The anamorphic transfer function is crucial for advancing optical technologies that rely on pixelated liquid crystal on silicon (PA-LCoS) devices. These devices are foundational in applications like holographic displays, optical communication systems, and high-resolution 3D imaging. The anamorphic transfer function measures how effectively these devices can accurately transfer spatial patterns of light, a critical requirement for achieving precision and reliability in these applications.
One of the central challenges is that, as pixel sizes decrease to enable higher resolutions, interpixel crosstalk and associated distortions increase, degrading performance. Crosstalk arises from the fringing electric fields between adjacent pixels and the elastic interactions within liquid crystal molecules. These factors lead to uneven modulation of light, creating spatial inhomogeneities that interfere with the device’s ability to maintain a uniform phase profile. This study aimed to quantify and mitigate these issues, focusing on devices with pixel sizes as small as 8 microns and numerically simulating even smaller sizes down to 4 microns.
Researchers in Spain combined experimental and numerical approaches to analyze performance. Experimentally, they tested a commercial PA-LCoS device with 8-micron pixels, employing high-frequency binary phase grating patterns with periods ranging from 2 to 16 pixels. They measured light diffraction efficiency using a radiometer and analyzed polarization states through Stokes polarimetry. For instance, they found that the diffraction efficiency for horizontal gratings could drop to approximately 33.8%, consistent with sinusoidal phase grating limits, while vertical gratings exhibited higher efficiencies closer to theoretical binary phase grating values of 40.5–42.7%, depending on the fill factor.
Numerical simulations complemented the experimental results. The split-field finite-difference time-domain (SF-FDTD) method enabled detailed 3D modeling of liquid crystal orientation and light interaction within the microdisplay. The researchers found that as pixel sizes decreased from 8 microns to 4 microns, performance challenges intensified. Smaller pixel sizes exacerbated polarization distortions, particularly in the ±1st diffraction orders, where the Stokes parameters deviated significantly from the input light’s linear polarization. This degradation was linked to out-of-plane liquid crystal reorientations caused by fringing fields. Despite these challenges, the degree of polarization (DOP) remained stable at nearly 100%, suggesting consistent polarization purity even with increased distortions.
Horizontal and vertical grating orientations revealed distinct behaviors due to their alignment with the liquid crystal’s pretilt direction. Horizontal gratings were more affected by fringing fields, leading to asymmetrical diffraction efficiencies between the ±1st orders, particularly for smaller fill factors (e.g., 0.2–0.6 microns interpixel gaps). Vertical gratings, in contrast, showed more robust diffraction efficiency but suffered from greater polarization degradation due to stronger out-of-plane effects.
The study demonstrates that horizontal and vertical grating orientations exhibit distinct performance behaviors, with horizontal patterns being more susceptible to fringing field effects and vertical patterns showing polarization degradation due to out-of-plane liquid crystal interactions. By leveraging the findings, developers can optimize pixel designs, grating configurations, and fill factors to balance resolution with performance.
Reference
Sánchez-Montes, A. R., Francés, J., Martínez-Guardiola, F. J., Márquez, A., Moya, A., Mena, E. J., Calzado, E. M., Neipp, C., & Gallego, S. (2025). Full polarimetric evaluation of the anamorphic transfer function for pixelated liquid crystal microdisplays. Optics and Lasers in Engineering, 184, 108670. https://doi.org/10.1016/j.optlaseng.2024.108670
