Ripple Structure Research Articles

Formation of slantwise orientated nanoscale ripple structures on a single-crystal 4H-SiC surface by time-delayed double femtosecond laser pulses

An effective control of the nanoscale periodic ripple structures on a single-crystal 4H-SiC surface is carried out by temporally delayed double collinear femtosecond laser pulse trains with different linear polarizations. The observed evolution of the ripple slant angle shows a dramatic decrease within an initial time delay range of ~10 ps between the two laser pulses, but keeps almost steady for larger time elapse. Through the detailed analyses, we propose a theoretical model to reveal the underlying physics: regarding the double femtosecond laser pulses, the first laser pulse transfers the semiconductor into nonequilibrium states associated with a transient refractive index grating on the surface, which subsequently tends to change the direction of the surface plasmon excitation of the second laser pulse. These behaviors can eventually result in the slantwise oriented laser intensity fringes and corresponding periodic ablation of grooves. Such investigations provide additional insights into the ripple formation processes, which is very important for control of nanomanufacturing materials by femtosecond lasers.

Fano-type spectra and other interference effects in an all-dielectric nanoshell

Large spectral variations in the extinction of nanostructures are important for many applications like optical antennas, cloaking materials, optical circuits and others. Such features in extinction spectra are typically attained using plasmonic particles and high-index or complex nanostructures. In this work, we show that large interference structures can manifest even in the extinction spectra of a single dielectric particle with a low relative refractive index (), and notably even as a spectral region of transparency and directional scattering of incident light. This is observed when the core and shell resonances of a layered spherical particle have overlaps with each other in the energy spectrum. Interference between the core and shell modes results in larger spectral features in magnitudes of the scattering constants an, bn i.e. a strong enhancement of what is traditionally called the ripple structure in the extinction spectra of a dielectric particle. This effect is in addition to any possible overlap of electric resonances of two different mode numbers or the overlap of an electric and magnetic resonance, which have been shown to result in sharp asymmetric (Fano) resonances in the forward or backscattering spectra of a high-index or metallic particle.

Submicron Laser-Induced Dot, Ripple and Wrinkle Structures and Their Applications

Submicron Laser-Induced Dot, Ripple and Wrinkle Structures and Their Applications

The Kelvin–Helmholtz instability in the Orion nebula: the effect of radiation pressure

The recent observations of rippled structures on the surface of the Orion molecular cloud (Bern\'{e} et al. 2010), have been attributed to the Kelvin-Helmholtz (KH) instability. The wavelike structures which have mainly seen near star-forming regions taking place at the interface between the hot diffuse gas, which is ionized by massive stars, and the cold dense molecular clouds. The radiation pressure of massive stars and stellar clusters is one of the important issues that has been considered frequently in the dynamics of clouds. Here, we investigate the influence of radiation pressure, from well-known Trapezium cluster in the Orion nebula, on the evolution of KH instability. The stability of the interface between HII region and molecular clouds in the presence of the radiation pressure, has been studied using the linear perturbation analysis for the certain range of the wavelengths. The linear analysis show that consideration of the radiation pressure intensifies the growth rate of KH modes and consequently decreases the e-fold time-scale of the instability. On the other hand the domain of the instability is extended and includes the more wavelengths, consisting of smaller ones rather than the case when the effect of the radiation pressure is not considered. Our results shows that for $\lambda_{\rm KH}>0.15\rm pc$, the growth rate of KH instability dose not depend to the radiation pressure. Based on our results, the radiation pressure is a triggering mechanism in development of the KH instability and subsequently formation of turbulent sub-structures in the molecular clouds near massive stars. The role of magnetic fields in the presence of the radiation pressure is also investigated and it is resulted that the magnetic field suppresses the effects induced by the radiation pressure.

Sub-Micro Laser Modifications of Tribological Surfaces

Abstract In order to address necessary improvements and optimizations of actual highly stressed tribological systems, their overall energy efficiency must be increased through the deployment of pinpointed surface solutions. For this purpose, not only wear, but also the friction behavior of these tribological systems has to be improved. Lubricants and greases are widely used in the industry to reduce adequately the friction and wear of a wide range of tribological systems, but for certain systems, they possess some drawbacks (reduced cleanliness, limited working temperatures, etc.). Therefore, alternative solutions such as laser texturing, which can modify the lubricant dynamics and the materials surface properties, offer new possibilities to decrease the friction and improve the wear properties of tribological systems. In the present study, channel and nanostructured ripple structures (also known as laser induced periodic surface structures or LIPSS) were produced, using an ultra-short pulse laser on mirror-polished 42CrMo4 discs, and characterized. The LIPSS distinguish themselves from the channel structure through their waveform with amplitude smaller than 1 μm. In order to investigate the effects of such surface structures on the overall tribological performance, oscillating wear tests (120 N, 90°C, 25 Hz, 4 mm, 2 h) with a SRV4-Tribometer using polished 100Cr6 convex cylinders as counter bodies were performed. The results obtained from these investigations with the 2 different surface structures were compared with a benchmark system (surfaces without structures). The tribological investigations showed that the nanostructured LIPSS tested against anti-friction coated cylinders significantly reduced the friction coefficient by a factor of 25 % in comparison to the benchmark, without influencing negatively the wear behavior.

Modulation of Electron-Phonon Coupling in One-Dimensionally Nanorippled Graphene on a Macrofacet of 6H-SiC.

Local electron-phonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron-phonon coupling to the out-of-plane phonon at the K̅ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron-phonon coupling in the surface graphene.

Ripple formation on silver after irradiation with radially polarised ultrashort-pulsed lasers

We report on the morphological effects induced by the inhomogeneous absorption of cylindrically polarized femtosecond laser irradiation of silver (Ag) in sub-ablation conditions. A theoretical prediction of the role of surface plasmon excitation and thermal effects in the production of self-formed periodic ripples structures is evaluated. To this end, a combined hydrodynamical and thermoelastic model is presented to account for the influence of temperature-related lattice movements in laser beam conditions that are sufficient to produce material melting. The results indicate that material displacements due to hydrodynamics are substantially larger than strain-related movements, which also emphasises the predominant role of fluid transport in surface modification. Moreover, theoretical simulations highlight the influence of the polarisation state in the size of ripple periodicity for a specialized case of cylindrically polarized beams, the radially polarized beams. The results show that the ripple periodicity is larger if linearly polarized beams are used. This is the opposite trend to the behaviour for materials with decreasing electron-phonon coupling constant g with increasing electron temperature, which highlights the significant role of g.

Identification of Whispering Gallery Mode (WGM) coupled photoluminescence and Raman modes in complex spectra of MoS2 in Polymethyl methacrylate (PMMA) microspheres

A ripples structure is observed in the photoluminescence (PL) spectra of MoS2 adsorbed on the surface of single polymethyl methacrylate (PMMA) microspheres in contrast to the broad spectra of nanosheets. Theoretical simulations and photonic nanojet technique have been used to identify the ripple structures. Part of these structures has been found to be due to the whispering gallery modes (WGMs) of the microcavity. The Raman modes of MoS2 and PMMA, on the other hand, are identified by the photonic nanojet induced enhancement. This work helps applications in photonics and sensing technique which require clean systems with well characterized spectral lines.

Characteristics of ripple structures revealed in OH airglow images

AbstractSmall‐scale ripple structures observed in OH airglow images are most likely induced by either dynamic instability due to large wind shear or convective instability due to superadiabatic lapse rate. Using the data set taken in the mesopause region with an OH all‐sky imager at Yucca Ridge Field Station, Colorado (40.7°N, 104.9°W), from September 2003 to December 2005, we study the characteristics and seasonal variations of ripple structures. By analyzing the simultaneous background wind and temperature observed by the nearby sodium temperature/wind lidar at Fort Collins, Colorado (40.6°N, 105°W), and a nearby medium‐frequency radar at Platteville, Colorado (40.2°N, 105.8°W), we are able to statistically study the possible relation between ripples and the background atmosphere conditions. Characteristics and seasonal variations of ripples are presented in detail in this study. The occurrence frequency of ripples exhibits clear seasonal variability, with peak in autumn. The occurrence of ripples shows a local time dependence, which is most likely associated with the solar tides. The lifetime and spatial scale of these ripples are typically 5–20 min and 5–10 km, respectively, and most of the ripples move preferentially either southward or northward. However, more than half of the observed ripples do not advect with background flow; they have higher Richardson numbers than those ripples that advect with background flow. It is possible that they are not instability features but wave structures that are hard to be distinguished from the real instability features.

In Situ Growth of Fe(Ni)OOH Catalyst on Stainless Steel for Water Oxidation

AbstractStainless steel contains the elements Fe, Ni, Cr and Mn, which are known as active centers of oxygen evolution reaction (OER) catalysts. The high conductivity of stainless steel also makes it an ideal substrate for OER. These facts imply that stainless steel should be a suitable candidate as an OER electrode. Here, we report a simple solution treatment approach that enables stainless steel to be an efficient and stable OER electrode. It was found that a uniform brown film with highly rippled sheet structure could be in situ grown on stainless steel at room temperature by immersing it in an alkaline oxidant solution containing NaOH and (NH4)2S2O8. The composition of the brown film was found to include Fe(Ni)OOH by Raman and X‐ray photoelectron spectroscopy (XPS) analyses. In 1 M KOH electrolyte, the prepared OER electrode exhibited good electrocatalytic performance with a relatively low overpotential of 300 mV at a benchmark current density of 10 mA/cm2 and a small Tafel slope of 34 mV/decade. Moreover, this OER electrode showed excellent long‐term stability. This work highlights the possibility of potentially converting accessible materials into useful catalysts through simple chemical treatments.

Fractal characterization and wettability of ion treated silicon surfaces

Fractal characterization of surface morphology can be useful as a tool for tailoring the wetting properties of solid surfaces. In this work, rippled surfaces of Si (100) are grown using 200 keV Ar+ ion beam irradiation at different ion doses. Relationship between fractal and wetting properties of these surfaces are explored. The height-height correlation function extracted from atomic force microscopic images, demonstrates an increase in roughness exponent with an increase in ion doses. A steep variation in contact angle values is found for low fractal dimensions. Roughness exponent and fractal dimensions are found correlated with the static water contact angle measurement. It is observed that after a crossover of the roughness exponent, the surface morphology has a rippled structure. Larger values of interface width indicate the larger ripples on the surface. The contact angle of water drops on such surfaces is observed to be lowest. Autocorrelation function is used for the measurement of ripple wavelength.

Femtosecond laser-induced modification at aluminum/diamond interface

We investigated femtosecond-laser-induced modification at an Al/diamond interface. The interface was irradiated from the backside through the diamond substrate, which is transparent to the laser beam. Extremely high pulse energies, i.e., 200 and 100 µJ/pulse, were used to irradiate the interface. The cross-section of the laser-irradiated line was observed with conventional and high-voltage transmission electron microscopy. The modification of the laser-irradiated interface was characterized by the formation of an amorphous phase sandwiched between the deformed Al film and the diamond substrate. The major chemical component of the amorphous phase was identified as carbon, blown from the diamond substrate. The newly formed interface between the amorphous phase and the diamond substrate was concave. In addition, a fine ripple structure with an average spacing one-quarter the wavelength of the laser light was formed only in the sample irradiated by the higher-energy pulses.

Anisotropic crystallization in solution processed chalcogenide thin film by linearly polarized laser

The low activation energy associated with amorphous chalcogenide structures offers broad tunability of material properties with laser-based or thermal processing. In this paper, we study near-bandgap laser induced anisotropic crystallization in solution processed arsenic sulfide. The modified electronic bandtail states associated with laser irradiation lead to a distinctive photoluminescence spectrum, compared to thermally annealed amorphous glass. Laser crystalized materials exhibit a periodic subwavelength ripple structure in transmission electron microscopy experiments and show polarization dependent photoluminescence. Analysis of the local atomic structure of these materials using laboratory-based X-ray pair distribution function analysis indicates that laser irradiation causes a slight rearrangement at the atomic length scale, with a small percentage of S-S homopolar bonds converting to As-S heteropolar bonds. These results highlight fundamental differences between laser and thermal processing in this important class of materials.

Description and reimplementation of real freeform surfaces

Freeform surfaces are becoming an increasingly exciting opportunity in optical design, in particular when correcting systems with off-axis geometries. Nevertheless, especially when coming to commercial use, the challenges for manufacturing are difficult to handle. The optical quality of a system is perturbed by typical deformations, such as localized figure errors and regular mid-spatial-frequency ripples, that come from the diamond-turning process. In this proposal, we investigated a workflow for analyzing the impact of real optical surfaces on the optical performance for even complex systems. Based on a simple and robust description, the surface is implemented back into the design. While the more localized deviations are analytically described by radial basis functions, the residual ripple structures are covered by a new approach based on the power spectral density. The reimport of optical surfaces back into the design software allows simple estimations for the requirements on manufacturing and the analysis of the realistic impact on system performance.

Surface Morphology Evolution Induced by Multiple Femtosecond Laser Ablation on 4H-SiC Substrate and Its Application to CMP

4H silicon carbide (SiC) substrates were processed by multiple femtosecond (fs) laser ablation. Its surface morphologies changed dramatically at different scanning pitches, different scanning velocities, different repetition rates and near-threshold fluence. The evolution of surface morphologies and the uniformity of rippled structures were respectively investigated and discussed. The overlapped zones of adjacent pulses were focused in discussion. In the case of high repetition rate, periodic ripples with average spatial periodicities ranging from 145 nm to 196 nm were induced; whereas in the case of lower repetition rate, coarse ripples (roughly 440 nm) and fine ripples (about 117 nm) were simultaneously formed. Large amount of agglomeration was fabricated in the case of scanning pitch 0.05 μm and scanning velocity ≤ 5 mm/s. At near-threshold fluence, 4H-SiC surface was partly ablated. When the rippled structures induced in Transverse Irradiation Mode were further ablated in Cross Scanning Mode, horizontal ripples changed into vertical ripples. X-ray diffraction and Raman spectra were utilized for surface analysis. Confirmatory experiment was carried out, demonstrating that chemical mechanical polishing (CMP) process could be promoted by fs laser ablation.

Spall behavior of copper under ultra-high strain rate loading

The spall behavior of copper at ultra-high strain rate is studied by molecular dynamics simulation combined with an experimental analysis of laser ablation of a bulk copper target by femtosecond laser pulses. In the molecular dynamics simulation, two-temperature model is used, shock wave and spallation characteristics of copper shock-loaded by femtosecond laser are analyzed in detail. It is concluded that the evolution of pressure indicates a triangular waveform of the shock wave, and the spall strength of copper is about 19 GPa at strain rates ranging from 109 s-1 to 1010 s-1, while higher pressure would melt the sample and the spall strength decreases to 14.89 GPa. Normally, the spallation is characterized by the sample free-surface undergoing alternately acceleration and deceleration, and the spallation mechanism could be explained by void nucleation, growth, coalescence that leads to the final fracture. An experiment is conducted to achieve high strain rate load on copper. The driving laser has a pulse width of 25 fs and central wavelength of 800 nm, the thickness values of the shocked copper foils are (5025) nm, fabricated by electron beam sputtering deposition onto 180 upm cover slip substrates. The driving laser beam with maximum intensity 5.51013 W/cm2, is focused on the front surface of the copper through the transparent substrate. Movements of the free rear surfaces of the copper foils are detected by chirped pulse spectral interferometry, and the theoretical time resolution is 1.3 ps. As a result, the free surface displacement and velocity evolution profile of the shocked area are obtained in a single measurement, and the results directly show that the maximum free surface velocity is 0.43 km/s and no alternately acceleration and deceleration appears. According to the shock wave relations, the maximum pressure near free-surface is 8.18 GPa. Meanwhile, derived from the velocity evolution profile, the strain rate is 7.3109 s-1. Combining with the above molecular dynamics simulation results, it is concluded that there is no spallation in the copper foil. Furthermore, we recover the sample targets and observe the microstructures by using scanning electron microscope. The copper foils are peeled off, but no spall scab is observed, indicating that the internal stress is between the copper spall strength and the bonding strength of copper foil with the transparent substrate. Ripple structure on copper surface demonstrates the femtosecond pulsed laser has ablated the copper film, and the propagation of the shock in fs regime is sensitive to microscopic defects.

Cavity induced fluorescence enhancement of graphitic carbon nitride submicron flakes

Graphitic carbon nitride (g-C3N4), which is structurally analogous to graphene, shows excellent fluorescent yield. Sharp ripple structure is observed in the fluorescence spectra of g-C3N4 flakes grafted on the surface of single polymethyl methacrylate (PMMA) microspheres. The intensities and the number of modes of these structures nonlinearly vary with the size of micro-cavity and the coupled power. Theoretical simulations carried out with the help of Mie theory show that the ripple structure is due to modulation of the fluorescence by the whispering gallery modes (WGMs) of the spherical microcavity.

Case study of convective instability observed in airglow images over the Northeast of Brazil

An intense activity of ripples during the nighttime was observed in airglow images over São João do Cariri (36.5°W, 7.4°S) on 10 October 2004 which lasted for two hours. Those ripples appeared simultaneously with the crossing of a mesospheric front and medium scale gravity waves. The ripples occurred ahead of the mesospheric front and their phase front were almost parallel to the phase of the mesospheric front and were almost perpendicular to the phase front of the gravity wave. Using wind measurements from a meteor radar located at São João do Cariri and simultaneous vertical temperature profiles from the TIMED/SABER satellite, on the night of the events and within the imager field of view, the atmospheric background environment in the mesosphere and lower thermosphere (MLT) was investigated in order to understand the instability process that caused the appearance of the ripples. Dynamic and convective instabilities have been pointed out as responsible for creation of ripples in the MLT. The observed ripples were advected by the neutral wind, they occurred into a region with negative lapse rate of the potential temperature and the Richardson number was negative as well. According to these characteristics, the ripple structures could be generated in the MLT region due to the predominance of convective instability.

Patterning of periodic ripples in monolayer MoS2 by using laser irradiation

We have investigated the effect of laser irradiation on monolayer MoS2 and observed the swellingup of the monolayer from the SiO2 substrate upon laser illumination. The mismatch in the thermal expansion between the substrate and MoS2 can result in the structural deformation. Employing this method, one can induce structural deformation in a desired pattern, and one can demonstrate the patterning of periodic ripples in monolayer MoS2 by using laser irradiation. The controlled fabrication of the ripple structure may be instrumental in understanding the effect of ripples on the interesting physical properties of monolayer MoS2.

Influence of substrate and film thickness on polymer LIPSS formation

Here we focus on the influence of both, substrate and film thickness on polymer Laser Induced Periodic Surface Structures (LIPSS) formation in polymer films. For this aim a morphological description of ripples structures generated on spin-coated polystyrene (PS) films by a linearly polarized laser beam with a wavelength of 266nm is presented. The influence of different parameters on the quality and characteristics of the formed laser-induced periodic surface structures (LIPSS) was investigated. We found that well-ordered LIPSS are formed either on PS films thinner than 200nm or thicker than 400nm supported on silicon substrates as well as on thicker free standing films. However less-ordered ripples are formed on silicon supported films with intermediate thicknesses in the range of 200–380nm. The effect of the thermal and optical properties of the substrate on the quality of LIPSS was analyzed. Differences observed in the fluence and number of pulses needed for the onset of surface morphological modifications is explained considering two main effects which are: (1) The temperature increase on polymer surface induced by the action of cumulative laser irradiation and (2) The differences in thermal conductivity between the polymer and the substrate which strongly affect the heat dissipation generated by irradiation.