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The unique diffractive properties of gratings have made them essential in a wide range of applications, including spectral analysis, precision measurement, optical data storage, laser technology, and biomedical imaging. With advancements in micro- and nanotechnologies, the demand for more precise and efficient grating fabrication has increased. This review discusses the latest advancements in grating manufacturing techniques, particularly highlighting laser interference lithography, which excels in sub-beam generation through wavefront and amplitude division. Techniques such as Lloyd's mirror configurations produce stable interference fringe fields for grating patterning in a single exposure. Orthogonal and non-orthogonal, two-axis Lloyd's mirror interferometers have advanced the fabrication of two-dimensional gratings and large-area gratings, respectively, while laser interference combined with concave lenses enables the creation of concave gratings. Grating interferometry, utilizing optical interference principles, allows for highly precise measurements of minute displacements at the nanometer to sub-nanometer scale. This review also examines the application of grating interferometry in high-precision, absolute, and multi-degree-of-freedom measurement systems. Progress in grating fabrication has significantly advanced spectrometer technology, with integrated structures such as concave gratings, Fresnel gratings, and grating–microlens arrays driving the miniaturization of spectrometers and expanding their use in compact analytical instruments. [ABSTRACT FROM AUTHOR]

Liquid crystalline (LC) poly(methacrylates) with azobenzene moieties in their side chains have been synthesized and their chemical structure has been confirmed with nuclear magnetic resonance spectroscopy and gel permeation chromatography. Azobenzene moieties are linked to polymer backbone via an alkyl spacer with length from 2 to 10 methylene units. Photooptical processes in thin amorphized polymer films are examined through measurements of optical anisotropy and holographic grating recording. Birefringence values induced with a linearly polarized green (532 nm) laser beam has strongly depended from the spacer length. Irradiation with two circularly polarized green laser beams leads to the formation of polarization and surface relief gratings, which have been visualized with polarized optical and atomic force microscopy, respectively. Diffraction efficiency of the recorded gratings is in correlation with the spacer length, namely, the shorter the spacer, the higher diffraction efficiency can be achieved. For polymers with short spacers polarization gratings can be easily erased by circularly polarized light, but only partial erasing of surface relief gratings is observed. The results presented offer the possibility of tailoring the chemical structure of azobenzene‐containing polymers for designing high‐efficiency media for the holographic recording. [ABSTRACT FROM AUTHOR]

The ability to unveil the spatial distribution of refractive index (RI) within volumetric holographic optical elements (HOEs) is critical for quantitating their diffractive behaviors. Angle‐resolved far‐field measurements of diffractive intensity have been prevalent toward this end. However, this century‐old approach is unable to directly visualize the spatial distribution of RI at mesoscopic scale. More significantly, visualization of molecular orientation within photoaddressable polymers (PAPs), which serve as standard recording media for polarization HOEs (pHOEs), remains uncharted territory. The recent advent of dielectric tensor tomography (DTT) has paved the way for full characterization of 3D anisotropic dielectric tensors, encompassing principal RIs and their optic axes. This study embarks on direct visualization of the 3D spatial distribution of anisotropic molecular orientations within holographically recorded PAPs. Illuminating these PAPs with polarized light at varying angles, the diffracted vector fields essential for reconstructing the dielectric tensor tomogram are captured. After diagonalizing the dielectric tensors, periodic rotations of the anisotropic molecule orientations can be visualized in the PAPs, which have never been achieved so far. The homogeneity of grating patterns produced under diverse manufacturing conditions is also examined and juxtaposed. [ABSTRACT FROM AUTHOR]

In this study, a simplified transformer model is used to predict the beam waist of 1,092 nm light coupled out from SiN-based mixed pitch gratings at various heights. The beam waists data at various heights above the grating is first compiled. Then, we used a sequence of the current beam waist values, z-positions, and the computed mathematical indicators (features) to predict the next beam waist value (labels). Optimized transformer model yields average percentage error (APE) of 6.6% between the predicted and actual beam waists, which corresponds to 93.4% prediction accuracy. This study provides a pioneering approach to using natural language processing model to perform predictive modelling on photonics data, and possible extrapolation of photonics data using transformer model.

This article presents the optical analysis on how the light coupling performance between optical channel waveguides and gold periodic nanoarrays in the visible spectrum can be improved using an indium tin oxide (ITO) thin film as buffer layer. Coupling efficiency, out-of-plane scattering and far-field projected beam focus and diffraction angle are evaluated using Finite-Difference Time-Domain (FDTD) method and scattering theory for different nanogratings and nanocylinders array patterns. The implementation of ITO enables up to a 12 fold increase in coupling efficiency, a 6.4 fold enhancement of light scattered out of the array plane and consequent improvements in the diffracted beam radiation intensity peaks, without compromising the beam's focus or altering its deflection angle. This configuration provides a cheap and simple solution to increase the device performance without changing the array features, for integrated biosensing and scattering-related applications.

The unique diffractive properties of gratings have made them essential in a wide range of applications, including spectral analysis, precision measurement, optical data storage, laser technology, and biomedical imaging. With advancements in micro- and nanotechnologies, the demand for more precise and efficient grating fabrication has increased. This review discusses the latest advancements in grating manufacturing techniques, particularly highlighting laser interference lithography, which excels in sub-beam generation through wavefront and amplitude division. Techniques such as Lloyd’s mirror configurations produce stable interference fringe fields for grating patterning in a single exposure. Orthogonal and non-orthogonal, two-axis Lloyd’s mirror interferometers have advanced the fabrication of two-dimensional gratings and large-area gratings, respectively, while laser interference combined with concave lenses enables the creation of concave gratings. Grating interferometry, utilizing optical interference principles, allows for highly precise measurements of minute displacements at the nanometer to sub-nanometer scale. This review also examines the application of grating interferometry in high-precision, absolute, and multi-degree-of-freedom measurement systems. Progress in grating fabrication has significantly advanced spectrometer technology, with integrated structures such as concave gratings, Fresnel gratings, and grating–microlens arrays driving the miniaturization of spectrometers and expanding their use in compact analytical instruments.

The current paper studies gap solitons with parabolic-nonlocal form of self-phase modulation. The soliton solutions for this model are revealed with the successful application of the extended auxiliary equation approach. The parameter constraints ensure the existence of such gap solitons. [ABSTRACT FROM AUTHOR]

Bound states in the continuum (BICs) are theoretically known to possess infinite lifetimes and Q factor. However, due to the difficulties in achieving it in reality, symmetry breaking is often introduced in the structure to transform symmetrically protected BICs into quasi-BICs (q-BICs) with extremely high Q factor. Therefore, q-BICs can be utilized to enhance the Q factor of optical sensors. In this paper, we propose the design of a double-layer composite one-dimensional grating with a high Q factor. The structure consists of a double-layers silicon (Si) grating on a silicon dioxide (SiO2) substrate. By introducing a displacement in the upper-layer grating to break the symmetry, q-BICs are induced. The induced q-BICs achieve a Q factor of 2248 for transverse magnetic (TM) wave, enabling enhanced optical sensing capabilities. The proposed q-BICs sensor, exhibiting anisotropy for both TM and transverse electric wave (TE), holds great potential for narrowband polarizers and sensing applications. [ABSTRACT FROM AUTHOR]

This study introduces a sensor consisting of a metal-insulator-metal (MIM) structure, having an integration of gold nanorods and gratings, to perform various functions including sensing refractive index, detecting blood components, monitoring temperature, and measuring magnetic fields. The sensor's spectral properties are studied analytically in the MIR region implementing finite element method. Furthermore, the potential in the RI sensing is extensively examined by modulating the geometrical parameters, ultimately establishing a direct relationship with the transmittance profile. Following computational experiments, the best possible configuration exhibits the maximum sensitivity of 6118.685 nm/RIU. This research shows the biosensing applications by the detection of near-infrared and visible blood components such RBC, Hb, WBC, plasma, and water with a high sensitivity of 3185.05 nm/RIU. Moreover, ethanol is employed to evaluate the proffered sensor's temperature sensing feature and a maximum sensitivity 1.18 nm/°C was attained. Furthermore, this sensor can be used for magnetic field detection as it shows a high magnetic field sensitivity of 354.66 pm/Oe. The structural model is simple. Besides, this design has the potential to serve as a substitute for sensors that rely on silver. It is chemically stable and performs well in refractive index measurement, biosensing, temperature sensing and magnetic field detection.

In the field of X-ray phase-contrast imaging, a time-saving approach and preservation of details are crucial factors for obtaining phase-contrast images. In this manuscript, a single grating imaging system is proposed to perform the X-ray phase-contrast imaging. Instead of the time-consuming phase-stepping method, this system uses a single-shot algorithm to retrieve the distribution of samples' attenuation and phase gradient. Unlike the single-shot Fourier transform algorithm, which truncates the high-frequency component of the image and reduces the spatial resolution, our method can retrieve the attenuation and phase information images with the same spatial resolution as the images acquired directly by the X-ray detector used. Furthermore, by using a large-size X-ray detector (29 cm × 23 cm), the imaging system can be configured as either a microscopic instrument or a normal large field-of-view imaging system. Finally, a series of experiments were performed to validate the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

The optical nanostructures with high-integration have attracted considerable attentions in the function expansion for the organic light-emitting devices (OLEDs). However, these nanostructures are usually fabricated by hard brittle materials, such as Si, and applied as attached equipment into device system. The traditional integration strategy gradually cannot meet the requirement for ultrathin, high-flexibility and multifunction in the new generation OLEDs. In this work, the electron beam lithography (EBL) has been applied to integrated the optical nanostructures into metallic electrode, realizing the high functional integration into ultrathin OLEDs. After carefully designed the structured electrode, we have successfully integrated the function of charge injection, trapped photons extraction and light collimation into one. And this attempt may provide a feasible strategy for the fabrication of next-generation ultrathin and multifunctional photoelectric devices. [ABSTRACT FROM AUTHOR]

An efficient light management technique is proposed and investigated by developing a TCAD model for microcrystalline silicon based thin film solar cell. The model includes physical insight related to some optical phenomenon like scattering and diffusive reflection of light as well as some electrical phenomena like carrier transport at the heterointerfaces. Two different structures, which comprises of sawtooth and grating-type back nanotextured in combination with random front nano-textured absorber layers are considered in this work. It has been observed that sawtooth or grating type back nano-textured absorber layer results in enhanced absorption of incident photons as compared to the planar back absorber layer. Also, the grating type back-textured absorber provides better performance compared to its sawtooth counterparts. Finally, the depth and period of grating are optimized in order to maximize the power conversion efficiency of grating-back-textured with random-front-textured absorber. The optimized structure results in an efficiency of 12.325% which corresponds to an improvement of 3.975% compared to the random front and back nano-textured thin film solar cell. [ABSTRACT FROM AUTHOR]

A surface homogenization method for acoustic waves over thin microstructured surfaces in the presence of a fluid in a potential flow is presented. Sound hard surfaces are considered, the flow is considered two-dimensional and slow and a low Mach approximation is introduced. We consider acoustic waves with a typical wavelength 1/k much larger than the array spacing h and thickness e. Owing to the small parameter ε=kh , with e/h=O(1) , a matched asymptotic expansion technique is applied to the low Mach potential wave equation in the frequency domain. A boundary condition is obtained on an equivalent flat wall, which links the acoustic velocity to its normal and tangential derivatives (of the Myers type). The accuracy of the effective model is tested numerically for various periodic shapes and the accuracy of the model in O(ε2) is validated. [ABSTRACT FROM AUTHOR]

Accurate probing, monitoring, and controlling the pH level is critical in different fields. In the last two decades, optical fiber/waveguide-based pH sensors have been widely investigated with the aim of further enhancement of sensitivity and miniaturization. Here, we propose a novel and cost-effective pH sensor based on polymer waveguide Bragg grating, addressing main aspects like sensitivity enhancement, miniaturization, narrow band response, and stable performance. The large evanescent field due to metal under cladding and high index coating combined with excellent penetration depth match with pH-sensitive polymer coating of poly-acrylic acid/poly-allylamine hydrochloride (PAA/PAH) in an aqueous medium, ensures a sensitivity of 10.4 nm/pH for a pH range of 4 to 7, two orders higher than all reported Bragg grating pH sensors to our knowledge. A narrow band (∼1 nm) response has been ensured by low propagation loss of the TE mode. Further, the variations in sensitivity and peak reflectivity are within 3% for a ±10% inaccuracy in various structural parameters, reflecting excellent fabrication tolerances. The proposed sensor is an important development, especially for the perishable food industry in which a high sensitivity in a pH range of 4 to 7 is highly desirable to accurately determine the freshness of foods.

We report a class of all-dielectric, additively-manufactured polarization converters with tailored temporal frequency responses within the Ku and Ka microwave bands (15– 40 GHz). These multi-layer devices consist of cascaded, subwavelength, high-contrast gratings with different fill fractions and orientations, providing control over the effective anisotropic properties of each layer. In design, the subwavelength gratings are modeled as homogeneous anisotropic layers. This allows the overall metastructure to be treated as a stratified dielectric medium. Therefore, it can be analyzed and optimized using plane-wave transfer matrix techniques that fully account for multiple reflections between layers. Using this cascaded grating geometry, a variety of high-efficiency microwave polarization converters can be realized with broadband, multiband, or multifunctional behavior. The transmissive metastructures do not require anti-reflection layers since impedance matching is incorporated into their design. Three example devices based on alumina/air gratings have been monolithically fabricated using ceramic stereolithography: a broadband reflective half-wave plate, a broadband isotropic polarization rotator, and a dual-band linear-to-circular polarization converter.

ABSTRACTFor single-walled carbon nanotubes (SWCNTs) with a length of 1–50 µm, the surface plasmon-polariton (SPP) resonance is within the terahertz frequency range; therefore, SWCNT lattices can be used to design frequency-selective surface (FSS). A numerical model of electromagnetic wave diffraction on a two-dimensional periodic SWCNT lattice can be described by an integro-differential equation of the second-order with respect to the surface current along SWCNT. The equation can be solved by the Bubnov–Galerkin method. Frequency dependence of reflecting and transmitting electromagnetic waves for FSSs near the SPP resonance is studied numerically. It is shown that the resonances are within the lower-frequency part of the terahertz range. We also estimate the relaxation frequency of an individual SWCNT and demonstrate the applicability of the Kubo formula for graphene conductivity to array of strips similar in size to SWCNTs under consideration.

Surface plasmon resonance (SPR) is a common and useful measurement technique to perform fast and sensitive optical detection. SPR instrumentations usually comprise optical systems of mirrors and lenses which are quite expensive and impractical for point-of-care applications. In this work, we presented a novel and compact SPR device called SPECTRA, designed as a spectrophotometer add-on with a grating coupling configuration. The device is conceived as a marketable solution to perform quick SPR measurements in grating configuration without the requirement of complex instrumentation. The device can be customized either in a vertical structure to reach lower incident light angles, or in a horizontal configuration, which is suitable for SPR analysis using liquid solutions. The SPECTRA performance was evaluated through SPR measurements in typical applications. The vertical SPECTRA system was employed to detect different functionalization molecules on gold 720 nm-period grating devices. Meanwhile, the horizontal SPECTRA configuration was exploited to carry out fluid-dynamic measurements using a microfluidic cell with glycerol solutions at increasing concentrations to account for different refractive indexes. The experimental tests confirmed that the SPECTRA design is suitable for SPR measurements, demonstrating its capability to detect the presence of analytes and changes in surface properties both in static and dynamic set-ups. [ABSTRACT FROM AUTHOR]

For single-walled carbon nanotubes (SWCNTs) with a length of 1–50 µm, the surface plasmon-polariton (SPP) resonance is within the terahertz frequency range; therefore, SWCNT lattices can be used to design frequency-selective surface (FSS). A numerical model of electromagnetic wave diffraction on a two-dimensional periodic SWCNT lattice can be described by an integro-differential equation of the second-order with respect to the surface current along SWCNT. The equation can be solved by the Bubnov–Galerkin method. Frequency dependence of reflecting and transmitting electromagnetic waves for FSSs near the SPP resonance is studied numerically. It is shown that the resonances are within the lower-frequency part of the terahertz range. We also estimate the relaxation frequency of an individual SWCNT and demonstrate the applicability of the Kubo formula for graphene conductivity to array of strips similar in size to SWCNTs under consideration. [ABSTRACT FROM AUTHOR]

The strong coupling of molecules with surface plasmons results in hybrid states which are part molecule, part surface-bound light. Since molecular resonances may acquire the spatial coherence of plasmons, which have mm-scale propagation lengths, strong-coupling with molecular resonances potentially enables long-range molecular energy transfer. Gratings are often used to couple incident light to surface plasmons, by scattering the otherwise non-radiative surface plasmon inside the light-line. We calculate the dispersion relation for surface plasmons strongly coupled to molecular resonances when grating scattering is involved. By treating the molecules as independent oscillators rather than the more typically considered single collective dipole, we find the full multi-band dispersion relation. This approach offers a natural way to include the dark states in the dispersion. We demonstrate that for a molecular resonance tuned near the crossing point of forward and backward grating-scattered plasmon modes, the interaction between plasmons and molecules gives a five-band dispersion relation, including a bright state not captured in calculations using a single collective dipole. We also show that the role of the grating in breaking the translational invariance of the system appears in the position-dependent coupling between the molecules and the surface plasmon. The presence of the grating is thus not only important for the experimental observation of molecule-surface-plasmon coupling, but also provides an additional design parameter that tunes the system.

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Stimulated by the extraordinary power conversion efficiency (PCE) of hybrid organic-inorganic perovskites (HOIP) based solar cells (SCs), the derivative studies on inorganic perovskites (IOP) based SCs have been intensely investigated. In order to overcome the disadvantages of CsPbBr3, most prominently the unfavorable larger band gap (2.3eV), a grating layer of mesoporous anatase TiO2(mp-TiO2) has been inserted into the conventional configuration of SCs. The grating layer acts as the electron transfer layer (ETL) and light absorption strengthening layer at the same time. Due to the combined effects, the increased contacting area increased the fill factor (FF) and enhanced light trapping in the grating layer increased the short-current density, the average PCE of IOP based SCs has increased from 5.67% to 7.58%, which is a ca. 34% increase relatively. Furthermore, research on traditional HOIP-based SCs is also conducted. Interestingly, the increasing PCE mechanism is quite different from their inorganic counterparts, which should be attributed to the strain effect of different film structures. Thus, the strain-induced defect charge-state transitions of MAPbI3 by the grating layer increased the open-circuit voltage (Voc); and similarly, the increased contacting area also increased the FF, resulting in a 13% increase for PCE.

Light reflection plays a crucial role in a number of modern technologies. In this paper, analytical expressions for maximal reflected power in any direction and for any polarization are given for generic planar structures made of a single material represented by a complex scalar susceptibility. The problem of optimal light‐matter interactions to maximize reflection is formulated as the solution of an optimization problem in terms of the induced currents, subject to energy conservation and passivity, which admits a global upper bound by using Lagrangian duality. The derived upper bounds apply to a broad range of planar structures, including metasurfaces, gratings, homogenized films, photonic crystal slabs, and more generally, any inhomogeneous planar structure irrespective of its geometrical details. These bounds also set the limit on the minimum possible thickness, for a given lossy material, to achieve a desired reflectance. Moreover, the results allow the discovery of parameter regions where large improvements in the efficiency of a reflective structure are possible compared to existing designs. Examples are given of the implications of these findings for the design of superior and compact reflective components made of real, imperfect (i.e., lossy) materials, such as ultra‐thin and efficient gratings, polarization converters, and light‐weight mirrors for solar/laser sails. [ABSTRACT FROM AUTHOR]

Accuracy becomes progressively important in the wake of development in advanced industrial equipment. A key position sensor to such a quest is the optical linear encoder. Occasionally, inappropriate mounting can cause errors greater than the accuracy grade of the optical linear encoder itself, especially for open-type optical linear encoders, where the mounting distance between the reading head and main scale must be accurately controlled. This paper analyzes the diffraction fields of a traditional scanning reticle made by amplitude grating and a newly designed combined grating; the latter shows a more stable phase in mathematical calculation and simulations. The proposed combined gratings are fabricated in a laboratory and assembled into the reading heads. The experimental results indicate that the mounting tolerance between the reading head and the main scale of the optical linear encoder can be improved. [ABSTRACT FROM AUTHOR]

This paper retrieves an optical 1–soliton solution to a model that is written as a concatenation of the Lakshmanan–Porsezian–Daniel model and Sasa–Satsuma equation. The method of undetermined coefficients obtains a full spectrum of 1–soliton solutions. The multiplier approach yields the conserved densities, which subsequently lead to the conserved quantities from the bright 1–soliton solution. [ABSTRACT FROM AUTHOR]

Pd-based alloy coating on optical fiber is widely employed for hydrogen concentration sensing. However, it is difficult to distinguish the Pd-based alloy expansion from hydrogen adsorption or thermal effect. In this article, a novel fiber-optic hydrogen sensor based on $\pi $ -shifted fiber Bragg grating (FBG) with locally coated Pd/Ta alloy layer, which enables an in situ temperature compensation, is proposed and experimentally demonstrated. The hydrogen sensitivity difference between the extremely narrow peak of $\pi $ -shifted FBG and the sidelobe wavelength is employed to realize the hydrogen concentration detection with temperature compensation. Theoretical simulation and experiment show that the hydrogen sensitivity of the structure can be enhanced by optimizing the partially coating length. When the alloy length is 2 mm for a 10-mm grating, the wavelength shifts of central and sidelobe 19.2 pm and 6.0 pm are achieved, respectively, at 1% hydrogen concentration. Maximum hydrogen sensitivity after temperature compensation of 13.2 pm is obtained. The proposed local-compensated method based on partially coating on $\pi $ -shifted FBG is particularly potential for oxygen-free environments with high requirement on hydrogen accuracy. [ABSTRACT FROM AUTHOR]

Soft contact lenses with built-in sensors have shown great potential for real-time monitoring of intraocular pressure (IOP). This paper reports a contact lens IOP sensor based on the moiré patterns. One concentric ring grating (G1) is fabricated on a commercial contact lens, the other line grating (G2) is stored in a computer. Images of G1 are captured using a smartphone camera. Then moiré patterns are generated by superimposing the images of the two gratings in a computer. The central angle $\theta $ formed between two innermost adjacent moiré patterns is extracted as the IOP transducing signals. The performance of the sensor has been evaluated using a home-made hollow hemispherical silicone model eye. The sensitivity of the contact lens IOP sensor is ~0.15°/mmHg with high repeatability. Additionally, the sensor sensitivity is essentially independent of the initial IOP exerted on the contact lens. Furthermore, the virtual G2 provides great flexibility for the design and fabrication of the sensor, the fabrication of G1 directly on commercial contact lenses paves the way for clinic use. All these results indicate the suitability of this type of sensor for monitoring IOP. [2022-0111] [ABSTRACT FROM AUTHOR]

To avoid mixing of the undesired frequencies in a free‐space, the point sources of gratings, defined by a spatially varying refractive index, should be sinusoidally rather than binarily arranged. This long‐lasting but gold standard lesson attainable from a classical Fourier optics, however, is underutilized in the practical materialization of gratings, mainly because most of the fabrication methods, developed thus far, are intrinsically compatible with a binary rather than sinusoidal grating. Recently, such design concept of optical Fourier elements has been implemented into the real surface gratings (referred to as optical Fourier surfaces) by taking advantage of advanced nanofabrication, whereas their volumetric equivalents (optical Fourier volumes, OFVs) have yet to be conceptually and experimentally elucidated. In this work, the key characteristics of OFVs and their structural requirements are systematically exploited with the assistance of analytical and numerical calculations. Especially, the dynamic range and thickness of volume gratings, required for OFVs, are newly defined. Given these theoretical blueprints, both the holographic photopolymers and photoaddressable polymers are then revisited and they are experimentally validated as easy‐to‐craft mediums of OFVs. The landscape of volume gratings is extended by providing an integrative pipeline of OFVs across their theoretical design and practical materialization. [ABSTRACT FROM AUTHOR]

Plane-wave imaging (PWI) with angular compounding has gained in popularity over recent years, because it provides high frame rates and good image properties. However, most linear arrays used in clinical practice have a pitch that is equal to than the wavelength of ultrasound. Hence, the presence of grating lobes is a concern for PWI using multiple transmit angles. The presence of grating lobes produces clutter in images and reduces the ability to observe tissue contrast. Techniques to reduce or eliminate the presence of grating lobes for PWI using multiple angles will result in improved image quality. Null subtraction imaging (NSI) is a nonlinear beamforming technique that has been explored for improving the lateral resolution of ultrasonic imaging. However, the apodization scheme used in NSI also eliminates or greatly reduces the presence of grating lobes. Imaging tasks using NSI were evaluated in simulations and physical experiments involving tissue-mimicking phantoms and rat tumors in vivo. Images created with NSI were compared with images created using traditional delay and sum (DAS) with Hann apodization and images created using a generalized coherence factor (GCF). NSI was observed to greatly reduce the presence of grating lobes in ultrasonic images, compared to DAS with Hann and GCF, while maintaining spatial resolution and contrast in the images. Therefore, NSI can provide a novel means of creating images using PWI with multiple steering angles on clinically available linear arrays while reducing the adverse effects associated with grating lobes. [ABSTRACT FROM AUTHOR]

The image-type angular displacement measurement method based on linear image recognition has garnered attention because of its higher frequency response, strong fault tolerance, and high robustness. In the small-size image angular displacement measuring device, due to the limited pixel size, the circular grating cannot place more lines when realizing single-channel absolute coding recognition on a small diameter grating disk. This leads to larger errors in angular displacement measurement when the line density of the grating disk is low. To improve the measurement accuracy of small-scale displacement, the present work aimed to reduce this error by studying the error compensation method involving low-density grating disks. First, the mechanism of linear image-type angular displacement measurements is described, and a measurement algorithm based on linear scan images is proposed. Second, the measurement error model for low-density grating disks is established according to the proposed measurement algorithm. Third, a simplified error compensation algorithm based on a harmonic model is developed. Finally, simulations and experiments are performed to verify the performance of the proposed algorithm. Simulation results show that the developed harmonic compensation algorithm can effectively reduce the error caused by the low-density circular grating. When the proposed error compensation algorithm is applied to a grating disk with a 62 mm diameter and 2N lines, the measurement accuracy is improved from 8.14” to 4.78”. The proposed error compensation algorithm can significantly improve the accuracy of linear image-type angular displacement measurements involving low-density grating disks, and the results presented herein laid a foundation for improving the accuracy and engineering applicability of angular displacement measurement technology. [ABSTRACT FROM AUTHOR]

In this letter, a novel design concept to achieve a low-cost wide-angle beam-scanning transmitarray (TA) using lens-loaded patch (LLP) elements is presented. In this design, each TA element consists of a beam switchable transmitting element, a group of receiving elements, and a signal via. The received powers of a 2×2 subarray are first combined and then transmitted to the corresponding LLP element, where the phases of the transmitted signals are manipulated for beamforming. Compared with the conventional designs, the phase shifters (PSs) can be multiplexed and the number of PSs is effectively eliminated by 75%, which significantly decreases the hardware cost. By employing multiple patches as the feed of the lenses, reconfigurable radiation patterns are obtained, and by properly choosing the transmitting patches, the developed TA achieves a wide-angle two-dimensional beam scanning within ±60° with stable gains. To validate the design concept, passive prototypes with a center frequency of 12.5 GHz are simulated, fabricated, and tested. The measured results show that the aperture efficiency is 30.1%, 19.4%, and 14.1% for beams steered to 0°, 30°, and 60° at 12.5 GHz, respectively. The beam coverage of ±60° with a less than 3.5 dB scanning loss in the H-plane is obtained. [ABSTRACT FROM AUTHOR]

In this letter, an effective method is proposed to suppress the overall sidelobes for period-modulated leaky-wave antennas (PM-LWAs). The radiation mechanism of the high sidelobes in PM-LWAs is thoroughly investigated by space harmonic (SH) analysis of the aperture field. It is found that the $n = 0$ SH in PM-LWAs occupies a larger proportion of power than that in periodic LWAs (P-LWAs), resulting in a higher sidelobe level (SLL) in PM-LWAs than that in P-LWAs. An SLL suppression method for PM-LWAs is proposed based on even SH cancellation, which is realized by aligning inversed period modulations in a pair of PM-LWAs. Exciting the PM-LWA pair with equal amplitudes and reversed phases, all radiations from the even SHs are cancelled out and hence the overall sidelobes including the first sidelobe and grating lobe are efficiently suppressed. Prototypes are designed, simulated, fabricated, and experimentally measured. Measured results agree well with simulations, which verify the theoretical predictions. The overall sidelobes are suppressed by more than 10 dB in almost the entire forward region. The radiation of $n = - 2$ SH (grating lobe) and the first sidelobe is reduced by more than 20 and 3.4 dB, respectively. [ABSTRACT FROM AUTHOR]

In this study, we proposed an electrical readout surface plasmon resonance (SPR) chemical sensor coupled with visible light to avoid significant light absorption of water. A gold (Au) diffraction grating with a submicrometer pitch, designed using the rigorous coupled-wave analysis (RCWA) method to have a submicrometer pitch and 30-nm-tall Au grating, was fabricated on an n-type silicon (n-Si) substrate by nanofabrication/microfabrication. The hot electrons generated by SPR on the grating are converted to photocurrent at the Schottky barrier at the Au/n-Si interface. The Schottky barrier height was estimated based on the $I-V$ characteristics of the device to be $q{\Phi}_{\mathrm{B}} = 0.72$ eV. The peak shift of photocurrent was observed to be consistentwith the SPR theory when the concentration of glucose aqueous solution changed the refractive index on the Au grating. Assuming that the incident wavelength and angle were fixed, a noise analysis revealed a refractive index resolution of $3.4-4.0 \times 10^{-5}$ refractive index unit (RIU). Since this structure eliminated bulky optics in SPR experiments, it would contribute to the realization of a thin single-chip label-free chemical sensor. [ABSTRACT FROM AUTHOR]

A simple thin film effective index analysis for first-order gratings in Si photonic waveguides is shown to provide highly accurate results for reflected and transmitted power spectrums as long as the waveguide remains single mode and non-radiating. A cover layer can be added to the grating region of a Si photonic waveguide to increase the strength of the grating, modify transition losses from the input waveguide to the grating waveguide region, and/or modify the width of the reflectivity spectrum. For a given grating period, the peak reflection and spectral width of the reflectivity decrease as the duty cycle is decreased or increased from ∼50%. For both radiating and multimode structures, the coupling between all modes, power radiated towards the superstrate (upwards), power radiated downwards (substrate) and transmitted power analyzed by Floquet-Bloch, Eigenmode Expansion and Finite Difference Time Domain methods show excellent agreement. Coupling coefficients calculated using analytic formulas are shown to be accurate only for shallow grating depths.

In this paper, the method of classical vector potential analysis along with the asymptotic strips boundary conditions (ASBC) is used to treat conducting strip grated dielectric pipes wrapped over core dielectric cylindrical rods. By virtue of the periodically-textured surface, this structure is able to exhibit plasmonic behaviors that facilitate strong surface field localization and wave propagation, thereby offering itself as cylindrical surface-wave waveguides or antennas. It can also be configured to exhibit electromagnetic band-gap (EBG) properties, yielding what is known as cylindrical EBG structures. As opposed to a typical counterpart whose core rod is instead conducting, the core dielectric is herein demonstrated to be vital in realizing wide total band-gaps in which neither slow surface nor fast space waves exist, something that the conducting core rod version is incapable of, being only able to offer surface-wave band-gaps, as do many others in the literature that have neglected the suppression of fast space waves as well. By coaxially connecting various such rod structures, each with its own distinct total band-gap, multi-frequency, multi-functional cylindrical antennas with mitigated inter-band interference even under simultaneous operations can be achieved when the pass-band of any one rod (in which its operating frequency lies) falls within the total band-gaps of all others, as herein investigated. Manufactured prototypes are also measured, yielding experimental results that agree well with theoretical predictions.

Reducing carrier leakage is the key to obtain laterally coupled distributed feedback (DFB) lasers with high injection efficiency. We proposed a device structure with slots fabricated between the ridge and gratings to form a good confinement on carriers. The incorporation of slots also makes the grating morphology easier to fabricate. To evaluate the dependence of coupling efficient on main structural parameters, four grating samples with different sizes are studied with FDTD Solutions. Calculation results indicate that slots with a width of $0.1\!-\!0.3\;\mu{m}$ is suitable for the device. And, changing the lateral width of the grating can be used as a direct method to adjust the coupling strength. In addition, the carrier distribution and optical field distribution of the device were simulated with Crosslight PICS3D, and the results show that compared with the traditional laterally-coupled gratings, this structure can strengthen the competitive advantage of fundamental mode and enhance the mode discrimination of the device. This structural design provides ideas and references for improving the injection efficiency and mode stability of laterally-coupled DFB lasers.

Grating couplers (GCs) are important for light to couple between optical fibers and photonic integrated circuits (PICs). The Bound State in Continuum (BIC) waveguides offer new functionalities for PIC. High-Q resonators, electro-optical modulator, and photodetectors can be built on it without etching steps. Due to the special waveguide geometry of BIC waveguides, a high efficiency GC is still missing. In this work, we demonstrated a subwavelength grating (SWG) GC for BIC waveguide with a coupling efficiency of 8 dB and a 1 dB bandwidth >70 nm. The device shows ultra-broadband behavior which is extremely useful for BIC-based PICs.

This paper describes a Parallel-Double-Grating Spectrometer (PDGS) with ultrahigh resolution. Two plane reflection gratings are placed in parallel and staggered to form a dispersive component, through which the light passes multiple times, which greatly improves the dispersion capability of the spectrometer, and therefore the resolution. The diffraction characteristics of the dispersive component and the mathematical model of the PDGS are studied. The results of simulation show that using two reflection gratings with a groove density of 168 gr/mm, combined with spectral splicing technology, the PDGS can achieve a measurement waveband from 450 nm to 610 nm with a resolution better than 30.88 pm. Modifying the structural parameters of the PDGS extends the applicability of the optical path structure to spectral bands from ultraviolet to infrared.

As a universal physical scheme, effective light couplings to waveguides favor numerous applications. However, the low coupling efficiency at wide angles prohibits this fundamental functionality and thus lowers the performance levels of photonic systems. As previously found, the transmission gratings patterned on waveguide facets could significantly improve the large-angle-inputted efficiency to the order of 10 − 1 . Here, we continue this study with a focus on a common scenario, i.e., a grating-modified waveguide excited by the Gaussian beam. A simplified 2D theoretical model is firstly introduced, proving that the efficiency lineshape could be well flattened by elaborately arranged diffractive gratings. For demonstration, subsequent explorations for proper grating geometries were conducted, and four structural configurations were selected for later full-wave numerical simulations. The last comparison studies showcase that the analytical method approximates the finite element method-based modelings. Both methods highlight grating-empowered coupling efficiencies, being 2.5 bigger than the counterparts of the previously reported seven-ring structure. All in all, our research provides instructions to simulate grating effects on the waveguide's light-gathering abilities. Together with algorithm-designed coupling structures, it would be of great interest to further benefit real applications, such as bioanalytical instrumentation and quantum photon probes. [ABSTRACT FROM AUTHOR]

This letter presents a folded reflectarray (FRA) capable of electrical beam steering in the Ka-band. The FRA consists of a low-profile feeding source, a dual-layer wire grid, and 36×36 polarization twisting reflective elements. An element of center shorted patch with two plated through holes is proposed where only two switches are required for polarization twisting and 1-bit encoding. A cavity is then introduced surrounding the element to improve the beam-steering performance. To verify the proposed element, two prototypes with 0° and 60° beam directions are designed and experimented where the metal stub and open circuit are used to model “on/off” states of PIN diodes for simplification. The measured peak gain of 0°FRA is 27.5 dBi with 2 dB gain bandwidth of 28–32 GHz (13.3%). A measured gain drop of 3.25 dB is achieved by the 60°FRA, which verifies the wide-angle beam-steering ability. The measured reflection coefficient of the feeding source is less than −15 dB over the operating band. The proposed FRA has a good potential to realize beam-steering with real PIN diodes for the satellite communications. [ABSTRACT FROM AUTHOR]

This articledemonstrates the developments toward a high-performance monolithic wheeled structure horizontal dual-axis [pitch (X-axis) and roll (Y-axis)] microelectromechanical system (MEMS) gyroscope based on off-plane coupling suppression silicon gratings. Gyroscopes have been fabricated with a silicon-on-glass fabrication process with vacuum sealed in metal packaging. The featured gyroscope operates in split mode with the bandwidths of two axes devices both of ∼70 Hz. By unitizing two individual active restraining control loops that adjust the quadrature amplitude via differentially biasing the silicon gratings in real time, the off-plane mechanical couplings of the two sense modes are significantly reduced by ∼95 and ∼30 times, which enables ∼25-fold and ∼56-fold improvements in the bias instability (BI) for a typical dual-axis device. Tactical-grade performances are achieved for both the X-axis and Y-axis gyroscopes with angle random walk of 0.02 deg/√h and 0.045 deg/√h along with in-run BI of 0.26 deg/h and 0.65 deg/h, respectively, without any temperature compensation. The current preliminary results show great potential to achieve navigation grade precision by employing the mode-matching technique in the future. The use of silicon gratings for out-of-plane coupling reduction also provides an effective way to build a high-performance planar monolithic chip inertial measurement unit. [ABSTRACT FROM AUTHOR]

Good light trapping is essential to make high efficiency InGaN-based solar cells. As InGaN wafers are being made increasingly, thinner, light trapping becomes even more important. In this study, we propose a structure of one-dimensional InGaN grating for the InGaN-based solar cells is proposed. The solar energy absorption characteristics of this structure are studied by the the Finite element method (FEM) method. By alternately altering the grating depth and the filling factor, a new type of grating structure is proposed. For such a structure, different gratings are studied. Numerical computation shows that the absorptance of the InGaN grating structure is over 0.88 throughout the entire computational band. The optimum parameters of the proposed structure are period (a = 480 nm), a filling factor (ff = 50 %) and depth (d=210 nm), which indicates the proposed structured surface may have potential applications in solar cells manufacturing. [ABSTRACT FROM AUTHOR]

The use of fifth-generation (5G) new radio (NR) spectrum around 26 GHz is currently raising the quest on its compatibility with the well-established Earth exploration-satellite service, which may be blinded by the spurious radiation emitted above the horizon (AtH) by base station (BS) antennas. Indeed, AtH grating lobes are often present during cell scanning due to the large interelement spacing in BS array antennas for achieving higher gains with a reduced number of RF chains. In this letter, we propose an approach based on an electrically thin metasurface-based dome for the reduction of AtH grating lobes in 5G-NR BS antennas. The proposed scanning range shifting approach exploits the natural lower amplitude of the grating lobes when the antenna array scans in an angular region closer to the broadside direction. The grating lobe reduction is here demonstrated considering a 1×4 phased linear antenna array operating under dual-liner ±45°-slant polarization. A simple design procedure for designing the metasurface dome is reported, together with the antenna performances, evaluated through a proper set of numerical experiments. It is shown that the grating lobe radiation toward the satellite region is significantly reduced, whereas the overall insertion loss is moderate. [ABSTRACT FROM AUTHOR]

Grating lithography has been successfully employed to fabricate the gratings of optical encoders, which is increasingly unsuitable for the fast and mass production of optical encoders. Due to the advantages of high efficiency, low cost, and simple process requirements, we make the first attempt to introduce additive manufacturing with perovskite quantum dots (QDs) to fabricate a novel linear encoder, which is named quantum dot encoder. First, to better print coding patterns, a new printing ink is created by combining perovskite quantum dots with polycaprolactone (PCL). Then, an innovative coding pattern is printed on the glass substrate by the near-field direct writing with the new ink, which can achieve an actively produced optical signal directly instead of the passively produced optical signal in traditional optical encoders. Finally, a new measurement system for quantum dot encoder is established and an image processing method is elaborately designed to implement the displacement measurement. Simulations indicate that the quantum dot encoder can avoid the thermal deformation caused by the overheated light source, which is beneficial for the decoding precision. Measurement results indicate that the designed quantum dot encoder with a low-speed motion platform and a low-frame charge coupled device (CCD) can achieve a fair measurement precision at a reasonable displacement velocity, which validates our innovative approach for the fabrication of the linear encoder by means of additive manufacturing with perovskite quantum dots. [ABSTRACT FROM AUTHOR]

X-ray phase contrast imaging (XPCI) is an emerging imaging modality for industrial inspections and biomedical applications. A polycapillary semilens is utilized in the grating-based XPCI to reduce the imaging time or improve the image quality of the reconstructed images in this work. The proposed system is proved to be effective for improving the contrast-to-noise ratio (CNR) of the reconstructed images compared with using an aperture diaphragm in the system under the same exposure time. The polycapillary semilens concentrates the X-rays on the region of interest (ROI) of the sample, which reduces the unnecessary radiation in other areas. [ABSTRACT FROM AUTHOR]

We demonstrate a $1\times 256$ optical phased array based on the tri-layer Si3N4-on-Si platform. By optimizing the thickness and vertical position of the Si3N4 overlay, the grating antenna exhibits a ultra-low emission rate of 13.9 dB/cm at 1550 nm and a large bandwidth of 120 nm. Based on the optimized Si3N4 assisted grating antenna, the OPA realizes an ultra-sharp beam divergence of $0.154^{\circ }\times 0.0435$ , a two-dimensional beam steering range of $45.6^{\circ }\times 16.1$ , and a side lobe suppression ratio better than 10.8 dB. [ABSTRACT FROM AUTHOR]

A wideband dual-polarized slot array antenna based on hollow waveguide (HW) is presented in this article. Its wide bandwidth and high performance are enabled by two separate feeding networks, from the side and the bottom, using square coaxial line and ridge waveguide structures, respectively. The full-corporate-feed networks excite the radiation slots in a one-to-one fashion without using backed cavities. Tight element spacing is made possible, suppressing the grating lobe and maintaining high cross-polarization discrimination (XPD) across the wideband. This resolves a usual tradeoff between bandwidth and grating lobe. A double-ridge rectangular waveguide cavity is devised as an orthogonal mode splitter for isolation enhancement. Several design features are used to minimize the signal path length and facilitate routing. An $8\times8$ element prototype in K-band is designed, fabricated, and measured. Experimental results show that the antenna has a wide bandwidth of 23.4% (from 17 to 21.5 GHz) free from grating lobes. More than 25.5 dBi peak gain higher than 51.2 dB isolation and over 43.6 dB XPD are also obtained for the dual polarizations. [ABSTRACT FROM AUTHOR]

Geometric approximation errors of periodically distributed surface facets of cable-network reflector antennas can result in grating lobes undesired in some space missions. As the surface is shaped by the tensioned flexible cable-network structure, the electromagnetic (EM) and structural parameters are highly correlated, and the well-designed surface obtained only concerning the EM performance may not be shaped by the tension-only cable network with specified cable-tension constraints. To address the problem, this study proposes an integrated structural-EM optimization method to enable low grating lobe cable-network antenna design. In this method, force densities of the cables, which can determine both the cable tensions and surface node positions that decide the boundary conditions of the EM field, are selected as the design variables. An adaptive objective/constraint function is proposed, which enables the grating lobe level (GLL) to be minimized with constrained cable tensions, antenna gain, and first sidelobe level (SLL) in a simultaneous, feasible and relatively fast way. To demonstrate the feasibility and effectiveness of the proposed method, it was implemented on a 12 m-diameter cable-network antenna. The results indicate that the grating lobes are greatly degraded and the achieved surface can be formed by well tensioned cables. [ABSTRACT FROM AUTHOR]

Angle of arrival (AoA) for target echo signal detection is crucial in vehicular radar systems, where the estimation of signal parameters via rotational invariance (ESPRIT) provides a high-resolution estimation. A subarray decomposition strategy is presented in this article to create two ESPRIT antenna arrays with enhanced echo signal strengths, applicable to multi-input–multioutput (MIMO) operations for creating a large virtual array. The concept intends to maximize the echo signal’s strength with minimum signal interferences by increasing the aperture sizes of the two ESPRIT antenna arrays. The decomposition produces two virtually overlapped subarrays to increase the subarray aperture sizes in the spatial or spectral representations. It is applied to the MIMO-based arrays for two-way radiation patterns for ESPRIT estimation of AoA. Numerical simulation examples are shown to validate the feasibility and demonstrate the applications. [ABSTRACT FROM AUTHOR]

Millimeter-scale implants using ultrasound (US) for power and communication have been proposed for a range of deep-tissue applications, including neural recording and stimulation. However, published implementations have shown high sensitivity to misalignment with the external US transducer. Ultrasonic beamforming using a phased array to these implants can improve tolerance to misalignment, reduce implant volume, and allow multiple implants to be operated simultaneously in different locations. This article details the design of a custom planar phased array US system, which is capable of steering and focusing US power within a 3-D volume. Analysis and simulation is performed to determine the choice of array element pitch, with special attention given to maximizing the power available at the implant while meeting FDA limits for diagnostic US. Time reversal (TR) is proposed as a computationally simple approach to beamforming that is robust despite scattering and inhomogeneity of the acoustic medium. This technique is demonstrated both in active drive and pulse-echo modes, and it is experimentally compared with other beamforming techniques by measuring energy transfer efficiency. Simultaneous power delivery to multiple implants is also demonstrated. [ABSTRACT FROM AUTHOR]

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is normally treated by RF ablation. Intracardiac echography (ICE) is widely employed during RF ablation procedures to guide the electrophysiologist in navigating the ablation catheter, although only 2-D probes are currently clinically used. A 3-D ICE catheter would not only improve visualization of the atrium and ablation catheter, but it might also provide the 3-D mapping of the electromechanical wave (EW) propagation pattern, which represents the mechanical response of cardiac tissue to electrical activity. The detection of this EW needs 3-D high-frame-rate imaging, which is generally only realizable in tradeoff with channel count and image quality. In this simulation-based study, we propose a high volume rate imaging scheme for a 3-D ICE probe design that employs 1-D micro-beamforming in the elevation direction. Such a probe can achieve a high frame rate while reducing the channel count sufficiently for realization in a 10-Fr catheter. To suppress the grating-lobe (GL) artifacts associated with micro-beamforming in the elevation direction, a limited number of fan-shaped beams with a wide azimuthal and narrow elevational opening angle are sequentially steered to insonify slices of the region of interest. An angular weighted averaging of reconstructed subvolumes further reduces the GL artifacts. We optimize the transmit beam divergence and central frequency based on the required image quality for EW imaging (EWI). Numerical simulation results show that a set of seven fan-shaped transmission beams can provide a frame rate of 1000 Hz and a sufficient spatial resolution to visualize the EW propagation on a large 3-D surface. [ABSTRACT FROM AUTHOR]