Ripple Period Research Articles

Tuning the period of femtosecond laser induced surface structures in steel: From angled incidence to quill writing

Exposure of metal surfaces to multiple ultrashort laser pulses under certain conditions leads to the formation of well-defined periodic surface structures. We show how the period of such structures in steel can be tuned over a wide range by controlling the complex interaction mechanisms triggered in the material. Amongst the different irradiation parameters that influence the properties of the induced structures, the angle of incidence of the laser beam occupies a prominent role. We present an experimental and theoretical investigation of this angle dependence in steel upon irradiation with laser pulses of 120 fs duration and 800 nm wavelength, while moving the sample at constant speed. Our findings can be grouped into two blocks. First, we observe the spatial coexistence of two different ripple periods at the steel surface, both featuring inverse scaling upon angle increase, which are related to forward and backward propagation of surface plasmon polaritons. To understand the underlying physical phenomena, we extend a recently developed model that takes into account quantitative properties of the surface roughness to the case of absorbing metals (large imaginary part of the dielectric function), and obtain an excellent match with the experimentally observed angle dependence. As second important finding, we observe a quill writing effect, also termed non-reciprocal writing, in form of a significant change of the ripple period upon reversing the sample movement direction. This remarkable phenomenon has been observed so far only inside dielectric materials and our work underlines its importance also in laser surface processing. We attribute the origin of symmetry breaking to the non-symmetric micro- and nanoscale roughness induced upon static multiple pulse irradiation, leading to non-symmetric modification of the wavevector of the coupled surface plasmon polariton.

Ultrafast dynamics of single-pulse femtosecond laser-induced periodic ripples on the surface of a gold film

The ultrafast dynamics of periodic ripples are studied during their formation on the surface of a gold film with a prefabricated nanogroove. These transient ripples are induced by a single 800-nm, 50-fs laser pulse and are observed by a collinear pump-probe imaging method. When the laser polarization is parallel to the nanogroove, transient ripples begin appearing after an elapsed time of 25--80 ps, and become clear and regular at 400--600 ps. The ripple period increases from 685 to 770 nm as the laser fluence $F$ increases from 0.73 to $3.42\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$. The evolution of temperature and lattice temperature are theoretically studied using the two-temperature model. When the laser fluence $F$ is above $0.73\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$, the electron temperature rises to several ${10}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the collision frequency rises above ${10}^{16}/\mathrm{s}$, which further causes the localization of hot electrons. Moreover, the $d$-band electrons can be excited through two-photon absorption and become free electrons. Using the dielectric constant of the excited states, which includes the effects of hot-electron localization and $d$-band transitions, the period predicted by the surface-plasmon-polarization (SPP) model accords well with experimental results. Both theory and experiment give support to SPP excitations playing a prominent role in the formation of periodic ripples induced by femtosecond laser pulses.

Influence of DC-Link Voltage on Commutation Torque Ripple of Brushless DC Motors with Two-Segment Pulse-width Modulation Control Method

The commutation process causes current ripple to be generated in the drive system of brushless DC (BLDC) motor. This, in turn, leads to output torque ripple. Mechanical vibration and acoustic noise are its influences which are undesirable phenomenon in some applications. A new method is presented in this paper which reduces torque ripple and commutation period in the entire range of motor speed. This method is designed and implemented based on two-segment pulse-width modulation (PWM) and DC-link voltage doubling during commutation. Based on the presented theory and given the influence of DC-link voltage on ripple magnitude, some experiments are carried out in which simultaneous association of the above mentioned factors in reducing current ripple and commutation time in the entire speed range of rotor has been proved. The experimental results show that the current ripple magnitude in high speed range is almost 20 times less than conventional method based on H-PWM_L-ON technique.

Influence of ambient pressure on surface structures generated by ultrashort laser pulse irradiation

We report an experimental investigation on the surface structures induced by linearly polarized ≈ 900 fs laser pulses, at λ = 1055 nm, on silicon at different values of the ambient pressure, from ~ 10−4 mbar to one atmosphere. Our experimental findings address interesting influences of the surrounding pressure on: (1) the spatial period of ripples; (2) the formation of micro-grooves; (3) the shape of the structured area. Moreover, the effects of various states of polarization in vacuum as well as of circularly polarized pulses in air vs vacuum are also addressed. We identify as one possible key element of such experimental observations: the fact that as the pressure raises the ablated nanoparticles produced during the femtosecond ablation process of the target get deposited more and more on the sample surface covering the irradiated spot area and influencing the structuring process.

Investigation of femtosecond laser induced ripple formation on copper for varying incident angle.

The hydrodynamic mechanisms associated with the formation of femtosecond laser induced ripples on copper for two angles of incidence are reported. Laser pulse length used for this work is 35 fs. A revised two-temperature model is presented that comprises transient changes of optical characteristics during the irradiation with femtosecond pulses to model relaxation processes and thermal response in bulk copper. The theoretical model takes into account the fluid flow dynamics that result in ripple periods shorter than the wavelength of the surface plasmon polaritons. Theoretical and experimental results are reported for incident angles of 0° and 45° relative to the surface normal. There is agreement between the experimentally measured and the theoretically predicted ripple periodicity for 50 pulses at 0° incidence. By contrast, for 100 pulses at 0° incidence, and 50 and 100 pulses at 45° incidence, the experimentally measured ripples have a larger period than the one predicted by the model while the trends in period with increased incident angle, and increased fluence are in agreement between the experimental and the theoretical results.

Enhancement of surface area and wettability properties of boron doped diamond by femtosecond laser-induced periodic surface structuring

We demonstrate the formation of laser-induced periodic surface structures (LIPSS) in boron-doped diamond (BDD) by irradiation with femtosecond near-IR laser pulses. The results show that the obtained LIPSS are perpendicular to the laser polarization, and the ripple periodicity is on the order of half of the irradiation wavelength. The surface structures and their electrochemical properties were characterized using Raman micro-spectroscopy, in combination with scanning electron and atomic force microscopies. The textured BDD surface showed a dense and large surface area with no change in its structural characteristics. The effective surface area of the textured BDD electrode was approximately 50% larger than that of a planar substrate, while wetting tests showed that the irradiated area becomes highly hydrophilic. Our results indicate that LIPSS texturing of BDD is a straightforward and simple technique for enhancing the surface area and wettability properties of the BDD electrodes, which could enable higher current efficiency and lower energy consumption in the electrochemical oxidation of toxic organics.

Ultrafast photoionization and excitation of surface-plasmon-polaritons on diamond surfaces

Ultrafast plasmonics of novel materials has emerged as a promising field of nanophotonics bringing new concepts for advanced optical applications. Ultrafast electronic photoexcitation of a diamond surface and subsequent surface plasmon-polaritons (SPPs) excitation are studied both theoretically and experimentally. Following photoexcitation on the rising edge of the pulse, transient surface metallization was found to occur for laser intensity near 18TW/cm2 due to enhancement of the impact ionization rate. In this regime, the dielectric constant of the photoexcited diamond becomes negative in the trailing edge of the pulse thereby increasing the efficacy with which surface roughness leads to inhomogeneous energy absorption at the SPP wave-vector. These transient SPP waves imprint permanent fine and coarse surface ripples oriented perpendicularly to the laser polarization. The theoretical modeling is supported by the experiments on the generation of laser-induced periodic surface structure on diamond surface with normally incident 515-nm, 200-fs laser pulses. Sub-wavelength (Λ≈100nm) and near wavelength (Λ≈450nm) surface ripples oriented perpendicularly to the laser polarization emerged within the ablative craters with the increased number of laser shots. The spatial periods of the surface ripples decrease moderately with the increasing exposure. The comparison between experimental data and theoretical predictions demonstrates the role of transient changes of the dielectric permittivity of diamond during the initial stage of periodic surface ripple formation upon irradiation with ultrashort laser pulses.

Limited thermal transport in rippled graphene induced by bi-axial strain for thermoelectric applications

Among efforts made to improve thermoelectric efficiency, the use of structurally modified graphene nanomaterials as thermoelectric matter are one of the promising strategies owing to their fascinating physical and electrical properties, and these materials are anticipated to be less thermally conductive than regular graphene structures, as a result of an additional phonon scattering introduced at the modified surfaces. In this study, we explore the thermal conductivity behaviors of strain-induced rippled graphene sheets by varying the ripple amplitude, periodicity, and dimensions of the structure. We introduce a technique which enables creation of a graphene sheet with evenly distributed ripples in molecular dynamics simulation, and the Green-Kubo linear response theory is used to calculate the thermal conductivity of the structures of interest. The results reveal the reduction of thermal conductivity with the greater degree of strain, the smaller system dimension, and the shorter ripple wavelength, which, in turn, could lead to the thermoelectric efficiency enhancement. This work has significance in that it presents the capability of generating repeated and controllable patterns in molecular dynamics, and so, it enables the atomic-level transport study in the regularly patterned two-dimensional surface or in any structures with a specified degree of strain.

Laser-induced periodic surface structures on bismuth thin films with ns laser pulses below ablation threshold

We demonstrate the formation of laser-induced periodic surface structures (LIPSS) in bismuth (Bi) thin films by irradiation with nanosecond laser pulses. We report on the formation and the destruction of the LIPSS as a result of the delivered number of pulses; both the formation and destruction threshold were very well determined. Results show that the obtained LIPSS are perpendicular to the laser polarization, and their ripple periodicity is on the order of the irradiation wavelength. Although all the irradiation experiments were done in ambient air, Raman micro-spectroscopy indicates that the LIPSS are constituted by metallic bismuth, i. e. the LIPSS formation is oxidation free.

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.

Controlling periodic ripple microstructure formation on 4H-SiC crystal with three time-delayed femtosecond laser beams of different linear polarizations.

The control of laser-induced periodic ripple microstructures on 4H-SiC crystal surface is studied using temporally delayed collinear three femtosecond laser pulse trains linearly polarized in different directions. The ripple orientation appears to develop independent of the individual laser polarizations and exhibits non-monotonical change with variable time delays, whose variation tendency is also affected by the polarization intersection angles. Remarkably, the ripple period is observed to transfer from high- to low-spatial-frequency regions, accompanied by distinctly improved morphological uniformity and clearness. The results are satisfactorily interpreted based on a physical model of the surface wave excitation on a transient index metasurface, which is confirmed by further experiments. Our investigations indicate that transient noneqilibrium dynamics of the material surface provides an effective way to manipulate the laser-induced microstructures.

Traditional vs. advanced Bragg reflectors for oversized circular waveguide

This paper compares two types of distributed Bragg reflector, based on the periodic wall perturbation of an oversized circular waveguide. The first type is a traditional mirror, where wall ripples with a period of half a guided wavelength for the working mode couple forward and backward waves. The other type is an advanced reflector with a ripple period of about a guided wavelength, exploiting an intermediate conversion into a quasi-cutoff mode. The design of both reflectors has been optimized with a mode matching code to deliver a reflectivity >96% for the TE5,3 mode at 250GHz and a power to gun <0.5% in copper waveguides with a diameter of 15mm. Such specifications are relevant to the upstream mirror of a cyclotron auto-resonance maser under development at ENEA Frascati. The two types of reflector are compared in terms of mechanical dimensions, reflectivity, bandwidth and losses.

Investigation of Two Kinds of Periodical Surface Structures Induced by Femtosecond Laser on the Surface of Titanium Plate.

Two kinds of laser-induced periodical structures observed on metal titanium plate irradiated by femtosecond laser (fs-laser) are reported, including the fs-laser-induced concentric rings and periodical subwavelength ripples. The rings can be produced only when the titanium target is located before the focal plane. The period and distribution of the rings stay almost fixable with the change of laser fluence and pulse number. The average cross section area (ACSA) is introduced to investigate the material removal behavior with the growth of the rings. The rings grow with the accumulation of laser pulses and the increase of the laser fluence, but the growth rate will be moderated with the increase of the pulse number. The ripples are obtained in an annular region when powerful multi-pulses irradiate on the target surface. The ripple period varies with laser fluence and location. And the annular region migrates outward when the laser fluence increases. We suggest that the formations of ripples are attributed to the propagation of the laser-stimulated surface plasma wave (SPW) on the air/ti interface. The dependence of the ripple period on the pulse fluence calculated by the SPW model agrees well with our experimental observation. The investigation improves the understanding of the two kinds of periodical surface structures both in their appearances and mechanisms.

Formation of periodic ripples on silicon surface ablated by femtosecond laser

The formation mechanism of low-spatial-frequency laser-induced periodic surface structure (LSFL) on single-crystalline silicon irradiated by single femtosecond-laser pulse (pulse duration =150 fs and central wavelength =800 nm) in air is investigated theoretically based on the interference theory of Sipe-Drude model and surface plasmon polariton (SPP). In order to account for transient intrapulse changes in the optical properties of the material due to the excitation of a dense electron-hole plasma, we model the maximum of the electron density as a function of laser fluence by solving the generally accepted two-temperature equation and Drude model. The results show that both theories are applicable to explaining the LSFL formation on the high-excited silicon. In the Sipe-Drude theory, the factor (k) is used to describe the efficacy with which the surface roughness at position k leads to inhomogeneous absorption of radiation. We find that the value of (k) first increases until reaching a maximum at an electron density of 61021 cm-3 and then decreases with the laser fluence increasing. When the incident laser fluence is 0.38 J/cm2, which is the threshold for excited plasma, the period reaches a minimum value in a small range of the top. Besides, the law of period is calculated according to the relationship between the (k) and period. In the SPP theory, the ripple period on the highly excited silicon increases with the laser fluence increasing. Comparing the scopes of application of two theories, the Sipe-Drude theory is found to be suitable for the analysis of more extensive periodic surface structures, while the SPP theory is applicable only for the case that laser fluence is close to the damage threshold. Moreover, our results are capable of explaining that the delay direction of periodic ripples are always perpendicular to the incident laser polarization direction by using the Sipe-Drude theory. When laser fluence approaches to the damage threshold, the LIPSS period is calculated sightly to be below the laser wavelength. It also reveals that the periodic surface structures are approximately the same in the different polarization directions with the increase of incident angle. Taking into account a single pulse, the corrugation period decreases with the increase of angle of incidence in the S polarization direction. And under different polarizations, with the increase of incident angle, the changes of the ripple period show an opposite trend. The obtained dependence provides a way to better control the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material, which is of great significance for understanding the formation of periodic structure of silicon surface, caused by femtosecond laser, and its application in the field of silicon materials processing.

Femtosecond laser surface structuring of silicon with Gaussian and optical vortex beams

We report an experimental analysis of femtosecond laser induced surface structuring of silicon by exploiting both Gaussian and Optical Vortex beams. In particular, we show how different surface patterns, consisting of quasi-periodic ripples and grooves, can be obtained by using different states of polarization offered by optical vortex beams. Both for Gaussian and optical vortex beams, an increase of the number of laser pulses, N, or beam energy, E0, leads to a progressive predominance of the grooves coverage, with ripples confined in specific regions of the irradiated area at lower fluence. The average period of ripples and grooves shows a different dependence as a function of both E0 and N, underlying important differences in mechanisms leading to the formation of ripples and grooves. In particular, our experimental characterization allows identifying a preliminary stage of grooves generation with rudimental surface structures, preferentially directed parallel to the laser polarization. This supports the idea that one possible mechanism of grooves formation lies in the progressive aggregation of clusters of nanoparticles densely decorating the ripples. Our experimental findings provide important indications on the basic understanding of the processes involved in laser surface structuring with ultrashort pulses that can guide the design of the surface patterns.

Ripples in amorphous chalcogenide films under homogeneous laser illumination

Under homogeneous illumination of thin chalcogenide glass films by polarized light at a wavelength near the band gap ripples form, with a period of the order of 10–15µm, directed normal to the light polarization. The formation of the ripples cannot be explained by interference phenomena, which predict the ripple periods of the order of light wavelength. Our experimental and theoretical studies of the ripple formation in 1µm thick As10Se90 chalcogenide films show that the profile variation occurs due to lateral mass transport accelerated by light. The ripple formation is caused by competition between capillary forces and steady state electrostatic forces induced by redistribution of electrons and holes generated by light. Under these driving forces, each harmonic of the film roughness spectrum should exponentially grow or flatten, depending on its frequency. The average period of the ripples corresponds to those harmonics in the roughness spectra, which grow with maximum rate. Light-induced diffusion coefficients have been estimated from the kinetics of the ripple formation.

Strong terahertz radiation generation by beating of two spatial-triangular beams in collisional magnetized plasma

A scheme of terahertz (THz) radiation generation is proposed by beating of two spatial-triangular laser beams in plasma with a spatially periodic density when electron–neutral collisions have taken into account. In this process, the laser beams exert a ponderomotive force on the electrons of the plasma and impart the oscillatory velocity at the difference frequency in the presence of a static magnetic field which is applied parallel to the direction of the lasers. We show that higher efficiency and stronger THz radiation are achieved when the parallel magnetic field is used to compare the perpendicular magnetic field. The effects of beam width of lasers, collision frequency, periodicity of density ripples, and magnetic field strength are analyzed for strong THz radiation generation. The THz field of the emitted radiations is found to be highly sensitive to collision frequency and magnetic field strength. In this scheme with the optimization of plasma parameters, the efficiency of order 21% is achieved.

Infrared absorption of fs-laser textured CVD diamond

Nanoscale periodic texturing on polycrystalline CVD diamond surface was performed to obtain a significant increase in optical absorptance to visible and near-infrared radiation. Surface texturing, obtained by the use of fs-laser ultrashort pulses, has been demonstrated to induce a controlled periodicity of ripples of about 170 nm and length of several µm, able to drastically increase the diamond capability of interacting with solar radiation from its intrinsic visible blindness. Ultraviolet and visible Raman spectroscopy has been used to confirm the absence of non-diamond phases resulting from the process for the fs-laser-textured sample. Moreover, here we investigate the optical properties in the range 200 nm–25 µm. Absorbance of fs-laser-textured CVD diamond is considerably higher than the untreated one at every wavelength, resulting in a remarkable increase in the emittance: It points out the need for an optimization of process parameters to enhance the selective absorption capability.

Adhesion modification of neural stem cells induced by nanoscale ripple patterns

We have studied the influence of anisotropic nanopatterns (ripples) on the adhesion and morphology of mouse neural stem cells (C17.2) on glass substrates using cell viability assay, optical microscopy and atomic force microscopy. The ripples were produced by defocused ion beam sputtering with inert Ar ions, which physically remove atoms from the surface at the energy of 800 eV. The ripple periodicity (∼200 nm) is comparable to the thickness of the cytoplasmatic microspikes (filopodia) which link the stem cells to the substrate. All methods show that the cell adhesion is significantly lowered compared to the same type of cells on flat glass surfaces. Furthermore, the AFM analysis reveals that the filopodia tend to be trapped parallel or perpendicular to the ripples, which limits the spreading of the stem cell on the rippled substrate. This opens the perspective of controlling the micro-adhesion of stem cells and the orientation of their filopodia by tuning the anisotropic substrate morphology without chemical reactions occurring at the surface.

Threshold Dependence of Deep- and Near-subwavelength Ripples Formation on Natural MoS2 Induced by Femtosecond Laser.

Deep sub-wavelength ripples (DSRs) and near sub-wavelength ripples (NSRs) with uniform periods of ~160 nm and ~660 nm generated at the MoS2-vacuum interface is reported for the first time by the processing of femtosecond laser (800 nm, 120 fs, 1 kHz) in this paper. The DSRs and NSRs formation fluence thresholds are experimentally determined as 160 mJ/cm2 and 192 mJ/cm2, respectively. In addition, the ripple period is insensitive to the pulse number. Moreover, Raman analyses show that the MoS2 lattice in the irradiated area does not exhibit oxidation at room environment and the crystalline representation is well preserved in NSRs region. We attribute our result to the joint interactions of the spallation and sublimation of layered MoS2 together with the laser induced surface plasmon polaritons and propose an explanation to the threshold dependence of the ripple period. Our study provides some insights for ultrafast laser-matter interactions and indicates a simple effective method for future nano-fabrication of MoS2.