The high coherence of laser light sources is a key to the application of diffractive optics usable in holographic AR/VR displays. This can be combined with switchable diffractive elements, which are advantageous for several optical functions used in immersive holographic displays such as shutters, polarization filters, for rapid beam deflection and selection. We demonstrate a compact, effective and robust diffraction wide-angle switchable beam-deflecting device based on circular polarization gratings possessing Bragg-performances (Bragg-PG) and a polarization switch. Such grating/polarization switch pair may, for instance, be a discrete switchable deflection element or as a switching element for pre-deflection with field lenses for application in holographic AR/VR displays. Micrometer-thick circular polarization gratings characterized by high diffraction efficiency (DE > 95%), large diffraction angles (< 30°) and wide angular and wavelength acceptance were developed. In the presented embodiment, the output signal is controlled between the zero- and first-diffraction orders by the handedness of circular polarization of the incident light. Forming a stack of two such oppositely aligned gratings can double the deflection angle. These gratings are the result of a two-step photochemichal/thermal processing procedure of a photocrosslinkable liquid crystalline polymer (LCP). The holographic patterning provides a high spatial resolution (period < 700 nm) and the arbitrary orientation of the LC director as well as high optical quality and thermal and chemical stability of the final gratings. Highly efficient (diffraction efficiency, DE > 95% in the vis spectral range) and stable symmetric and slanted circular Bragg polarization gratings were fabricated using the developed material and processing technique. The high usable diffraction angles combined with high DE make the Bragg-PG attractive for HMD AR/VR applications because of the system inherent short focus lengths and large numerical apertures needed to meet the low space budget in HMD and other optical systems.
We present here two approaches for the fabrication of 2D and 3D optical structures. The first one is a step-by-step
fabrication process of 3D structures using thin relief gratings (stacks of thin 1D or 2D gratings). Azobenzene containing
materials for the surface relief inscription have been used in the step-by-step procedure, where after holographic
inscription of desired relief structure and coverage with spacer layer another correlated relief structure has been written
in the next active layer <i>etc</i>. The method provides full flexibility of the structure type and parameters including different
gratings in different layers (hierarchical structures). A technique to produce hexagonal relief gratings of enlarged
diameter which can be used for layer-by-layer photonic structures is developed. The second approach is a multi-beam
holographic recording using special phase masks. Such mask consists of three phase gratings arranged in plane
equilateral triangle geometry with gratings vectors at 120° to each other. A simple method of fabrication of well-adjusted
mask with rather high diffraction efficiency is developed. Hexagonal 2D surface relief and 3D volume phase structures
were fabricated by a single laser beam exposure using UV or visible wavelengths (depending on the material) through
the mask. Azobenzene containing materials and photopolymers, including new specially designed one, were used as
A new material for phase and phase-relief optical recording in the ultraviolet spectral region with postexposure diffusion
enhancement has been developed based on poly(methyl methacrylate) doped with aromatic ketones. Advantages of the
material are a high enhancement coefficient at significant final refractive index modulation amplitude and good stability.
The surface relieves shaped in processes of thermoannealing of exposed polymer layers containing photosensitive admixtures are investigated. Several ways of resolving ability improvement of the relief recording are proposed and implemented experimentally. They are based on: (1) retardation of deformation reate, (2) depression of glass transition temperature of a polymeric matrix, (3) considerable increase of annealing temperature of the samples and (4) overlapping of the recording and thermal developing processes under photorelief shaping by one laser pulse. The last method has made it possible to record surface gratings with spatial frequencies in the limits of 50-1500 mm<sup>-1</sup>.
Dynamic phase holograms recording in poly(methylmethacrylate) glasses containing anthracene derivatives and photochemically neutral component (residual solvent) is investigated and photoinduced diffusion of neutral molecules is proposed as a basis of the hologram evolution. The reversible recording under high temperatures and a sensitivity of the diffraction efficiency kinetics to external stresses are demonstrated. Some discrepancies between the experimental data and the offered model are discussed.
A model of phase hologram dynamics, caused by diffusion of small photochemically neutral molecules in polymeric medium is offered as an expansion of antiphase structures destruction concept. Results of experimental realization of the offered way are submitted. Variable real time dynamic regimes including hologram amplification and phase contrast inversion are demonstrated.
Surface photoreliefs, forming due to photoreduction of aromatic ketones and photodimerization of substituted anthracenes in poly(methylmethacrylate) layers are investigated. The photorelief is formed in a process of thermally activated structural relaxation of the material exposed extending a layer thickness. The relative photorelief height reaches several percent. Phototdimerization reversibility causes the reversibility of the relaxation relief and opens the prospects to the repeated relief hologram recording.
Free volume diffusion induced by photochemical reaction in glassy polymeric matrix is investigated. It causes significant density and refractive index variations leading to phase hologram enhancement in freshly prepared or heated photosensitive layers and opens the possibility of implementing photothermic optical recording. The calculating model of photoresponse formation is proposed and the results of numerical simulations are presented.