Oksana V. Sakhno

Surface modified ZrO2 and TiO2 nanoparticles embedded in organic photopolymers for highly effective and UV-stable volume holograms

The improved synthesis of large quantities of small (hydrodynamic diameter 6–8 nm) TiO2 and ZrO2 nanoparticles tailored as a high refractive index agent in holographic acrylate photopolymer composites has been developed. The surface of the particles was modified by p-dodecylbenzenesulfonic acid (organic shell weight fraction is about 50–70%) to render them highly compatible with organic monomer mixtures avoiding aggregation. The exposure to the interference pattern provides fast one-step formation of permanent highly efficient volume phase gratings with excellent transparency in the visible range. A high refractive index modulation amplitude n1 of about 0.0165 has been achieved in the composites containing 25 wt% of TiO2 nanoparticles. The level of light scattering in the gratings does not exceed 8–10% compared to the film without nanoparticles. The holographic grating based on acrylates doped with ZrO2 nanoparticles, first employed in this work, exhibited high photostability in contrast to holographic nanocomposites with TiO2 nanoparticles. Possible applications of such photocurable organic–inorganic nanocomposites are holographic diffractive elements for a number of optical and electro-optical applications or devices based on ordered arrays of functional nanoparticles within organic films.

Effective volume holographic structures based on organic?inorganic photopolymer nanocomposites

We demonstrate a practical approach for the development of a broad range of nanocomposites based on acrylate polymers and organically capped inorganic nanoparticles (NPs). The submicrometer scale volume patterning of the nanocomposites using holographic photopolymerization was investigated. The specific adjustment of both the material parameters (core?shell of the NP, monomer mixture, concentrations) and the patterning conditions led to materials that were suitable for the fabrication of effective optical diffractive elements and specific functional microdevices with light-emissive and nonlinear optical (NLO) properties. The nanocomposite preparation and properties, their holographic performance and some examples of functional polymer?NP structures are reported.

POLIPHEM: new type of nanoscale polymer-LC-switchable photonic devices

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.

Stable Selective Gratings in LC Polymer by Photoinduced Helix Pitch Modulation

A cholesteric mixture based on the nematic liquid crystalline side-chain polymer doped with a chiral-photochromic compound was prepared and used as an active medium for creation of stable polarization selective gratings by phototunable modulation of the helix pitch. Such modulation was fabricated in the polymer mixture by a nonpolarized UV-irradiation with spatially modulated intensity that causes E-Z isomerization of a chiral-photochromic dopant, decreasing its helical twisting power. It was shown that the gratings recorded by UV-exposure through a mask are strongly selective to the handedness of circular polarized light. The studied polymer film forms a right-handed helical structure and, correspondingly, the diffraction of only the right-circularly polarized light was found in the transmittance mode. The maximum diffraction efficiencies were found for the wavelength values between the maxima of selective light reflection. The films obtained open very interesting possibilities for further development of materials with stable gratings operating in the entire visible spectral range. Both the position and the width of the spectral range of an efficient diffraction can be easily controlled by the UV exposure and concentration of the dopant. The materials obtained and methods developed can be used for creation of specific diffraction elements for optics and photonics.

PPC: self-developing photopolymers for holographic recording

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.

Relief structures in the self-developing photopolymer materials

Summary Surface-relief structures are optically induced by visible laser exposure in the self-developing photopolymers. We describe the processes caused a relief formation, fabricating technologies, analyze the influence of recording process parameters (recording intensity, grating period, the value of adhesion between polymer film and substrate) and present the diffusive-polymerization theory to modeling a surface relief formation in photopolymers. With a holographic method the fan-out element that generates 18 and 25 beams of almost equal intensity is experimentally realized.

Holographic recording in thick photopolymer films

The peculiarities of holographic gratings recording in the thick layers of photopolymer self-processing materials have nbeen investigated. It is shown the main reason of limit of gratings thicknesses and, consequently, their angular nselectivity and diffraction efficiency is the dynamic amplification of holographic noises. The maximum diffraction nefficiency and angular selectivity with the minimum noises are achieved with use of pre-polymerization of layer and the npost-polymerizing self-amplification of holograms. This method allows to fabricate gratings with the thickness up to ninmi and angular selectivity about 6.

Temperature behavior and the optical damage threshold of holographic gratings based on photopolymerizable materials

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.

Kinetic peculiarities of holographic recording in photopolymers

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.

Bragg polarization gratings used as switchable elements in AR/VR holographic displays

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.