Grating Elements Research Articles

Advanced experimental applications for x-ray transmission gratings spectroscopy using a novel grating fabrication method

A novel fabrication method for soft x-ray transmission grating and other optical elements is presented. The method uses focused-ion-beam technology to fabricate high-quality free standing grating bars on transmission electron microscopy grids. High quality transmission gratings are obtained with superb accuracy and versatility. Using these gratings and back-illuminated CCD camera, absolutely calibrated x-ray spectra can be acquired for soft x-ray source diagnostics in the 100-3000 eV spectral range. Double grating combinations of identical or different parameters are easily fabricated, allowing advanced one-shot application of transmission grating spectroscopy. These applications include spectroscopy with different spectral resolutions, bandwidths, dynamic ranges, and may serve for identification of high-order contribution, and spectral calibrations of various x-ray optical elements.

Combined plasmonic gratings in organic solar cells

We propose an organic solar cell structure with combined silver gratings consisting of both a front and a back grating. This combination provides multiple, semi-independent enhancement mechanisms which act additively, so that a broadband absorption is obtained. Both gratings couple the incident light into various plasmonic modes, showing a more localized or propagating character respectively. In addition, some modes only appear for tilted incident light, and therefore present a complex angle-dependent behavior. We provide extensive numerical simulations, resulting in an optimized period of 490 nm, with front grating elements of 60 by 10nm and back elements of 60 by 30 nm. With these parameters an integrated absorption enhancement factor around 1.35 is observed, with absorption increasing from 48% to 65% under TM polarized light. In addition, the solar cell with combined gratings is much less sensitive to the angle of incident light than the single grating cases. Furthermore, the grating structure does not have a large influence on the TE polarized light absorption.

Analysis of diffraction of a plane wave on a grating consisting of impedance bodies of revolution

The problem of wave scattering by a grating consisting of coaxial impedance bodies of revolution is solved. The efficient numerical algorithm based on the modified null field method is offered. The method is applied both to scalar and vector formulations of the problem. The numerical results are obtained for various geometries of the grating elements.

Optimization of Zero-Reference Grating Considering Tip Distortions for Scanning Probe Microscopy Drift Measurement

A nonperiodic grating composed of unevenly distributed step elements on a flat substrate can serve as the calibration grating for scanning probe microscopy (SPM) drift measurements. The grating named zero-reference grating (ZRG) is characterized by the number of step elements t, the number of total grating elements n and the maximum correlation peak contrast D. To optimize the ZRG structures considering tip distortions in SPM images, mathematical morphology and genetic algorithms are adopted. Results show that the optimal step elements tend to distribute orderly under the influence of large tip radius and the contrast D decreases with the increase of tip radius when t < or = n/2. On the contrary, the optimal ZRG structures and the contrast D are insensitive to tip distortions when t > n/2. These differences can be interpreted by the effective code mechanism due to tip dilation. For quantitative drift measurements, the parameter t is suggested to be larger than n/2 since the performances of such optimal ZRGs are more robust to tip distortions.

A new polarimeter based on optical non-reciprocity in gratings with two-dimensional chirality

We have studied the optical transmission properties of a number of different dielectric diffraction gratings that each exhibit two-dimensional chirality in their grating elements. We have found that the diffracted beams transmitted through these structures exhibit large anisotropic polarization changes and polarization-dependent intensity modulations even though the chiral elements are arranged in square grating arrays. The strength and functional form of both of these effects differ for each diffracted beam. They are also dependent on the chirality of the surface patterning and the thickness of the patterned dielectric film. We show how these properties could be exploited to provide a new and more versatile form of polarimeter, and how the operation of such a device is inherently dependent on the optical non-reciprocity of these chiral gratings.

Decoding of coherent but not incoherent motion signals in early dorsal visual cortex

When several scattered grating elements are arranged in such a way that their directions of motion are consistent with a common path, observers perceive them as belonging to a globally coherent moving object. Here we investigated how this coherence changes the representation of motion signals in human visual cortex using functional magnetic resonance imaging (fMRI) and multivariate voxel pattern decoding, which have the potential to reveal how well a stimulus is encoded in different contexts. Only during globally coherent motion was it possible to reliably distinguish fMRI signals evoked by different directions of motion in early visual cortex. This effect was specific to the retinotopic representation of the visual field quadrant in V1 traversed by the coherent element path and could not simply be attributed to a general increase in signal strength. Decoding was more reliable for cortical areas corresponding to the lower visual field. Because some previous studies observed poorer speed discrimination when motion was grouped, we also conducted behavioural experiments to investigate this with our stimuli, but did not reveal a consistent relationship between coherence and perceived speed. Taken together, these data show that neuronal populations in early visual cortex represent information that could be used for interpreting motion signals as unified objects.

Lipid multilayer gratings

The interaction of electromagnetic waves with matter can be controlled by structuring the matter on the scale of the wavelength of light, and various photonic components have been made by structuring materials using top-down or bottom-up approaches. Dip-pen nanolithography is a scanning-probe-based fabrication technique that can be used to deposit materials on surfaces with high resolution and, when carried out in parallel, with high throughput. Here, we show that lyotropic optical diffraction gratings--composed of biofunctional lipid multilayers with controllable heights between approximately 5 and 100 nm--can be fabricated by lipid dip-pen nanolithography. Multiple materials can be simultaneously written into arbitrary patterns on pre-structured surfaces to generate complex structures and devices, allowing nanostructures to be interfaced by combinations of top-down and bottom-up fabrication methods. We also show that fluid and biocompatible lipid multilayer gratings allow label-free and specific detection of lipid-protein interactions in solution. This biosensing capability takes advantage of the adhesion properties of the phospholipid superstructures and the changes in the size and shape of the grating elements that take place in response to analyte binding.

Nonreciprocal amplitude-frequency resonant response of metasandwiches “ferrite plate-grating of resonant elements”

New microwave nonreciprocal properties are investigated in “ferrite plate – grating of resonant elements” metasandwiches arranged along the axis of a rectangular waveguide in a transverse constant magnetic field. Giant nonreciprocity in the transmission is observed at the ferromagnetic resonance frequencies at certain values of the magnetic field under conditions of a mutual influence between the ferromagnetic and the grating resonances. In addition, nonreciprocal splitting of the resonance in grating elements is observed under small magnetic field, which is much less than the field necessary to the ferromagnetic resonance excitation. The nonreciprocal transmission does not take place in the case of free ferrite in the absence of a grating. Sign reversal of the nonreciprocity is observed, when ferrite transfers to the opposite side of a grating as well as under certain values of the constant magnetic field, when the sign reversal of difference between frequencies of the ferromagnetic resonance and the grating resonance takes place. Nonreciprocal effects are explained by the interaction between precessing spins in ferrite and a magnetic field of the surface wave, formed by a grating, and by coupling between the resonances of grating elements. It has been shown theoretically that microwaves in waveguide with bianisotropic layer, simulating a grating of resonant elements, are elliptically or circularly polarized with frequency and spatially – dependent rotating sense of the microwave magnetic field. The nonreciprocal effects have been observed for different grating elements: for both electric dipoles and chiral elements.

Two dimensional polymer-embedded quasi-distributed FBG pressure sensor for biomedical applications

We report on the development of a flexible 2D optical fiber-based pressure sensing surface suitable for biomedical applications. The sensor comprises of highly-sensitive Fiber Bragg Grating elements embedded in a thin polymer sheet to form a 2x2 cm(2) sensing pad with a minimal thickness of 2.5mm, while it is easily expandable in order to be used as a building block for larger surface sensors. The fabricated pad sensor was combined with a low physical dimension commercially available interrogation unit to enhance the portability features of the complete sensing system. Sensor mechanical properties allow for matching human skin behavior, while its operational performance exhibited a maximum fractional pressure sensitivity of 12 MPa(-1) with a spatial resolution of 1x1cm(2) and demonstrated no hysteresis and real time operation. These attractive operational and mechanical properties meet the requirements of various biomedical applications with respect to human skin pressure measurements, including amputee sockets, shoe sensors, wearable sensors, wheelchair seating-system sensors, hospital-bed monitoring sensors.

Engineering surface plasmon grating couplers through computer simulation

Surface plasmon grating couplers are optimized by separately characterizing the collection efficiency, rescattering, and transmission effects of isolated grating elements with finite difference time domain methods and then using signal flow graph methods to assess the performance of arrays of N identical elements. Small bars, ridges, and trenches on silver at a wavelength of 700nm are shown to have different coupling patterns and efficiencies from near zero to the physical width of the element. The overall efficiency requires a suitable trade-off of coupling and surface wave transmission and an example structure exhibiting an equivalent 100% capture length over ten wavelengths is shown.

Transmission and directionality control of light emission from a nanoslit in metallic film flanked by periodic gratings

We investigate the transmission and directional properties of light emission from a nanoslit film flanked by periodic gratings, formed on a silver film, embedded in a high index dielectric medium. Using FDTD calculations it is demonstrated that the transmission has strong dependence on dimension of the dielectric film. The directional property of the emitted beam is controlled by tapering height of output grating elements and this effect is explained by standard theory of antenna array. We propose and examine the potential of such slit grating structure as a Plasmonic antenna in axial injection of light from single mode fiber to photonic crystal waveguide.

Grating coupling elements for high-Q resonators and rectangular waveguides of the millimeter wave band

Techniques for electrodynamic-level calculation of slot-grating wideband coupling elements for single-mode waveguides are developed. New coupling devices that ensure the obtaining of a voltage standing-wave ratio of 1.6 in the entire tuning range of a resonator without readjustment of the coupling element are designed and tested experimentally. It is found that application of new coupling devices in resonators do not deteriorate the loaded Q factor of the resonator.

Optical filter based on subtractive dispersion planar reflective gratings

We have successfully designed, fabricated, and tested an optical filter based on cascaded planar reflective gratings. The device uses a combination of two grating elements arranged in a subtractive dispersion configuration. The first grating demultiplexes a 300 nm wide band and drops optical channels at 1490 and 1550 nm, commonly used in passive optical networks. The second grating completely counter-balances the dispersion properties of the first grating and ultimately yields zero dispersion in the output waveguide. Such a configuration allows the transmission of optical signals though the device in an ultra-wide band spanning 1250–1410 nm. The filter was manufactured using an industry standard silica-on-silicon process which was augmented with grating facet formation and metallization. In spite of using low refractive index contrast waveguides (0.82%), the device had a remarkably low footprint of only 0.21 cm 2. Applications of the device in passive optical networks are discussed.

Optical fiber-fault surveillance for passive optical networks in S-band operation window

An S-band (1470 to 1520 nm) fiber laser scheme, which uses multiple fiber Bragg grating (FBG) elements as feedback elements on each passive branch, is proposed and described for in-service fault identification in passive optical networks (PONs). By tuning a wavelength selective filter located within the laser cavity over a gain bandwidth, the fiber-fault of each branch can be monitored without affecting the in-service channels. In our experiment, an S-band four-branch monitoring tree-structured PON system is demonstrated and investigated experimentally.

Analysis of directional grating-coupled radiation in waveguide structures

Radiation scattered from diffraction gratings on the surface of waveguides is analyzed using the volume current method. This technique allows the interpretation of the grating elements as scattering centers of the underlying waveguide mode, and radiating sources for the diffracted fields. The framework allows separation of the effects of the grating array global periodicity and the effects of the specific shape of the individual grating elements. A straightforward analogy between the effects of the grating element shape and the behavior of phased-antenna array systems allows a clear and intuitive understanding of the effects of blazed gratings on the directionality of grating-coupled radiation. The analysis is applied to some specific geometries.

Experimental demonstration of a laterally deformable optical nanoelectromechanical system grating transducer.

We experimentally demonstrate operation of a laterally deformable optical nanoelectromechanical system grating transducer. The device is fabricated in amorphous diamond with standard lithographic techniques. For small changes in the spacing of the subwavelength grating elements, lossy propagating resonant modes in the plane of the grating cause a large change in the optical reflection amplitude. An in-plane motion detection sensitivity of 160 fm/square root(Hz) was measured, exceeding that of any other optical microelectromechanical system transducer to our knowledge. Calculations predict that this sensitivity could be improved to better than 40 fm/square root(Hz) in future designs. In addition to having applications in the field of inertial sensors, this device could also be used as an optical modulator.

High-resolution pattern generation using the epoxy novolak SU-8 2000 resist by electron beam lithography

We report the fabrication of high-resolution sub 50 nm patterns by electron beam lithography using the epoxy novolak SU-8 2000 resist formulation. The minimum linewidth achieved is on the order of 30 nm and corresponds to a threefold reduction in minimum linewidth over previous reports describing similar resist chemistries. Our results also show that it is possible to fabricate dense linear grating elements without proximity correction. The dry etch resistance of native SU-8 2000 was found to be nearly twice that of poly(methylmethacrylate), making it ideal for applications that require pattern transfer. These studies are intended to explore the feasibility of SU-8 2000 as an electron beam resist for pattern generation on length scales below 50 nm.

Spatial aspects of object formation revealed by a new illusion, shine-through

When a vernier stimulus is presented for a short time and followed by a grating comprising five straight lines, the vernier remains invisible but may bequeath its offset to the grating (feature inheritance). For more than seven grating elements, the vernier is rendered visible as a shine-through element. However, shine-through depends strongly on the spatio-temporal layout of the grating. Here, we show that spatially inhomogeneous gratings diminish shine-through and vernier discrimination. Even subtle deviations, in the range of a few minutes of arc, matter. However, longer presentation times of the vernier regenerate shine-through. Feature inheritance and shine-through may become a useful tool in investigating such different topics as time course of information processing, feature binding, attention, and masking.

Shine-through: temporal aspects

If a vernier stimulus precedes a grating for a very short time, the vernier either remains invisible, but may bequeath some of its properties to the grating (feature inheritance), or might shine through keeping its features — depending on the number of grating elements [Herzog, M. H. & Koch, C., 2001. Seeing properities of an invisible element: feature inheritance and shine-through. Proceedings of the Natlional Academy of Science USA 98, 4271–4275]. Feature inheritance and shine-through represent two different states of feature binding [Herzog, M. H., Koch, C., & Fahle, M., Switching binding states. Visual Cognition (in press)], whereas shine-through depends in subtle ways on the spatial layout of the grating [Herzog, M. H., Fahle, M., & Koch, C., (2001). Spatial aspects of object formation revealed by a new illusion, shine-through Vision Research]. Here, we show that also temporal parameters of the grating influence shine-through. For example, a delayed presentation of certain grating elements can deteriorate performance dramatically.

Microstrip antenna coupling for quantum-well infrared photodetectors

We computed the expected performance of microstrip antennas as optical couplers in quantum-well infrared photodetectors (QWIPs) by the finite-difference time-domain technique. The microstrip antenna consisted of a metal square that is placed on one side of a thin QW stack, across from a continuous ground plane on the other side of the stack. The size of the square was one-half wavelength in the QW material, or ∼1.2 μm for a long-wavelength GaAs/AlGaAs QWIP. An array of these antennas can be used to couple to an arbitrary-size pixel. Unlike the elements of a diffraction grating, each antenna in the array acts as a nearly independent coupler whose peak wavelength is determined by the dimensions of the antenna, not by the period of the antenna array. Because of this local coupling, the array of microstrip antennas maintains absorption in small pixels much more effectively than do grating couplers. Furthermore, the microstrip antenna yields moderately high absorption over a useful spectral bandwidth with a very thin QW stack. Such a stack has a high photoconductive gain, which is useful for many applications.