterson Air Force Base, OH