Thin Stripes Research Articles

All electrical propagating spin wave spectroscopy with broadband wavevector capability

We developed an all electrical experiment to perform the broadband phase-resolved spectroscopy of propagating spin waves in micrometer sized thin magnetic stripes. The magnetostatic surface spin waves are excited and detected by scaled down to 125 nm wide inductive antennas, which award ultra broadband wavevector capability. The wavevector selection can be done by applying an excitation frequency above the ferromagnetic resonance. Wavevector demultiplexing is done at the spin wave detector thanks to the rotation of the spin wave phase upon propagation. A simple model accounts for the main features of the apparatus transfer functions. Our approach opens an avenue for the all electrical study of wavevector-dependent spin wave properties including dispersion spectra or non-reciprocal propagation.

UV-Written Long-Period Grating Based on Long-Range Surface Plasmon-Polariton Waveguide

This letter proposes a long-period grating based on coupling between the long-range surface plasmon mode and a cladding mode of a thin Au film stripe fully buried in polymer SU-8. Periodical surface corrugation and accompanied refractive index variation on the SU-8 upper-cladding were induced by ultraviolet-written technique. This configuration along the Au stripe gives rise to the observation of strong resonance in a 3-mm long periodic grating. A rejection band with a contrast of −23.5 dB at the wavelength 1561 nm was achieved, which can be tuned by $\sim 30$ nm with a temperature change of 6 °C. The grating offers potential applications in surface plasmon waveguide circuits and biosensors.

Nematoelastic crawlers.

A propagating "beam" triggering a local phase transition in a nematic elastomer sets it into a crawling motion, which may morph due to buckling. We consider the motion of the various configurations of slender rods and thin stripes with both uniform and splayed nematic order in cross-section and detect the dependence of the gait and speed on flexural rigidity and substrate friction.

Spatial structure of neuronal receptive field in awake monkey secondary visual cortex (V2)

Visual processing depends critically on the receptive field (RF) properties of visual neurons. However, comprehensive characterization of RFs beyond the primary visual cortex (V1) remains a challenge. Here we report fine RF structures in secondary visual cortex (V2) of awake macaque monkeys, identified through a projection pursuit regression analysis of neuronal responses to natural images. We found that V2 RFs could be broadly classified as V1-like (typical Gabor-shaped subunits), ultralong (subunits with high aspect ratios), or complex-shaped (subunits with multiple oriented components). Furthermore, single-unit recordings from functional domains identified by intrinsic optical imaging showed that neurons with ultralong RFs were primarily localized within pale stripes, whereas neurons with complex-shaped RFs were more concentrated in thin stripes. Thus, by combining single-unit recording with optical imaging and a computational approach, we identified RF subunits underlying spatial feature selectivity of V2 neurons and demonstrated the functional organization of these RF properties.

Cortical Dynamics of Figure-Ground Separation in Response to 2D Pictures and 3D Scenes: How V2 Combines Border Ownership, Stereoscopic Cues, and Gestalt Grouping Rules.

The FACADE model, and its laminar cortical realization and extension in the 3D LAMINART model, have explained, simulated, and predicted many perceptual and neurobiological data about how the visual cortex carries out 3D vision and figure-ground perception, and how these cortical mechanisms enable 2D pictures to generate 3D percepts of occluding and occluded objects. In particular, these models have proposed how border ownership occurs, but have not yet explicitly explained the correlation between multiple properties of border ownership neurons in cortical area V2 that were reported in a remarkable series of neurophysiological experiments by von der Heydt and his colleagues; namely, border ownership, contrast preference, binocular stereoscopic information, selectivity for side-of-figure, Gestalt rules, and strength of attentional modulation, as well as the time course during which such properties arise. This article shows how, by combining 3D LAMINART properties that were discovered in two parallel streams of research, a unified explanation of these properties emerges. This explanation proposes, moreover, how these properties contribute to the generation of consciously seen 3D surfaces. The first research stream models how processes like 3D boundary grouping and surface filling-in interact in multiple stages within and between the V1 interblob—V2 interstripe—V4 cortical stream and the V1 blob—V2 thin stripe—V4 cortical stream, respectively. Of particular importance for understanding figure-ground separation is how these cortical interactions convert computationally complementary boundary and surface mechanisms into a consistent conscious percept, including the critical use of surface contour feedback signals from surface representations in V2 thin stripes to boundary representations in V2 interstripes. Remarkably, key figure-ground properties emerge from these feedback interactions. The second research stream shows how cells that compute absolute disparity in cortical area V1 are transformed into cells that compute relative disparity in cortical area V2. Relative disparity is a more invariant measure of an object's depth and 3D shape, and is sensitive to figure-ground properties.

Bright single-mode random laser from a concentrated solution of π-conjugated polymers.

Using thin stripe excitation of a 10 ns pulsed laser, we observed robust and bright random laser (RL) emission in high concentrated solutions of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). In particular, within the proper excitation intensity range, single line RL emission is observed in both solutions, with full width at half-maximum of 0.17 nm and 0.4 nm for MEH-PPV and PCPDTBT, respectively. The reason for the random laser is that the refractive index fluctuation due to the aggregation in concentrated solution results in spatially extended random lasing modes, which are amplified through traveling light along the interface between solution and cuvette. Our work provides a simple but effective method to achieve bright single-mode RLs, with conversion efficiency on the order of 10%.

Inorganic alkali lead iodide semiconducting APbI<sub>3</sub> (A = Li, Na, K, Cs) and NH<sub>4</sub>PbI<sub>3</sub> films prepared from solution: Structure, morphology, and electronic structure

APbI3 alkali lead iodides were prepared from aqueous (A= Na, Cs, ammonium NH4+, and methyl­ammonium CH3NH3+) and acetone (A= Li, K) solutions by a self-organization low temperature process. Diffraction analysis revealed that the methylammonium-containing system (MAPbI3) crystallizes into a tetragonal perovskite structure, whereas the alkali and NH4+ systems adopt orthorhombic structures. Morphological inspection confirmed the influence of the cation on the growth mechanism: for A = Cs and NH4+, needle-like crystallites with lengths up to 3–4 mm; for A = K, thin stripes with lengths up to 5–6 mm; and for A = MA+, dodecahedral crystallites were observed. For A = Li and Na, the APbI3 systems typically resulted in polycrystalline aggregates. Optical absorption measurements demonstrated large energy band gaps for the alkali and ammonium systems with values between 2.19 and 2.40 eV. For electronic and chemical characterization by photoelectron spectroscopy, the as-prepared powders were dissolved in di-methylformamide and re-crystallized as thin films on F:SnO2 substrates by spin-coating. The binding energy differences between Pb4f and I3d core levels are highly similar in the investigated systems and close to the value measured for PbI2, indicating similar relative partial charges and formal oxidation states. The binding energies of the alkali ions are in accordance with oxidation state +1. The X-ray excited valence band spectra of the investigated APbI3 systems exhibited similar line shapes in the region between the valence band maximum and 4.5 eV higher binding energy due to common PbI6 octahedra which dominate the electronic structure. While the ionization energy values are quite similar (6.15 ± 0.25 eV), the Fermi-level positions of the unintentionally doped materials vary for different cations and different batches of the same material, which indicates that the position of the Fermi level can be influenced by changing the process parameters.

Length scale of interface heterogeneities selects propagation mechanism of frictional slip fronts

We present three-dimensional finite-element simulations showing the propagation of slip fronts at striped heterogeneous interfaces. The heterogeneous area consists of alternating stripes of weaker and stronger frictional properties, which is equivalent to a lower and higher fracture energy, respectively. By comparing the slip front propagation at interfaces that differ solely by the length scale of the heterogeneous pattern, we illustrate that two different propagation regimes exist. Interfaces with wide stripes present slip fronts with propagation speeds that transition from sub-Rayleigh to inter-sonic. Thinner stripes are, however, characterized by the propagation of sub-Rayleigh slip fronts, which are preceded by slip pulses of negligible slip in the weaker stripes. From a macroscopic point of view, an interface with a smaller heterogeneous pattern appears to be stronger than the equivalent coarser interface even though both have the same average properties. The numerical results as well as a theoretical approach based on fracture-mechanics considerations suggest that the origin of these two distinct propagation mechanisms lies in the interaction between the length scales of the cohesive zone and the heterogeneous configuration. We further show by estimating the relevant length scales that the occurring propagation mechanism is influenced by the friction weakening rate of the interface as well as the shear modulus of the bulk material.

Femtosecond laser pulse shaping at megahertz rate via a digital micromirror device.

In this Letter, we present a scanner and digital micromirror device (DMD)-based ultrafast pulse shaper, i.e., S-DUPS, for programmable ultrafast pulse modulation, achieving a shaping rate of 2 MHz. To our knowledge, the S-DUPS is the fastest programmable pulse shaper reported to date. In the S-DUPS, the frequency spectrum of the input pulsed laser is first spread horizontally, and then mapped to a thin stripe on the DMD programmed with phase modulation patterns. A galvanometric scanner, synchronized with the DMD, subsequently scans the spectrum vertically on the DMD to achieve a shaping rate up to 10 s MHz. A grating pair and a cylindrical lens in front of the DMD compensate for the temporal and spatial dispersion of the system. To verify the concept, experiments were conducted with the DMD and the galvanometric scanner operated at 2 kHz and 1 kHz, respectively, achieving a 2 MHz speed for continuous group velocity dispersion tuning, as well as 2% efficiency. Up to 5% efficiency of S-DUPS can be expected with high efficiency gratings and optical components of proper coatings.

Fluorinated photopolymer thermo-optic switch arrays with dielectric-loaded surface plasmon polariton waveguide structure

Novel polymer thermo-optic switch arrays were successfully designed and fabricated based on dielectric-loaded surface plasmon polariton waveguide (DLSPPW) structure. Highly fluorinated low-loss photopolymers and organic–inorganic grafting materials were used as the waveguide core and cladding, respectively. The low absorption loss characteristics and excellent thermal stabilities of the core and cladding materials were obtained. The proposed DLSPPW model was included of fluorinated polymer ridge with 4 × 4 μm2 size loaded on 60-nm thin gold stripe electrode heaters, organic-inorganic grafting material cladding and PMMA substrate. The operation of the device at signal wavelengths is controlled via the thermo-optic effect by electrically heating the gold stripes of dielectric-loaded surface plasmon polariton waveguides. The optimized structural properties of dielectric-loaded surface plasmon polariton waveguides were provided. The propagation loss of a 4-μm wide straight DLSPPW was measured as 0.55 dB∕cm at 1550 nm wavelength. The insertion loss of the device was measured to be about 4.5 dB. The switching rise and fall time of the device applied by 200 Hz square-wave voltage were obtained as 287.5 μs and 370.2 μs, respectively. The switching power was about 5.6 mW, and the extinction ratio was about 13.5 dB. The flexible low-loss multi-functional waveguide switch arrays are suitable for realizing large-scale optoelectronic integrated circuits.

Single-mode lasers and parity-time symmetry broken gratings based on active dielectric-loaded long-range surface plasmon polariton waveguides.

Single-mode distributed feedback laser structures and parity-time symmetry broken grating structures based on dielectric-loaded long-range surface plasmon polariton waveguides are proposed. The structures comprise a thin Ag stripe on an active polymer bottom cladding with an active polymer ridge. The active polymer assumed is PMMA doped with IR140 dye providing optical gain at near infrared wavelengths. Cutoff top ridge dimensions (thickness and width) are calculated using a finite element method and selected to guarantee single-mode operation of the laser. Several parameters such as the threshold number of periods and the lasing wavelength are determined using the transfer matrix method. A related structure based on two pairs of waveguides of two widths, which have the same imaginary part but different real part of effective index, arranged within one grating period, is proposed as an active grating operating at the threshold for parity-time symmetry breaking (i.e., operating at an exceptional point). Such "exceptional point" gratings produce ideal reflectance asymmetry as demonstrated via transfer matrix computations.

Discrete decreased activity in the lower thoracic spine on FDG PET/CT: another respiration-related artifact.

We observed that the diaphragmatic motion causing the well-known, so-called banana artifact on PET/CT images may additionally cause decreased FDG activity involving 1 or 2 lower thoracic vertebrae. The aim of this study was (1) to formally evaluate if this observation is indeed true and, if so, (2) to assess its frequency. One hundred fifty-four PET/CT scans (81 males and 73 females, age 61 ± 13 years) were analyzed. On the coronal CT images, the vertebra above where the hemi-diaphragmatic surfaces abut the vertebral column was defined as the target vertebra (TV). On sagittal PET images, FDG activity in TV was visually and quantitatively assessed. Then, the severity of banana artifact on coronal PET images was graded from 0 to 2: 0 if there was no artifact, 1 if a thin photopenic stripe, and 2 if a thick banana-shaped photopenic artifact in the vicinity of diaphragm. Visually decreased activity in TV was present in 0% (0/93), 3% (1/34), and 59% (16/27) of scans graded as 0, 1, and 2, respectively (grade 0 and/or 1 vs. grade 2, P < 0.001). The SUV ratio of TV to reference vertebrae were 0.99 ± 0.14, 0.99 ± 0.15, and 0.84 ± 0.19 in grade 0, 1, and 2 scans, respectively (P < 0.001). Error of attenuation correction from respiratory motion can lead to underestimation of FDG activity in the lower thoracic spine, and this should not be interpreted as possible marrow replacement process requiring further imaging if there is coexisting banana artifact.

Development of in-situ micro-debris measurement system

The in-situ debris environment awareness system has been developed. The objective of the system is to measure small debris (between 100μm and several cm) in orbit. The orbital distribution and the size distribution of the debris are not well understood. The size distribution is difficult to measure from the ground, although the size distribution is very important for the risk evaluation of the impact of debris on spacecraft. The in-situ measurement of the size distribution is useful for: (1) verification of meteoroid and debris environment models, (2) verification of meteoroid and debris environment evolution models, (3) real time detection of unexpected events, such as explosions and/or collisions on an orbit. This paper reports the development study of the in-situ debris measurement system and shows demonstration experiments and their results to describe the performance of the micro-debris sensor system. The sensor system for monitoring micro-debris with sizes ranging from 100μm to a few mm must have a large detection area, while the constraints of space deployment require that these systems be low in mass, low in power, robust and have low telemetry requirements. For this reason, we have been developing a simple trans-film sensor. Thin and conductive stripes (copper) are formed with fine pitch (100μm) on a thin film of nonconductive material (12.5-μm thick polyimide). A hypervelocity micro-particle impact is detected when one or more stripes are severed by perforation of the film. We designed a debris detector specialized for measuring the micro-debris size and collision rate. We then manufactured and calibrated the detector.

Self-healing patterns in ferromagnetic-superconducting hybrids

We study magnetic flux dynamic effects in a superconducting (SC) bridge with thin soft magnetic stripes placed either on top or under the bridge. Voltage–current (VI) measurements reveal that the edges of magnetic stripes oriented transvers or along the bridge introduce channels or barriers for vortex motion, resulting in the decrease or increase of the critical current, respectively. We propose a self-healing mechanism for the hybrid with longitudinal stripes whereby the magnetic pinning strength increases with current. The self-field of the current polarizes the magnetic stripes across their length, which enhances the stray fields at their long edges and creates a dynamic vortex pinning landscape to impede vortex flow. We show a qualitative confirmation of the proposed mechanism which offers new strategies to engineer adaptive pinning topologies in SC–ferromagnetic hybrids.

In-vivo visualization of color-biased stripes in monkey secondary visual cortex (V2) using high-resolution MRI

Event Abstract Back to Event In-vivo visualization of color-biased stripes in monkey secondary visual cortex (V2) using high-resolution MRI Xiaolian Li1, John Arsenault1, 2, Thomas Janssens1, 2, Qi Zhu1 and Wim Vanduffel1, 2, 3* 1 KU Leuven, Department of Neurosciences, Belgium 2 Massachusetts General Hospital, United States 3 Harvard Medical School, Department of Radiology, United States Cytochrome oxidase (CO) stainings revealed three types of stripes in macaque area V2. The color-selective cells are clustered in less than 1 mm wide thin dark stripes and separated from each other by about 3 to 5 mm [1]. These color-biased stripes are hardly visible using conventional resolution fMRI due to the small size of the columns. In this study, we aimed to visualize the color stripes in vivo using high-resolution fMRI. We used the same stimuli as in our double-label deoxyglucose (2L-DG) study [1]. Moving isoluminant color and achromatic grating stimuli were presented in separate blocks while the monkey fixated at the center of the stimulus during the scan. High-resolution contrast-enhanced fMRI data (0.6 mm isotropic voxels, EPI, TR = 3 s, TE = 21 ms) was acquired using implanted phased-array receive coils developed in our lab, which increase image SNR by more than 3-fold [2]. To minimize image artifacts caused by motion of the monkey, we implemented an optimized GRAPPA reconstruction algorithm [3]. Furthermore, a B-spline grid based nonlinear registration and fieldmap-based distortion correction algorithms were implemented to achieve better frame to frame and functional to structural image alignment. Finally, a denoising technique (GLMdenoise) [4] was used to reduce physiological noise and other nuisance artifacts caused by motion of monkey in the temporal domain. We found stripe-like color activations throughout the entire extent of retinotopically-defined area V2, which are reproducible across scan sessions (on different days) in the same monkey. Moreover, color-biased activities were found in retinotopically-defined V1, V3, V4 and V4A. We also found luminance-biased activity in area MT, which is consistent with the result of the previous 2L-DG study [1]. The results of this study show that high-resolution fMRI can be reliably used to study submillimeter-scale functional organization of the brain.

Spin scan tomographic array-based imager.

This work presents a novel imaging device based on tomographic reconstruction. Similar in certain aspects to the earlier presented tomographic scanning (TOSCA) principle, it provides several important enhancements. The device described generates a stream of one-dimensional projections from a linear array of thin stripe detectors onto which the (circular) image of the scene is rotated. A two-dimensional image is then reproduced from the one-dimensional signals using tomographic processing techniques. A demonstrator is presented. Various aspects of the design and construction are discussed, and resulting images and movies are presented.

Time-resolved optical measurement of thermal transport by surface plasmon polaritons in thin metal stripes

We investigate the thermal transport originating from the propagation of surface plasmon polaritons (SPPs) in a thin gold stripe. The SPPs are excited by a grating coupler on the Au stripe which was patterned onto a silicon membrane. The transmissivity changes of the Si membrane due to temperature-induced changes of the interference conditions enable measuring the temperature distribution with temporal and spatial resolution better than 1 μs and 1 μm. With this setup, we demonstrate that SPP excitation, propagation, and decay are accompanied by considerable heating and heat transport.

How visual illusions illuminate complementary brain processes: illusory depth from brightness and apparent motion of illusory contours.

Neural models of perception clarify how visual illusions arise from adaptive neural processes. Illusions also provide important insights into how adaptive neural processes work. This article focuses on two illusions that illustrate a fundamental property of global brain organization; namely, that advanced brains are organized into parallel cortical processing streams with computationally complementary properties. That is, in order to process certain combinations of properties, each cortical stream cannot process complementary properties. Interactions between these streams, across multiple processing stages, overcome their complementary deficiencies to compute effective representations of the world, and to thereby achieve the property of complementary consistency. The two illusions concern how illusory depth can vary with brightness, and how apparent motion of illusory contours can occur. Illusory depth from brightness arises from the complementary properties of boundary and surface processes, notably boundary completion and surface-filling in, within the parvocellular form processing cortical stream. This illusion depends upon how surface contour signals from the V2 thin stripes to the V2 interstripes ensure complementary consistency of a unified boundary/surface percept. Apparent motion of illusory contours arises from the complementary properties of form and motion processes across the parvocellular and magnocellular cortical processing streams. This illusion depends upon how illusory contours help to complete boundary representations for object recognition, how apparent motion signals can help to form continuous trajectories for target tracking and prediction, and how formotion interactions from V2-to-MT enable completed object representations to be continuously tracked even when they move behind intermittently occluding objects through time.

Ferromagnetic resonance micromagnetic studies in patterned permalloy thin films and stripes

We present micromagnetic simulations of ferromagnetic resonance in patterned permalloy films and isolated stripes. Films of the total thickness 20 nm or 40 nm are patterned in the form of 1D periodic structures with rectangular profile (rectangular grooves of depth varying from zero up to the film thickness) and in-plane period of 500 nm. The direction of the applied dc magnetic field is varied in the film plane from the direction parallel to the stripes to perpendicular one. The thickness of the patterned elements and direction of the bias field affect essentially the resonance peaks (changing their position, amplitude and number) and the corresponding dynamical magnetization profiles. We simulated from one up to three ferromagnetic resonance peaks and found the areas of microwave magnetization localization for them.

Surface sensitivity of straight long-range surface plasmon waveguides for attenuation-based biosensing

The sensing performance of straight long-range surface plasmon waveguides consisting of a thin gold stripe embedded in Cytop is explored theoretically as a function of the metal stripe cross-sectional dimensions and the length of the sensing channel, and as a function of the sensing medium refractive index. The surface sensitivity and detection limit of such waveguides for attenuation-based biosensing are assessed. We find that changes in coupling efficiency between the sensing waveguide and the access waveguides, and changes in attenuation constant, due to adlayer formation, can contribute additively to the sensing performance. We observed a trade-off between the insertion loss and the change in insertion loss occurring during sensing. Optimum designs leading to compact, sensitive, and cost-effective biosensors are reported.