Periodic Corrugation Research Articles

Low-Frequency Instabilities of Electron Beams in Weakly Irregular Transportation Channels

Evolution of low-frequency instabilities degrades electron beams in beam transportation channels and, not less importantly, deteriorates the coherence of the output radiation produced by microwave generators based on the use of such beams. These instabilities also determine the beam dynamics, including that associated with transitions of the beams from one stationary state to another. In this work, we use the method of coupled waves to study the stability of stationary states of the beams with high electron densities in weakly irregular vacuum beam transportation channels, specifically, in channels with shallow periodic wall corrugations. It is shown that in this case, the Bursian instability of the absolute type can evolve, which leads to a jump-wise transition of the beam to a state with a lower electron density.

Analog signal processing in the terahertz communication links using waveguide Bragg gratings: example of dispersion compensation.

We study the possibility of analog signal processing for the upcoming terahertz (THz) high-bitrate communications using as an example the problem of dispersion compensation in the THz communication links. In particular, two Waveguide Bragg Grating devices (WBGs) operating in the transmission mode are detailed. WBGs are designed by introducing periodic corrugation onto the inner surface of the metalized tubes. The resultant devices operate in a single mode regime either in the vicinity of the modal cutoff or in the vicinity of a bandgap edge, featuring large negative group velocity dispersions (GVD). We fabricate the proposed WBGs using 3D stereolithography, and metallize them using wet chemistry. Optical properties of the fabricated WBGs are investigated both theoretically and experimentally. The results confirm single mode guidance, relatively high coupling efficiency, as well as large negative group velocity dispersions in the range of several -100's ps/(THz · cm) in the vicinity of 0.14THz. This makes the short sections of proposed WBGs suitable for compensating positive dispersion incurred in the THz wireless links or fiber-assisted THz interconnects for signals of several-GHz bandwidth. Finally, we comment on the challenges associated with the analog signal processing in the THz spectral range.

Design Study of GW-THz Wave Transmission Without Mode Competition in an Oversized Relativistic Backward Wave Oscillator

Researchers have investigated plasma-related phenomena and internal breakdowns in the gigawatt–terahertz relativistic backward wave oscillator (RBWO) caused by field emissions from the surfaces of the waveguide circuits in these oscillators. In order to increase the power-handling capability of relativistic electron beam devices, an RBWO is proposed in the form of an oversized structure in which the diameter ( $D$ ) of the electrodynamic structure is increased to several times the free-space wavelength ( $\lambda )$ . An optimal periodic rectangular corrugation structure of which the axial mode is matched to the $\pi $ mode is designed in order to determine only the TM01 mode without mode competition in a slow-wave structure (SWS). The oversized RBWO has a fairly large diameter and uses a resonant reflector to achieve optimum efficiency while also preventing mode competition in the SWS. Absorption of a decelerated electron beam is achieved by the collector, which protects against breakdown and damage. The results of 0.5–0.1 THz from 500 kV–5 kA in the relativistic region are observed by means of a particle-in-cell simulation, and the dispersion relationship is determined by a finite-difference time-domain simulation. The outcomes show that a high-power oversized RBWO can control the selection of the mode and the interaction efficiency.

Corrugation-assisted metal-coated angled fiber facet for wavelength-dependent off-axis directional beaming.

We propose a fiber-optic-plasmonic hybrid device that is based on a corrugation-assisted metal-coated angled fiber facet (CA-MCAFF) for wavelength-dependent off-axis directional beaming (WODB). The device breaks into two key structures: One is the MCAFF structure, which is a modified Kretschmann configuration implemented onto a fiber platform, thereby being able to generate a unidirectional surface plasmon with dramatically enhanced properties in terms of non-confined diffracted radiation loss and operational bandwidth. The other is the periodic corrugation structure put on the MCAFF, thereby enabling WODB functionality out of the whole structures. The corrugated metal surface out-couples the surface plasmon mode to free-space optical radiation into a direction that varies with the wavelength of the optical radiation with excellent linearity. We perform extensive numerical investigations based on the finite-element-method and analyze the out-coupling efficiency (OCEout) and spectral bandwidth (SBout) of the proposed device for various designs and conditions. We determine the seven structural parameters of the device via taking sequential optimization steps. We deduce two optimal conditions particularly for the fiber-facet angle, in terms of the averaged OCEout or the SBout in the whole visible wavelength range (400 - 700 nm), which eventually leads to OCEout = 30.4% and SBout = 230 nm or to OCEout = 24.5% and SBout = 245 nm, respectively. These results suggest substantial enhancements in both OCEout and SBout, in comparison with the performance properties of a typical nano-slit-based device having a similar type of WODB functionality. The proposed CA-MCAFF is a simple, compact and efficient WODB device that is fully compatible with the state-of-the-art optical fiber technology.

Band Gap Opening Induced by the Structural Periodicity in Epitaxial Graphene Buffer Layer.

The epitaxial graphene buffer layer on the Si face of hexagonal SiC shows a promising band gap, of which the precise origin remains to be understood. In this work, we correlate the electronic to the atomic structure of the buffer layer by combining angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and high-resolution scanning transmission electron microscopy (HR-STEM). We show that the band structure in the buffer has an electronic periodicity related to the structural periodicity observed in STM images and published X-ray diffraction. Our HR-STEM measurements show the bonding of the buffer layer to the SiC at specific locations separated by 1.5 nm. This is consistent with the quasi 6 × 6 periodic corrugation observed in the STM images. The distance between buffer C and SiC is 1.9 Å in the bonded regions and up to 2.8 Å in the decoupled regions, corresponding to a 0.9 Å corrugation of the buffer layer. The decoupled regions are sp2 hybridized. Density functional tight binding (DFTB) calculations demonstrate the presence of a gap at the Dirac point everywhere in the buffer layer, even in the decoupled regions where the buffer layer has an atomic structure close to that of graphene. The surface periodicity also promotes band in the superperiodic Brillouin zone edges as seen by photoemission and confirmed by our calculations.

Acoustic Wood anomaly in transmitted diffraction field

In acoustics, the term Wood anomaly, in analogy to the Wood anomaly in optics, has so far referred to the anomalies observed in the specular reflection spectra of acoustic waves perpendicularly incident on periodic surfaces. Inspired by the pioneering work of Jungman et al. on the study of the transmission field of a solid-fluid periodic interface, this work attempts to provide a complete experimental investigation of the transmission fields of a broadband sound pulse transmitted through a periodic liquid-solid interface as well as a periodic solid-liquid interface. At different frequencies, two types of anomalies are observed: a spectral tip and a spectral dip, which correspond, respectively, to the brighter band and the darker band in optical Wood anomalies. The search for their physical origin suggests that the type and location of the observed spectral anomalies are strongly related to the generation and the diffraction of pseudosurface waves on the interface having superimposed periodic corrugations and time-domain windowing in spectral analysis. To compare with the surface waves on a plane surface, the properties of the pseudosurface waves are also investigated through examining their phase and by comparing their amplitudes.

The “ tension transformation zone” model of amorphous alloys

An amorphous alloy is a glassy solid that is formed through the supercooling of a melt. As the melt cools via the glass transition, its atoms freeze into a long-range disordered structure. Amorphous alloys represent a relatively young class of materials, having been first reported in 1960 when Duwez and co-workers produced Au-Si alloys by developing the rapid-quenching technology. The advent of amorphous alloys, especially the bulk samples with their characteristic size in excess of 1 mm, has aroused much interests in the basic science of glass transition, glass structure, and their intriguing properties. For crystalline metals, their structure can be well described by the period lattices and lattice defects including dislocations, twins, stacking faults, grain doundaries, etc. However, these traditional structural defects are not defined in amorphous alloys. Therefore, this type of atomic-disordered alloys manifest a series of excellent mechanical properties, including extraordinary strength, high hardness, large elastic limit and relatively high fracture toughness, making them attractive candidates for many potential applications as structural materials. At temperatures far below the glass transition temperature, the failure of amorphous alloys is generally induced by 10 nm thick shear banding with the single-dominated or multiple mode. It is well known that the shear banding is an instability mode of plastic flow from homogeneous to localized feature. Although the precise mechanism for amorphous plasticity is not well discovered, it is widely accepted that the shear-banding-mediated plasticity originates from a cascade of inelastic shear rearrangements of local atomic groups, called shear transformation zones (STZs). The STZs are thermally activated events with the transient nature, driven by shear stress and giving rise spatially to Eshelby fields. However, many recent works have shown that the failure of amorphous alloys is not always dominated by the shear banding; instead, a brittle failure will take place with a tension mode. The latter is usually accompanied with a new type of fracture surface morphology: fine dimples and/or nanoscale periodic corrugations. In order to understand such a dissipation process of fracture energy, we proposed the “tension transformation zone (TTZ)” model of amorphous alloys in 2008. The TTZ describes the brittle nucleation-controlled cavitation of local atomic groups that can be activated by shear-induced dilatation or direct hydrostatic tension. Here, we review how the TTZ model was developed, including its inherent nature, activation conditions, atomistic simulations and relevant experiments. The difference and relationship between the proposed TTZ and the classical cavitation are extensively discussed. Therefore, the energy dissipation in fracture of amorphous alloys is determined by two competing elementary processes, via. STZs and TTZs ahead of the crack tip. Based on this STZ vs. TTZ picture, the ductile-to-brittle transition of amorphous alloys can be understood as the change in the nature of transformation zones from shear-dominated STZs to dilatation-dominated TTZs. This review ends with the key aspects that deserve further study regarding the TTZ model. These aspects, at least, include (1) the experimental capture of TTZs, (2) the dynamics properties, (3) the spatio-temporal evolution, and (4) the theoretical construction from TTZs to brittle failure in amorphous alloys.

Silicon Add-Drop Filter Based on Multimode Grating Assisted Couplers

We demonstrate the design, fabrication, and characterization of a two-mode silicon add-drop filter, which is based on multimode grating-assisted contra-directional couplers. The mode conversions between two asymmetry strip waveguides are realized by using spatially periodic corrugations in one side wall of a multimode waveguide at the phase-match condition. Experimental results show that the fundamental mode of a single-mode waveguide is successfully coupled to the first two modes of the multimode waveguide in the opposite direction. The corresponding mode conversion bandwidths and extinction ratios are also measured. The demonstrated add-drop filter may find their applications in a hybrid multiplexing system by combing wavelength-division multiplexing technology and mode-division multiplexing technology for on-chip optical interconnect.

Measurements of terahertz radiation generated using a metallic, corrugated pipe

A method for producing narrow-band THz radiation proposes passing an ultra-relativistic beam through a metallic pipe with small periodic corrugations. We present results of a measurement of such an arrangement at Brookhaven's Accelerator Test Facility (ATF). Our pipe was copper and was 5cm long; the aperture was cylindrically symmetric, with a 1mm (radius) bore and a corrugation depth (peak-to-peak) of 60µm. In the experiment we measured both the effect on the beam of the structure wakefield and the spectral properties of the radiation excited by the beam. We began by injecting a relatively long beam compared to the wavelength of the radiation, but with short rise time, to excite the structure, and then used a downstream spectrometer to infer the radiation wavelength. This was followed by injecting a shorter bunch, and then using an interferometer (also downstream of the corrugated pipe) to measure the spectrum of the induced THz radiation. For the THz pulse we obtain and compare with calculations: the central frequency, the bandwidth, and the spectral power—compared to a diffraction radiation background signal.

Optical transmission theory for metal-insulator-metal periodic nanostructures

Abstract A semi-analytical formalism for the optical properties of a metal-insulator-metal periodic nanostructure using coupled-mode theory is presented. This structure consists in a dielectric layer in between two metallic layers with periodic one-dimensional nanoslit corrugation. The model is developed using multiple-scattering formalism, which defines transmission and reflection coefficients for each of the interface as a semi-infinite medium. Total transmission is then calculated using a summation of the multiple paths of light inside the structure. This method allows finding an exact solution for the transmission problem in every dimension regime, as long as a sufficient number of diffraction orders and guided modes are considered for the structure. The resonant modes of the structure are found to be related to the metallic slab only and to a combination of both the metallic slab and dielectric layer. This model also allows describing the resonant behavior of the system in the limit of a small dielectric layer, for which discontinuities in the dispersion curves are found. These discontinuities result from the out-of-phase interference of the different diffraction orders of the system, which account for field interaction for both inner interfaces of the structure.

Shear Waves in an Initially Stressed Elastic Plate with Periodic Corrugations

The influence of the combined effect of initial stress and periodic surface corrugation on ultrasonic SH wave propagation in an elastic plate is investigated. Considering weak surface corrugations, a small parameter is defined using the ratio between the corrugation amplitude and the average thickness of the plate and the perturbation method of strained parameters is used up to the second order. An equation for evaluating the wave phase velocity in the periodically corrugated plate is realized. Numerical examples are provided to graphically illustrate the variations of the phase velocity in periodic plates, with various corrugation amplitudes and wavelengths, under compressive and tensile initial stresses over a frequency spectrum. The study reveals the dispersive nature of SH waves in corrugated plates. It is found that the wave phase velocities are slightly higher (lower) in plates under initial tension (compression) than initially stress-free plates. The influence of initial stresses on SH phase speed decreases as the frequency increases. It is shown that periodic surface roughness has a stronger effect on reducing wave phase speeds than a shallow surface waviness.

Directional and monochromatic thermal emitter from epsilon-near-zero conditions in semiconductor hyperbolic metamaterials.

The development of novel thermal sources that control the emission spectrum and the angular emission pattern is of fundamental importance. In this paper, we investigate the thermal emission properties of semiconductor hyperbolic metamaterials (SHMs). Our structure does not require the use of any periodic corrugation to provide monochromatic and directional emission properties. We show that these properties arise because of epsilon-near-zero conditions in SHMs. The thermal emission is dominated by the epsilon-near-zero effect in the doped quantum wells composing the SHM. Furthermore, different properties are observed for s and p polarizations, following the characteristics of the strong anisotropy of hyperbolic metamaterials.

Manipulating frequency-dependent diffraction, the linewidth, center frequency and coupling efficiency using periodic corrugations

In this paper, we demonstrate an approach to couple freely propagating THz pulses onto a metal wire waveguide utilizing grating as various shaped grooves; rectangular, triangular and circular grooves fabricated directly into the metal. Using broadband THz pulses incident on the wire, we use metal segments containing number of uniformly, symmetric spaced grooves around subwavelength aperture to launch surface propagating multi-cycle pulses along the wire. THz radiation detected at the end of the waveguide with grooves showed overall improved signal amplitude and increased bandwidth as compared to that for the non-machined waveguide. We observe a one-to-one correspondence between the groove number and the number of oscillations in the THz wave form radiated from the end of the waveguide. We further demonstrate that this coupled radiation is radially polarized. Although the alteration of cross-sectional parameters of the grooves, the shape of the individual grooves, in a controlled manner should allow for arbitrarily shaped THz pulses to be launched on the waveguide, all of which compared to rectangular grooves’ output signal and the efficient shaped groove has been chosen.

Numerical analysis of the laminar forced convective heat transfer in coiled tubes with periodic ring-type corrugation

Wall curvature and wall corrugation represent two of the most used passive techniques to enhance convective heat transfer. The effectiveness of wall curvature is due to the fact that it gives origin to a secondary fluid motion orthogonal to the main flow, while wall corrugation is used to disrupt the development of the boundary layers, by enhancing the convective heat transfer mechanism. The compound use of the two techniques has been investigated in literature, mainly experimentally, but further investigation is still needed. In particular, it has been experimentally observed that this compound enhancement technique brings an additional heat transfer augmentation in the majority of applications whereas in the very low Reynolds number range the surface average performances of corrugated coils are lower than the one shown by smooth wall coils.This paper deepened the knowledge on this phenomenon presenting a numerical investigation of the effect induced by a periodic ring-type corrugation on the laminar convective heat transfer in coiled tubes. The study considered the laminar flow in the Reynolds and Dean number range 25-100 and 6-24 respectively. The investigation was particularly focused on the Dean's vortices destruction mechanism, induced by the wall corrugation and on the consequent breakdown of the average Nusselt number.

Plasmonic mesostructures with aligned hotspots on highly oriented mesoporous silica films

Gold mesostructures are fabricated by oblique angle deposition on highly oriented mesoporous silica (MPS) thin films utlizing the periodic surface corrugation on the scale of 10 nm as a prepattern for controlled deposition. Scanning electron microscopy analysis clarified that the mesostructures comprised an array of interconnected gold nanorods oriented in plane to form meso gratings. We measured the surface enhanced Raman scattering (SERS) to demonstrate that the nanosized gaps between the rods act as hotspots. Although the sample was as thin as 8.0 nm, large SERS signals appeared because of the very narrow gaps (< 2 nm). The spatial mapping confirmed the uniform distribution of hotspots over the sample.

Adhesion mechanics of graphene on textured substrates

Graphene, the two dimensional form of carbon, has excellent mechanical, electrical and thermal properties and a variety of potential applications including nano-electro-mechanical systems, protective coatings, transparent electrodes in display devices and biological applications. Adhesion plays a key role in many of these applications. In addition, it has been proposed that the electronic properties of graphene can be affected by elastic deformation caused by adhesion of graphene to its substrate. In light of this, we present here a continuum mechanics based theoretical framework to understand the effect of nanoscale morphology of substrates on adhesion and mechanics of graphene. In the first part, we analyzed the adhesion mechanics of graphene on one and two dimensional periodic corrugations. We carried out molecular statics simulations and found the results to be in good agreement with our theory. We modeled adhesive interactions as surface forces due to a Lennard–Jones 6–12 potential in both our analysis and simulations and in principle any other interaction potential can be used with our methodology. The results show that graphene adheres conformally to substrates with large curvatures. We showed in principle that the theory developed here can be extended to substrates with arbitrary textures that can be represented by a Fourier series.In the second part, we study the mechanics of peeling of graphene ribbons from one dimensional sinusoidally textured substrates. In the molecular statics simulations, we observed two key features in the peel mechanics of the ribbons - the ribbons slide over the substrate and undergo adhesion and peeling near the crack front in an oscillatory manner, the frequency of which reveals the wavelength of the underlying substrate. Our theory qualitatively captures these features of the peel mechanics and is general enough that it can be extended to other two dimensional materials like Molybdenum Disulphide (MoS2), Boron Nitride (BN) or other thin films and different kinds of interaction potentials.

Ductile to brittle transition of fracture of a Zr-based bulk metallic glass: Strain rate effect

Quasi-static and dynamic tensile experiments were conducted on a Zr-based bulk metallic glass at room temperature. A significant ductile-to-brittle transition was identified with increasing strain rate, based on the changes in the macroscopic fracture mode from shear to normal tension and in the microscopic fracture feature from vein patterns to fine dimples and/or nanoscale periodic corrugations. According to the Mohr-Coulomb criterion, it is revealed that such a transition is due to the competition between the intrinsic critical shear and tensile strengths at different strain rates. Microscopically, the strain-rate-induced transition is attributed to the change in the motion of local atomic groups from shear transformation zone to tension transformation zone, in which the characteristic volume of shear transformation zone is a key parameter.

Surface Modulation of Graphene Field Effect Transistors on Periodic Trench Structure.

In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6-3.74) was extracted from the current-voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench.

Effect of periodic corrugation, reinforcement, heterogeneity and initial stress on Love wave propagation

This paper investigates the impact of corrugated boundary surfaces, reinforcement on the propagation of Love-type wave in prestressed corrugated heterogeneous fiber-reinforced layer resting over a void pores half-space. The heterogeneity in the upper corrugated layer is caused due to exponential variation in the elastic constants with respect to the space variable pointing positively downwards. The dispersion equation in the complex form has been derived using method of separation of variables. The real and imaginary parts of the complex dispersion equation were separated and found in well agreement with the classical Love wave equation. Also, the attenuation of the Love waves has been discussed. The study reveals that such a medium transmits two fronts of Love waves. The first front depends upon the change in volume fraction of the pores and the second front depends upon the modulus of rigidity of the elastic matrix of the medium. The substantial influence of corrugation parameters, reinforcement, undulatory parameter, initial stress, heterogeneity parameter and position parameter on the phase velocity, and attenuation of Love-type wave have been observed and depicted by means of graph. It has been observed that the phase velocity decreases with the increase in initial stress parameters, heterogeneity, and reinforcement in upper layer.

Bandwidth Study of the Microwave Reflectors with Rectangular Corrugations

The mode-selective microwave reflector with periodic rectangular corrugations in the inner surface of a circular metallic waveguide is studied in this paper. The relations between the bandwidth and reflection coefficient for different numbers of corrugation sections were studied through a global optimization method. Two types of reflectors were investigated. One does not consider the phase response and the other does. Both types of broadband reflectors operating at W-band were machined and measured to verify the numerical simulations.