Subwavelength Ripples Research Articles

Femtosecond double-pulse fabrication of hierarchical nanostructures based on electron dynamics control for high surface-enhanced Raman scattering

This Letter presents a simple, efficient approach for high surface-enhanced Raman scattering by one-step controllable fabrication of hierarchical structures (nanoparticles+subwavelength ripples) on silicon substrates in silver nitrate solutions using femtosecond double pulses based on nanoscale electron dynamics control. As the delays of the double pulses increase from 0 fs to 1 ps, the hierarchical structures can be controlled with (1) nanoparticles--the number of nanoparticles in the range of 40-100 nm reaches the maximum at 800 fs and (2) ripples--the subwavelength ripples become intermittent with decreased ablation depths. The redistributed nanoparticles and the modified ripple structures contribute to the maximum enhancement factor of 2.2×10(8) (measured by 10(-6) M rhodamine 6G solution) at the pulse delay of 800 fs.

The influence of ultra-fast temporal energy regulation on the morphology of Si surfaces through femtosecond double pulse laser irradiation

The effect of ultrashort laser-induced morphological changes upon irradiation of silicon with double pulse sequences is investigated under conditions that lead to mass removal. The temporal delay between twelve double and equal-energy pulses (Ep=0.24J/cm2 each, with pulse duration tp=430fs, 800nm laser wavelength) was varied between 0 and 14ps and a decrease of the damaged area, crater depth size and periodicity of the induced subwavelength ripples (by 3-4%) was observed with increasing pulse delay. The proposed underlying mechanism is based on the combination of carrier excitation and energy thermalization and capillary wave solidification and aims to provide an alternative explanation to the control of ripple periodicity by temporal pulse tailoring. This work demonstrates the potential of pulse shaping technology to improve nano/micro processing.

Adjustments of dielectrics craters and their surfaces by ultrafast laser pulse train based on localized electron dynamics control

A quantum model with the consideration of laser wave-particle duality based on the plasma model is employed for the femtosecond laser pulse train processing of fused silica. Effects of the key pulse train parameters, such as the pulse separation time and the number of pulses per train on the distributions of free electron are discussed. The calculations show that the spatial/temporal distributions of free electron can be adjusted by transient localized electron dynamics control using femtosecond laser pulse train design; the results are ablation shapes of craters and subwavelength ripples. It is also found that the first pulse separation time (Δt1) can be used for rough adjustments of ablated structures, while the second pulse separation time (Δt2) can be used for the fine tuning of ablated structures, especially the shapes of craters.

Periodic subwavelength ripples on lithium niobate surfaces generated by tightly focused sub-15 femtosecond sub-nanojoule pulsed near-infrared laser light

On exposure to 85 MHz sub-15 fs pulsed 800 nm Ti:sapphire laser light in the focal spot of a high-numerical aperture oil immersion objective, characteristic periodic nanostructures emerged on the surface of z-cut congruent LiNbO3 crystals. Close to the ablation threshold shallow ripples oriented parallel to the laser polarization appeared on the surface at a periodicity Λ∥ ≈ 200 nm as well as ripples running perpendicular to the laser polarization at almost the same period (Λ⊥ ≈ 190 nm). Nanostructure formation involved melting and resolidification of LiNbO3 as evidenced by scanning electron microscopy images of structural peculiarities. Micro-Raman spectroscopy demonstrated that the resolidified material crystallized in the original structural phase. Ripple formation is attributed to plasma wave interference patterns that arise in the electron–hole plasma generated by multiphoton and avalanche collisional excitation.

Simulation of rippled structure adjustments based on localized transient electron dynamics control by femtosecond laser pulse trains

This study investigates the effects of pulse energy distributions on subwavelength ripple structures (the ablation shapes and subwavelength ripples) using the plasma model with the consideration of laser particle–wave duality. In the case studies, the laser pulse (800 nm, 50 fs) trains consist of double pulses within a train with the energy ratios of 1:2, 1:1, and 2:1. Localized transient electron densities, material optical properties, and surface plasmon generation are strongly affected by the energy distributions. Hence, the adjustment of the ablation shape and subwavelength ripples can be achieved based on localized transient electron dynamics control during femtosecond laser pulse train processing of dielectrics. The simulation results show that better, more uniform structures, in terms of ablation shapes and subwavelength ripples, can be easily formed at a lower fluence or subpulse energy ratio of 1:1 with a fixed fluence. It is also found that pulse trains at a 1:1 energy ratio are preferred for drilling high-aspect-ratio microholes or microchannels.

Nonlinear optical mechanism of forming periodical nanostructures in large bandgap dielectrics

Nonlinear excitation mechanisms of plasmons and their influence on femtosecond-laser induced sub-wavelength ripple generation on dielectric and semiconducting transparent materials are discussed. The agreement of theoretical and experimental data indicates the relevance of the model.

Adjustment of ablation shapes and subwavelength ripples based on electron dynamics control by designing femtosecond laser pulse trains

A quantum model is proposed to investigate femtosecond laser pulse trains processing of dielectrics by including the plasma model with the consideration of laser particle-wave duality. Central wavelengths (400 nm and 800 nm) strongly impact the surface plasmon field distribution, the coupling field intensity distribution (between the absorbed intensity and the surface plasma), and the distribution of transient localized free electron density in the material. This, in turn, significantly changes the localized transient optical/thermal properties during laser materials processing. The effects of central wavelengths on ablation shapes and subwavelength ripples are discussed. The simulation results show that: (1) ablation shapes and the spacing of subwavelength ripples can be adjusted by localized transient electron dynamics control using femtosecond laser pulse trains; (2) the adjustment of the radii of ablation shapes is stronger than that of the periods of subwavelength ripples.

Influence of irradiation dose on laser-induced surface nanostructures on silicon

We report on the dependence of femtosecond laser-induced periodic surface structures on an increase of incident pulse number. On silicon, the patterns evolve from linear, parallel sub-wavelength ripples, grossly perpendicular to the laser polarization, via coalesced wider features parallel to the polarization, to a crater with periodically structured, pillar-like walls. Closer inspection of the patterns indicates that the different features always continue to exhibit reminiscence to the preceding lower-dose patterns, suggesting that, indeed, all patterns can be created by ONE single GENERAL formation process, as in self-organized structure formation, and the different structures/feature sizes are NOT due to DIFFERENT mechanisms.

Creation of periodic subwavelength ripples on tungsten surface by ultra-short laser pulses

Formation of periodic subwavelength ripples on a metallic tungsten surface is investigated through a line-scribing method under the irradiation of 800 nm, 50 fs to 8 ps ultra-short laser pulses. The distinctive features of the induced ripple structures are described in detail with different laser parameters. Experimental measurements reveal that with gradual decrease of the laser fluence, the pulse duration or the scanning speed, the ripple period is inclined to reduce but the ripple depth tends to become pronounced. Theoretical analyses suggest that the transient dielectric function change of the tungsten surface mainly originates from the nonequilibrium distribution of electrons due to the d-band transitions. A sandwich-like physical model of air–plasma–target is proposed and the excitation of a surface plasmon polaritonic (SPP) wave is supposed to occur on the interface between the metallic target and the electron plasma layer. Formation of ripples can be eventually attributed to the laser–SPP interference. Theoretical interpretations are consistent with the experimental observations.

Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in subablation conditions

An investigation of ultrashort pulsed laser--induced surface modification due to conditions that result in a superheated melted liquid layer and material evaporation are considered. To describe the surface modification occurring after cooling and resolidification of the melted layer and understand the underlying physical fundamental mechanisms, a unified model is presented to account for crater and subwavelength ripple formation based on a synergy of electron excitation and capillary wave solidification. The proposed theoretical framework aims to address the laser--material interaction in subablation conditions and thus the minimal mass removal in combination with a hydrodynamics-based scenario of the crater creation and ripple formation following surface irradiation with single and multiple pulses, respectively. The development of the periodic structures is attributed to the interference of the incident wave with a surface plasmon wave. Details of the surface morphology attained are elaborated as a function of the imposed conditions, and results are tested against experimental data.

Subwavelength ripples adjustment based on electron dynamics control by using shaped ultrafast laser pulse trains

This study reveals that the periods, ablation areas and orientations of periodic surface structures (ripples) in fused silica can be adjusted by using designed femtosecond (fs) laser pulse trains to control transient localized electron dynamics and corresponding material properties. By increasing the pulse delays from 0 to 100 fs, the ripple periods are changed from ~550 nm to ~255 nm and the orientation is rotated by 90°. The nearwavelength/subwavelength ripple periods are close to the fundamental/second-harmonic wavelengths in fused silica respectively. The subsequent subpulse of the train significantly impacts free electron distributions generated by the previous subpulse(s), which might influence the formation mechanism of ripples and the surface morphology.

Surface plasmon polariton model of high-spatial frequency laser-induced periodic surface structure generation in silicon

In recent years, high-spatial frequency laser-induced surfaces structures have been generated in a large variety of dielectrics. In silicon subwavelength ripples, some of which featured periodicities below 100 nm, were formed using ultrafast lasers. We demonstrate for Si(100) surfaces that generation of a dense electron-hole plasma in the focal spot of ultrashort-pulsed laser light followed by massive excitation of plasma waves provides an explanation for the formation of such high-spatial frequency surface structures. The applied Drude-like model includes carrier-carrier collisions and is in excellent agreement with the experimentally observed ripple period.

Ablation effects of femtosecond laser functionalization on steel surfaces

In this work, the topographical and chemical effects of femtosecond laser irradiation on industrial 100Cr6 steel surfaces are analyzed. The irradiated areas present cavities with conical spikes at their center and an outer ring zone with periodical structures called sub-wavelength ripples. The number of pulses drastically influences the morphology of both zones. Abbott–Firestone curves show that 1μm depth cavities have the best load capacity and might increase the lubrication efficiency due to their homogeneous topographic profile. The elemental depth profile obtained by X-ray photoelectron spectroscopy reveals that the laser irradiation produces a chemical carbide shift and increases the iron oxide layer thickness by a factor of 1.7 compared to the native layer. Raman analysis reveals the presence of metallic oxide and carbon bands. The distribution of amorphous and crystalline carbon depends on the distribution of the laser intensity. The increase of the number of pulses leads to the amorphization of the irradiated zones, according to the melt-quenching phenomena. These novel results permit a better understanding of the ablation phenomena in steels.

Formation mechanisms of sub-wavelength ripples during femtosecond laser pulse train processing of dielectrics

A new model is proposed to investigate femtosecond laser pulse train processing of dielectrics by including the laser wave properties in the photon particle properties based plasma model. In the case studies, the pulse duration is 50 fs and the pulse delays are 0, 25, 50 and 75 fs. Both the laser wave–particle duality and transient localized changes of material properties are considered in the proposed model, and the formation mechanism of sub-wavelength ripples is revealed. This study shows that the interference between surface plasmons and laser field plays a key role in the formation of sub-wavelength ripples for which the excitation of surface plasmons is necessary. The predicted period of sub-wavelength ripples is in agreement with the experiments.

Femtosecond laser-induced subwavelength ripples on Al, Si, CaF2 and CR-39

The formation of self-organized subwavelength ripples on Al, Si, CaF2 and CR-39 induced by 25fs laser pulses at central wavelength of 800nm has been observed under certain experimental conditions. In case of Al subwavelength gratings with periodicities ranging from 20 to 220nm are reported. For CaF2 the periodicity goes up to 625nm. In case of Si, nano-gratings have the periodicity of 10–100nm. The interspacing of these gratings is 60nm in case of CR-39. These features which are significantly shorter than incident laser wavelength are observed at the irradiation fluence slightly higher than the ablation threshold regardless of the target material. In addition to these nanoripples, classical or microripples with an average spacing of 1–2μm have also been registered on irradiated surfaces of Al and Si. These microripples have appeared at fluence higher than that is required for nanoripple-formation. It has been found that the formation of the laser-induced ripples is strongly dependent and quite sensitive to the incident laser fluence and the selection of material.

Properties of High-Frequency Sub-Wavelength Ripples on Stainless Steel 304L under Ultra Short Pulse Laser Irradiation

The paper concentrates on surface texturing on sub-micro meter scale with ultra short laser pulses that has several applications, e.g. changing the hydrophilic/hydrophobic performance, optical or tribological properties of materials. In general, the formations of wavy structures, or ripples on a surface irradiated by short pulse lasers has been observed experimentally since 1965, and are usually referred to as Laser Induced Periodic Surface Structures (LIPSS). Generally Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL) are observed. The existing theoretical models do not describe the origin, nor growth of the ripples satisfactorily. That is why the experimental approach still plays a leading role in the investigation of ripple formation. In this paper we study the development of HSFL and LSFL as a result of picosecond laser pulses on a surface of stainless steel. Influences of number of pulses and pulse overlap on ripples growth were examined.

Formation of periodic surface nanostructures on Ti 3+:Al 2O 3 crystals using femtosecond laser pulses

Periodic surface nanostructures are observed on Ti 3+:Al 2O 3 single crystals that have been irradiated by a single focused beam from a femtosecond pulsed laser (wavelength: 800 nm; pulse duration: 130 and 152 fs). Atomic force microscopy images of single-ablated zones and modified structures created by fixing and translating samples through the focal region of a linearly polarized laser beam reveal self-organized periodic surface nanostructures (ripples) with a subwavelength spacing, which are oriented perpendicular to the electric-field vector of the laser beam. The period of the subwavelength ripples obtained by linearly polarized laser irradiation varies from ∼ λ/5 to 2 λ/5 ( λ: incident laser wavelength) depending on the laser pulse energy. This phenomenon can be explained by assuming that the incident light field interferes with the electric field of electron plasma waves propagating inside the material; this interference periodically modulates the electron plasma density and modifies the surface ablation. In addition, for the first time, we observe screw-shaped nanostructures in the focal spot of circularly polarized beam irradiation. The morphology of these nanostructures appears to reflect the circular polarization of the laser light.

Direct fabrication of periodic patterns with hierarchical sub-wavelength structures on poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) thin films using femtosecond laser interference patterning

A simple optical interference method for the fabrication of simply periodic and periodic with a substructure on poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) using femtosecond laser interference patterns is demonstrated. The femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Homogeneous periodic arrays could be fabricated even using a single laser pulse. In addition, multipulse irradiation resulted in reproducible sub-wavelength ripples oriented perpendicularly to the laser polarization with spatial period from 170 to 220 nm (around one-fourth of the laser wavelength). In addition, the observed size of the spatial period was not affected by the number of incident laser pulses or accumulated energy density. Using high energy pulses it was possible to completely remove the PEDOT:PSS layer without inducing damage to the underneath substrate.

Sub-wavelength Ripple Formation on Silicon Induced by Femtosecond Laser Radiation

Periodic microstructures on silicon bulk are formed by the irradiation of the femtosecond laser with the laser wavelength of 800 nm and the pulse length of 130 fs. We investigate the surface periodic ripple structures produced by femtosecond laser treatment. The effects of feedrate of sample, v, on laser-induced surface topography are studied. We find that the femtosecond laser produce periodic ripples of the sub-micron level on silicon surface. At the same time, we realize the optimal conditions to produce these surface structures. When choosing N A = 0.3, and v = 2000 μm/s or 3000 μm/s, we find a series of periodic-structure ripples where the spacing is about 120 nm and the width is about 450 nm. The experimental results indicate that femtosecond laser treatment can produce line arrays on the sub-micron level, which is a positive factor for fabricating grating and other optical applications in nanoscales.

Sub-wavelength ripples in fused silica after irradiation of the solid/liquid interface withultrashort laser pulses

Laser-induced backside wet etching (LIBWE) is performed using ultrashort 248 nm laserpulses with a pulse duration of 600 fs to obtain sub-wavelength laser-induced periodicsurface structures (LIPSS) on the back surface of fused silica which is in contact with a0.5 mol l−1 solution of pyrene in toluene. The LIPSS are strictly one-dimensionalpatterns, oriented parallel to the polarization of the laser radiation, andhave a constant period of about 140 nm at all applied laser fluences(0.33–0.84 J cm−2) and pulse numbers (50–1000 pulses). The LIPSS amplitude varies due tothe inhomogeneous fluence in the laser spot. The LIPSS are examined withscanning electron microscopy (SEM) and atomic force microscopy (AFM). Theirpower spectral density (PSD) distribution is analysed at a measured area of10 µm × 10 µm. The good agreement of the measured and calculated LIPSS periods stronglysupports a mechanism based on the interference of surface-scattered and incidentwaves.