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 etc. 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
We present here novel easily processible organic-inorganic nanocomposites suitable for holographic fabrication of
diffraction optical elements (DOE). The nanocomposites are based on photocurable acrylate monomers and inorganic
nanoparticles (NP). The compatibility of inorganic NP with monomers was achieved by capping the NP surface with
proper organic shells. Surface modification allows to introduce up to 50wt.% of inorganic NP in organic media.
Depending on the NP nature (metal oxides, phosphates, semiconductors, noble metals) and their properties, the materials
for both efficient DOE and multifunctional elements can be designed. Organic-inorganic composites prepared have been
successfully used for the effective inscription of periodic volume refractive index structures using the holographic
photopolymerization method. The nanocomposite preparation procedure, their properties and optical performance of
holographic gratings are reported. The use of functional NP makes it possible to obtain effective holographic gratings
having additional physical properties such as light-emission or NLO. Some examples of such functional polymer-NP
structures and their possible application fields are presented. The combination of easy photo-patterning of soft organic
compounds with physical properties of inorganic materials in new nanocomposites and the flexibility of the holographic
patterning method allow the fabrication of mono- and multifunctional one- and multi-dimensional passive or active
optical and photonic elements.
A new type of volume holographic gratings based on polymer-liquid crystal composite named POLIPHEM (POlymer LIquid Crystal Polymer Holograms Electrically Manageable) is presented. The new composite material in combination with the proper holographic fabrication results in switchable holographic gratings with high diffractive and electro-optical parameters. Periodic structures consisting of alternating polymeric and LC-rich regions with aligned mono-domain morphology of the LC are formed due to photopolymerisation and phase separation of the initially homogenous film of photo-curable monomers and liquid crystals under illumination with an interference field (λrec=364 nm) at room temperature. Compared to typical holographic polymer-dispersed liquid crystals (H-PDLCs) POLIPHEM films are characterized by the absence of light-scattering, strong anisotropy after holographic exposure, low driving voltages and fast electro-optic time-response. The kinetics of the holographic recording under different irradiation intensities were investigated at λtest=632.8 nm, the microstructure and the electro-optical response of POLIPHEM transmission gratings have been analyzed. POLIPHEMs were realized in the pitch range of 0.28-6 μm. The diffraction efficiency of more than 96% was achieved for p-polarized probe light (for λtest=632.8 nm). Possible mechanism of POLIPHEM formation is discussed briefly.
The peculiarities of holographic gratings recording in the thick layers of photopolymer self-processing materials have
been investigated. It is shown the main reason of limit of gratings thicknesses and, consequently, their angular
selectivity and diffraction efficiency is the dynamic amplification of holographic noises. The maximum diffraction
efficiency and angular selectivity with the minimum noises are achieved with use of pre-polymerization of layer and the
post-polymerizing self-amplification of holograms. This method allows to fabricate gratings with the thickness up to
inmi and angular selectivity about 6'.
The performance of the developed family of highly efficient self-processing photopolymer compositions (PPC) for holographic recording in real time and use of these materials as the recording media for some applications is investigated. We consider the mechanism of holographic recording, peculiarities of recording process in self- developing photopolymers, dependence of holographic characteristics on the thermodynamic properties of compositions. ON the basis of obtained data we offer the method of PPC modification that results in increasing of the materials light-sensitivity, the efficiency of recording and the threshold of the gratings optical damage. We also cite the instances of use of PPC for production of different holographic optical elements.
The methods of sensibilization of self-processing photopolymer for holographic recording in the spectral range 500 - 700 nm have been developed. Two new compositions PPC- 520 and PPC-650 were developed in a result of change of the initiator system and modification of monomer component in the basis PPC-488. New PPC are offered to manufacture of various holographic elements: gratings, lenses and spatial optical filters for the pattern recognition schemes.
The kinetic of holographic recording in photopolymer compositions is considered in the terms of Abrami-Erofeev equation. It has been shown for composition with polymerization-diffusion mechanism of recording it is possible to realize the conditions, at which the kinetic of recording and consequently, light sensitivity of a material and efficiency of recording will be determined by the parameters of the polymerization process.
Results of investigations of diffraction efficiency (DE) temperature dependence and optical damage threshold of volume phase holograms-transmission gratings are presented. DE remains constant in the range +10 < T < +110 degree(s)C for holograms on the base of PPC-488. Changes of DE are reversible in +110 < T < +200 degree(s)C and if T > +200 degree(s)C the irreversible change takes place because of specific cracking of films along phase planes. The reversible change of DE can be described by diagrams of phase balance in the system: polymer-diffusant. Optical damage threshold of under study gratings is defined by the threshold of photo- and thermo-decay of neutral diffusant 1-naphthalene bromide. Different compositions, which contain the diffusant with high threshold of photo- and thermal-dissociation processes have been tested. Some modifications of basic photopolymer composition with the threshold of optical damage more than 200 MW/sm2 were proposed.