Embossing Reactive Mesogens: A Facile Approach to Polarization‐Independent Liquid Crystal Devices

Abstract

Reactive mesogens (RMs) are used in a wide variety of applications, from retardation plates for organic light‐emitting display screens, liquid crystal phase stabilization, optical or electrically active elastomers to novel optic components. They can be processed as standard liquid crystals and subsequently polymerized to stabilize both shape and anisotropic properties. However, creating complex shapes while maintaining good alignment of the RM optical axis can be a challenge. In the present work, embossing is used to replicate a wide variety of RM structures in several electro‐optic devices. A novel device geometry is proposed where a polarization‐independent lens is formed by opposing birefringent Fresnel zone plates embossed in aligned RM. The RM forms the optical elements and alignment layers for an index‐matched liquid crystal, arranged to produce the twisted nematic configuration. Low voltages switch the device between nonfocusing and focusing states. After characterizing lens efficiency and beam properties, the method is used to fabricate a switchable multilevel Fresnel zone plate with optical efficiencies beyond 50%, and theoretically able to produce polarization‐independent lenses with efficiencies approaching 100% from a single structure. Finally, manufactured polarization‐independent gratings and microlens arrays are presented using the method to illustrate the wide range of applicability.

Polarisation independent liquid crystal lenses and contact lenses using embossed reactive mesogens

“…The objective of the present work is to design an arrangement for a conventional nematic liquid crystal that could produce polarisation independent operation for either multi‐level diffractive or refractive optical structures, producing high efficiency lenses operating at lower voltages. The approach taken was to form optical structures on the internal sides of the liquid crystal device from birefringent materials that are index matched to the contacting liquid crystal and orthogonal to each other . Each refractive or diffractive lens structure is formed in a parallel‐aligned reactive mesogen on to the internal surfaces of the liquid crystal lens, deposited directly onto the electrodes.…”

“…Operating principal of the proposed lenses . A simple 2‐level diffractive Fresnel lens is shown on a flat substrate in both the A, field OFF and B, field ON states.…”

“…The approach taken was to form optical structures on the internal sides of the liquid crystal device from birefringent materials that are index matched to the contacting liquid crystal and orthogonal to each other. 16 Each refractive or diffractive lens structure is formed in a parallel-aligned reactive mesogen on to the internal surfaces of the liquid crystal lens, deposited directly onto the electrodes. The device is constructed with two such lenses opposing each other, and the optic axes of the birefringent material are arranged to be orthogonal, with both ordinary and extraordinary refractive indices matched to those of the contacting nematic.…”

“…Other polarisation independent modes F I G U R E 3 Operating principal of the proposed lenses. 16 A simple 2-level diffractive Fresnel lens is shown on a flat substrate in both the A, field OFF and B, field ON states. The operating principal for an adaptive contact lens operating in this same fashion is shown for the field OFF,C, and ON,D, states.…”

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“…The use of surface relief gratings as alignment layers for liquid crystal devices is well established, with commercial success for Zenithal Bistable Displays (ZBD) [6]. In this work, sub-micron amplitude and micron pitched surface-relief gratings are embossed onto ITO (indium-tin oxide) coated glass IDEs using a polymer film imparted with the required surface relief structure copied from a photolithographically defined master via an electro-forming step [7,8]. This geometry has led to a working prototype of a device that demonstrates both resistance to mechanical shock and submillisecond switching times.…”