Simple Gratings Research Articles

Multi focus acoustic field generation using Dammann gratings for phased array transducers

Phased array transducers can shape acoustic fields for versatile manipulation; however, generating multiple focal points typically involves complex optimization. This study demonstrates that Dammann gratings – binary phase gratings originally used in optics to generate equal-intensity spot arrays – can be adapted for acoustics to create multiple equal-strength focal points with a phased array transducer. The transducer elements were assigned phases of 0 or π, based on a Dammann grating defined by its transition points. Simulations show that simple gratings with two transition points can generate fields with up to 12 focal points of nearly equal acoustic pressures. Compared to conventional multi-focus phase optimization techniques, the Dammann grating approach offers computational efficiency and facile reconfiguration of the focal pattern by adjusting the grating hologram. We tested this approach in numerical simulations with a hypothetical high-resolution array, achieving up to 12 focal points, and validated the efficacy of the Dammann grating in a conventional 16x16 transducer array through both simulations and experiments. This comparison highlights that while Dammann gratings effectively generate multi-focus fields, the recreation ability of these gratings in a conventional array shows a lower resolution than the hypothetical array. This study underlines the potential of adapting binary phase functions from photonics to enhance ultrasound-based acoustic manipulation for tasks requiring parallel actuation at multiple points.

Photonic-Enhanced Perovskite Solar Cells: Tailoring Color and Light Capture.

The current exponential growth of solar electricity technologies toward consumer-oriented applications, as in building- or vehicle-integrated photovoltaics (B/VIPV), is calling for improved solar cells, not only in cost-effectiveness, but also with better adaptability and aesthetics. Here, using perovskite solar cells (PSCs) as test bed, we demonstrate an unprecedented photonic method to generate any color on a cell layout, while also increasing PV efficiency. To this end, photonic surface features were designed for PSCs, which filled the dual purpose of light-trapping (LT) and modulation of reflected light interference. A variety of geometries, from simple gratings to complex semispheroids, were optically optimized for two of the most challenging colors, magenta and green, while assuring the generation of their maximum feasible photocurrent. The best results corresponded to a current density of 22.07 mA/cm2, obtained for the magenta solar cell with top domes, exhibiting an increase of 6.68%, relative to an optimized planar reference cell. In turn, the same type of geometry was able to generate the leading green cell, with up to 21.40 mA/cm2 (a relative increase of 3.44%). Additionally, the uniformity of the optical output of the optimal solar cells was tested under a range of incident light angles, between 0◦ and 60◦, where the current density suffered relative losses only down to 6.65%.

A study of strontium aluminates for all optical contactless sensing applications using smartphone interrogation

In the present paper we study the spectral and time responses of Eu and Dy doped strontium aluminates and compare the results obtained by means of a standard spectrometer and a smartphone. The advantages of each measurement tool are outlined. It is shown that smartphones equipped with simple transmission diffraction grating can efficiently be used in contactless sensing applications and as measurements tools for the study of the spectral and time dependent responses of phosphors. We also show that smartphones are up to an order of magnitude faster in measuring rise and decay time responses and allow a more detailed analysis of luminescence dynamics compared to spectrometers. A method of duty cycle scanning of the time responses is proposed to study the spectrally and time dependent luminescence structure dynamics which is related to the individual distribution of traps in the phosphors. These smartphone capabilities justify their use as affordable interrogation instruments for time-decay encoded phosphorescent sensors.

Form birefringent polymeric structures realized by 3D laser printing.

The 3D laser printing of form birefringent structures promises fast prototyping of polarization-sensitive photonic elements. However, achieving the quarter- and half-wave phase retardation levels needed in applications still remains a challenge, especially at visible wavelengths. Thickness of the birefringent region, usually consisting of simple 1D gratings, must be sufficiently large to ensure the required retardance, making the 3D laser-printed gratings prone to mechanical collapse. Here we demonstrate 3D laser-printed mechanically robust form birefringent 3D structures whose thickness and phase retardation can be increased without loss of mechanical stability, and report on the realization of compact self-supporting structures exhibiting quarter- and half-wave phase retardation at visible wavelengths.

Coarse-to-fine interaction on perceived depth in compound grating.

To encode binocular disparity, the visual system uses a pair of left eye and right eye bandpass filters with either a position or a phase offset between them. Such pairs are considered to exit at multiple scales to encode a wide range of disparity. However, local disparity measurements by bandpass mechanisms can be ambiguous, particularly when the actual disparity is larger than a half-cycle of the preferred spatial frequency of the filter, which often occurs in fine scales. In this study, we investigated whether the visual system uses a coarse-to-fine interaction to resolve this ambiguity at finer scales for depth estimation from disparity. The stimuli were stereo grating patches composed of a target and comparison patterns. The target patterns contained spatial frequencies of 1 and 4 cycles per degree (cpd). The phase disparity of the low-frequency component was 0° (at the horopter), -90° (uncrossed), or 90° (crossed), and that of the high-frequency components was changed independent of the low-frequency disparity, in the range between -90° (uncrossed) and 90° (crossed). The observers' task was to indicate whether the target appeared closer to the comparison pattern, which always shared the disparity with the low-frequency component of the target. Regardless of whether the comparison pattern was a 1-cpd + 4-cpd compound or a 1-cpd simple grating, the perceived depth order of the target and the comparison varied in accordance with the phase disparity of the high-frequency component of the target. This effect occurred not only when the low-frequency component was at the horopter, but also when it contained a large disparity corresponding to one cycle of the high-frequency component (±90°). Our findings suggest a coarse-to-fine interaction in multiscale disparity processing, in which the depth interpretation of the high-frequency changes based on the disparity of the low-frequency component.

Tunable narrowband and broadband coexisting absorber enabled by a simple all-metal grating for sensing applications

We realize a tunable narrowband and broadband coexisting absorber based on a simple step-shaped all-metal grating structure. The absorber presents an ultra-narrow absorption band of 1.5 nm and a relatively broad absorption band of 29.8 nm, both with nearly 100% absorption in the infrared region. The mechanism underlying the dual-band perfect absorption is the interaction between two diffraction coupled surface plasmon polariton (SPP) modes with one of them modulated by a cavity resonance. Influences of structure parameters on the absorption performance are numerically investigated. It is found that the positions of the two perfect absorption peaks can be easily tuned both independently and together by changing the structural parameters. In addition, the designed grating structure presents excellent sensing performance with sensitivity and figure of merit as high as 2,514 nm/RIU and 1,600 RIU−1, respectively. The excellent sensing performance, flexible tenability and simple structure design endow this grating absorber with great potential in high-precision biochemical sensing applications.

Interrogation Method with Temperature Compensation Using Ultra-Short Fiber Bragg Gratings in Silica and Polymer Optical Fibers as Edge Filters.

The use of simpler and less bulky equipment, with a reliable performance and at relative low cost is increasingly important when assembling sensing configurations for a wide variety of applications. Based on this concept, this paper proposes a simple, efficient and relative low-cost fiber Bragg grating (FBG) interrogation solution using ultra-short FBGs (USFBGs) as edge filters. USFBGs with different lengths and reflection bandwidths were produced in silica optical fiber and in poly(methyl methacrylate) (PMMA) microstructured polymer optical fiber (mPOF), and by adjusting specific inscription parameters and the diffraction pattern, these gratings can present self-apodization and unique spectral characteristics suitable for filtering operations. In addition to being a cost-effective edge filter solution, USFBGs and standard uniform FBGs in silica fiber have similar thermal sensitivities, which results in a straightforward operation without complex equipment or calculations. This FBG interrogation configuration is also quite promising for dynamic measurements, and due to its multiplexing capabilities multiple USFBGs can be inscribed in the same optical fiber, allowing to incorporate several filters with identical or different spectral characteristics at specific wavelength regions in the same fiber, thus showing great potential to create and develop new sensing configurations.

Enhancing light emission near low-refractive-index contrast nanostructures via mirror-symmetry breaking

A platform of recent interest and large application potential is one in which the light emitters are directly integrated with an optical metasurface. Plasmonic metals and high-refractive-index, dielectric Mie metasurfaces have been explored in this connection but have their challenges. We propose low-refractive-index, contrast nanostructured thin films for studying metasurface-emitter systems, specifically focusing on two configurations of practical importance: LED white light generation using color converting polymers and fluorescence-based sensing in aqueous media. To achieve light emission enhancement in a low-index contrast, low optical absorption setting, we exploit the excitation of quasi-bound states in the continuum modes in a mirror-symmetry broken grating. Our numerical study predicts widely tuneable sharp-linewidth emission enhancements, near-zero quenching, and large and controllable active volume in the grating vicinity, which are significant improvements in comparison with both plasmonic and high-index contrast, all-dielectric platforms. When compared with simple gratings, mirror-symmetry broken gratings give four times larger radiative enhancement. Our results are of interest in furthering experimental activity and realizing applications such as light converter for efficient white LEDs and smart detection electronics-integrated substrates for sensing.

High-power broadband supercontinuum generation through a simple narrow-bandwidth FBGs-based fiber laser cavity

The generation of supercontinuum (SC) often requires ultrashort pulsed lasers with high peak power and gain media with large nonlinear coefficients, such as a long piece of fiber or photonic crystal fiber. In this Letter, we propose and demonstrate that high-power SC can be generated through a simple narrow-bandwidth fiber Bragg gratings (FBGs)-based laser cavity without any modulation, based on the mechanism of intense nonlinear effects induced by the inherent self-pulsation generated inside the cavity. In the experiment, an ∼80 W SC laser with the spectrum range from <600 nm to 1600 nm was achieved. To the best of our knowledge, this is the first report about SC generation through a simple fiber laser cavity. This work enriches the research content of SC and provides a cost-effective method for high-power SC lasers.

Ultra-precision trapezoidal grating mixture of photonic crystal’s effect on enhancing the optoelectronic properties of thin film GaAs solar cell

The IV semiconductor materials have a pivotal role in photovoltaic (PV) systems. Research on the optimal grating structure has attracted a lot of interest because of its potential application in light-trapping in thin film PV cells. The Photonic Crystals (PCs) with their nano-engineered feature which allowed them to obtain high efficiency solar cells in particular from a parametric aspect. By using the Rigorous Coupled Wave Analysis (RCWA) method by RSoft CAD software, a Gallium arsenide (GaAs) thin film is studied with a simple trapezoidal grating of PCs to improve optical absorption with height, width and the mixed ultra-precision grating variations taking into account the J-V solar cell characteristics. According to our simulation results and compared to the planar grating, an enhancement in the power conversion efficiency (η) of 53.72% with a one dimensional PCs grating, and a 115.36% improvement with a simple trapezoidal grating. Furthermore, by mixing the ultra-precision values of the gratings we have an enhancement of 147.25% in height variation and 151.04% in width variation, with a noticeable improvement in the width effect compared to the height that reached up to 58.22%.

Terahertz dual-band polarization control and wavefront shaping over freestanding dielectric binary gratings with high efficiency

Metasurfaces for multi-band or wideband polarization control are usually achieved by cascading or nesting several elements in a period, which complicate the structure and introduce crosstalk. In this work, independent polarization control is achieved around two terahertz frequencies, 0.14 THz and 0.3 THz, through a simple binary one-dimensional grating. The correlation of the phase retardation with frequency is broken, since it is determined by the propagation of a single mode at the lower frequency and by the interference of two modes at the higher frequency. Four gratings capable of different types of dual-band polarization control, a half-half wave plate, a half-quarter wave plate, a quarter-half wave plate and a quarter-quarter wave plate, are designed and experimentally tested. All the samples are made of polymers and 3D-printed as freestanding lightweight structures. The experimental results of all the samples validate polarization conversion with state-of-the-art high efficiency. Simulation results indicate their favorable bandwidth of more than 30%. The half-half wave plate gratings are further applied to build a metalens for efficient dual-band beam focusing based on the geometric phase. They show great potential in developing multifunctional terahertz elements using simple configurations.

FBG Tactile Sensor for Surface Thickness and Shape Measurement

In recent years, numerous measurement techniques have been employed to characterize surface textures in the micrometer and nanometer range. Although these techniques have enabled significant advances in surface metrology, larger surface details are not easily measurable with such techniques. We propose a simple and reliable fiber Bragg grating (FBG) based tactile displacement sensor to measure a wide range of displacements that vary from $0.2~\mu \text{m}$ to 2mm. The device adopts a novel multi-pivot mechanical lever-based amplification mechanism to achieve a wide measurement range. Experimental results show that the sensor can measure both 219nm thick, thin-film coating and 1.22mm glass slide by selecting an appropriate pivot position. Further, the device is employed to identify topographies of three different surfaces of different dimensions. The sensor’s optimized configuration demonstrates an excellent displacement sensitivity of 63926pm/mm with a high resolution of 15.64nm. The repeatability of the sensor in this configuration is 2.44%.

Design of a microstructured surface for infrared radiation regulation based on structural combinations

There has been an increased interest on the development of infrared radiation (IR) regulation based on microstructured surfaces with desired radiative properties. Here design of a microstructured surface is proposed in terms of structural combinations to mediate the IR characteristics of targets. The designed microstructured surface is a combined structure, which is a two-dimension periodic composite grating obtained by combining two periodic simple gratings with different parameters and taking the union. Remarkably, the simple grating is also a combined structure obtained by combining and taking the intersection of two different single structures, a dielectric/metal/dielectric (DMD) structure and a metal grating structure. The results show that on the premise of having the structural characteristics of both constituent structures, the designed combined structures can not only inherit the spectral characteristics of both constituent structures but also have some new effects. For the adjusted composite grating, it is found that three absorption peaks with the maximal absorbance up to 89% are achieved in 5–8 μm range. Additionally, the average reflectance is greater than 80% in 3–5 μm and 8–14 μm bands. The adjusted composite grating fulfills the requirement for multiband regulation and the method of structural combinations suggests a feasible means to regulate the IR characteristics of targets.

Bound States in the Continuum in One-Dimensional Dimerized Plasmonic Gratings**Supported by the National Natural Science Foundation of China (Grant Nos. 11804288 and 91750102) and the Projects of President Foundation of Chongqing University (Grant No. 2019CDXZWL002).

A simple one-dimensional subwavelength plasmonic grating can support symmetry protected bound states in the continuum (BICs), but not necessarily for the non-symmetry protected BICs. By dimerizing the lattice, non-symmetry protected BIC can be supported on the dimerized grating and can be tuned readily. The mechanism for the BICs in the dimerized grating is interpreted in the viewpoint of interference between the electromagnetic multipoles.

Content-based Dissociation of Hippocampal Involvement in Prediction.

Recent work suggests that a key function of the hippocampus is to predict the future. This is thought to depend on its ability to bind inputs over time and space and to retrieve upcoming or missing inputs based on partial cues. In line with this, previous research has revealed prediction-related signals in the hippocampus for complex visual objects, such as fractals and abstract shapes. Implicit in such accounts is that these computations in the hippocampus reflect domain-general processes that apply across different types and modalities of stimuli. An alternative is that the hippocampus plays a more domain-specific role in predictive processing, with the type of stimuli being predicted determining its involvement. To investigate this, we compared hippocampal responses to auditory cues predicting abstract shapes (Experiment 1) versus oriented gratings (Experiment 2). We measured brain activity in male and female human participants using high-resolution fMRI, in combination with inverted encoding models to reconstruct shape and orientation information. Our results revealed that expectations about shape and orientation evoked distinct representations in the hippocampus. For complex shapes, the hippocampus represented which shape was expected, potentially serving as a source of top-down predictions. In contrast, for simple gratings, the hippocampus represented only unexpected orientations, more reminiscent of a prediction error. We discuss several potential explanations for this content-based dissociation in hippocampal function, concluding that the computational role of the hippocampus in predictive processing may depend on the nature and complexity of stimuli.

Grating deployed total-shear 3-beam interference microscopy with reduced temporal coherence.

Interference microscopy is a powerful optical imaging technique providing quantitative phase distribution information to characterize various type technical and biomedical objects. Static and dynamic objects and processes can be investigated. In this paper we propose very compact, common-path and partially coherent diffraction grating-based interference microscopy system for studying small objects like single cells with low densities being sparsely distributed in the field of view. Simple binary amplitude diffraction grating is the only additional element to be introduced into a conventional microscope optical system. By placing it at a proper distance in front of the microscope image plane the total-shear operation mode is deployed resulting in interferograms of the object-reference beam type. Depending on the grating to image plane separation distance two or three-beam interferograms are generated. The latter ones are advantageous since they contain achromatic second harmonics in the interferogram intensity distributions. This feature enables to use reduced temporal coherence light sources for the microscope to reduce coherent noise and parasitic interference patterns. For this purpose we employ the laser diode with driving current below the threshold one. Results of conducted experiments including automatic computer processing of interferograms fully corroborate analytical description of the proposed method and illustrate its capabilities for studying static and dynamic phase objects.

Comparison Study of Optical Sensing Property of Gratings with and without Lamellae Layers

Local plasmatic resonance existing around nanoscale metallic structures is closely related to the refractive index of the surrounding media. Gold Lamellae layers embedded in gratings are ideal to populate the plasmatic resonances locally with much higher concentration than the simple gratings, leading to enhanced sensitivity to the testing materials. Based on our earlier success in replicating dielectric lamellae layers, this paper reports our further work in converting the dielectric lamellae layers into gold ones and the study of their optical property as refractive index sensors. To prove the local surface plasmatic resonance be responsible for absorbing the light, the gratings without the lamellae layer were also tested for comparison. Both numerical simulation using the finite difference time domain method and optical measurements have demonstrated the enhanced sensitivity by the gratings with lamellae layers, although structural optimization is still needed. With the further improvement in both structural configuration, material and nanofabrication technique, it is believed that the proposed lamellae layers should be a promising candidate for highly sensitive environmental sensors.

Complex trapezoid grating for light trapping in thin-film solar cells: super-fine structure

The research of the optimal surface structure has attracted considerable interest because of its potential application in light trapping in thin-film solar cells (TFSCs). In this paper, a super-fine structure named complex trapezoid grating is proposed to improve the optical absorption comparing to the conventional simple trapezoid grating in a-Si:H TFSCs. The numerical calculation by utilizing rigorous coupled-wave analysis (RCWA) is conducted to obtain the optical absorption of the structured surface. The results demonstrate that, compared to a planar slab, the optimized-simple trapezoid grating shows 97% enhancement of power conversion efficiency η while the complex trapezoid grating shows 131% enhancement. Obviously, the complex trapezoid grating exhibits a better performance than the simple grating, which is due to the perfect antireflective effect and microcavity resonance effect. The angular response of the optical absorption in a-Si:H TFSCs was also investigated. The results further indicate that it is a better way to select the complex trapezoid grating in improving the optical absorption of silicon-based TFSCs.

Design and Analysis of FBG Sensor for Explosive Detection Applications

In this paper, a simple uniform fiber Bragg grating (FBG) sensor is designed, and maximum reflection of 7.1 dB at 1.55 μm is obtained. As this FBG shows a better reflection, it deserves well for the purpose of sensing and communication predominantly as they are resistant to electromagnetic radiations, light weight, low cost, firm size, and ease of handling. The proposed FBG is used for explosive sensing applications to detect explosives like trinitrotoluene (TNT), nitroglycerin, and royal demolition explosive (RDX). The different strain and temperature wavelength shifts of the explosives also analyzed and plotted.

Rectangular and sinusoidal Au-Grating as plasmonic sensor: A comparative study

Sensitivity of surface plasmon resonance sensor (SPRS) is mainly associated to efficient excitation of surface plasmon polaritons (SPPs) waves. In this work, effect of 50 nm thick Au film designed as rectangular and sinusoidal grating structure on glass substrate has been explored by finite element analysis (FEA) using COMSOL Multiphysics 5.3a. The comparison between the rectangular and sinusoidal gratings for efficient sensing has been presented here. Slit width (SW) for grating device has been optimized by applying far-field and near-field analysis while periodicity of grating structure is kept constant. Far field transmission spectra for both devices have been analyzed by illuminating the grating device through excitation port from the substrate side. Resonance wavelength (RW) dips for plasmonic grating device has been examined in each transmission spectrum for different values of refractive indices (RI). The sensitivity of plasmonic grating device has been calculated by measuring the shift in resonance wavelength by varying refractive index of analyte. The sinusoidal grating supporting fundamental plasmonic mode is found to be more sensitive (717 nm/RIU) as compared to simple rectangular grating (693 nm/RIU). Being highly sensitive to minute change in refractive index, such devices are efficiently applicable in real life.