Time-dependent phase modulation in liquid crystal photonics

Thesis

Liquid crystal (LC) spatial light modulators (SLMs) have become essential components in various optical applications, enabling real-time and high-precision control over light properties such as phase, amplitude, and polarization. This thesis focuses on the optimization of LC-based optical phase modulators, aiming to enhance their response time, phase modulation depth, and polarization independence while maintaining low-voltage operation.

The study investigates two distinct nematic LC devices: a pi-cell-based phase modulator and a super-twisted nematic (STN) LC-based polarization-independent phase modulator. By applying optimized voltage waveforms, both devices achieve full 2π rad phase modulation within 1 ms, with the pi-cell operating in a reflective configuration and the STN device employing a four-pass configuration. To characterize and analyse the dynamic phase modulation behaviour, phase-shifting interferometry techniques are employed. Two phase-shifting interferometers are designed and constructed to provide high-resolution, accurate time-resolved phase measurements.

The theoretical framework is built upon Frank continuum theory and Ericksen-Leslie theory, which describe the elastic and hydrodynamic properties of nematic LCs. The free-energy density of the LC device is formulated and solved through Euler-Lagrange equations to determine the director profile, which is then used for phase modulation calculation. For twisted LC devices, Jones matrix formalism is utilized to compute phase modulation characteristics.

Furthermore, this work develops a computational optimization algorithm that integrates Jones matrix formalism and particle swarm optimization (PSO) to determine the optimal configuration of LC devices combined with additional waveplates. This approach results in an improved performance of phase modulation range and polarization independence beyond what is achievable with a single LC device. The optimization results validate that strategic combinations of LC devices and passive optical elements can significantly improve modulation performance within a voltage range or time scale.

The findings of this thesis contribute to the advancement of LC-based phase modulators by addressing the fundamental trade-offs between response time, phase modulation range, and applied voltage. The developed methodologies provide a robust foundation for designing next-generation LC optical modulators, with potential applications in adaptive optics, beam steering, holography, and AR/VR systems.

Authors: Xue, L

• DOI: 10.5287/ora-jb75p8doy
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: optical phase modulators; Liquid crystal devices; optical phase measurement system
• Subjects: liquid crystal devices; optical phase modulator