Christopher M. Walker

Laterally Azo-Bridged H-Shaped Ferroelectric Dimesogens for Second-Order Nonlinear Optics: Ferroelectricity and Second Harmonic Generation

Two classes of laterally azo-bridged H-shaped ferroelectric liquid crystals (FLCs), incorporating azobenzene and disperse red 1 (DR-1) chromophores along the FLC polar axes, were synthesized and characterized by polarized light microscopy, differential scanning calorimetry, 2D X-ray diffraction analysis, and electro-optical investigations. They represent the first H-shaped FLC materials exhibiting the ground-state, thermodynamically stable enantiotropic SmC* phase, i.e., ground-state ferroelectricity. Second harmonic generation measurements of one compound incorporating a DR-1 chromophore at the incident wavelength of 1064 nm give a nonlinear coefficient of d(22) = 17 pm/V, the largest nonlinear optics coefficient reported to date for calamitic FLCs. This value enables viable applications of FLCs in nonlinear optics.

Charge controlled, fixed optic axis analog (“v-shaped”) switching of a bent-core ferroelectric liquid crystal

Recently discovered anticlinic ferroelectric liquid crystals (FLCs) offer a mode of fast fixed-optic-axis electro-optic modulation useful for a variety of applications. In addition, the high spontaneous polarization of a member of this class of FLCs (∼500–600nC∕cm2) suggests that it is a good candidate for electrostatically controlled thresholdless analog switching, whereas conventional SmC* FLCs have a binary response. Using a simple charge control drive circuit, we have found that this FLC is in fact capable of hysteresis-free analog switching. An optical model suggests that this class of FLCs might be engineered to have a much larger electro-optic effect than seen in this example.

High-Tilt, High-PS, de Vries FLCs for Analog Electro-Optic Phase Modulation

Four new de Vries I-A-C* ferroelectric liquid crystal mixtures have been formulated to have high tilt (45°), high polarization, and low smectic layer shrinkage for use in analog electro-optic phase modulators. The mixtures are based on a high-tilt anti-ferroelectric compound which, when combined with other compounds, results in ferroelectric I-A-C* materials. Three of the mixtures include a new germanium-containing liquid crystal compound. X-ray diffraction measurements show layer contraction as small as 7%, while the temperature-dependence of birefringence shows a sharp increase at the A → C* transition as expected of a de Vries material. Charge control drive produced V-shaped switching with low hysteresis.

Anticlinic Ferroelectric Bananas for Electro-Optic Modulators

Five bent-core compounds structurally analogous to 8OPIMB6* were synthesized using three types of non-imine linkages for better stability. Only two showed liquid crystal phases, and neither possessed the SmC A P F phase. Four compounds were synthesized with the Tschierske core to determine whether or not fluorinated tails can be used to improve stability while maintaining the SmC A P F phase. All four exhibited polar smectic C phases, and two showed the SmC A P F phase. Racemic 8OPIMB6* was synthesized to test the importance of chirality in SmC A P F compounds, it was found to have properties identical to the chiral 8OPIMB6*.

Spatial Light Modulators with Integrated Phase Masks for Holographic Data Storage

A phase mask formed as a planarized relief structure within a few microns of the pixel plane of a ferroelectric liquid crystal SLM eliminates the DC Fourier-plane hotspot, eliminating the need for precision-aligned relay optics

Microdisplay wafer-flatness metrology by optical interferometry

We report a microdisplay wafer-flatness metrology technique based on digital high-pass filtering of topography data obtained from a commercial optical interferometer. This technique discriminates against both wafer-scale bow/warp and pixel-scale roughness to reveal die-scale flatness variations that are the most relevant to microdisplay gap uniformity. We report flatness measurements of a variety of live and test silicon wafers supporting VLSI microdisplay circuitry, and show how these measurements correlate with the performance of liquid-crystal microdisplays assembled from similar wafers. The technique is sensitive to cross-die flatness variations as small as 25 nm in the presence of wafer bow of tens of microns. The wafer flatness variations that make the greatest contribution to liquid-crystal cell-gap non-uniformity arise from interactions between the chemical mechanical planarization (CMP) process and the VLSI circuit layout. Our metrology technique can help the VLSI designer optimize microdisplay layout, and provides an objective flatness specification for wafers purchased from third-party foundries.

Highly integrated single-chip optical correlator

Barriers to commercialization of optical correlators include the complexity and cost of their manufacture, their large size compared to typical electronic processors, and the cost of their components. Using sub-micron CMOS VLSI fabrication processes it is possible to build the two SLMs and photodetector array of a Vander Lugt correlator on a single silicon die. The correlators lenses can be fabricated on a single piece of glass using diffractive optics technology and then attached to the CMOS die to form a monolithic assembly. This approach greatly reduces the mechanical degrees of freedom that must be controlled by the correlators housing thus lowering cost, reducing size, and improving reliability. Here we report on the design and performance of a prototype.

Highly integrated compact optical correlators using FLC-VLSI spatial light modulators and diffractive optics

Due to advances in spatial light modulator technology and high-speed imagers, optical correlators are becoming viable for a variety of high-speed image processing applications. However, conventional approaches to assembling correlators produce systems which are costly to manufacture, and which are too large for some uses. We are investigating approaches to the construction of compact, highly integrated correlators that combine ferroelectric liquid-crystal-on- VLSI spatial light modulators, a CMOS imager, and diffractive optical elements.