Recording Of Holographic Gratings Research Articles

Comparison of holographic recording options for reflection-format and transmission-format coupler-type diffractive optical elements: theoretical exploration and experimental validation

The holographic recording of gratings that have very large diffraction angles and/or slants is challenging because recording requires beam angles that are not possible without the use of prisms. However, by using a recording wavelength that is different from (usually shorter than) the intended operating wavelength, it is possible to record with less challenging beam angles. In this paper, a recently developed model that allows systematic investigation of the potential and limits of this wavelength-shift recording technique is extended to include a reflection format. Transmission and reflection recording options are compared systematically for the first time, and it is shown that in reflection recording, some couplers can be recorded more easily when the recording wavelength is longer than the operating wavelength. This opens up new design options for previously challenging regions of the spectrum. Experimental validation of the reflection version of the model is carried out by holographic recording in reflection mode at 532 nm in Bayfol HX 200 photopolymer, demonstrating the coupling of blue light.

Freeform surfaces that enable high-quality Offner-type imaging spectrometers with holographic grating.

In this paper, the impact of introducing freeform surfaces into the recording and imaging paths of holographic gratings on system performance is quantitatively investigated, and the performance boundaries of various systems are demonstrated. These performance parameters encompass numerical aperture, spectral resolution, spectral band, and slit length, among others. The results indicate that introducing a freeform surface in the recording path can significantly improve performance, surpassing the introduction of a freeform surface in the imaging path. Besides, the performance improvement is several times that brought by an aspherical surface. Therefore, by incorporating a freeform surface in the recording path and using simple spherical elements in the imaging path, a series of high-performance and low-cost imaging spectrometers can be achieved. Additionally, a high-quality imaging spectrometer with holographic grating is introduced, where all surfaces are freeform.

Holographic Recording of Unslanted Volume Transmission Gratings in Acrylamide/Propargyl Acrylate Hydrogel Layers: Towards Nucleic Acids Biosensing.

The role of volume hydrogel holographic gratings as optical transducers in sensor devices for point-of-care applications is increasing due to their ability to be functionalized for achieving enhanced selectivity. The first step in the development of these transducers is the optimization of the holographic recording process. The optimization aims at achieving gratings with reproducible diffraction efficiency, which remains stable after reiterative washings, typically required when working with analytes of a biological nature or several step tests. The recording process of volume phase transmission gratings within Acrylamide/Propargyl Acrylate hydrogel layers reported in this work was successfully performed, and the obtained diffraction gratings were optically characterized. Unslanted volume transmission gratings were recorded in the hydrogel layers diffraction efficiencies; up to 80% were achieved. Additionally, the recorded gratings demonstrated stability in water after multiple washing steps. The hydrogels, after functionalization with oligonucleotide probes, yields a specific hybridization response, recognizing the complementary strand as demonstrated by fluorescence. Analyte-sensitive hydrogel layers with holographic structures are a promising candidate for the next generation of in vitro diagnostic tests.

780 nm near-infrared photorefractive holograms recorded in undoped Bi12TiO20 crystal under the action of external electric field

In this paper, we studied recording and erasure of photorefractive holographic gratings recorded at 780 nm in an undoped Bi12TiO20 (BTO) crystal sample under the action of an externally applied electric field. The diffraction efficiency was measured by directly chopping the diffracted beams by the gratings recorded in a two-wave mixing setup without using of any phase-modulation technique of the recording beams or feedback mechanism. An enhancement of 3.5-fold in the diffraction efficiency was observed when the applied electric field increased from 0 to 6 kV/cm. The theoretical dependence of the diffraction efficiency on the external electric field considering the effect of screening of charges near the electrodes was investigated, and the result is supported by good fit to the experimental data, allowing us to compute characteristic parameters of the material, including the values of photoactive donor concentrations (ND1)eff ≈ 0.12 × 1016 cm−3 and (ND2)eff ≈ 0.8 × 1015 cm−3, respectively, for the levels placed at 1.6 eV and 1.3 eV below the conduction band responsible for photorefractive recording in the BTO sample.

Polymer composition with phenanthrenequinone for recording relief holographic gratings

The formation of periodic reliefs of the thickness of photosensitive polymer layers after recording holographic gratings in them and stimulating material deformations by reversible plasticization in a non-solvent liquid is considered. The phenomenon was studied for the composition of a copolymer with side anthracene groups — phenanthrenquinone. Phenantrenquinone transfers the energy of electronic excitation to oxygen molecules entering through the open surface of the polymer layer which then cause the oxidation of anthracene fragments. Holographic gratings with a period of 2–5 μm were recorded by laser radiation at a wavelength of 532 nm in layers about 1 μm thick. The photoreliefs were formed during the subsequent swelling of the layer in the medium of a hydrocarbon developer. The photosensitized oxidation of the anthracene groups of the new polymer under the action of optical radiation in the spectral range 408–532 nm was studied using the electron absorption spectra. It is shown that approaching the excitation wavelength to the long-wavelength absorption maximum of phenanthrenquinone (410 nm) makes it possible to increase the sensitivity of the material layer by a factor of 15 compared to layers with methylene blue as a photosensitizer. It has been experimentally established that the amplitude of weak periodic reliefs of a deformation nature (height less than 0.01 μm), which appear immediately after the recording of holographic gratings, increases many times during the treatment of the layer with liquid hydrocarbon. Their maximum value reaches 25 % of the thickness of the recording layer. Presumably, the deformation of the inhomogeneously irradiated layer is stimulated by the transfer of the polymer material into a highly elastic state during its swelling. Photoreliefs are stable after drying. Their strength can be increased by photocrosslinking the material as a result of photodimerization of residual anthracene groups under uniform irradiation with light at a wavelength of 365 nm. The non-sinusoidality of the photorelief reduces the diffraction efficiency achievable with total reflection to values less than 0.20. The studied polymer composition can be used to form relief-phase diffractive optical elements by radiation in the blue-green region of the spectrum, provided by a number of high-power laser sources.

Characterisation of Holographic Recording in Environmentally Stable Photopolymerisable Glass

Photopolymerisable glasses are holographic recording materials which provide good recording capability, improved dimensional stability, and negligible shrinkage. Recently, a novel photopolymerisable hybrid sol-gel (PHSG) for holographic recording of volume gratings has been reported. The PHSG has significantly improved gelation time and high water resistance, both of which make it an attractive material for mass production of holographic optical elements (HOEs) with no sensitivity to ambient humidity. In order to achieve full control over the performance of the material and further improve its properties, a study of grating formation under holographic patterning is essential. This paper reports characterisation of the grating recording in PHSG. The approach is based on the analysis of grating parameters during exposure and post-recording dark processes. The obtained results suggest that photopolymerisation of the methacrylate groups is the main contributor to the creation of refractive index modulation during exposure. During the dark process, the enhancement of the refractive index modulation is observed, probably due to further polycondensation. The observations made facilitate controlled and predictable diffraction efficiency of gratings recorded on the PHSG, thereby furthering the prospect of the development of HOEs with customisable specification.

Evolution of surface relief gratings in As20Se80 amorphous chalcogenide films under laser illumination

Variation of grating amplitudes on a surface of amorphous chalcogenide films (ACF) As20Se80 has been studied under illumination by a band-gap light with the purpose to understand mechanism of photo-induced (PI) mass transfer. After holographic recording of surface relief gratings (SRGs) of various periods Λ (from 3 to 15 µm) they were illuminated by a diode laser (λ = 660 nm) and their profile variation was analyzed using optical microscopy, atomic force microscopy, light scattering, and optical profilometry. The SRGs with Λ < 8 µm exponentially flattened with time of illumination, whereas amplitudes of the SRGs with Λ > 8 µm exponentially grew. Theoretical analysis of the kinetics of PI mass transfer shows that the SRG profile variation occurs by bulk diffusion of As and Se atoms as a result of competition between capillary forces and electrostatic forces created by redistribution of electrons and holes generated by light. Our theory predicts both flattening and growth of the SRG amplitudes under uniform light illumination. The PI diffusion coefficients have been estimated both from the flattening kinetics of the SRG amplitude and from its growth and found to be in good agreement with the theory. It is also observed that the profile evolution is accompanied by dewetting of ACFs and roughening of the initial SRGs.

Refractive index modulation in media with saturable photosensitivity

Permanent change of the refractive index of photo-thermo-refractive glass can be achieved by exposure to ultraviolet light followed by thermal development. The dependence of the change on the dosage is nonlinear because the photochemical process of photosensitivity involves saturation. With a holographic recording of Bragg gratings inside glass, a sinusoidally modulated dosage is imprinted into distorted modulation of the refractive index. We investigate this distorted modulation and derive analytical expressions for the amplitudes of its spatial harmonics.

Influence of gold nanoparticles in polymer nanocomposite on space-temporal-irradiation dependent diffraction grating recording

Following the modern and extremely useful direction of research and development of polymer nanocomposites with combined effects of photopolymerization superimposed by plasmon-induced processes, we investigated acrylate-based nanocomposites with broadband sensitive Irgaqure784 initiator and silicon oxide as well as gold nanoparticles in different regimes of illumination. The details of initiator photodecomposition and polymer chain formation mechanisms at the presence of the gold nanoparticles' (GNPs) plasmon fields and different combinations of irradiation wavelengths or intensities are discussed. These are related to the mechanism and efficiency of holographic volume and surface grating recording based on photopolymerization and mass-transport, diffusion processes of the organic matrix, and nanoparticles. GNP's presence in a monomer nanocomposite influences the recording parameters in a combined way, making preferable the double-wavelength irradiation mode, which may increase the efficiency of the recorded gratings up to two times at decreased exposures. Results may be used for planning fabrication processes of more complex, multifunctional photonic elements.

Effect of Corona Discharge on the Optical Properties of Thin-Film Cu–As2Se3 Structures

The effect that corona discharge has on holographic recording of diffraction gratings in a thin-film Cu−As2Se3 structure deposited on glass by thermal vacuum evaporation is studied. The use of negative corona discharge during holographic recording enhances the holographic sensitivity of the Cu−As2Se3 structure and the diffraction efficiency of inscribed gratings. Irradiation of the Cu−As2Se3 structure with actinic light results in photodarkening in the spectral region in which the structure exhibits a strongly absorption, while the region characterized by a weaker absorption becomes more transparent. The effect of photoinduced transparency is enhanced if samples are treated using positive corona discharge and with negative corona discharge the effect is weakened. The experimental results are explained qualitatively, assuming the diffusion of Cu+ ions into the As2Se3 layer.

Влияние коронного разряда на оптические свойства тонкопленочных структур Cu-As-=SUB=-2-=/SUB=-Se-=SUB=-3-=/SUB=-

Effect of corona discharge on holographic recording of diffraction gratings in Cu-As2Se3 thin film structure was studied. Thin film structures of Cu-As2Se3 were prepared on glass substrates by sequential deposition of Cu and As2Se3 layers using vacuum thermal evaporation technique. Applying of negative corona discharge during recording procedure caused an enhancing both of the holographic sensitivity and diffraction efficiency of the grating. Exposure to actinic light illumination resulted in photodarkening effect in Cu-As2Se3 thin film structure in the spectrum region of strong absorption. At the same time the fotoinduced bleaching of Cu-As2Se3 film structure in the spectrum region of weak absorption was observed. Negative corona discharge applied during the light exposure enhanced the bleaching, while positive corona discharge caused its weakening. The photoinduced diffusion of Cu+ ions into As2Se3 film was considered to explain qualitatively the experimental results.

Фотоиндуцированное поглощение и импульсна запись динамических голограмм в кристаллах силиката висмута

Dynamics of the photoinduced absorption and holographic-grating recording in photorefractive bismuth silicate crystal has been studied. It has been shown that, with the use of nanosecond laser pulses and at the intensity above 1 МW/cm2, the induced absorption takes place due to population of the short-lived trapping levels having the characteristic relaxation times from milliseconds to a few tens of milliseconds. Recording of dynamic holograms in bismuth silicate has been realized in these conditions. Two mechanisms of holographic grating recording have been established, with the lifetimes differing by three orders of magnitude. Though the intensities are relatively low, below or on the order of 1 MW/cm2, the medium response is determined by a photorefractive nonlinearity mechanism with relaxation times at a level of several seconds. The intensities above 5 MW/cm2 are associated with fast components (ms relaxation times) appearing due to population of short-lived traps. It has been demonstrated that a contribution of each mechanism is dependent on the intensity of laser radiation and that, for the intensities above 10-15 MW/cm2, the decisive part is played by the short-lived traps with lifetimes on the order of milliseconds.

Polarization Gratings in Azobenzene-Based Fully Liquid Crystalline Triblock Copolymer.

Photosensitive liquid crystalline (LC) block copolymers are a universal platform for designing functional materials for optics and photonics. Here, azobenzene LC ABA triblock copolymer containing mesogenic groups in each block is studied as a medium for the recording of polarization holographic gratings. The fast recording of polarization gratings is successfully performed with two orthogonally circularly polarized light beams of a 532-nm laser. It is shown that annealing of the recorded polarization grating increases diffraction efficiency considerably, by a factor of 70, and provokes simultaneous erasing of the surface relief grating if it has been recorded. Holographic behavior of the triblock copolymer is compared with that of an azobenzene homopolymer and a random copolymer, which has the same monomer ratio as the triblock copolymer. The obtained results are expected to allow designing new materials for the polarization of optical elements and holographic data storage.

LED-Cured Reflection Gratings Stored in an Acrylate-Based Photopolymer.

The storage of volume holographic reflection gratings in low-toxicity photopolymers represents a challenge at present since they can be used in many important applications such as biosensors and holographic optical elements. In this context, an acrylate-based photopolymer developed in our research group was employed to study the recording of unslanted holographic reflection gratings at high spatial frequencies. The optimal preparation conditions of the photopolymer layers were determinated. The diffraction efficiencies are measured in both recording and curing stage and a comparative study of these values was realized. In addition, a theoretical study using Kogelnik’s coupled wave theory was carried out with the aim of understanding the diffraction efficiency behaviour of both processes. In this work, a maximum diffraction efficiency of 14.1% was reached after a curing process in 150 µm layers at a recording wavelength of 488 nm. This value represents a good result compared to that reported in the literature and opens the way to reflection mode holography research using low-toxicity material.

Recording and erasure of photorefractive holograms in undoped BTO crystal at moderate to high intensities of 639.7 nm laser under action of 532 nm laser pre-illumination.

We investigate recording and erasure of photorefractive holographic gratings in an undoped Bi12TiO20 crystal in a moderate to high intensity regime of the recording beams at 639.7nm without and with the action of laser pre-illumination at 532nm. The detected hologram without pre-illumination indicates the participation of two photorefractive electronic gratings in its recording process, and the diffracted signal by itself exhibits a fivefold enhancement when the total intensity increases from 38.4 to 214.5 mW/cm2. The dependence of the measured total diffraction efficiency on intensity was investigated and showed linear behavior. At least three gratings are present in the regime of pre-illumination and participate in the writing and erasure of holographic mechanisms. Two of them are electronic, and one is hole-based, with a phase difference of Δϕ between them. The theoretical approach used to analyze the total diffraction efficiency based upon the photorefractivity standard model, and considering the presence of the three gratings, showed good agreement with the holographic erasure experimental data and permitted us to compute Δϕ, which exhibited strong and unusual dependence on the total intensity.

Reversible Polarization Recording in As2S3–Se Multilayer Nanostructures

Polarization holographic recording of diffraction gratings and their subsequent erasure have been investigated on As2S3–Se multilayer nanostructures. The work investigates how preexposure to actinic laser radiation up to complete photoinduced changes in the optical properties affects the formation of diffraction gratings in the studied structure. It is shown that the preexposure of an As2S3–Se multilayer nanostructure (MNS) leads to photobleaching, and the maximum achievable diffraction efficiency (DE) of 35% does not change; however, the required exposure value is increased. It is also shown that exposure using one laser beam results in complete erasure of the diffraction grating recorded up to the maximum. Seven recording–erasure cycles show that the kinetics of the increase in diffraction efficiency and its maximum value do not change, which indicates that the As2S3–Se multilayer structure is capable of reversible holographic recording under orthogonal circular polarization. Study of the gratings recorded with an atomic-force microscope shows that the main factor determining the diffraction efficiency value is modulation of the relief, the depth of which is greater than 200 nm.

LTL type nanozeolites utilized in surface photonics structures for environmental sensors

Environmental monitoring has been continuously increasing due to an emerging global emphasis on the importance of sustainable development. Improving water quality monitoring and control is a challenge that calls for innovative solutions. Current methods of water monitoring are costly and analytical techniques suitable for field use are limited. There is a need for accurate, long–term monitoring of environmental contaminants using sensors that can be operated on site. The aim of our research is to theoretically model, fabricate and characterise holographic sensors for water quality monitoring that are simple to operate, capable of sensing copper present in fresh water and have relatively low cost. The sensors are created by holographic recording of surface relief gratings (SRG) in a self-processing photopolymer material. Interrogation of these structures by light allows indirect measurements of ion concentration in real time. The SRG structures are modified by coating with porous LTL-nanoparticles (nanosized zeolites) which selectively adsorb copper ions. The suitability of the sensors for detection of copper (II) present in water at concentration levels 1–4 mM is reported. The current detection limit of the sensor is 63 ppm.

Effect of corona discharge on formation of holographic diffraction gratings in the Cu–As2Se3 thin-film structure

Application of negative corona discharge in the course of recording leads to an increase in the holographic sensitivity of the Cu–As2Se3 structure and diffraction efficiency of the recorded holographic gratings and the relief gratings fabricated with the aid of chemical etching. Positive corona discharge impedes the recording of holographic gratings in the Cu–As2Se3 structure. The Cu–As2Se3 structure exhibits the properties of a negative photoresist under irradiation in the presence of the negative corona discharge. In addition, the dissolution selectivity substantially increases due to the corona discharge.

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.

High Performance Photoinitiating Systems: Application to Holographic Grating Recording

Photopolymerization reaction can be used in self-developing photopolymerizable resin to recording holographic gratings. In this work, the efficiencies of two and three component photoinitiating system for holographic recording are analyzed in term of photopolymerization and grating diffraction yield kinetics. The selected systems are based on a visible dye, electron donor and electron acceptor. Time resolved and steady state spectroscopic studies of the photoinitiators are presented. This detailed photochemical study outlines the possible existence of photocyclic initiating systems where the dye is regenerated during the photochemical process. To study the impact of photoinitaiting system on the hologram grating formation, diffraction curves are compared to those of monomer to polymer conversion. This work outlines the importance of the photochemistry on fi nal holographic material properties.