Grating Formation Research Articles

Domain dynamics in coercive-field engineered sub-µm periodically poled Rb-doped KTiOPO4

Nonlinear optical interactions involving counter-propagating photons are of great interest for both classical and quantum optical applications. However, their use is sparsely spread due to the fact that they require quasi-phase-matched devices with sub-µm periods. A recent breakthrough has been the fabrication of bulk sub-µm domain gratings in Rb-doped KTiOPO4 by creating a grating of low- and high- coercive field regions in the crystal via periodic ion exchange, so-called coercive-field engineering. Here, we investigate the physical mechanisms behind this method and study the interplay between the ion-exchanged grating properties and the ferroelectric domain dynamics. Furthermore, we investigate the scalability of the method by studying the domain morphology of sub-µm periodically poled crystals with periods ranging from 755 to 433 nm and correlating it to that of the ion-exchanged regions. We show that the formation of sub-µm domain gratings is governed by the depth, shape, and critical ion-concentration of the ion-exchanged volume and that it is independent of the poling period. These findings are crucial for further scaling the coercive field engineering technique to even shorter poling periods and larger aperture periodically poled crystals.

Formation of diffraction gratings in optically patternable hydrogel films

AbstractLocal light‐induced material response forms the basis for volume holographic patterning of light‐responsive materials. This enables applications as multifunctional biomedical materials capable of integrating optical functions, such as for advanced intraocular lens (IOL) implants. Therefore, hydrogel films based on 8‐arm PEG and azobenzene‐functionalized acrylates are prepared. A local change in optical properties is induced by UV exposure: a decrease in refractive index is detected by ellipsometric examination. One‐dimensional diffractive surface gratings, generated using a photomask, show optical functionality and are visualized in the atomic force microscope. The results show that a local change in optical properties can be induced in functionalized hydrogel films by photochemical crosslinking, making them suitable for volume holographic patterning.

Terahertz Properties of Polymers for 2D Nonlinear Grating Formation

Terahertz Properties of Polymers for 2D Nonlinear Grating Formation

On-chip symmetrically and asymmetrically transformed plasmonic Bragg grating formation loaded with a functional polymer for filtering and CO2 gas sensing applications

Herein, two configurations of transformed plasmonic Bragg grating (p-BG) formation loaded with a functional polymer are suggested for filtering and gas sensing applications. The symmetrically transformed p-BG formation is well suited for the development of a wide bandwidth (BW) bandstop filter. Desired filter specifications such as extinction ratio, stopband region, and stopband BW can be optimized by adjusting the geometric dimensions of the device. Polyhexamethylene biguanide (PHMB) is a functional polymer that is linearly correlated to the CO2 concentration. Therefore, it is possible to employ such material for the development of CO2 sensors. The asymmetrically transformed p-BG formation is created by moving the gold nano-blocks that are periodically arranged in the MIM waveguide away from the line of symmetry. As a result, an additional narrowband MZI resonance dip appears in the transmission continuum and is obtained along with the broadband Bragg reflection. The spectral features of the suggested devices are analyzed via the finite element method (FEM). The CO2 sensitivity of the suggested device is ∼ 226 pm/ppm for the gas concentration range of 215 ppm to 434 ppm which is considerably higher than most of the formerly suggested sensor designs.

Analysis of light diffraction by azobenzene-based photoalignment layers.

Photoalignment materials, such as the azobenzene-based PAAD series studied here, are becoming increasingly important in liquid crystal-based optical devices and displays. Yet their properties and, in particular, their response to light, are still not fully understood. We investigate, experimentally and theoretically, the photoinduced birefringence, the order parameter and the formation of surface relief gratings, as well as the diffraction caused by them. We show that some of the azobenzene PAAD materials are suitable for the formation of surface relief gratings with high modulation depth, while others exhibit strong photoinduced birefringence. The two effects are inversely correlated: the stronger the surface relief grating is, the weaker is photoinduced birefringence. Analytical formulas based on the Raman-Nath approximation and numerical simulations of Maxwell's equations are used to quantify the diffraction caused by the induced diffraction gratings, showing excellent agreement between theory and experiment.

Diffraction gratings formed spontaneously by a two-dimensional undulation of the pseudo-layer structure of a twist-bend nematic phase

ABSTRACT Structure and optical transmission properties of spontaneously formed diffraction gratings by bent-core liquid crystalline materials that exhibit a twist-bend nematic (NTB) phase transition, when confined in thin planar cells are reported. We begin with experimentally measuring the polarisation properties of diffraction peaks up to the second-order and observe a generalised behaviour of polarisation of the first-order peaks. Moreover, we show that the study of the second-order diffraction peaks combined with the proposed preliminary model can be an effective tool to predict the spatial variation of the optic axis. Then, we build a continuum model that describes the formation of gratings as a result of competition between surface conditions and bulk strain due to the shrinking of pseudo-layers and determine the threshold conditions on the onset of a 2D pseudo-layer structure of the NTB phase. We use the beam propagation method and transfer matrix method to calculate the transmissivity of diffracted light by using the spatial variation of the optic axis determined from the modelled structure. It is established that the beam propagation method is superior to the transfer matrix method.

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.

Spatial modulation spectroscopy of semiconductors using dynamic gratings

Subject of study. Several characterization methods that utilize recording and reading out of dynamic gratings to analyze functional materials are proposed. Method. The methods are based on pulsed recording of thin and volume dynamic gratings followed by reconstruction using continuous laser emission to facilitate the detection of the formation and relaxation dynamics of the recorded diffraction grating. Nonlinear optical, thermo-optic, and kinetic properties of condensed media were obtained based on the measured kinetics of the diffracted signals. Main results. The performance characteristics of a dynamic grating method were analyzed for characterization and parameter measurements of functional materials. In particular, the efficacy of the measurement of the kinetic properties for pulsed recordings of thin and volume holographic gratings was demonstrated. The lifetimes of the short- and long-lived trap levels involved in recording of the dynamic grating were determined in a photorefractive bismuth silicate crystal and the dependence of the diffraction efficiency of the gratings on the intensity of the recording radiation was determined. The advantages of using an additional grating (homodyne) to enable the separation of the contributions of different simultaneously manifesting nonlinearity mechanisms were demonstrated. This method was applied to monocrystalline germanium. A method for assessing the thickness of a film on the surface of a sample based on the amplitude of an oscillating component related to a sound wave propagating in air in the vicinity of the film’s surface was also proposed. A silicon dioxide film was considered as an example. The application of the diffraction to different orders on volume dynamic gratings was proposed for the measurement of the nonlinear optical susceptibility of the fifth and higher orders. The proposed approach was evaluated on photorefractive crystals and semiconductor media. The kinetic and thermo-optic properties, including the time of the population of trap levels, thermo-optic coefficient, and thermal diffusivity, of the media were determined. Practical significance. The proposed characterization methods facilitate the isolation of different nonlinearity mechanisms involved in the formation of dynamic gratings and the measurement of nonlinear optical, thermo-optic, and kinetic properties (optical susceptibilities of different orders, thermo-optic coefficient, thermal diffusivity, and lifetimes of free-charge carriers and trap levels).

Photodesign and fabrication of surface relief gratings on films of polyimide-based supramolecular systems obtained using host-guest strategy

New supramolecular structures were attained by an original approach based on mixing the concepts of polyamidic acid (PAA) host and an azochromophore guest with hydrogen bonding intermolecular interactions. The materials have been obtained and characterized for use as liquid crystal (LC) alignment layers. The azochromophore was introduced in the PAA solution in different ratio followed by casting on glass plate and thermal treatment to realize a film with imide structure, confirmed by Fourier-transform infrared spectroscopy (FTIR). Thermal analyses indicated that the dye loading in the polymer host slightly reduces the temperature of thermal decomposition from 508 to 466 °C, while glass transition temperature ranges between 244 and 213 °C. UV–Vis data proved that the samples exposure at the wavelengths of 355 nm is beneficial for surface relief gratings (SRGs) formation. Atomic force microscopy (AFM) evidenced the surface structuring of the samples and the changes in the spatial and functional contact area characteristics as a function of azo-dye amount. The attained grating periodicity was about 1 μm, while the depth ranged between 35 and 100 nm depending of the system composition. Surface anisotropy, bearing properties and fluid retention are enhanced upon chromophore incorporation. LCs anchoring energy increases with azochromophore percent up to 22.9 ∙ 10 −16 μJ/m 2 , which is desirable for LC display applications. • Supramolecular structures, based on a polymer matrix and an azo-dye, are achieved. • Transparent flexible polyimide films with heat resistance up to 466 °C are attained. • As the azo-dye loading increases LASER-made SRGs depth increases from 37 to 81 nm. • Anisotropy, bearing and fluid retention are enhanced upon azo-dye incorporation. • LC anchoring energy increases with azochromophore percent up to 22.9 ∙ 10 −16 μJ/m 2 .

Kinetics of dynamic refractive index gratings in nematic liquid crystals in spatially inhomogeneous electric fields

The kinetics of director reorientation in a nematic LC cell and the corresponding change in the extraordinary refractive index for a transmitted light are calculated. The simulation is carried out in order to determine the regularities in the director reorientation times depending on the elastic and viscous properties of the LC medium, the magnitude of the electric field strength, and spatial confinement effects. We have used the Ericksen-Leslie model to describe the reorientation of the LC director under the action of constant electric fields. The simulation has been carried out for a spatially uniform electric field, as well as for a spatially modulated sinusoidal field. Our modeling predicts the formation of fast dynamic refractive index gratings under the action of a constant spatially modulated sinusoidal electric field. With the increase of the LC viscosity, the dynamic grating becomes quasi-stationary.

Local Direction of Optomechanical Stress in Azobenzene Containing Polymers During Surface Relief Grating Formation

AbstractIn this work, it is revealed how the photoinduced deformation of azobenzene containing polymers relates to the local direction of optomechanical stresses generated during irradiation with interference patterns (IPs). It can be substantiated by the modeling approach proposed by Saphiannikova et al., which describes the directional photodeformations in glassy side‐chain azobenzene polymers, and proves that these deformations arise from the reorientation of rigid backbone segments along the light polarization direction. In experiments and modeling, surface relief gratings in pre‐elongated photosensitive colloids of few micrometers length are inscribed using different IPs such as SS, PP, ±45, SP, RL, and LR. The deformation of colloidal particles is studied in situ, whereby the local variation of polymer topography is assigned to the local distribution of the electrical field vector for all IPs. Experimentally observed shapes are reproduced exactly with modeling azopolymer samples as visco‐plastic bodies in the finite element software ANSYS. Orientation approach correctly predicts local variations of the main axis of light‐induced stress in each interference pattern for both initially isotropic and highly oriented materials. With this work, it is suggested that the orientation approach implements a self‐sufficient and convincing mechanism to describe photoinduced deformation in azopolymer films that in principle does not require auxiliary assumptions.

Formation of liquid crystal gratings with crystal/quasicrystal patterns through photoalignment

ABSTRACT In this study, a simple method for generation of liquid crystal (LC) grating with crystal/quasicrystal/kaleidoscopic patterns via photoalignment is proposed. The crystal/quasicrystal/kaleidoscopic patterns are created using multiple-beam interference when the recording beam is divided into multiple beams by passing a multiple-aperture mask and focused by a lens. Two-dimensional (2D) patterns are recorded on LC cells based on the laser-induced adsorption of azo dyes that guide the orientation of the LC molecules. Measurements indicate that the 2D LC gratings with crystal/quasicrystal patterns are electro-optically tunable and possess polarisation-independent diffraction efficiency.

Improving performance oftwo‐stagephotopolymers for volume holographic recording by fluorinatedepoxy‐amine cross‐linkedmatrices

AbstractHost matrices play an important role in the volume holographic gratings formation of photopolymers. In this study, a series of fluorinated epoxy resin (FTGE) with low refractive index (1.44–1.46) are synthesized and characterized by nuclear magnetic resonance spectra (1H NMR,19F NMR) and fourier transform infrared spectra (FTIR). Fluorinated epoxy‐amine cross‐linked matrices are formed at room temperature and their thermal polymerization properties are investigated through real‐time FTIR and thermogravimetric analyses (TGA). Using the fluorinated epoxy‐amine as host matrices, holographic photopolymers are fabricated. The influence of different kinds (Prop‐FTGE, Buta‐FTGE, and Penta‐FTGE) and weight ratios (0%, 8%, 16%, and 23%) of FTGEs on holographic performances of the photopolymers are explored. The results show that the sample with 23% Prop‐FTGE has the most excellent holographic optical characteristics. 0.5 mm thick sample with 23% Prop‐FTGE reaches 91% diffraction efficiency within 80 mJ/cm2exposure dosage, and gets a 2000 lp/mm Bragg volume grating with 0.24° angular selectivity. Meanwhile, under a single pulse exposure mode (150–200 ps pulse width, 25 mJ/cm2exposure dosage), a grating with 4.4% diffraction efficiency is obtained, which proves the ultrafast response and recording capability of the sample.

Four-wave mixing by phase-amplitude holographic gratings in the photorefractive piezocrystal of 43m symmetry class

A system of coupled-wave equations for calculating the vector amplitudes of linearly polarized light waves at four-wave mixing by phase-amplitude holographic gratings in a cubic photorefractive semiconductor of an arbitrary cut belonging to the 43m symmetry class is presented. The dependences of the intensities of the polarization components of the reversed light wave on the orientation angle for GaAs crystal of (110)-cut are calculated on the basis of the numerical solution of the system of coupled-wave equations. The obtained dependences are compared with the known theoretical and experimental data. It is shown that the best agreement between the results of theoretical modeling and experimental data at calculating the counterpropagating four-wave mixing in GaAs crystal of (110)-cut is achieved if formation of several phase-amplitude holographic gratings is allowed, and the contribution of the photoelastic and inverse piezoelectric effects are taken into account together with absorption of the crystal. Keywords: four-wave mixing, photorefractive semiconductor, light wave, holographic grating, coupled wave equations.

Четырехволновое взаимодействие на фазово-амплитудных голографических решетках в фоторефрактивном пьезокристалле класса симметрии 43m

A system of coupled-wave equations for calculating the vector amplitudes of linearly polarized light waves at four-wave mixing by phase-amplitude holographic gratings in a cubic photorefractive semiconductor of an arbitrary cut belonging to the 4 ̅3m symmetry class is presented. The dependences of the intensities of the polarization components of the reversed light wave on the orientation angle for GaAs crystal of (110)-cut are calculated on the basis of the numerical solution of the system of coupled-wave equations. The obtained dependences are compared with the known theoretical and experimental data. It is shown that the best agreement between the results of theoretical modeling and experimental data at calculating the counterpropagating four-wave mixing in GaAs crystal of (110)-cut is achieved if formation of several phase-amplitude holographic gratings is allowed, and the contribution of the photoelastic and inverse piezoelectric effects are taken into account together with absorption of the crystal.

Three modes of the nonstationary holographic current excitation in a gallium oxide crystal

We report the nonstationary holographic current excitation in a β-Ga2O3 crystal at light wavelength . The material demonstrates insulating properties and high transparency for visible light, but this, however, does not prevent the dynamic space-charge grating formation and the holographic current observation for various external electric fields —zero, dc and ac ones. The signal amplitude is measured and analyzed vs. the frequency of phase modulation, spatial frequency and electric field value. The main photoelectric parameters such as specific photoconductivity, sensor responsivity and diffusion length of carriers are determined for the blue region of spectrum.

Unconventional Approaches to Light-Promoted Dynamic Surface Morphing on Polymer Films

Abstract Recent progress in research of light-promoted surface morphing on polymer materials is reviewed. Photoinduced mass transport motions in azobenzene polymers leading to surface relief gratings (widely called SRGs) formation are well-known and extensively discussed. Among many efforts, this article focuses on unconventional approaches, most of which are seemingly less highlighted. The first part introduces investigations using azobenzene polymers; introducing supramolecular approaches with tunable and removable azobenzene units, self-structuring of surface morphology upon single laser beam irradiation, photo-triggered migration via Marangoni flow, SRG formation and other functions of polymer brushes etc. The second part overviews the mass transport motions driven by other photoreaction classes such as photopolymerization, photocrosslinking, and photoisomerization of non-azobenzene components. The final part describes new dynamic surface morphing processes observed in fingerprint texture of liquid crystals and liquid crystal polymer networks, which can be applied to reversible friction control and self-cleaning. These uncommon attempts at photo-assisted dynamic morphing extend the possibilities of microfabrication on polymer films and are expected to find new opportunities to create new surface functions in soft materials.

Accelerated formation of persistent holographic gratings based on one-directional electron transfer in KCl-Ag/Ta2O5 nanocomposites

Permanent preservation of data is essential for massive information recording. Combination of semiconductor with plasmonic nanoparticles has been applied in multicolor display and high-density optical storage. However, bidirectional electron transfer occurs at the Schottky interface under UVA irradiation, resulting in reversible photochemical reaction, information erasure, low recording efficiency and writing rate. To address these issues, a novel Schottky heterostructure of Ag/Ta2O5 modified with alkali halide is developed to realize photoinduced one-directional electron transfer from metal to semiconductor. The recorded information in such a medium of KCl-Ag/Ta2O5 presents excellent holographic storage stability even under exposure of a strong UVA ray (360 nm, 385 mW/cm2). Meanwhile, grating growth rate and efficiency are significantly enhanced by optimizing Ag particle distance and Cl− anion loading amount. This work provides an important strategy for fast and persistent data storage.

High-precision point-by-point fiber Bragg grating inscription

The paper presents a novel multi-pass point-by-point method for high-precision inscription of fiber Bragg gratings with a femtosecond laser radiation. The dynamics of the gratings formation is demonstrated in comparison with the classical method of UV-radiation inscription. It is shown that due to the feasibility of the inscription dynamics control, it is possible to fabricate high-precision Bragg gratings with specified parameters and a high degree of reproducibility. The proposed technique is demonstrated to be a convenient tool for studying the processes occurring in glass during multiphoton absorption of laser radiation.

Formation of holographic diffraction gratings in thin films of chalcogenide glassy semiconductors

The paper presents a study of the formation of holographic diffraction gratings in thin films of chalcogenide glassy semiconductors. The recording process of holographic gratings at the argon-laser radiation wave length 488 nm and the process of chemical etching that enables the formation of а relief holographic grating are analysed. The optimum conditions for the formation of diffraction gratings in films of arsenic sulfide As2S3 are defined. It is shown that at the 488 nm wave length of an argon laser the optimum exposure comes to ∼5–8 J/cm2. At the recording stage a quasi-phase (relief-phase) grating is formed, with the diffraction efficiency on the order of a few per cent. Etching of the exposed sample with a solution of NaOH alkali in deionised water and isopropanol makes it possible to increase considerably the relief depth and to improve the diffraction efficiency of a thin diffraction grating approximately up to 20 % for the red spectral region, and to approach the maximal value ∼34 % for the near infra-red region. The results of the study considered look promising for the creation of relief holographic gratings which are essential in present-day optical instrument building (production of spectral devices, holographic sights, and the like).