Holographic Recording Research Articles

Computational multi-wavelength phase synthesis using convolutional neural networks [Invited

Multi-wavelength digital holographic microscopy (MWDHM) provides indirect measurements of the refractive index for non-dispersive samples. Successive-shot MWDHM is not appropriate for dynamic samples and single-shot MWDHM significantly increases the complexity of the optical setup due to the need for multiple lasers or a wavelength tunable source. Here we consider deep learning convolutional neural networks for computational phase synthesis to obtain high-speed simultaneous phase estimates on different wavelengths and thus single-shot estimates of the integral refractive index without increased experimental complexity. This novel, to the best of our knowledge, computational concept is validated using cell phantoms consisting of internal refractive index variations representing cytoplasm and membrane-bound organelles, respectively, and a simulation of a realistic holographic recording process. Specifically, in this work we employed data-driven computational techniques to perform accurate dual-wavelength hologram synthesis (hologram-to-hologram prediction), dual-wavelength phase synthesis (unwrapped phase-to-phase prediction), direct phase-to-index prediction using a single wavelength, hologram-to-phase prediction, and 2D phase unwrapping with sharp discontinuities (wrapped-to-unwrapped phase prediction).

Multi-Wavelength Optical Patterning for Multiscale Materials Design

Laser interferometry is a consolidated technique for materials structuring, enabling single step and large area patterning. Here we report the investigation of the morphological modification encoded on a thin film of a photosensitive material by the light interference pattern obtained from a laser operating in multiline mode. Four lines with equal intensity are retained, with the same p linear polarization. An azopolymer is exploited as medium for the holographic recording. Optical microscopy and profilometer measurements analyze the modification induced in the bulk and on the surface of the irradiated area. We show that the intensity profile of the interference patterns of two laser beams is the one obtained assuming each line of the laser as an independent oscillator of given intensity and wavelength, and how these light structures are faithfully replicated in the material bulk and on the topography of the free surface. Patterns at different length scales are achievable in a single step, that can be traced back to both interference fringes and wave envelopes. The proposed multi-wavelength holographic patterning provides a simple tool to generate complex light structures, able to perform multiscale modifications of photoresponsive materials

Reconstruction resolution enhancement of EPISM based holographic stereogram with hogel spatial multiplexing

We investigate how the splicing mode of a holographic element (hogel) affects the reconstruction of a 3D scene to improve the reconstruction resolution of a holographic stereogram fabricated using the effective perspective image segmentation and mosaicking method (EPISM). First, the effect of hogel spatial multiplexing on holographic recording and reconstruction is studied based on the mechanism of recording interference fringes in the holographic recording medium. Second, combined with the influence of multiple exposures on the hologram’s diffraction efficiency, the diffraction efficiency of the holographic stereogram is analyzed in the spatial multiplexing mode. The holographic stereogram is then regarded as a special optical imaging system. The theory of spatial bandwidth product is adopted to describe the comprehensive resolution of the holographic stereogram, which explains why hogel spatial multiplexing can significantly improve the reconstruction resolution of a holographic stereogram. Compared with the traditional printing method under the same parameters in optical experiments, hogel spatial multiplexing has a lower diffraction efficiency but a higher quality of reconstructed image, consistent with the theoretical analysis.

Versatile approach to laser beam shaping and analyzing by holographic phase masks

A new technology that includes a flexible holographic recording system using photo-thermo-refractive (PTR) glass is demonstrated for creating so-called ‘holographic phase masks’ or HPMs. These diffractive optical elements are permanently recorded in the PTR glass, can be multiplexed as with normal holograms, do not require electric power to operate, and can perform near arbitrary beam phase transformations. The holographic setup includes a spatial light modulator that enables recording transmitting volume Bragg gratings (VBGs) with arbitrary phase patterns encoded into them. As a result, the desirable phase pattern is introduced in a beam that is diffracted by the VBG. These HPMs are tunable within the visible and near IR spectral regions and can be made achromatic, with spectral widths of up to 50 nm. Furthermore, they tolerate laser power densities on the order of several kW cm−2. Applications of HPMs include high-power, broadband laser beam shaping and modal analysis of complex beams profiles, both of which are demonstrated here.

Acoustic to Electronic and Public to Private: An Introduction to Music in the Great Expansion (1700–2100 CE)

Acoustic to Electronic and Public to Private: An Introduction to Music in the Great Expansion (1700–2100 CE)

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.

Theoretical and experimental investigations of photo-sensitivity and polarization-sensitivity in TI/PMMA polymers

A new kind of cationic initiator IRGACURE 784 (TI, BASF) was dispersed into PMMA matrix to fabricate TI/PMMA polymer, which exhibited excellent absorptive and polarization sensitivities under 532 nm wavelength laser beam exposure. Theoretical investigations on the grating formation mechanisms under two different time scales of exposure were performed (short time duration: millisecond level, long time duration: second level). We introduced an analogical four-energy-level rate equation on analyzing the photo-induced process of TI molecules. Furthermore, an absorptive diffusion polymerization (ADP) model resulting from the combination of the non-local diffusion model and the analogical four-energy-level model was proposed to describe the grating formation in TI/PMMA polymers with different conditions (sample thickness and exposure duration level). In addition, intensity (two p-polarized recording beams) and polarization holographic recording (two orthogonally polarized recording beams) approaches were applied to compare the grating enhancement process and multiplexed storage ability in TI/PMMA polymers. We achieved the maximum single grating strength with 0.86 and 0.63 in 3 mm thick sample, respectively. And, cumulative grating strength of 2.06 and 1.23 by angular multiplexing five gratings and polarized-angular multiplexing ten gratings in 3 mm thick sample, respectively. Our current work supplemented and summarized the comprehensive holographic performance in TI/PMMA polymer by investigating its photo-sensitivity and polarization-sensitivity.

Characterization of Polarization Holographic Gratings Obtained on Azopolymer Thin Films by Digital Holographic Microscopy

Polarization diffraction gratings are formed by one-step polarization holographic recording in azopolymer thin films. The evolution of the gratings parameters, such as the modulation of diffraction efficiency and relief depth with regard to different exposure dose is analyzed. Phase-shifting digital holographic microscopy is applied for the measurement of the light-induced polarization diffraction gratings. For the accurate hologram acquisition and reconstruction of the complex amplitude transmitted by the gratings, we performed all-optical (without moving components) phase-shifting implemented within in the imaging system of the digital holographic microscope. The experimental measurement results and theoretical predictions were compared and analyzed.

Lloyd’s mirror interferometer for holographic recording in photorefractive crystals

We report the use of Lloyd’s mirror interferometer for holographic recording in photorefractive material. The experimental results show that perturbations in the diffraction efficiency considerably decrease due the feature of that system; and, as a consequence, fast (electron-based) and slow (hole-based) gratings were recorded in a nominally undoped photorefractive Bi12TiO20 crystal. Measurement of the response times were determined as a function oflightintensity for the 0-1.2 mW/cm2 range using the 532 nm laser wavelength. Parameters that characterize the crystal were also determined and compared with available data in the literature in order to assess the reliability of this technique.

Morphology and position measurement of irregular opaque particle with digital holography of side scattering

Morphology measurement is essential for a better understanding of irregular particle dynamics. Digital holography of side scattering is applied to measure the morphology and 3D position of pulverized coal particles. The digital holographic recording of particle side scattering is modelled and the formula is verified by experiments. A side scattering holography system is established to record the hologram of pulverized coal particles. The hologram of particles with several scatterers has uneven chirp fringes which is mainly influenced by scatterer shape, while the hologram of particles with multiple scatterers is a superposition of single scatterer fringes with a speckle background which is influenced by both scatterer shape and location. The Moiré pattern which appears easily in digital holography of side scattering is also analyzed. The proposed technique is validated by in-line holography and is proven to have an advantage in the identification of irregular particle morphological and surface features simultaneously.

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.

Digital holographic polarimeter using dual reference beam interferometry.

An off-axis digital holographic imaging polarimeter was developed to estimate the Jones matrices of an object. The Jones vector image of the electric field returned from the object is determined from a single holographic recording using the interference between the dual, nearly orthogonal, reference beams. The technique compensates for phase variations in the optical beam paths between the recorded holograms and relaxes the need to generate orthogonal illumination polarization states. A minimization algorithm was developed to compute an estimation of the Jones matrix image of an object based on a set of measured Jones vector images. A proof-of-concept demonstration was performed to compute an estimated Jones matrix image of a polarimetrically complex object using digital holograms recorded with 6 different illumination polarizations.

Optical Vortices Generation by Azopolymeric Relief Gratings

The energy and spectral conditions for single-stage holographic recording of a diffraction optical element based on the carbazole-containing azo polymer, that forms singular light beams (optical vortices), have been established. With the atomic-force microscopy (AFM), the surface morphology of the recorded relief holograms was studying, and their diffraction efficiency has been estimated. The topology of the generated optical phase singularities has been studied and the stability range of an optical vortex having the topological charge l = 2 has been found. The possibility of using the developed diffractive optical element in the scheme of optical tweezers for manipulating micro-objects is demonstrated.

Enhanced electrical and optoelectronic performance of SnS crystal by Se doping

In the current decade, Metal chalcogenides have achieved immense importance due to their tunable bandgaps, excellent optical absorption and outstanding optoelectronic behavior. Specifically, pristine and doped SnS (tin sulfide) crystals have fascinated noteworthy consideration mainly due to their potential applications in photodetectors, photovoltaic devices, holographic recording devices and field-effect transistors. In that view, selenium doped tin sulfide crystals were synthesized by direct vapor transport (DVT) technique. The elemental composition was confirmed by energy dispersive analysis of X-ray (EDAX). The structural characterization reveals a layer type growth mechanism of grown SnS0.25Se0.75 crystals. The powder X-ray diffraction reveals the orthorhombic phase and high crystallinity of synthesized crystals. X-ray photoelectron spectroscopy analysis confirmed the formation of SnS0.25S0.75 structure and binding energy of the elements in the samples. The temperature dependent in-plane and out-of-plane resistivity measurements determined the anisotropic charge conduction of grown crystals. The carrier concentration and mobility came out to be 6.19 × 1015 cm−3 and 31.48 cm2 V−1 s−1 calculated from hall effect measurement. The photosensitive properties of SnS0.25Se0.75 crystal-based photodetector were also investigated under different biasing conditions. The device exhibited an excellent response time of 0.6 s along with good photocurrent and responsivity as compared with previous reports. Additionally, air stability after 1 month under 0 V and 1 mV biasing have been reported. This study assigned that Se doping played a major role in enhancing optoelectronic properties and the composition can be used in potential optoelectronic devices for future applications.

Development and Testing of a Dual-Wavelength Sensitive Photopolymer Layer for Applications in Stacking of HOE Lenses

Diffractive optical elements (DOEs) have been in development for many years and are an exciting technology with the capability to re-direct light, using diffraction rather than refraction. Holographic Optical Elements (HOEs) are a subset of diffractive optical elements for which the photonic structure is created holographically, i.e., by recording a specific interference pattern in a suitable, photosensitive optical material. Volume HOEs are of particular interest for some applications because of their very high diffraction efficiency and single diffracted order; however, high dispersion and angular wavelength selectivity still present significant challenges. This paper explores a method for producing a compound DOE useful for situations where elements designed for two separate target wavelengths can be advantageously combined to achieve a highly efficient HOE with reduced dispersion. A photopolymer material consisting of two independently sensitized laminated layers is prepared and used in sequential holographic recording at two different wavelengths. The photonic structures recorded are investigated through examination of their diffraction peaks and comparison with the structure predicted by modeling. Finally, the device is illuminated with an expanded diverging beam at both target wavelengths and with white light, and a strong diffracted beam is observed.

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.

Effect of Glycerol on an N-Vinylpyrrolidone-Based Photopolymer for Transmission Holography.

N-vinylpyrrolidone (NVP) has a large molecular structure, so it is difficult to diffuse during holographic recording, especially at low spatial frequencies. We used glycerol to promote the diffusion of NVP, and successfully improved the holographic performance of the photopolymer at low spatial frequencies. As the concentration of glycerol increases, the holographic performance first increases and then remains stable. The optimal concentration of glycerol is 0.21 mol/L. At this concentration, the maximum diffraction efficiency of the photopolymer is 84%, the refractive index modulation is 1.95 × 10−3, and the photosensitive sensitivity is 7.91 × 10−4 cm2/mJ. Compared with the control group, the maximum diffraction efficiency, maximum refractive index modulation and photosensitivity at low spatial frequencies (800 lp/mm) have increased by 11.19 times, 4.69 times and 1.71 times, respectively. Using the optimized photopolymer for transmission holographic recording and reproduction, we have obtained a clear and bright transmission hologram. The photopolymer modified with glycerol is expected to be applied to the fields of holography, diffractive optics, and so on.