Periodic Corrugation Research Articles

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.

Formation of Periodically Arranged Nanobubbles in Mesopores: Capillary Bridge Formation and Cavitation during Sorption and Solidification in an Hierarchical Porous SBA-15 Matrix.

We report synchrotron-based small-angle X-ray scattering experiments on a template-grown porous silica matrix (Santa Barbara Amorphous-15) upon in situ sorption of fluorinated pentane C5F12 along with volumetric gas sorption isotherm measurements. Within the mean-field model of Saam and Cole for vapor condensation in cylindrical pores, a nitrogen and C5F12 sorption isotherm is well described by a bimodal pore radius distribution dominated by meso- and micropores with 3.4 and 1.6 nm mean radius, respectively. In the scattering experiments, two different periodicities become evident. One of them (d1 = 11.5 nm) reflects the next nearest neighbor distance in a 2D-hexagonal lattice of tubular mesopores. A second periodicity (d2 = 11.4 nm) found during in situ sorption and freezing experiments is traced back to a superstructure along the cylindrical mesopores. It is compatible with periodic pore corrugations found in electron tomograms of empty SBA-15 by Gommes et al. ( Chem. Mater. 2009, 21, 1311 - 1317). A Rayleigh-Plateau instability occurring at the cylindrical blockcopolymer micelles characteristic of the SBA-15 templating process quantitatively accounts for the superstructure and thus the spatial periodicity of the pore wall corrugation. The consequences of this peculiar morphological feature on the spatial arrangement of C5F12, in particular the formation of periodically arranged nanobubbles (or voids) upon adsorption, desorption, and freezing of liquids, are discussed in terms of capillary bridge formation and cavitation in tubular but periodically corrugated pores.

Study on light extraction efficiency of light-emitting diodes having periodically corrugated enhancement structures with different duty cycles

The duty cycle effect on the light extraction efficiency of light-emitting diodes having different periodically corrugated enhancement structures placed opposite to the emission surface was studied. The experimental results were compared with numerical simulation results. The numerical simulation was performed using a rigorous-coupled-wave-analysis method. In the case of common structures such as a patterned-sapphire structure, we found that, in general, extraction efficiency increased drastically when duty cycle was increased from zero, and then it varied relatively slowly as the duty cycle was increased further; after that, the extraction efficiency dropped drastically as the duty cycle approached 100%. However, for a structure that was composed of autocloned-photonic crystals super-imposing on the periodical corrugation, the efficiency increased monotonically with the duty cycle and reached 83.4% (corresponding to 589% enhancement efficiency) when the duty cycle approached 100%. A method for the fabrication of such high-efficiency structures was proposed.

Ordering and order-disorder phase transition in the (1x1) monolayer chemisorbed on the (111) face of an fcc crystal

In this paper we have considered a simple lattice gas model of chemisorbed monolayer which allows for the harmonic fluctuations of the bond length between the adsorbate atom and the surface site. The model also involves a short-ranged attractive potential acting between the adsorbed atoms as well as the surface periodic corrugation potential. It has been assumed that the adsorbed atoms are bonded to the uppermost layer of the substrate atoms. In particular, using Monte Carlo simulation method we have focused on the orderings appearing in the dense monolayer formed on the (111) face of an fcc solid. Within the lattice gas limit, the chemisorbed layer forms a (1x1) structure. On the other hand, when the bonds are allowed to fluctuate, three other different ordered phases have been found to be stable in the ground state. One of them has been found to be stable at finite temperatures and to undergo a phase transition to the disordered state. The remaining two ordered states have been found to be stable in the ground state only. At finite temperatures, the ordering has been demonstrated to be destroyed due to large entropic effects.

Outcoupling efficiency of OLEDs with 2D periodical corrugation at the cathode

We study theoretically the optical performance of organic light-emitting diodes (OLEDs) with 2D periodical corrugation at the cathode. We show how emergence of radiative surface plasmon resonances at the 2D corrugated cathode leads to the enhancement of the outcoupling efficiency of the OLED, which is primarily due to the outcoupling of emission generated by vertically oriented emitting excitons in the emission layer. We analyze the outcoupling efficiency of the OLED as a function of geometrical parameters of the corrugation and establish design rules for optimal outcoupling enhancement with the 2D corrugation at the cathode.

Enhanced Terahertz Transmission Through Bullseye Plasmonics Lenses Fabricated Using Micromilling Techniques

Imaging applications at terahertz frequencies are, in general, limited to relatively low spatial resolution due to the effects of diffraction. By using a subwavelength aperture in the near-field, however, it is possible to achieve subwavelength resolution, although low transmission through the aperture limits the sensitivity of this approach. Plasmonic lenses in the form of bullseye structures, which consist of a circular subwavelength aperture surrounded by concentric periodic corrugations, have demonstrated enhanced transmission, thereby increasing the utility of near-field imaging configurations. In this paper, the design, fabrication, and experimental performance of plasmonic lenses optimized for 300 GHz are discussed. While nanofabrication techniques are required for optical applications, microfabrication techniques are sufficient for terahertz applications. The process flow for fabricating a double-sided bullseye structure using a precision micromilling technique is described. Transmission and beam profile measurements using a customized terahertz testbed are presented.

High-Gain Linearly Tapered Antipodal Slot Antenna on LCP Substrate at E- and W-Bands

In this letter, we present a high-gain linearly tapered antipodal slot antenna (LTASA) on a thin-film liquid crystal polymer (LCP) substrate for E- and W-band applications. The proposed LTASA consists of a microstrip (MS) line feed, an MS-to-antipodal parallel strip line (APSL) balun, a linearly flared antipodal slot line (ASL), and periodic corrugations. An ultrawideband of from 47 to 110 GHz was demonstrated by full-wave simulation and measurement. To characterize the far fields of the LTASA, we developed an optical far-field measurement system in which the RF signal captured by the LTASA will be upconverted to the optical spectrum for power detection. By utilizing the optical approach, the developed system addresses flexibility, light weight, low profile, and low interconnection loss. A high gain of $16.5 + 1 ~\hbox{dBi}$ , narrow beamwidth, low sidelobes, and low cross polarization were demonstrated for the proposed LTASA from 75 to 95 GHz.

Polarization-rotating, Bragg-grating filters on silicon-on-insulator strip waveguides using asymmetric periodic corner corrugations.

We report on a wavelength-selective, polarization-rotating filter using a partially etched asymmetric Bragg grating on a compact single-mode, silicon-on-insulator (SOI) strip waveguide. The asymmetric Bragg grating rotates and reflects the fundamental transverse electric (TE) and transverse magnetic (TM) modes at selected wavelengths. Our device offers wavelength-selective, polarization-rotating reflection at its reflection port and wavelength-selective, polarization-independent notch filtering at its through port. The device is fabricated using a CMOS-compatible process with 193 nm-deep ultraviolet lithography on an SOI platform. We present results for a 294.4 μm long device that has a 3 dB bandwidth of 2.63 nm and a low-excess loss of less than 1 dB. This device also has a maximum polarization-extinction-ratio of greater than 27 dB.

Superlubric-pinned transition in sliding incommensurate colloidal monolayers

Two-dimensional (2D) crystalline colloidal monolayers sliding over a laser-induced optical lattice providing the periodic ``corrugation'' potential recently emerged as a new tool for the study of friction between ideal crystal surfaces. Here, we focus in particular on static friction, the minimal sliding force necessary to depin one lattice from the other. If the colloid and the optical lattices are mutually commensurate, the colloid sliding is always pinned by static friction; however, when they are incommensurate, the presence or absence of pinning can be expected to depend upon the system parameters, like in one-dimensional (1D) systems. If a 2D analogy to the mathematically established Aubry transition of one-dimensional systems were to hold, an increasing periodic corrugation strength ${U}_{0}$ should turn an initially free-sliding, superlubric colloid into a pinned state, where the static friction force goes from zero to finite through a well-defined dynamical phase transition. We address this problem by the simulated sliding of a realistic model 2D colloidal lattice, confirming the existence of a clear and sharp superlubric-pinned transition for increasing corrugation strength. Unlike the 1D Aubry transition, which is continuous, the 2D transition exhibits a definite first-order character, with a jump of static friction. With no change of symmetry, the transition entails a structural character, with a sudden increase of the colloid-colloid interaction energy, accompanied by a compensating downward jump of the colloid-corrugation energy. The transition value for the corrugation amplitude ${U}_{0}$ depends upon the misalignment angle $\ensuremath{\theta}$ between the optical and the colloidal lattices, superlubricity surviving until larger corrugations for angles away from the energetically favored orientation, which is itself generally slightly misaligned, as shown in recent work. The observability of the superlubric-pinned colloid transition is proposed and discussed.

Dual-periodic-corrugation-induced broadband light absorption enhancement in organic solar cells

We demonstrate an efficient approach to realize broadband light trapping in organic solar cells (OSCs) by integrating two-dimensional dual periodic corrugations. The dual-periodic corrugations consist of two set different periodic gratings, and have the effect to excite the surface plasmon-polariton (SPP) resonance associated with the cathode/organic interface in broadband wavelength region by adjusting befitting periods of the microstructures. Experimental and numerical results demonstrate that light absorption in OSCs has been enhanced in broadband owing to the effect of SPP-induced field enhancement. The short-circuit photocurrent density is improved from 4.072 to 5.365 mA/cm2, and a 31% enhancement of power conversion efficiency has been obtained compared to the conventional planar devices.

Generation of Nanoscale Stripes at Failure of Amorphous Metals

We analyzed the failure characteristics of the metallic glass Co43Fe20Ta5.5B31.5 (at.%) deformed in bending. The nanoscale fracture surface morphology respects the micromechanisms of the failure of the amorphous structure. The fracture surfaces consist of nanosized dimples (40 nm) arranged in the lines respecting the periodic corrugation zones oriented perpendicular to the crack propagation direction. The corrugation topology exhibits the point nature of the generation site, the concentric form of the stress waves and their interference.

Topologically confined states at corrugations of gated bilayer graphene

We investigate the electronic and transport properties of gated bilayer graphene with one corrugated layer, which results in a stacking AB/BA boundary. When a gate voltage is applied to one layer, topologically protected gap states appear at the corrugation, which reveal as robust transport channels along the stacking boundary. With increasing size of the corrugation, more localized, quantum-well-like states emerge. These finite-size states are also conductive along the fold, but in contrast to the stacking boundary states, which are gapless, they present a gap. We have also studied periodic corrugations in bilayer graphene; our findings show that such corrugations between AB- and BA-stacked regions behave as conducting channels that can be easily identified by their shape.

Lamb wave band gaps in one-dimensional radial phononic crystal plates with periodic double-sided corrugations

In this paper, we present the theoretical investigation of Lamb wave propagation in one-dimensional radial phononic crystal (RPC) plates with periodic double-sided corrugations. The dispersion relations, the power transmission spectra, and the displacement fields of the eigenmodes are studied by using the finite element method based on two-dimensional axial symmetry models in cylindrical coordinates. Numerical results show that the proposed RPC plates with periodic double-sided corrugations can yield several band gaps with a variable bandwidth for Lamb waves. The formation mechanism of band gaps in the double-sided RPC plates is attributed to the coupling between the Lamb modes and the in-phase and out-phases resonant eigenmodes of the double-sided corrugations. We investigate the evolution of band gaps in the double-sided RPC plates with the corrugation heights on both sides arranged from an asymmetrical distribution to a symmetrical distribution gradually. Significantly, with the introduction of symmetric double-sided corrugations, the antisymmetric Lamb mode is suppressed by the in-phase resonant eigenmodes of the double-sided corrugations, resulting in the disappearance of the lowest band gap. Furthermore, the effects of the geometrical parameters on the band gaps are further explored numerically.

On propagation of plane symmetric waves in a periodically corrugated straight elastic layer

Abstarct The classical Bernoulli−Euler model is employed for analysis of the plane wave propagation in an infinitely long elastic layer with a periodically varying thickness. The Floquet theory is applied to derive asymptotic formulas defining location and broadness of frequency stop bands for several corrugation shapes with different levels of discontinuity. For a layer with the perfectly smooth periodic corrugation, the equation of the axial wave motion is solved by the method of multiple scales in vicinity of critical frequencies. Then it is transformed to the canonical Mathieu equation, and its stability diagram is compared with predictions of the Floquet theory. The qualitative differences between the shapes of stop bands are discussed. The Wentzel−Kramers−Brillouin approximation is employed to solve the Bernoulli−Euler equation for the layer with perfectly smooth periodic corrugation, and the results are discussed in view of existence of pass and stop bands and in view of the levels of approximation used in alternative asymptotic methods.

Using surface impedance for calculating wakefields in flat geometry

Beginning with Maxwell's equations and assuming only that the wall interaction can be approximated by a surface impedance, we derive formulas for the generalized longitudinal and transverse impedance in flat geometry, from which the wakefields can also be obtained. From the generalized impedances, by taking the proper limits, we obtain the normal longitudinal, dipole, and quad impedances in flat geometry. These equations can be applied to any surface impedance, such as the known dc, ac, and anomalous skin models of wall resistance, a model of wall roughness, or one for a pipe with small, periodic corrugations. We show that, for the particular case of dc wall resistance, the longitudinal impedance obtained here agrees with a known result in the literature, a result that was derived from a very general formula by Henke and Napoly.

Errata: Enhancement of light absorption efficiency of amorphous-silicon thin-film tandem solar cell due to multiple surface-plasmon-polariton waves in the near-infrared spectral regime

The reflectances of a thin-film solar cell were computed, using the rigorous coupled-wave approach, as functions of the angle of incidence and the free-space wavelength for illumination by linearly polarized plane waves. A tandem solar cell made of amorphous-silicon alloys was considered. The metallic back-reflector was taken to be periodically corrugated. Both the simple and the compound periodic corrugations of the metallic back-reflector (surface-relief gratings) were investigated. Low-reflectance bands in the reflectance spectrums were correlated with the solutions of the underlying canonical boundary-value problem to delineate the excitation of multiple surface-plasmon-polariton (SPP) waves. For the standard AM1.5 solar irradiance spectrum, we found that the light absorption efficiency in the near-infrared spectral regime can be increased up to 100% when multiple SPP waves of both linear polarization states are excited.

Differential microstrip lines with reduced crosstalk and common mode effect based on spoof surface plasmon polaritons.

We apply the concept of spoof surface plasmon polaritons (SPPs) to the design of differential microstrip lines by introducing periodic subwavelength corrugations on their edges. The dispersion relation and field distribution of those lines are analyzed numerically. And then through designing practical coupling circuits, we found that compared with conventional differential microstrip lines, the electromagnetic field can be strongly confined inside the grooves of the corrugated microstrip lines, so the crosstalk between the differential pair and the adjacent microstrip lines is greatly reduced, and the conversion from the differential signal to the common mode signal can also be effectively suppressed. The propagation length of those lines is also very long in a wide band. Moreover, the experimental results in time domain demonstrate those lines perform very well in high-speed circuit. Therefore, those novel kinds of spoof SPPs based differential microstrip lines can be widely utilized in high-density microwave circuits and guarantee signal integrity in high-speed systems.

Fracture morphology pattern transition dominated by the crack tip curvature radius in brittle metallic glasses

The dynamic evolutions of different fracture morphologies in the fracture surface of brittle Mg-based metallic glasses (MGs) were systematically and quantitatively studied by SEM and AFM. It shows that the evolution of the fracture morphology pattern is dominated by an effective dynamic parameter of critical crack tip curvature radius. Based on crack tip plastic zone and shear transformation zone (STZ) theories, a theoretical model for the evolution of the crack tip curvature radius by considering the activation of STZs during fracture was proposed. The model can simulate the crack propagation process, and predict the critical crack tip curvature radius and the fracture pattern transition between the dimples and the periodic corrugations solely occurring in brittle MGs. These results have implications for understanding the microscopic fracture mechanism and the structural origin of fracture in MGs.

Cryogenic-temperature-induced transition from shear to dilatational failure in metallic glasses

At temperatures well below the glass transition temperature, the failure of metallic glasses is generally induced by shear banding, which is a result of the self-organized shear transformation zones (STZs). Here, we demonstrate that, upon cooling down to liquid helium temperature (4.2K), a Zr-based bulk metallic glass under quasi-static uniaxial tension can fracture via cavitation, rather than by shear banding, showing a transition from shear- to dilatation-dominated failure. This transition is supported by the breakdown of low-temperature strengthening of materials, as well as the changes in the macroscopic failure mode from shear to tension and in the microscopic fracture morphology from vein patterns to fine dimples or nanoscale periodic corrugations. According to the Mohr–Coulomb criterion, it is revealed that the capability of this glass to dilatation is enhanced with decreasing temperature, indicating the temperature-dependent normal stress sensitivity of failure. Our result implies that the shear-dominated STZs will convert into dilatation-dominated operations at very low temperatures.

Nanoscale Fracture Morphologies of Soft Magnetic CoFeTaB Amorphous Alloy

We analyzed the failure characteristics of the bulk metallic glass Co43Fe20Ta5.5B31.5 (at.%) deformed by compression and by bending of the ribbons. Under the load the amorphous structure can store high elastic energy. The surface morphology of nanoscale fracture is in accord with the micromechanisms of the failure of the amorphous structure. The fracture surface consists of a smooth mirror cleavage zone and a river pattern zone with nanosized dimples, arranged in lines respecting the periodic corrugation zones, oriented perpendicular to the crack propagation direction. The presence of the failure initiation centers in the ribbon in uences the dynamics of crack propagation and also the surface morphology of the fracture.