Holographic Recording Material Research Articles

The Interplay of Processing-Related Influences on the Formation of Volume Holographic Gratings in a Free-Surface Epoxy-Based Recording Material

Understanding the formation processes of holographic gratings in polymers as a function of material composition and processing is important for the development of new materials for holography and its associated applications. Among the processing-related factors that affect grating formation in volume holographic recording material, pre-exposure, prebaking and dark storage, as well as the associated variations in layer thickness and composition, are usually underestimated. This study highlights the influence and interaction of these factors and shows that they should not be neglected. This is of particular importance for samples with a free surface. Here, one such epoxy-based free-surface material is investigated. To determine the influence of prebaking on the holographic grating formation, as well as on the achieved refractive index contrast, angular resolved analysis of volume holographic phase gratings is applied through point-by-point scanning of the local material response. Grating characteristics are determined by comparison with simulations based on rigorous coupled wave theory. Thus, the optimal dose for prebaking can be determined, as well as the optimal exposure time, depending on the dose. The influence of dark storage on the material response is investigated over a period of 12 weeks and shows a strong dependence on the deposited energy density.

P‐7.3: Unidirectional Observable Window Signage Using Structural Colors Which Enable to Make Images Invisible from Back Side

It stands to reason that someone observes mirrored imaging according to the direction of viewing if you draw something on the transparent material. The authors dislike this mirrored imaging problem, although it is not a fatal defect. In 2021, our research group proposed window signage using dye‐doped thin film layer to avoid this problem of mirrored imaging. But our 2021 model had a defect that the transparency of a glass window with half mirror layer was less than the general glass material without half mirror because of image generating mechanism. To solve this “reflect surroundings” problem, we needed to find new technology for generating color using a transparent material. This paper shows a unidirectional observable window signage using holographic imaging or holographic optical elements for unidirectional color generating by light controls, how to make structural colors using holographic recording technology and material development.

Practical method for dynamic color holographic display.

A practical method for dynamic color holographic display by using a computer-generated hologram (CGH) with a high space-bandwidth product is proposed, and a dynamic color holographic display system is designed by a space-division method. First, three primary color CGHs of different frames from a color movie are fabricated on holographic recording material by a self-made CGH microfilming system. Secondly, the CGH is fixed on an X-Y moving stage, which is controlled by the system in order to bring the CGH to the appointed position. Thirdly, three primary color lasers are used to reconstruct the CGH. The switch of the lasers is controlled by the system synchronous with the X-Y moving stage. The color video with high quality can be obtained after filtering the three primary color reconstructed wavefronts. The experimental results demonstrate that the proposed dynamic color holographic display method is effective. It has practical application value in high-quality CGH display.

Improving the Holographic Recording Characteristics of a Water-Resistant Photosensitive Sol–Gel for Use in Volume Holographic Optical Elements

Continual improvements to holographic recording materials make the development of volume holographic optical elements increasingly more attainable for applications where highly efficient, lightweight diffractive optical elements can replace conventional optics. A fast-curing, water resistant photosensitive sol–gel capable of volume holographic recording has recently drawn attention for its extreme environmental and physical robustness, in particular its water/moisture and scratch resistance. However, to date, the refractive index modulation has been limited. While water-resistant properties are invaluable in the face of the weathering which many practical systems for outdoor applications will endure, high refractive index modulation is also important in order to facilitate high diffraction efficiency holograms recorded in relatively thin layers. Lower grating thickness ensures a large angular and wavelength range of operation-properties that are critical for many applications of holographic optical elements such as solar light harvesting, optical displays and illumination management. For any application where low-cost mass production is envisaged, sensitivity/writing speed is also a crucial factor. In this research, we studied the recording properties of these water-resistant photosensitive sol–gel layers at two different recording wavelengths (532 and 476 nm) and investigated methods for improving these properties. We report more than two-fold improvement of the refractive index modulation from 1.4×10−3 to 3.3×10−3 in layers of thickness ranging from 40–100 μm and more than an order of magnitude increase in photosensitivity/recording speed through better matching between recording wavelength and layer absorption, chemical alterations and thermal post-processing techniques.

Influence of Pre-Exposure on the Material Response of Epoxy-Based Volume Holographic Recording Material.

The formation of volume holograms in photosensitive polymers is a complex process under the influence of many interacting factors: material composition and processing, exposure conditions, and pre-exposure affect the development and final characteristics of holographic gratings. In order to better understand the interplay of these influencing factors, the detailed investigations of holographic recording in a new organic material are performed and the results are presented here. The material response and performance of an epoxy-based free surface material designed for volume holography are investigated. For this purpose, time-resolved investigation of volume holographic grating growth is performed on the one hand. Spatially resolved analysis of volume holographic phase gratings by point-by-point scanning of the local material response to the Gaussian intensity distribution of the recording beams is carried out on the other hand. Thus, the influence of pre-exposure on the temporal grating formation, as well as on the final obtained refractive index contrast, was determined. The various effects observed can be explained by the consumption of photosensitive compounds and prior crosslinking in the course of pre-exposure. Rather unexpected effects are that, on the one hand, pre-exposed gratings emerge with ever more complete null diffraction at the transition point and, on the other hand, a stabilizing effect of some degree of pre-exposure on regions exposed with low intensity was identified.

Numerical optimization of writing computer-generated holograms in threshold media

Recent developments in binary photo-magnetic materials showed efficient and ultra-fast rewriting of holograms where the intensity threshold allows for a dense, sub-diffraction limit packing of hologram points. This paper describes the numerical optimization of the process of writing and reconstructing of 2-D images in a binary-phase computer-generated holograms stored in the said threshold-like medium. Global optimization of the free parameters of the writing process is shown, including the intensity threshold level, propagation distance, hologram spot size and the shape of the boundary regions of the written spots. We present the optimal set of parameters for the best possible writing quality. Full Text: PDF ReferencesA. Stupakiewicz, K. Szerenos, D. Afanasiev et al., "Ultrafast nonthermal photo-magnetic recording in a transparent medium", Nature 542, 71 (2017). CrossRef J. Starobrat, A. Frej, J. Bolek, R. Trybus, A. Stupakiewicz, and M. Makowski, "Photo-magnetic recording of randomized holographic diffraction patterns in a transparent medium", Opt. Lett. 45, 5177 (2020). CrossRef V. Ostroverkhov, et al., "Micro-Holographic Storage and Threshold Holographic Recording Materials", Jap. J. App. Phys. 48.3S1, 03A035 (2009). CrossRef K. Matsushima, T. Shimobaba, "Band-Limited Angular Spectrum Method for Numerical Simulation of Free-Space Propagation in Far and Near Fields", Opt. Express 17, 19662 (2009). CrossRef F. Wyrowski, O. Bryngdahl, "Iterative Fourier-transform algorithm applied to computer holography", JOSA A 5.7, 1058 (1988). CrossRef I. Ducin, T. Shimobaba, M. Makowski, K. Kakarenko, A. Kowalczyk, Jaroslaw Suszek, M. Bieda, A. Kolodziejczyk, M. Sypek, "Holographic projection of images with step-less zoom and noise suppression by pixel separation", Opt. Comm. 340, 131 (2015). CrossRef M. Makowski, "Minimized speckle noise in lens-less holographic projection by pixel separation", Opt. Express 21, 29205 (2013). CrossRef

A New Three-Component Photo-Initiating System for Visible Light Recording of Volume Holograms with Single-Pulsed Laser.

Versatile substituted electron-deficient trichloromethylarenes can easily be synthesized and combined with a Safranine O/triarylalkylborate salt to form a highly efficient three-component photo-initiation system that starts free radical polymerization to finally form holographic gratings with a single-pulsed laser. The mechanism of this photo-initiation most likely relies on an electron transfer from the borate salt into the semi-occupied HOMO of the excited dye molecule Safranine O, which after fragmentation generates an initiating alkyl radical and longer-lived dye radical species. This dye radical is most probably oxidized by the newly introduced trichloromethylarene derivative as an electron acceptor. The two generated radicals from one absorbed photon initiate the photopolymerization and form index gratings in a suitable holographic recording material. This process is purely photonic and does not require further non-photonic post treatments.

Use of forbidden singlet–triplet electron transitions in photopolymer material for holographic recording with high intensity nanosecond laser pulses

The goal of this research is recording of volume holograms in thick photopolymer material, based on the use of forbidden singlet–triplet transitions of the dye-sensitizer with high intensity (up to 12 GW cm−2) nanosecond pulses at λ=532nm. The material has threshold character of recording, that can lead to improved three-dimensional (3D) control of photochemical or photophysical processes. This is especially important for the microholographic approach to 3D multilayer optical data storage. The concentrations of the dye-sensitizer and electron donor in the photopolymer was optimized, and high value of refractive index change of 3.1⋅10−3 was achieved. The sensitivity of the material under research is much higher than the sensitivity of materials using two-photon absorption, which made it possible to record microholograms by a single laser pulse. Using the method of heterodyne detection, we also studied the spatial distribution of the optical inhomogeneity in photopolymer material.

Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display.

Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefractive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second.

Water Resistant Cellulose Acetate Based Photopolymer for Recording of Volume Phase Holograms

The development of environmentally robust photosensitive materials for holographic recording is crucial for applications such as outdoor LED light redirection, holographic displays and holographic sensors. Despite the progress in holographic recording materials development, their sensitivity to humidity remains a challenge and protection from the environment is required. One approach to solving this challenge is to select substrate such as cellulose acetate, which is water resistant. This work reports the development of a cellulose-based photopolymer with sensitivity of 3.5 cm2/mJ and refractive index modulation of 2.5 × 10−3 achieved in the transmission mode of recording. The suitability for holographic recording was demonstrated by recording gratings with the spatial frequency of 800 linepairs/mm. The intensity dependence of the diffraction efficiency of gratings recorded in 70 μm thick layers was studied and it was observed that the optimum recording intensity was 10 mW/cm2. The robustness of the structures was studied after immersing the layer in water for one hour. It was observed that the diffraction efficiency and the surface characteristics measured before and after exposure to water remain unchanged. Finally, the surface hardness was characterized and was shown to be comparable to that of glass and significantly higher than the one of PVA-based acrylamide photopolymer.

Design of a waveguide eye-tracking system operating in near-infrared with holographic optical elements

Recent research in augmented reality (AR) eyewear has prompted interest in using volume holographic optical elements for this application. However, many sensing operations in AR systems require the use of wavelengths in the near-infrared (NIR) (750 to 900 nm). These wavelengths typically exceed the sensitivity range of available commercial holographic recording materials (450 to 650 nm), which complicates the design of optical elements with power since significant aberrations result when the reconstruction wavelength differs from the construction wavelength. Several methodologies for designing a waveguide hologram imaging system in NIR are reviewed and evaluated. The design approach presented in our work integrates the most effective practices such as fabrication point source location optimization and aberration analysis to realize effective holographic waveguide couplers formed with visible wavelength light and reconstructed in the NIR. The technique is demonstrated by designing and fabricating an input waveguide hologram in conjunction with a multiplexed output coupling hologram. The resulting input/output waveguide holograms can achieve an image resolution of (∼3 lp / mm) with a 0.6-mm-thick glass substrate that has a refractive index of 1.8.

Tunable Waveguides Couplers Based on HPDLC for See-Through Applications.

Photopolymers have become an important recording material for many applications, mainly related to holography. Their flexibility to change the chemical composition together with the optical properties made them a versatile holographic recording material. The introduction of liquid crystal molecules in a photopolymer based on multifunctional monomer provides us the possibility to generate tunable holograms. The switchable holographic elements are a key point for see-through applications. In this work, we optimize the holographic polymer-dispersed liquid crystals composition to improve the performance of tunable waveguide couplers based on transmission gratings and specifically their response under an applied electric field. A variation around 60% in the transmission efficiency was achieved.

Red-sensitive organic nanoparticle-polymer composite materials for volume holographic gratings with large refractive index modulation amplitudes

We demonstrate volume holographic recording at a wavelength of 640 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. We employ a new photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find the optimum composition of the system for volume holographic recording. We show that recorded volume gratings of 0.5-µm spacing possess the saturated peak-to-mean refractive index modulation amplitudes as large as 3×10−2 at a readout wavelength of 640 nm. Our results show the usefulness of photopolymerizable HBP-dispersed NPCs for volume holographic recording materials for various photonic applications including security and color holograms, and volume Bragg grating devices in head-mounted displays.

Thermal Properties of Bayfol® HX200 Photopolymer.

Bayfol® HX200 photopolymer is a holographic recording material used in a variety of applications such as a holographic combiner for a heads-up display and augmented reality, dispersive grating for spectrometers, and notch filters for Raman spectroscopy. For these systems, the thermal properties of the holographic material are extremely important to consider since temperature can affect the diffraction efficiency of the hologram as well as its spectral bandwidth and diffraction angle. These thermal variations are a consequence of the distance and geometry change of the diffraction Bragg planes recorded inside the material. Because temperatures can vary by a large margin in industrial applications (e.g., automotive industry standards require withstanding temperature up to C), it is also essential to know at which temperature the material starts to be affected by permanent damage if the temperature is raised too high. Using thermogravimetric analysis, as well as spectral measurement on samples with and without hologram, we measured that the Bayfol® HX200 material does not suffer from any permanent thermal degradation below C. From that point, a further increase in temperature induces a decrease in transmission throughout the entire visible region of the spectrum, leading to a reduced transmission for an original 82% down to 27% (including Fresnel reflection). We measured the refractive index change over the temperature range from C to C. Linear interpolation give a slope for unexposed film, with the extrapolated refractive index at C equal to . This refractive index change decreases to when the material is fully cured with UV light, with a C refractive index equal to . Spectral properties of a reflection hologram recorded at 532 nm was measured from C to C. A consistent 10 nm spectral shift increase was observed for the diffraction peak wavelength when the temperature reaches C. From these spectral measurements, we calculated a coefficient of thermal expansion (CTE) of by using the coupled wave theory in order to determine the increase of the Bragg plane spacing with temperature.

Spatial Frequency Response of Epoxy-Based Volume Holographic Recording Material.

Holographic volume phase gratings are recorded in an epoxy-based, free-surface, volume holographic recording material. Light-induced gratings are formed by photo-triggered mass migration caused by component diffusion. The material resolution enables a wide range of pattern spacings, to record both transmission and reflection holograms with many different spatial frequencies. An optimum spatial frequency response is found between the low spatial frequency roll-off and the high spatial frequency cut-off. The influence of the energy density of exposure on the spatial frequency response is investigated. Secondary volume holographic gratings (parasitic gratings) are observed in the high frequency range. The possibility of distinguishing the regular grating from the secondary grating is discussed in the form of probe wavelength detuning.

Holographic waveguides in photopolymers

The possibilities that offer the holographic optical elements for photovoltaic and "see through display" applications open new windows for holographic recording materials. In this sense, some specific characteristics are required for each particular application. Waveguides are one of the key elements for these applications. Photopolymers are one of the most competitive candidates for waveguide fabrication. In this work, we evaluate the performance of one example from each of three families of photopolymer material in fabrication of a 633nm waveguide. Firstly, polyvinyl alcohol acrylamide, PVA/AA, the second one, a nanoparticle-thiol-ene, NPC, and on the last place a penta/hexa-acrylate based polymer with dispersed nematic liquid crystal molecules, PDLC. We study the critical role of the material and in particular, spatial resolution for this application.

Polarisation-free and high-resolution holographic grating recording and optical phase conjugation with azo-dye doped blue-phase liquid crystals

ABSTRACTWe present the results of a detailed study of the mechanism and dynamics of holographic Bragg grating formation in azo-dye doped blue-phase liquid crystals. The principal mechanism leading to refractive index modulation is lattice distortion caused by photo-activated trans–cis isomerism of the azo-dye dopant. High diffraction efficiency bulk (Bragg) gratings with grating spacing as small as 1 μm can be written with low optical power and do not require specific optical field polarisation. Furthermore, the written grating can be prolonged by uniform illumination with another laser, instead of being erased. Azo-doped blue-phase liquid crystals thus present themselves as highly promising photosensitive materials for high-resolution holographic recording and image processing application. An experimental demonstration of holographic image reconstruction via optical phase conjugation with excellent aberration correction capability is presented.

Volume Polarization Holographic Recording in Phenanthrenequinone Doped Poly(MMA-co-BzMA) Photopolymer

Phenanthrenequinone (PQ) doped poly-(methyl methacrylate) (PMMA) have been proven to be available material for volume polarization holographic recording. In our work, we introduced benzyl methacrylate (BzMA) into the PQ/PMMA to increase the concentration of PQ (up to 1.15wt%) within the polymeric media. The enhancement of concentration of PQ brings a mean for improving performance of material. Compared with the pure PQ/PMMA, photoinduced birefringence and diffraction intensity were multiplied by introducing BzMA. This investigation can provide a significant foundation for improving polarization holographic characteristics of PQ/PMMA by modifying the chemical composition of the material.

N-isopropylacrylamide-based photopolymer for holographic recording of thermosensitive transmission and reflection gratings

In recent years, functionalized photopolymer systems capable of holographic recording are in great demand due to their potential use in the development of holographic sensors. This work presents a newly developed N-isopropylacrylamide (NIPA)-based photopolymer for holographic recording in reflection and transmission modes. The optimized composition of the material is found to reach refractive index modulation of up to 5×10-3 and 1.6×10-3 after recording in transmission and reflection mode, respectively. In addition to fulfilling the requirements for holographic recording materials, the NIPA-based photopolymer is sensitive to temperature and has lower toxicity than acrylamide-based photopolymers. Possible application of the NIPA-based photopolymer in the development of a holographic temperature sensor is discussed.

Study on permanent holographic recording in trimethylol propane triacrylate-based photopolymer films with high diffraction efficiency

We investigate permanent hologram recording in a photosensitive material with high diffraction efficiency (DE) of ∼90%. We successfully recorded three-dimensional (3D) object information, gratings, and “hogel” images in this photopolymer film without postprocessing. In addition, we study multiplexed hologram storage in this material. Because of its high resolution, high DE, and lack of need for electric field and postprocessing, this photopolymer may be suitable for analog hologram, digital hologram, and big data permanent storage. Our study proposes a potential holographic recording material to apply in large size static 3D holographic displays, including analog hologram storage, digital hologram prints, holographic anticounterfeiting, and big data holographic disks.