Phase Gratings Research Articles

Diffraction-optimized aperiodic surface structures for enhanced current density in organic solar cells.

Novel diffractive surface structures are designed, evaluated, and optimized to achieve a significant increase in current density in organic solar cells. The surface relief phase gratings are patterned based on concentric rings, a Fermat's spiral, and an Archimedean spiral, respectively, and thus diffract incident light independent of its azimuthal angle, as they all exhibit circularly symmetric diffraction patterns. The grating pillars are arranged on the rings or spirals according to periodic or deterministic aperiodic Thue-Morse and Rudin-Shapiro sequences, tailoring the desired diffraction patterns. After evaluating the surface patterns against each other and further parameter optimization, a final current density enhancement of 5 % is achieved for one of the most promising patterns, the Thue-Morse sequence on an Archimedean spiral.

Polarized induced phase grating in a quantized four-level graphene monolayer system.

We discuss the electromagnetically induced grating (EIG) and electromagnetically induced phase grating (EIPG) in a four-level quantized graphene monolayer system. By using the density matrix technique and perturbation theory, we first obtain the self-Kerr nonlinear susceptibility of the graphene system; afterwards, we study the amplitude and phase modulations of the probe light. We discovered that the EIG and EIPG can be found by controlling the elliptically polarized coupling fields that interact with the monolayer graphene system. Owing to the phase modulation of the transmitted light beam, we recognized that the probe strength can also additionally switch from zeroth-order to high-order diffraction. Moreover, we found that the diffraction performance of the grating may be adjusted through tuning the polarization of the coupling light.

Space-time analogy and its application to design schemes borrowed from Fourier optics for processing ultrafast optical signals

Square-wave pulse generation with a variable duty ratio can be realized with the help of the ideas of Talbot array illuminators formulated for binary phase gratings. A binary temporal phase modulation of continuous wave (CW) laser field propagating through a group-delay-dispersion circuit of the fractional Talbot length results in a well defined sequence of square-wave-form pulses. When P = 1 a duty ratio of the pulses D is for Q = 4 and for Q = 3 and 6. Maximum intensity of the pulses doubles and triples compared to the CW intensity for and , respectively. These pulses can be used for return-to-zero laser field modulation in optical fiber communication. For extra features between the pulses are found originating from a finite rise and drop time of phase in a binary phase modulation. A similar effect to the benefit of the time-space analogy is predicted for binary phase gratings and interpreted as gleams produced by imperfect edges of the components of the rectangular phase gratings.

Cortical mechanisms of visual brightness.

SUMMARYThe primary visual cortex signals the onset of light and dark stimuli with ON and OFF cortical pathways. Here, we demonstrate that both pathways generate similar response increments to large homogeneous surfaces and their response average increases with surface brightness. We show that, in cat visual cortex, response dominance from ON or OFF pathways is bimodally distributed when stimuli are smaller than one receptive field center but unimodally distributed when they are larger. Moreover, whereas small bright stimuli drive opposite responses from ON and OFF pathways (increased versus suppressed activity), large bright surfaces drive similar response increments. We show that this size-brightness relation emerges because strong illumination increases the size of light surfaces in nature and both ON and OFF cortical neurons receive input from ON thalamic pathways. We conclude that visual scenes are perceived as brighter when the average response increments from ON and OFF cortical pathways become stronger.

Physics-enhanced neural network for phase retrieval from two diffraction patterns.

In this work, we propose a physics-enhanced two-to-one Y-neural network (two inputs and one output) for phase retrieval of complex wavefronts from two diffraction patterns. The learnable parameters of the Y-net are optimized by minimizing a hybrid loss function, which evaluates the root-mean-square error and normalized Pearson correlated coefficient on the two diffraction planes. An angular spectrum method network is designed for self-supervised training on the Y-net. Amplitudes and phases of wavefronts diffracted by a USAF-1951 resolution target, a phase grating of 200 lp/mm, and a skeletal muscle cell were retrieved using a Y-net with 100 learning iterations. Fast reconstructions could be realized without constraints or a priori knowledge of the samples.

Shaping of Optical Wavefronts Using Light‐Patterned Photothermal Metamaterial

AbstractHaving full control of light propagation in free space represents the ultimate goal for an imaging system. Spatial Light Modulators (SLMs), featuring the ability to actively control the spatial distribution of light components, are usually limited either to performing small and continuous adjustments to imaging aberrations or to rapidly shifting discrete segments of the wavefront with severe diffraction artifacts. Here a photothermally modulated optical structure is introduced as a highly responsive SLM capable of producing arbitrarily shaped wavefront patterns with smooth as well as step‐like features. The phase‐shift inducing temperature profile within the plasmonic metamaterial at the core of the SLM structure replicates closely the pattern of illuminating light intensity avoiding the common speed and spatial gradient limitations. As a result, the SLM concept is insensitive to polarization, free of diffraction artifacts, and offers sub‐millisecond response time and high transmittance >80%. The dynamic generation of a set of common optical functions is demonstrated, including low‐order Zernike polynomials patterns, phase grating, or vortex, which build an essential phase modulation toolbox across different imaging applications.

HKUST-1-Doped High-Resolution Volume Holographic Gratings

We report on transmission holographic gratings doped with metal organic frameworks (MOFs). As a first attempt, we focused on MOF-199, also known as HKUST-1, which is an efficient adsorbent of VOCs. HKUST-1 is not soluble in the pre-polymerized holographic mixture. For this reason, samples containing HKUST-1 show high light scattering. In this work, the recording of HKUST-1-doped one-dimensional transmission phase gratings is demonstrated. The optical properties of the recorded structures, such as diffraction efficiency and average refractive index changes, are reported by using angular analysis measurements. A first attempt to demonstrate the possibility of using the doped gratings as sensors is also reported.

Inscription of lateral superlattices in semiconductors using structured light

We propose a non-destructive, all-optical technique to imprint embedded lateral superlattices near semiconductor heterostructures by illuminating the samples with a stable interference pattern generated by a phase diffraction grating. We demonstrate the technique on an ultrahigh mobility GaAs/AlGaAs sample with a Si δ-doping by inducing a persistent charge redistribution at cryogenic temperatures in the doping layer containing DX-centers. Weiss commensurability oscillations in the magnetoresistance of the light-induced superlattice are observed and analyzed to obtain its characteristics.

Grating-based in-line geometric-phase-shifting incoherent digital holographic system toward 3D videography.

Incoherent digital holography (IDH) with a sequential phase-shifting method enables high-definition 3D imaging under incoherent lights. However, sequential recording of multiple holograms renders IDH impractical for 3D videography. In this study, we propose grating-based in-line geometric-phase-shifting IDH. Our method divides orthogonal circularly polarized lights into four copies with a fabricated phase grating and subsequently creates self-interference holograms with geometric phases introduced by a segmented linear polarizer. This enables single-shot recording of holograms without the need for a specially designed image sensor, such as a polarization-sensitive sensor. Moreover, the achievable spatial resolution is higher than that of off-axis methods. As a proof-of-principle experiment, we demonstrated snapshot and video recording of 3D reflective objects using our IDH method. The results confirmed the feasibility of the proposed method.

Evaluation of Diffraction Orders Efficiency of Laguerre-Gaussian Beam Generated by the Phase and Amplitude Fork Grating Applied on Spatial Light Modulator

Evaluation of Diffraction Orders Efficiency of Laguerre-Gaussian Beam Generated by the Phase and Amplitude Fork Grating Applied on Spatial Light Modulator

Efficient Storage and Encryption of 32-Slice CT Scan Images Using Phase Grating.

Medical images are treated as sensitive as it carries patients’ confidential information and hence must be protected from unauthorized access. So, a strong encryption mechanism is a primary criterion to transmit these images over the internet to protect them from intruders. In many existing algorithms, noise affection in the extracted images is high, hence not suitable for medical data encryption. Here, we present a new method using phase grating to multiplex as well as encrypting 32 cross-sectional CT scan images (slices) in a single canvas for optimization of storage space and improvement of security. The entire process is divided into a few steps. Before transmission, the main canvas is encrypted with the help of a random phase matrix. The main canvas is further encrypted by the transposition method to enhance security. After decryption, inverse Fourier transform is applied at the proper location of the decrypted canvas to extract the images from the spectra. Quality is measured with peak-signal-to-noise ratio and correlation coefficient methods. Here, it is greater than 38 and the correlation coefficient is close to 1 for all images, thereby indicating of good quality of extracted images. The effect of three common cyber-attacks (viz. known-plaintext attack, chosen-plaintext attack, and chosen-ciphertext attack) is also presented here. The correlation coefficient during cyber-attacks is found to be close to zero, which implies the robustness of the algorithm against cyber-attacks. Finally, a comparison with existing techniques shows the effectiveness of the proposed method.

The effect of transverse wavefront width on specular neutron reflection.

In the analysis of neutron scattering measurements of condensed matter structure, it normally suffices to treat the incident and scattered neutron beams as if composed of incoherent distributions of plane waves with wavevectors of different magnitudes and directions that are taken to define an instrumental resolution. However, despite the wide-ranging applicability of this conventional treatment, there are cases, such as specular neutron reflectometry, in which the structural length scales of the scattering object require that the wavefunction of an individual neutron in the beam be described by a spatially localized packet - in particular with respect to the transverse extent of its wavefronts (i.e. normal to the packet's mean direction of propagation). It is shown in the present work that neutron diffraction patterns observed for periodic transmission phase gratings, as well as specular reflection measurements from patterned thin films with repeat units of the order of micrometres, can be accurately described by associating an individual neutron with a wave packet and treating a beam as a collection of independent packets. In these cases, accurate analysis requires that the transverse spatial extent of a neutron packet wavefront be accounted for in addition to the angular divergence of the beam that is characterized by a distribution of packet mean wavevector directions. It is shown how a measure of the effective transverse spatial extent of the neutron packet - over which its wavefronts are of sufficient uniformity to produce coherent scattering - can be determined by employing reference diffraction gratings and patterned thin films of known structure and composition.

Propagation of a multi-vortex beam: far-field diffraction of a Gaussian beam from a multi-fork phase grating.

In this work, the far-field propagation of multi-vortex beams is investigated. We consider diffraction of a Gaussian wave from a spatial light modulator (SLM) in which a multi-fork grating is implemented on it at the waist plane of the Gaussian wave. In the first-order diffraction pattern a multi-vortex beam is produced, and we consider its evolution under propagation when different multi-fork gratings are implemented on the SLM. We consider two different schemes for the phase singularities of the implemented grating. A topological charge (TC) equal to l1 is considered at the center of the grating, and four similar phase singularities all having a TC equal to l2=l14 (or l2=-l14) are located on the corners of a square where the l1 singularity is located on the square center. Some cases with different values of l1, and consequently l2, are investigated. Experimental and simulation results show that if signs of the TCs at the corners and center of the square are the same, the radius of the central singularity on the first-order diffracted beam increases, and it convolves the other singularities. If their signs are opposite, the total TC value equals zero, and at the far-field, the light beam distribution becomes a Gaussian beam. For determining the TCs of the resulting far-field beams, we interfere experimentally and by simulation the resulting far-field beams with a plane wave and count the forked interference fringes. All the results are consistent.

Terahertz passive amplification via temporal Talbot effect in metamaterial-based Bragg fibers

A fully passive terahertz (THz) pulse amplification device is proposed based on leveraging the temporal Talbot effect (TTE) on a highly dispersive silicon-based metamaterial Bragg fiber. To maximize the TTE passive gain, we introduce and explore three different strategies, denoted as coherent pulse addition (CPA), forward Talbot illuminator (FTAI), and backward Talbot illuminator (BTAI). The BTAI strategy allows additional degrees of freedom for controlling the output pulse shape. Moreover, by using a continuous phase profile, we overcome the hurdle of implementing discrete phase gratings in limited time response phase modulators. We explore different pulse shapes and chirped pulses with CPA, and different gain factors with both BTAI and FTAI. Numerical simulation results show a 5.8 dB gain for a 9.8 cm long fiber with the CPA method, a 9.9 dB gain with FTAI (1.37 cm long), and a 8.8 dB gain with BTAI (1.25 cm long). These results indicate the potential of the approaches presented here, which can be used as a springboard for further developments toward high gain passive amplification THz devices.

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.

Tailored diffraction asymmetries from spatially odd-symmetric phase gratings

Phase gratings, each period of which consists of even numbers of equal-width and equal-thickness elements, can be devised to attain asymmetric diffraction patterns. We show that engineering of offset refractive indices in different elements exhibiting a spatially odd symmetry leads to the elimination of a single diffraction order (directional elimination), while further manipulations on offset refractive indices may lead to the elimination of all odd or even orders (grouped elimination) or all orders but one or two selected (directional selection). These intriguing effects arise from destructive interference between diffracted amplitudes contributed by paired or successive elements, and violation of Friedel's law in such transparent gratings is an effect of higher order multiple scattering.

Analysis of circular gratings as image plane coronagraph filters

Coronagraphy is an efficient technique for identifying and characterizing extra-solar planets orbiting in the habitable zone of their parent star. An important family of coronagraphs is based on amplitude or phase filters placed at an intermediate image plane of the optical system, spreading starlight outside of the so-called “Lyot stop” located at the exit pupil plane of the instrument. This article explores the potential of circular amplitude and phase gratings employed as image plane coronagraph filters. It presents a theoretical analysis of the simplest case of an amplitude circular grating and introduces an inversion paradigm with respect to classical Lyot coronagraph, by exchanging its image and pupil masks. Various types of circular gratings are considered and their performance is evaluated with the help of numerical simulations. The most promising solutions are presented and discussed. The main conclusion is that high attenuation ratios of the parent star are feasible, provided that the system is properly optimized with a Lyot stop especially tailored to the grating filter.

Recording of dynamic and permanent gratings in composite LC cells containing gold nano-island films

The nonlinear optical properties of composite liquid crystal (LC) cells containing gold nano-island film (AuNIF) deposited on one of the cell substrates are studied with the help of dynamic holographic techniques. A dynamic phase grating of a photoinduced change in the refractive index arises in a composite LC cell when the two-wave mixing of laser beams of a cw Nd:YAG laser is used. The nonlinear optical response is caused due to a surface-induced photorefractive effect observed under the simultaneous action of a periodic light interference pattern and a constant voltage. The result of this impact is appearance of an internal electric field (Eint) at the AuNIF-LC interface. This electric field defines the local reorientation of LC molecules. Our experimental studies prove that the Eint value can be controlled by the size and shape of Au nanoparticles in AuNIF. As a result, we obtained a record of either fast dynamic gratings or quasi-stationary gratings in the composite LC cells depending on the morphology of AuNIF.

Model parameters of a horizontal seismometer with an optical diffraction vibration sensor

Subject of study. This study investigates the model of a horizontal seismometer composed of a physical pendulum and an optical diffraction sensor for angular vibrations that comprises a laser, an assembly of two relief phase diffraction gratings (DGs), a diaphragm isolating the radiation of the first diffraction order after the diffraction grating assembly, and a photodetector. The physical pendulum comprises a disk mounted on the rotation axis, pullback springs, and an additional weight on the disk edge. The assembly of DGs was positioned on the disk. The signal from the photodetector output was proportional to the small linear displacements of the additional weight owing to the inertial forces under horizontal vibrations of the seismometer base. Model parameters. The diameter and mass of the disk are 250 mm and 2 kg, respectively. The free swing period of the pendulum is 3 s. The mass of the additional weight is 0.1 kg. The laser power is 5 mW. Two relief phase DGs with a groove depth of 0.32 µm and a period of 0.1 mm are positioned on the opposite facets of a glass block of DGs with a thickness of 13 mm. The grooves of the first DG are parallel to the grooves of the second DG. An FD-24k photodiode, a load resistor, an amplifier, and a filter with a bandwidth of 1800 Hz are connected to the output. Test results. The model of the optical diffraction seismometer and SM-3 reference seismometer were mounted on a single platform. Periodic oscillations of the platform with a period of 0.475 s were applied in the first series of experiments. The dependence of the signals from the outputs of seismometers of both types were similar; however, a 90° phase shift of the output signal from the investigated model with respect to the SM-3 output signal was present. This is because the output signal of the sensor in the optical diffraction seismometer is proportional to the angular displacement of the physical pendulum, whereas the signal from the coil of the SM-3 sensor is proportional to the speed of pendulum movement. A slow weak linear force was applied to the platform with seismometers in the second experiment series. In this case, the SM-3 output signal did not exceed the noise level, whereas the output signal of the optical seismometer model was triple the noise level. The sensitivity of the seismometer model significantly exceeds the sensitivity of the SM-3 seismometer under slow perturbations. Practical significance. The output signal from the optical diffraction vibration sensor is proportional to the angular displacement of the pendulum instead of the speed of its movement. Thus, the sensitivity of the seismometer model does not decrease in the range of low frequency vibrations. A seismometer with negative feedback for investigation of weak low-frequency vibrations of the Earth’s surface can be designed based on this scheme.

Fast fabrication of diffractive patterns on glass by excimer laser ablation

ArF excimer laser ablation is used for the periodic patterning of glass surfaces. Projection of phase gratings provides the parallel generation of high resolution line patterns with structure periods in the µm-range. A fast and efficient process is possible, if laser pulse repetition and work piece motion are synchronized, so that multiple pulses can be applied to each work piece position, while continuously moving the work piece. A writing speed of 0.1 cm2/s is demonstrated and the scalability to 1 cm2/s is presented. Sub-micron periods are obtained by using Schwarzschild objectives. The process is applied for high contrast diffractive marking of glass.