Grating Elements Research Articles

Wide-range angle sensing based on mixed variable line spacing gratings

Variable line spacing (VLS) gratings are core components in spatial spectrum and synchrotron radiation facilities due to their merits of self-focusing, aberration correction, and flat focal field. However, optimal design and fabrication of VLS gratings are still required for matching various applications. Scratch-induced selective wet etching has been proven as a robust way for fabricating various nanostructures because of its simplicity, flexibility, and controllability. In this paper, we proposed convenient and maskless methods to fabricate VLS grating based on scratch-induced selective wet etching. During the etching, key factors, including normal load for the scratching, etching time, and types of etchants, were investigated systematically toward optimizing the process. The shapes including sidewall angle and the bottom curvature of the grating elements can be controlled by adjusting the normal load, etching time and/or etching etchants. It is found that the VLS gratings present a wide-range sensing for angle detection based on the reflection spectrum. Furthermore, combining transferring process, the VLS master gratings could be replicated on polydimethylsiloxane (PDMS) surface, which showed great optical performance. This work opened new light to some extent for the application of angle sensing and structural color showing.

Critical Analysis of the Development of the Design of Lattice Tube Concrete Bridges with a Ride on Top

Purpose. The paper aims to highlight and substantiate the need to find rational design schemes for lattice tube concrete bridges with a ride on top based on the analysis of recent research and regulatory documents. Methodology. The current scientific research is analyzed to determine the current state of development of pipe concrete lattice structures. Methods for improving structures are presented. Combinations of filling the grating elements with concrete, variants of cross-sections of the grating elements, their advantages and disadvantages are analyzed. The state of building codes of Ukraine and other countries is considered in order to determine possible options for the design of pipe concrete bridge structures. Due to the lack of detailed research on this issue, the feasibility of implementing optimization studies for these structures and the steps necessary for this are determined. Findings. The optimization of pipe-concrete bridge structures is a relevant area of research, but it requires a multicomponent approach and the use of modern computer facilities. The method of linear optimization is proposed and its general steps for finding economic models are determined. It was found that the base of Ukrainian SCSs in the field of pipe and concrete structures is limited, but can be expanded by using European standards and other international regulations. Originality. The necessity of global development and improvement of pipe concrete gratings of bridge spans is highlighted. Attention is focused on the advantages of this area, which contributes to decision-making at the stage of selecting the type of bridge and detailed design of pipe-concrete lattice bridges. A methodology for finding the optimal grids is proposed, which can integrate existing methods of structural improvement and the requirements of regulatory documents. Practical value. The results of the study can be used to improve the design of pipe concrete bridges at the design stage. Optimization of gratings can help to increase the efficiency of construction and reliability of this type of bridge structure.

Relationship between machining accuracy and optical properties of convex blazed grating in ultra-precision cutting.

For ultra-precision machining of convex blazed grating elements there are inevitable machining errors, surface defects, and surface roughness, all of which can have an impact on their diffraction efficiency. In this paper, we use PCGrate software based on the integration method to establish the machining error model, surface defect model, and surface roughness model of convex spherical blazed grating with a curvature radius of 41.104 mm, a substrate diameter of 14 mm, a grating density of 53.97 line/mm and a blazed angle of 3.86° as the basic specification. To investigate the effect of base curvature radius error, grating period error, blazed angle error, grating ridge and valley passivation radius, Poisson burr height, and blaze surface roughness on their -1 order diffraction efficiency in the 0.95-2.5 µm spectral range. The results show that when the curvature radius error of the spherical base is less than ±80µm, the influence on diffraction efficiency can be ignored. Among the three groups of grating microstructure parameters, the influence of blazed angle on grating diffraction efficiency is the largest, followed by a grating period, and the influence of grating apex angle is the smallest, among which when the error of blazed angle is less than ±0.1° and the error of grating period is less than ±0.1µm, the influence on diffraction efficiency can be ignored. The effect of the passivation radius of the grating valley on the diffraction efficiency is smaller than that of the passivation radius of the grating ridge, and the maximum reduction of diffraction efficiency is 0.096 and 0.144 when the grating ridge and valley passivation radius are 50nm∼650 nm, respectively. The diffraction efficiency decreases significantly in the wavelength range of 1.9-2.5 µm with the increase of Poisson burr height and blaze surface roughness, and its effect on the diffraction efficiency can be neglected when the Poisson burr height is less than 0.5 µm and the blaze surface roughness value is less than RMS 1 nm. The machining error, surface defect, and surface roughness models of the convex blazed grating are optimized to provide a comprehensive machining accuracy basis for ultra-precision cutting of convex grating components.

Inverse design of a Si-based high-performance vertical-emitting meta-grating coupler on a 220 nm silicon-on-insulator platform

Efficient extraction of light from a high refractive index silicon waveguide out of a chip is difficult to achieve. An inverse design approach was employed using the particle swarm optimization method to attain a vertical emitting meta-grating coupler with high coupling efficiency in a 220-nm-thick silicon-on-insulator platform. By carefully selecting the figure of merit and appropriately defining parameter space, unique L-shape and U-shape grating elements that boosted the out-of-plane radiation of light were obtained. In addition, a 65.7% (−1.82 dB) outcoupling efficiency and a 60.2% (−2.2 dB) fiber-to-chip vertical coupling efficiency with an 88 nm 3 dB bandwidth were demonstrated by numerical simulation. Considering fabrication constraints, the optimized complex meta-grating coupler was modified to correspond to two etching steps and was then fabricated with a complementary metal-oxide-semiconductor-compatible process. The modified meta-grating coupler exhibited a simulated coupling efficiency of 57.5% (−2.4 dB) with a 74 nm 3-dB bandwidth in the C-band and an experimentally measured coupling efficiency of 38% (−4.2 dB).

Optical strain sensor with dual fibre Bragg grating topology

The paper presents the design, operation, and proof of principle realisation and validation of a relatively cheap fibre optic strain sensor based on two fibre Bragg grating (FBG) elements with different spectral responses. Its performance is compared with the measurement capabilities of a FBG-based sensor that uses an optical interrogator.

Switching the sign of the topological charge of vortex beams via changing the fork grating resolution

We demonstrate the effect of the resolution of forked-diffractive optical elements (DOEs) on the helicity of the topological charge (TC) of the orbital angular momentum (OAM) beams. To aim that, the forked-DOEs are designed with the same TC while they are different in resolution. Changing the resolution allows the fork direction to be switched. Interestingly, switching the sign of the TC is the consequential achievement for the OAM beams, generated by the fork grating elements with specific resolutions. Furthermore, changing the resolution make the fork gratings to be designed for generating petal-like beams in which the magnitude of the TC tends to zero. Finally, three criteria are calculated for all intensity distributions to quantify the generated beams’ quality and investigate where the helicity of the TC changes. Consequently, we demonstrate the emphasis on selecting the resolution of DOEs which plays a pivotal role in optical systems, for instance in optical tweezers where the OAM beams need to be generated with a desired sign of TC.

Ultra-Precision Cutting and Characterization of Reflective Convex Spherical Blazed Grating Elements.

In this work, based on the diffraction principle of reflective blazed grating, the structure size of the convex spherical blazed grating unit is determined, the machining accuracy of the convex spherical blazed grating is formulated, the effects of tool nose radius and Poisson burr on the diffraction efficiency of the convex spherical blazed grating are analyzed, and the performances of cutting convex gratings with microcrystalline aluminum RSA6061 and RSA6061+ chemically plated NiP for two workpiece materials are compared. A convex spherical blazed grating with a radius of curvature R = 41.104 mm, substrate diameter 14 mm, grating density 53.97 line/mm, and blaze angle of roughly 3.8° is turned by a four-axis ultra-precision machining system by adjustment of the cutting tool, workpiece material, and cutting parameters, as well as modification of the layouts of the blazed grating on the convex sphere. The results of the testing of convex spherical blazed grating elements in both layouts show that the size error of the grating period is close for both layouts, the size error of grating height is smaller in the equal-along-arc layout, the blaze angle error in the equal-along-projection layout is only 0.74%, and the average roughness of the blazed surface is less than 5 nm to meet the processing quality requirements of the reflective convex spherical blazed grating. The greater the blaze angle accuracy of the blazed grating, the higher its diffraction efficiency, so the grating element with an equal-along-projection layout has a higher diffraction efficiency than the grating element with an equal-along-arc layout. RSA6061+ chemically plated NiP material is superior to RSA6061 material in Poisson burr height and blazed surface roughness, which is more suitable for Offner-type imaging spectrometers in the spectral range 0.95–2.5 μm (SWIR).

Single-Pulse Measurement of Orbital Angular Momentum Generated by Microring Lasers.

Optical beams with helical phase fronts carry orbital angular momentum (OAM). To exploit this property in integrated photonics, micrometer-scale devices that generate beams with well-defined OAM are needed. Consequently, lasers based on microring resonators decorated with azimuthal grating elements have been investigated. However, future development of such devices requires better methods to determine their OAM, as current approaches are challenging to implement and interpret. If a simple and more sensitive technique were available, OAM microring lasers could be better understood and further improved. In particular, despite most devices being pulsed, their OAM output has been assumed to be constant. OAM fluctuations, which are detrimental for applications, need to be quantified. Here, we fabricate quantum-dot microring lasers and demonstrate a simple measurement method that can straightforwardly determine the magnitude and sign of the OAM down to the level of individual laser pulses. We exploit a Fourier microscope with a cylindrical lens and then investigate three types of microring lasers: with circular symmetry, with "blazed" grating elements, and with unidirectional rotational modes. Our results confirm that previous measurement techniques obscured key details about the OAM generation. For example, while time-averaged OAM from our unidirectional laser is very similar to our blazed grating device, single-pulse measurements show that detrimental effects of mode competition are almost entirely suppressed in the former. Nevertheless, even in this case, the OAM output exhibits shot-to-shot fluctuations. Thus, our approach reveals important details in the underlying device operation that can aid in the improvement of micrometer-scale sources with pure OAM output.

High-speed Stokes vector receiver enabled by a spin-dependent optical grating

Stokes vector direct detection is a promising, cost-effective technology for short-distance communication applications. Here, we design and fabricate a spin-dependent liquid crystal grating to detect light polarization states. By separating the circular and linear components of incident light, the polarization states can be resolved with accuracy of up to 0.25°. We achieved Stokes vector direct detection of quadrature phase-shift keying (QPSK), 8PSK, and 16-ary quadrature amplitude modulation signals with 32, 16, and 16 GBd rates, respectively. We integrated the system, including the grating, photodetectors, and optical elements, on a miniaturized printed circuit board and demonstrated high-speed optical communications with 16 GBd rate QPSK signals.

Springtail coloration at a finer scale: mechanisms behind vibrant collembolan metallic colours.

The mechanisms and evolution of metallic structural colours are of both fundamental and applied interest, yet most work in arthropods has focused on derived butterflies and beetles with distinct hues. In particular, basal hexapods-groups with many scaled, metallic representatives-are currently poorly studied and controversial, with some recent studies suggesting either that thin-film (lamina thickness) or diffraction grating (longitudinal ridges, cross-ribs) elements produce these colours in early Lepidoptera and one springtail (Collembola) species. Especially the collembolan basal scale design, consisting of a single lamina and longitudinal ridges with smooth valleys lacking cross-ribs, makes them an interesting group to explore the mechanisms of metallic coloration. Using microspectroscopy, Raman spectroscopy, electron microscopy and finite-difference time-domain optical modelling, we investigated scale colour in seven springtail species that show clear metallic coloration. Reflectance spectra are largely uniform and exhibit a broadband metallic/golden coloration with peaks in the violet/blue region. Our simulations confirm the role of the longitudinal ridges, working in conjunction with thin-film effects to produce a broadband metallic coloration. Broadband coloration occurs through spatial colour mixing, which probably results from nanoscale variation in scale thickness and ridge height and distance. These results provide crucial insights into the colour production mechanisms in a basal scale design and highlight the need for further investigation of scaled, basal arthropods.

Анализ влияния быстрых и медленных мультипликативных помех на искажения диаграммы направленности системы "решетка-приемник".

The effect of fast and slow multiplicative noise on the distortion of the beam pattern of the «array-receiver» system in the conditions of the smoothing effect of the receiver and taking into account the time of relative lag from the grating elements was analyzed. Expressions are obtained for instantaneous beam patterns of the «array-receiver» system in the absence and presence of multiplicative noise. It is shown that in the case of action of multiplicative noise the beam pattern of the «array-receiver» system is obtained by averaging the instantaneous pattern on the grating correctness at a time interval equal to the signal duration. Influence of periodic multiplicative noise on distortion of beam pattern of system «array-receiver» is investigted. It is shown that for all signals except frequency-modulated, the beam pattern expression is maximized at the time t0=0. It is also shown that the beam pattern distortion of the «array-receiver» system when exposed to periodic multiplicative noise is less than for the separately considered phased antenna array. The degree of reduction of distortion from smoothing action of matched receiver filter is determined; and the distortion is less, the greater the ratio of the spectrum width of the noise modulation function to the signal spectrum width. Analysis of the influence of fluctuation multiplicative noise on the distortion of beam pattern of «array-receiver» system for the case of matching with signal of receiver filter is performed. The power-average beam pattern of the «array-receiver» system is determined through the energy spectrum of the noise modulation function. It is noted that multiplicative noise, all other things being equal, has less effect on the beam-receiver pattern of the system when using broadband pulse signals without intra-pulse modulation and at the same time resolution in range. It is noted that in case of deep phase distortions for gratings with reversible phases, the distortion of the beam pattern of the «array-receiver» system caused by multiplicative noise is significantly more than those distortions associated with the extremity of the signal spectrum width. The newly smoothing effect of the receiver reduces the expansion of the beam lobe caused by multiplicative noise. An expression is obtained for the average power of the beam pattern of the «array-receiver» system with normally distributed phase distortions of the signal. The effect of the signal duration on beam distortions caused by multiplicative noise was quantified.

Generation of focusing ring of metalens and its application in optical trapping of cold molecules

Metasurface can precisely control degrees of freedom of the phase, polarization, and amplitude of the incident light field. It provides a new way to develop the next generation of the experimental platform of quantum-state manipulation on-chip, which has important application prospects. This paper proposes a new type of metasurface structure, that is, a metalens composed of silicon grating elements with different duty ratios that can form a focusing ring on the focal plane. The intensity distribution of the ring light field in the focal plane and the focusing characteristics of metalens with different numerical apertures are studied. An optical storage ring of magnesium fluoride (MgF) molecule is constructed by using this kind of metalens focusing ring. The optical potential and dipole force of the MgF molecule in the focused light field are calculated, and the dynamic process of MgF molecule motion in the storage ring is simulated by the Monte-Carlo method. The research results show that for the incident light of 1064-nm radially polarized light, the designed metasurface structure has good focusing characteristics, and the light field intensity of the focusing ring is 55.1 times stronger than that of the incident light. The focal length of the annular light field is 22 μm and the full width at half maximum of the light intensity distribution in the focal plane is 0.8 μm, and the numerical aperture of the hyperlens is 0.69. The maximum dipole potential of MgF molecules in the light field is 32 μK, which can realize the loading of MgF molecules and trap them in the surface storage ring.

THE RESEARCH OF THE RAILWAY RAIL FOR ANALYSIS OF SURFACE INITIATED ROLLING CONTACT FATIGUE CRACKS

Problems. Under the influence of dynamic loading from trains and natural conditions, the railway track deteriorates, characterized by the appearance of defects and residual deformations of the railway track, which lead to accelerated wear and malfunction of other elements of the rail. The appearance of the specified defects, failures and destructions of the elements of the rail grating is associated with the action of additional local stresses arising in the rails of the railway track. On a low-quality track grating with deviations of the parameters of the rail track, when the train is moving, there is an unaccounted interaction of the track and rolling stock with local overload of the interaction elements along the path. As a rule, the unaccounted interaction of track and rolling stock is accompanied by the impact of the wheel and the rail with overloading the contact area. When the impact of the wheel and rail strikes, stresses exceeding the endurance limit occur in the rail head. In this case, the rails are born in the head and begin to develop defects of contact-fatigue origin in the form of horizontal vertical and transverse cracks.The aim of the study. Conducting practical studies of the origin and development of contact-fatigue cracks by wear of the rails in the laboratory.Methods of implementation. An experimental unit was designed and manufactured to investigate the rails for contact damage. The unit is mounted on a mechanical single-cylinder press of the K 2322 model with a nominal force of 16 tons at a nominal stroke frequency of 120 per minute. The bulk of the experimental setup contains a wheel-rail friction unit. The force of clamping the rail to the wheel. Provides a hydraulic system. For testing, a test program was developed, which provides for two-hour tests, after which tests for the presence of defects (micro cracks) on the rail are conducted. Research on the presence of defects is carried out by the method of magnetic powder non-destructive testing.Research results. Studies have shown that contact-fatigue cracks in the rail head occurred after approximately 200 thousand load cycles. This generally coincides with the results presented in other papers, which show the occurrence of similar damage after 190-290 thousand cycles, depending on the properties of rail steel (hardness, surface roughness, etc.).Conclusions. The main cause of the origin and development of contact fatigue defects in the rail head is insufficient contact fatigue strength of the rail steel. For carrying out practical researches in laboratory conditions the methodology was developed, the experimental unit was constructed and made. Experimental studies have shown that the occurrence of defects on the surface of the rail is observed after 200-250 thousand load cycles. With the further increase in the number of load cycles, there is a rapid development of existing contact-fatigue cracks and the formation of new cracks in the contact zone.

Frequency-doubled vortex beam emitter based on nonlinear Cherenkov radiation

In this work, we propose a new scheme to generate frequency-doubled vortex beams from a radially poled LiNbO3 micro-ring resonator based on nonlinear Cherenkov radiation. The near-infrared fundamental wave is resonant in the micro-ring, while the second harmonic is emitted from the resonator along the Cherenkov phase-matching direction. The topological charge of the emitted second-harmonic vortex beam is determined by both the azimuthal order of the whispering galley modes and the number of nonlinear grating elements. The field distribution and the conversion efficiency of the emitted vortex beam are investigated.

The Role of Particle Size in the Dispersion Engineering of Plasmonic Arrays

Grazing diffraction orders on metal gratings give rise to peculiar optical effects that were contemplated by Wood, Rayleigh and Fano. With plasmonic nanoparticles as resonant grating elements, the phenomenology of such surface lattice resonances becomes quite rich, including spectrally narrow extinction peaks and optical band gap formation. It has been observed that at perpendicular incidence either the higher or lower energy branch corresponding to the first grazing diffraction orders is bright, i.e., couples strongly to light. Reviewing the literature, it appears that particle size is the factor determining which dispersion branch lights up. However, a consistent explanation for this effect is lacking. After revisiting the effect experimentally and by numerical simulation, we clarify the underlying physics by analyzing nanoparticle gratings in terms of, first, an oscillator model and, second, a photonic crystal description. Both approaches reveal the central role of a particle-size-dependent phase shift...

Investigating the Performance of Metal-Assisted Guided Mode Resonance Based Structures for Biosensing Applications

The sensing abilities of a metal-assisted guided mode resonance (MAGMR)-based structure are compared with that of a conventional guided mode resonance (GMR)-type structure operating at the same resonance wavelength. By studying the electric field intensities at resonance, it has been found that the MAGMR sensor offers a superior and more uniform sensitivity over the entire sensor surface as compared to the GMR sensors, where the sensing takes place primarily at the trenches between the grating elements. The MAGMR sensor also shows an asymmetric field profile with 44.96% of the total field extending into the sensing region at resonance in comparison with that value of 25.62% for the GMR sensor. The bulk and surface sensitivity values have also been evaluated for a range of refractive indices and analyte thicknesses, and it has been observed that the MAGMR sensor offers superior sensitivity values over the GMR sensor for all the considered cases. The limit of detection has then been calculated for both the structures, where the MAGMR structure shows a 9-fold improvement in comparison with the GMR structure for an identical readout scheme.

Multichannel remote polarization control enabled by nanostructured liquid crystalline networks

In this article, we demonstrate that a grating fabricated through nanoscale volumetric cross-linking of a liquid crystalline polymer enables remote polarization control over the diffracted channels. This functionality is a consequence of the responsivity of liquid crystal networks upon light stimuli. Tuning of the photonic response of the device is achieved thanks to both a refractive index and a shape change of the grating elements induced by a molecular rearrangement under irradiation. In particular, the material anisotropy allows for nontrivial polarization state management over multiple beams. The absence of any liquid component and a time response down to 0.2 ms make our device appealing in the fields of polarimetry and optical communications.

Near-infrared bilayer nanowire grid polarizer array fabricated using soft nanoimprint lithography

We fabricated a cost-effective and tractable near-infrared nanowire grid polarizer array with six different directional grating elements on a Borofloat 33 glass substrate at once using a soft nanoimprint and metal thermal evaporation process. Each element consists of a 200-nm period bilayer Al grating with an area of 1.3 × 1.3 mm2. Scanning electron microscopy images reveal that each unit of the polarizer array has an intact nanostructure without large-area damage. Meanwhile, the TM transmittance of each element exceeds 65% in the wavelength range of 1 to 2.5 μm, and of particular note, it is >70 % in the wavelength range of 1.2 to 2.5 μm. The extinction ratio is more than 20 dB in 1.38- to 2.5-μm wavelength range, proving that the polarizer array has good polarization characteristics. Such a polarizer array has low costs and better compatibility with microfabrication processes, which can be used as an element of near-infrared polarization image detection equipment.

Strong and weak couplings in molecular vibration–plasmon hybrid structures

Molecular vibration-plasmon couplings in a hybrid structure, which are composed of a silver grating filled with polymethyl methacrylate (PMMA) molecules (SG-PMMA), have been investigated theoretically. It is found that the interaction between the vibrational transitions and plasmons can transform from weak coupling into strong coupling by reducing the distance between the elements. When the space between grating elements is large, the localized surface plasmon resonance (LSP) of the silver elements greatly enhances the absorption of the PMMA molecules. As the gap between elements becomes small, the plasmonic nanocavity (NC) mode emerges and couples strongly with the molecular vibrational mode of PMMA. The strong coupling results in two new hybridized modes and the Rabi splitting energy is about 15 meV. Our work provides an effective way to alter the coupling strength of the molecular vibration-plasmon hybrid system and may be beneficial to the further biochemical and biophysical applications.

Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides.

Subwavelength grating (SWG) structures are an essential tool in silicon photonics, enabling the synthesis of metamaterials with a controllable refractive index. Here we propose, for the first time to the best of our knowledge, tilting the grating elements to gain control over the anisotropy of the metamaterial. Rigorous finite difference time domain simulations demonstrate that a 45° tilt results in an effective index variation on the fundamental TE mode of 0.23 refractive index units, whereas the change in the TM mode is 20 times smaller. Our simulation predictions are corroborated by experimental results. We furthermore propose an accurate theoretical model for designing tilted SWG structures based on rotated uniaxial crystals that is functional over a wide wavelength range and for both the fundamental and higher order modes. The proposed control over anisotropy opens promising venues in polarization management devices and transformation optics in silicon photonics.