Enhance of LCOS Phase Modulation in Visible Wavelength Range Based on Metasurface

Abstract

This thesis explores the integration of metasurface with liquid crystal on silicon (LCOS) technology to enhance the phase modulation capability while reducing the thickness of the liquid crystal layer, addressing key challenges in high-performance holographic displays. LCOS devices are known for their high-resolution light modulation, but their phase depth and response time are limited due to the proportional relationship between liquid crystal thickness and switching speed. To overcome this problem, we propose a new approach in which a metasurface (composed of a subwavelength TiO₂ nanopillar array with a size gradient) partially transfers the phase depth modulation requirements from the liquid crystal layer. The design exploits the electrically controlled birefringence (ECB) mode for continuous tuning of the refractive index in nematic liquid crystals, combined with a metasurface designed and optimized via finite difference time-domain (FDTD) simulations. Die-level assembly techniques were used to fabricate the LCOS prototype, while advanced nanofabrication methods, including electron beam lithography and reactive ion etching, were used to achieve high-aspect ratio metasurface structures. Simulation results show that the phase modulation depth is improved by 37.5% compared to the pure liquid crystal system, achieving full 2π coverage in the visible spectrum (440-720 THz). In our 1000nm thick pure liquid crystal LCOS system, the maximum phase modulation capability in the visible light range is 1.45π on average. Then after increasing by 37.5%, it becomes 1.45π × (1 + 0.375) ≈ 2.0π. In optical devices, if the phase modulation range does not reach 2π, that is, one complete cycle, the wavefront cannot be modulated arbitrarily, which limits the device's capabilities in imaging, holography, beam shaping, etc. Compared to the base system without optimization, this realization means that the design has full wavefront control capability, which is a performance threshold. This is a significant improvement because the phase response of liquid crystal is inherently dispersive and changes with wavelength, making it difficult to achieve over a wide bandwidth. 3 The experimental results show that under dynamic voltage control, the resonance effect at 600 nm enhances the phase modulation by about 1π radians, verifying the potential of metasurfaces in achieving thinner liquid crystal layers without affecting their performance. Key innovations include highly error-tolerant metasurface design, novel nanoscale processing for LCOS-metasurface integration, and solution of processing challenges such as metal adhesion and PMMA cross-linking. This work advances the development of compact phase-only LCOS devices for use in augmented reality, holography, and optical communications, while providing a scalable framework for future metasurface-enhanced electro-optical systems.