Holographic Gratings Research Articles

Вырожденное четырехволновое взаимодействие на пропускающих голографических решетках в кристалле Bi-=SUB=-12-=/SUB=-TiO-=SUB=-20-=/SUB=- среза (110)

The regularities of the stationary degenerate four-wave mixing on transmission holographic gratings formed in the Bi12TiO20 crystal of the (110)-cut are analyzed. The system of differential equations has been obtained; this system that can be used to find the components of the vector amplitudes of linearly polarized light waves in the case of four-wave mixing on phase and phase-amplitude holographic gratings. The theoretical model take into account the linear electrooptic, the photoelastic and the inverse piezoelectric effects, as well as natural optical activity, circular dichroism and crystal absorption. The values of the orientation angle and the crystal thickness where the reflection coefficient can reach maximum values are determined. It was experimentally found that the reflection coefficient can reach 2.4 with an optimal choice of the orientation angle in the Bi12TiO20 crystal of (110)-cut with the thickness of 7.7 mm. It is shown that the best agreement between theoretical and experimental data is achieved if the phase-amplitude structure of transmission holographic gratings formed in the Bi12TiO20 crystal is taken into account in the mathematical model of diffraction.

Holographic grating fabrication for wide angular bandwidth using polymer thin films

To increase the angular bandwidth of volume holographic grating, we fabricate holographic gratings based on grating multiplexing technique by using thin films of photopolymers and polymer dispersed liquid crystals. Experimental results confirm that the liquid crystal materials increase the refractive index modulation of the grating, enabling high diffraction efficiency with wide angular response compared to pure polymer materials. We observe that the fabricated holographic grating has near 80% of diffraction efficiency and about 18° of angular bandwidth, which can be further improved by modifying the liquid crystal/polymer mixtures and the grating multiplexing technique. The grating can be used to fabricate holographic waveguide structures for emerging applications in the near-eye display systems.

Регистрация высокочастотных рельефно-фазовых голографических структур на фотоматериале ПФГ-04

A new variant of processing photographic plates for holography based on dichromed gelatin PFG-04 (produced by JSC "Slavich Company", Pereslavl-Zalessky) for the manufacture of high-frequency relief–phase holographic gratings with a spatial frequency up to 1500 mm-1 is proposed. The technology is based on the selective destructive effect of short-wave UV radiation on gelatin and subsequent etching of the layer with various reagents. For the first time, relief-phase high-frequency holographic gratings with a maximum diffraction efficiency of 67% were obtained on PFG-04 photographic plates.

大角带宽高衍射效率体全息光栅的设计和制备

大角带宽高衍射效率体全息光栅的设计和制备

LED-based chromatic and white-light vortices of fractional topological charges

Based on an LED light source, we present an experimental scheme to produce chromatic and white-light optical vortices carrying fractional topological charges. In theory, we develop a general formula to simulate the propagation dynamics of such fractional chromatic and white-light vortices based on the eigenmode decomposition of orbital angular momentum within the Gaussian–Schell model. In the experiment, for the generation of the chromatic and white-light optical vortices, we use a phase-only spatial light modulator to prepare the desired holographic gratings and a prism to compensate for the dispersion for three primary colors (red, green, and blue). The experimental observations, e.g., the seal-healing effect of dislocation lines, are in good agreement with the simulation results. The present work extends the studies of fractional optical vortices not only from highly coherent to partially coherent region but also from monochromatic to chromatic (white-light) source, which may find potential applications in LED-based visible light communication and optical displaying.

Design of a compact waveguide eyeglass with high efficiency by joining freeform surfaces and volume holographic gratings.

In this paper, a compact waveguide eyeglass integrating freeform surfaces and volume holographic gratings (VHGs) is proposed for full-color display with high energy utilization. The in-coupler with four freeform surfaces collimates the light emitting from the micro image source (MIS) and couples them into the waveguide. The six-layer VHGs as an outcoupler are designed to modulate the light propagating toward the user's eye. The chromatic aberrations and aberrations are well optimized and compensated by the in-coupler. The diffraction angular bandwidth of the gratings matches the angular range of the light propagating in the waveguide. The simulation results show that our proposed eyeglass achieves a diagonal field of view (FOV) of 39.5°, the average diffraction efficiency of the outcoupler achieves 95.22%, and the diffraction uniformity is about 0.95. Because of the integrated designs and compact stable structures, the optimized display system is expected to be flexibly used in various applications.

Design of Tunable Holographic Liquid Crystalline Diffraction Gratings.

Tunable diffraction gratings and phase filters are important functional devices in optical communication and sensing systems. Polarization gratings, in particular, capable of redirecting an incident light beam completely into the first diffraction orders may be successfully fabricated in liquid crystalline cells assembled from substrates coated with uniform transparent electrodes and orienting layers that force a specific molecular distribution. In this work, the diffraction properties of liquid crystal (LC) cells characterized by a continually rotating cycloidal director pattern at the cell substrates and in the bulk, are studied theoretically by solving a relevant set of the Euler-Lagrange equations. The electric tunability of the gratings is analyzed by estimating the changes in liquid crystalline molecular distribution and thus in effective birefringence, as a function of external voltage. To the best of our knowledge, such detailed numerical calculations have not been presented so far for liquid crystal polarization gratings showing a cycloidal director pattern. Our theoretical predictions may be easily achieved in experimental conditions when exploiting, for example, photo-orienting material, to induce a permanent LC alignment with high spatial resolution. The proposed design may be for example, used as a tunable passband filter with adjustable bandwidths, thus allowing for potential applications in optical spectroscopy, optical communication networks, remote sensing and beyond.

Multiplane differential phase contrast imaging using asymmetric illumination in volume holographic microscopy.

.Significance: Differential phase contrast (DPC) is a well-known imaging technique for phase imaging. However, simultaneously acquiring multidepth DPC images is a non-trivial task. We propose simultaneous multiplane DPC imaging using volume holographic microscopy (VHM).Aim: To design and implement a new configuration of DPC-VHM for multiplane imaging.Approach: The angularly multiplexed volume holographic gratings (AMVHGs) and the wavelength-coded volume holographic gratings (WC-VHGs) are used for this purpose. To obtain asymmetric illumination for DPC images, a dynamic illumination system is designed by modifying the regular Köhler illumination using a thin film transistor panel (TFT-panel).Results: Multidepth DPC images of standard resolution chart and biosamples were used to compare imaging performance with the corresponding bright-field images. An average contrast enhancement of around three times is observed for target resolution chart by DPC-VHM. Imaging performance of our system is studied by modulation transfer function analysis, which suggests that DPC-VHM not only suppresses the DC component but also enhances high-frequency information.Conclusions: Proposed DPC-VHM can acquire multidepth-resolved DPC images without axial scanning. The illumination part of the system is adjustable so that the system can be adapted to bright-field mode, phase contrast mode, and DPC mode by controlling the pattern on the TFT-panel.

Volume holographic optical element for light sheet fluorescence microscopy.

Three-dimensional (3D) imaging of living organisms requires fine optical sectioning and high-speed image acquisition, which can be achieved by light sheet fluorescence microscopy (LSFM). However, orthogonal illumination and detection arms in the LSFM system make it bulky. Here, we propose and demonstrate the application of a volume holographic optical element (photopolymer-based volume holographic grating) for designing a compact LSFM system, called a volume holographic LSFM (VHLSFM). Using the VHLSFM, we performed in vivo imaging of Caenorhabditis elegans (C. elegans) and observed high-contrast optically sectioned fluorescence images of the oocytes and embryonic development in real time for 3D imaging.

Two-photon fluorescence imaging of subsurface tissue structures with volume holographic microscopy.

.Significance: Two-photon (2P) fluorescence imaging can provide background-free high-contrast images from the scattering tissues. However, obtaining a multiplane image is not straightforward. We present a two-photon volume holographic imaging (2P-VHI) system for multiplane imaging.Aim: Our goal was to design and implement a 2P-VHI system that can provide the high-contrast optically sectioned images at multiple planes.Approach: A 2P-VHI system is presented that incorporates angularly multiplexed volume holographic gratings and a femtosecond laser source for fluorescence excitation for multiplane imaging. A volume hologram with multiplexed gratings provides multifocal observation, whereas nonlinear excitation using a femtosecond laser helps in significantly enhancing both depth resolution and contrast of images.Results: Standard fluorescent beads are used to demonstrate the imaging performance of the 2P-VHI system. Two-depth resolved optical-sectioning images of fluorescently labeled thick mice intestine samples were obtained. In addition, the optical sectioning capability of our system is measured and compared with that of a conventional VHI system.Conclusions: Results demonstrated that 2P excitation in VHI systems provided the optical sectioning ability that helps in reducing background noise in the images. Integration of nonlinear fluorescence excitation in the VHI provides some unique advantages to the system and has potential to design multidepth optical sectioned spatial–spectral imaging systems.

Simultaneous multi-color optical sectioning fluorescence microscopy with wavelength-coded volume holographic gratings.

Optical sectioning fluorescence microscopy provides high contrast images of volumetric samples and has been widely used for many biological applications. However, simultaneously acquiring multi-color fluorescence images require additional optical elements and devices, which are bulky, wavelength specific, and not cost-effective. In this paper, wavelength-coded volume holographic gratings (WC-VHGs) based optical sectioning fluorescence microscopy is proposed to simultaneously offer multi-color fluorescence images with fine out-of-focus background rejection. Due to wavelength degeneracy, multiplexed WC-VHGs are capable of acquiring multi-wavelength fluorescence images in a single shot, and displaying the laterally separated multi-wavelength images onto CCD. In our system optical sectioning capability is achieved through speckle illumination and HiLo imaging method. To demonstrate imaging characteristics of our system, dual-wavelength fluorescence images of both standard fluorescent microspheres and ex vivo mT/mG mice cardiac tissue are presented. Current results may find important applications in hyperspectral imaging for biomedical research.

Recording optical vortices in azo polymer films by applying holographic method

In this paper the optical vortices with topological charge <i>q</i> = –1, 1, 2, 4 are recorded in azo polymer films by using holographic technology. The forked holographic gratings formed by the Gaussian beam and optical vortex beam are recorded in the sample films, the original forked holographic grating and the recording rate are analyzed. The vortex beam is reconstructed by illuminating the sample film with a reference beam, and the recording quality is analyzed. Also the erasability and durability of the sample are tested. The experimental results show that the recording rates of vortex beams with different topological charges are relatively uniform, which means that the optical vortices with different topological charges can be recorded at the same speed. The forked holographic grating of the high-order optical vortex splits in the recording process due to the disturbances, such as anisotropic nonlinear light, atmospheric turbulence, and background light field. However, the split vortex beam still maintains a stable ring structure. The reconstructed optical vortex and the original optical vortex are highly consistent in morphology, and the interference fringes of the reconstructed optical vortices are highly consistent with the original vortex holographic gratings, indicating that the topological charge information in the optical vortices can be effectively recorded and read out. The recorded information can be erased by heating the sample to about 97 ℃, and new information can be re-recorded after cooling. There appears no fatigue in the sample after the information has been erased 100 times and good durability is still retained. Optical vortices theoretically have infinite states of topological charges, based on which great success is achieved in optical communication and information encoding. Therefore, storing and reading information of topological charges in optical vortices may have potential applications in optical information storage.

Effect of molecular weight of PEG polymer matrix on the diffraction efficiency of Methyl Orange holographic media

Abstract Recording media for diffraction gratings formation based on polymer films of polyethylene glycol doped with the photoisomerisable azo-dye Methyl Orange have been developed. Holographic recording of the plane wave front for parallel and orthogonal orientation of polarization vectors of light beams have been performed at room temperature. In order to optimize the recording conditions and obtain high-efficient polarization holographic gratings, in the present work we study the dependence the average molecular weight of polymer matrix and solvent system for thin layers formation on the diffraction efficiency. Five PEG polymers were used as host for azo-dye incorporation −1500, 4000, 6000, 10,000 and 40,000, and two solvent systems, EtOH–H2O and THF-H2O, were examined to investigate film-forming properties. It was concluded, that structure of films as well as diffraction efficiency value and curves character strongly depend on PEG molecular weight.

Modification and Polishing of the Holographic Diffraction Grating Grooves by a Neutralized Ar Ion Beam

We describe the method for increasing the efficiency of holographic diffraction gratings (HDGs) intended for operation in the X-ray wavelength range. We describe the procedure of processing of HDG surface for reducing the amplitude and roughness of grooves. For reducing the groove amplitude, we have used Xe ions with an energy of 600 eV, while ion polishing was performed with Ar ions having an energy of 800 eV. It is shown that, after ion polishing, the diffraction efficiency of the grating at a wavelength of 4.47 nm increases by almost four times.

Fabrication of nanodiamond-dispersed composite holographic gratings and their light and slow-neutron diffraction properties

We demonstrate the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. Nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. We describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Results of slow-neutron diffraction from such gratings are also presented.

Polarization holographic gratings with enhanced parameters recorded in azopolymer based nanocomposite materials

We present study of the parameters of polarization holographic gratings recorded in thin films of the azopolymer PAZO (Poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) and in a composite PAZO-based organic/inorganic material with incorporated TiO2 nanoparticles with size 21 nm and different concentrations. The gratings are inscribed by two plane waves with left and right circular polarization from a He-Cd gas laser (442 nm) at recording angle 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that the diffraction efficiency for the hybrid samples is enhanced with respect to the pure azopolymer films. Comparison is also made between polarization response of samples deposited from water and from methanol solution.

Multifunctional holographic gratings for simultaneous coupling and beam splitting applications in photonic integrated circuits

A novel holographic grating for simultaneous coupling of power and optical beam splitting application is designed. The proposed design incorporates two identical holo-gratings separated by a distance D. A microscopic object such as micro-sphere or micro-ring is introduced in between these two gratings at a distance $$ \frac{D}{2} $$ and a Gaussian signal is allowed to pass through the first holo-grating which emerges out of the second one. With the proposed design eleven different splitting ratios could be achieved successfully by varying dimension of microscopic objects and the orientations of holo-gratings with respect to plane of incidence. For coupling applications the simulation of the proposed design shows 100% Transmission Efficiency (TE). This leads to an immense flexibility for the use of proposed model in Photonic Integrated Circuits. We demonstrate the possibility of such a multi-functional Holographic device with the aid of Finite Difference Time Domain (FDTD) simulations. To the best of our knowledge, this is the first investigation where simultaneous beam splitting and coupling applications are realized in a holographic optical element.

Full-color see-through near-eye holographic display with 80° field of view and an expanded eye-box.

A full-color see-through near-eye holographic display is proposed with 80° field of view (FOV) and an expanded eye-box. The system is based on a holographic optical element (HOE) to achieve a large FOV while the image light is focused at the entrance to human pupil and the image of entire field enters human eye. As we know, one of the major limitations of the large FOV holographic display system is the small eye-box that needs to be expanded. We design a double layer diffraction structure for HOE to realize eye-box expansion. The HOE consists of two non-uniform volume holographic gratings and a transparent substrate. Two fabricated holographic gratings are attached to front and back surfaces of the substrate to multiplex image light and achieve eye-box expansion. Simultaneously, the HOE is also manufactured for RGB colors to realize full-color display. The experiment results show that our proposed display system develops 80° round FOV and an enlarged eye-box of 7.5 mm (H) ×5 mm (V) at the same time. The dynamic display ability is also tested in the experiments. The proposed system provides a new solution for the practical application of augmented reality display.

Multi-functional two-dimensional holographic grating based on silver pattern coated by azopolymer

A two-dimensional diffraction device with the properties of polarization modulation, angular sensitivity, polarization-independent diffraction efficiency and repeatable recording has been designed and fabricated in an azopolymer-silver double-layer film by means of two-step holographic exposure. A silver pattern is formed in the light field with the sinusoidally distributed intensity, and then a polarization holographic grating is recorded in the azobenzene liquid-crystalline layer through the interference of two orthogonally linearly polarized lasers. We report here that the polarization states of the diffraction beams from the recorded two-dimensional grating are orthogonally modulated; the diffraction efficiency is related to the incident angle of the probe light and the rotation angle between the two-step recording, while independent on the incident polarization state. Furthermore, the experimental results are analyzed through the scalar diffraction theory and Jones matrices.

Very high contrast volume holographic gratings recorded in photopolymerizable nanocomposite materials.

Volume holographic phase gratings possessing the saturated refractive index modulation amplitudes as large as 4.5×10-2 were recorded at a wavelength of 532 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. This prominent result was achieved by a combination of the HBP nanoparticles with triazine and aromatic ring units and an electron donor/acceptor photo-initiator system doped in an acrylate monomer blend with low viscosity. As a result, efficient mutual diffusion of HBP nanoparticles and monomer having their very large refractive index difference took place. Obtained results suggest a potentiality of our newly developed HBP-dispersed NPC gratings as efficient volume holographic optical elements for various photonic applications including wearable headsets for augmented and mixed reality.