Subwavelength Ripples Research Articles

2D compound structures with deep subwavelength period on silicon fabricated by double time delayed femtosecond laser beams

In this work, two orthogonally polarized femtosecond laser beams are employed to irradiate a p-doped silicon wafer with an electrical resistivity of 0.008 Ω·cm. It is interesting to find that 2D compound structures composed of sub-wavelength periodic ripples and deep sub-wavelength nanodot array can be produced when proper laser fluence and time delay between the dual laser beams are used. The formation of the periodic ripples can be explained by the interference between the preceding incident laser and it induced surface plasmon polaritons (SPPs). The periodic nanodot array has a period down to ∼200 nm and the radius of the nanodot is ∼30 nm, most of which appear at the boundary between the ditch and ridge of the ripple. During the ripples’ formation, the residual melting silicon is most probably located at the boundary between the ditch and ridge of the ripple. Furthermore, the period of the nanodot array is roughly equal to the perimeter of the nanodot. Therefore, it is considered that the dot array may be generated due to the Rayleigh-Taylor instability of the melting silicon. It is also noted that these nanodots are all uniformly arranged along vertical lines, indicating that the subsequent incident laser may break the stochastic characteristic of the Rayleigh-Taylor instability and produce the 2D periodic dot array. The thermo-hydrodynamical process combined with the interference effect between SPPs and the incident laser can benefit the formation of complex surface structures with versatile functions.

Femtosecond Laser‐Induced Periodic Surface Structuring of the Topological Insulator Bismuth Telluride

AbstractSurface structuring of topological insulator Bi2Te3 single crystals with femtosecond laser by varying pulses N and energy E is reported. Interesting effects related to laser‐induced periodic surface structures formation in this class of materials are evidenced. At low pulse energy, a clear formation of periodic, subwavelength ripples oriented orthogonally to the laser polarization is observed; those are restricted to an annular region surrounding a featureless central disk as the laser energy progressively increases. The structural analysis shows that some degree of crystallinity is preserved in the rippled area, but the central disk is amorphous resembling what is observed for germanium (Ge) and is associated with the hindering of surface structure formation due to a thick melted surface layer. Interestingly, at larger fluence or number of pulses, a transition to suprawavelength grooves occurs within the annular region covered by surface structures. The findings demonstrate a clear incubation behavior, suggesting that the formation of laser‐induced periodic surface structures is coherent with the general features of the process already reported for other materials. However, the disappearance of these structures in the central area, possibly resulting from the influence of the depth of the melt layer, indicates a mixed behavior for Bi2Te3.

Periodic Surface Structuring of Copper with Spherical and Cylindrical Lenses

The use of a cylindrical lens in femtosecond laser surface structuring is receiving attention to improve the processing efficiency. Here, we investigate the structures produced on a copper target, in air, by exploiting both spherical and cylindrical lenses for beam focusing, aiming at elucidating similarities and differences of the two approaches. The morphological features of the surface structures generated by ≈180 fs laser pulses at 1030 nm over areas of 8 × 8 mm2 were analyzed. For the spherical lens, micron-sized parallel channels are formed on the target surface, which is covered by subwavelength ripples and nanoparticles. Instead, the cylindrical lens leads to a surface decorated with ripples and nanoparticles with a negligible presence of micro-channels. Moreover, the morphological features achieved by focusing ≈180 fs laser pulses at 515 nm with the cylindrical lens and varying the scanning parameters were also studied. The experimental results evidence a direct effect of the hatch distance used in the scanning process on the target surface that contains dark and bright bands corresponding to regions where the rippled surface contains a richer decoration or a negligible redeposition of nanoparticles. Our findings can be of interest in large area surface structuring for the selection of the more appropriate focusing configuration according to the final application of the structured surface.

Effects of supra-wavelength periodic structures on the formation of 1D/2D periodic nanostructures by femtosecond lasers

Femtosecond laser-induced periodic surface structure (fs-LIPSS) is a high-throughput parallel laser patterning method to fabricate various surface structures of sub-micrometer scale; the morphology and regularity of the structured surface determine its function. In this paper, single and double femtosecond laser beams are used to fabricate nanostructures on a silicon substrate. In the experiments, it is found that the appearance of supra-wavelength structures with periods longer than the laser wavelength not only deteriorate the regularity of the sub-wavelength periodic ripples but also hinder the fabrication of the deep sub-wavelength ripples when a single femtosecond laser is used. In addition, when double time-delayed femtosecond laser beams are used, 2D nanostructures, such as hexagonally arranged nano-hole array, cannot be fabricated once the supra-wavelength structures appear. The formation mechanism of the supra-wavelength periodic structures, which cannot be explained by the interference between the incident laser and surface plasmon polaritons, may be attributed to the hydrothermal wave on the molten surface layer. These findings provide insights to understand the mechanisms of 1D and 2D nanostructures produced by single and dual femtosecond laser beams, which is critical for fabricating functional surfaces.

Ripple patterns over oblique Ar+ sputtered SiC/Si(1 1 1) surfaces: Role of preferential sputtering

Controlled surface structuring of amorphous silicon carbide (a-SiC) thin films as grown on silicon substrates Si(111) by Radio Frequency (RF) sputtering and irradiation of obliquely incident 80 keV Ar+ ion beam has been investigated. Sub-wavelength ripple patterns with wave-vector parallel to the ion beam projection are found to evolve on argon sputtered surfaces. Studies reveal that the temporal parameters such as ripple wavelength and amplitude, ordering and homogeneity of these patterns vary non-linearly with argon ion fluence. The formation of such surface structures is attributed to the preferential sputtering of silicon in comparison to carbon. Our results reveal that ion irradiation can be exploited to fabricate controlled surface nanostructures by optimizing the irradiation parameters.

Dynamic Structural Color Display Based on Femtosecond Laser Variable Polarization Processing

AbstractVivid structural colors, inspired by butterflies, which can display iridescent colors dynamically when white light illuminates the surface with subwavelength period ripples, have attracted significant interest for its potential application in the fields of information storage, anti‐counterfeiting, etc. However, there is still a huge demand for the development of multipattern dynamically transformable display surfaces using a versatile method. A new scheme based on femtosecond laser combined with variable polarization processing has been proposed. This scheme enables a surface with a structural color function that can be easily prepared by adjusting the polarization of the laser via a half‐wave plate. Four types of structural units with designated periodic orientations are written on the same sample to exhibit four different colorful images. Through experiment and analysis, it is demonstrated that different colors can be displayed by changing the observation angle, and a multi‐pattern display can be realized by rotating the sample. The lithography‐free inscribed patterns show a strong azimuthal angular sensitivity to color and its brightness. This work demonstrates that femtosecond laser surface structuring provides a versatile platform to create various micro/nanostructures on a solid surface, and this technique can be used for the biomimetic lithography‐free processing of any material using ablation.

Periodic subwavelength ripples on a Si surface induced by a single temporally shaped femtosecond laser pulse: enhanced periodic energy deposition and reduced residual thermal effect

In the fabrication of regular, uniform periodic ripples produced by femtosecond laser pulses, enhancing the periodic energy deposition and reducing the residual heat are two basic goals. This paper reports the formation of periodic subwavelength ripples on a silicon surface induced by a single shaped 800 nm femtosecond laser pulse. Using periodic π-phase step modulation, a Gaussian pulse is shaped into a pulse train with varying temporal intervals in the range of 0.1–7.2 ps. The single shaped pulse is applied to a silicon surface to produce periodic subwavelength ripples. The results show that when the interval between two subpulses is greater than 0.9 ps, periodic ripples can be observed at the center of the ablation crater. Increasing the subpulse interval can effectively reinforce the formation of periodic ripples. The two-temperature model and Drude model (TTM–Drude model) are applied to study the evolution of the electron density, electron temperature, and lattice temperature after irradiation by a single shaped pulse. The results show that the temporally shaped femtosecond laser pulse can enhance the excitation of surface plasmon polaritons and the periodic energy deposition while reducing residual thermal effects on the silicon surface, eventually resulting in periodic ripples at the center of the ablation area.

Adjustment of Subwavelength Rippled Structures on Titanium by Two-Step Fabrication Using Femtosecond Laser Pulses

An effective approach is proposed to adjust the surface morphology induced by using a femtosecond laser, including the area and period of rippled structures. The effect of the processing steps and laser polarization on the surface morphology of rippled structures on a titanium surface was experimentally investigated in this study. A processing sequence was designed for two series of femtosecond laser pulses that irradiate a titanium surface, for example, N = 50(0°) + 50(90°). The experimental results show that the area and period of rippled structures can be simultaneously adjusted by following a two-step method. Due to the enhancement of energy absorption and SP-laser coupling of the initial rippled structures, large area surface structures with small periods are fabricated using two series of femtosecond laser pulses with the same polarization direction. By changing the polarization direction of the two series of femtosecond laser pulses, the recording, erasing, and rewriting of subwavelength ripples is achieved. During the rewriting process, material removal and the formation of new ripples simultaneously occur.

Periodic nanopatterning and reduction of graphene oxide by femtosecond laser to construct high-performance micro-supercapacitors

Reduction and patterning of graphene oxide (GO) have been extensively applied in the field of electronic devices such as sensor, transistors and power source. Surface morphology and reduction level of reduced graphene oxide (rGO) directly influence the performance of devices. Herein, we put forward a new method of one-step periodic nanopatterning and reduction of GO using femtosecond laser pulses in air environment. Three-dimensional (3D) laser induced periodic surface structures (LIPSSs) with subwavelength ripples and flakes were obtained at the irradiation of laser fluences higher than the threshold of GO reduction (∼9 mJ/cm2). Meanwhile, the change in chemical compositions and phase structure transformation of femtosecond laser reduced GO (Fs-LRGO) was identified through X-ray photoelectron spectroscopy, Raman spectroscopy and X-ray diffraction. The excited surface plasma polaritons (SPPs) and photochemical effect were put forward to reveal the mechanism in the formation and reduction of Fs-LRGO with nanopatterns. By virtue of the periodic nanopatterns of Fs-LRGO, we also constructed the all-solid-state interdigital rGO micro-supercapacitors (MSCs) with ultralow electrode interspace of ∼10 μm and different connection ways, indicative of superior flexibility and cycle stability. This work provides new insights into the fundamental mechanism and utility of ultrafast periodic nanopatterning and reduction of GO for the application of multifunctional electronic device engineering.

Large-area commercial-grating-quality subwavelength periodic ripples on silicon efficiently fabricated by gentle ablation with femtosecond laser interference via two cylindrical lenses

This paper presents a new strategy for efficiently fabricating large-area subwavelength periodic ripples with commercial grating quality on a silicon surface. Two laser beams of 800 nm, 50 fs at a repeat frequency of 1 kHz were focused on a silicon wafer via two cylindrical lenses. The spacing of the interference fringes was adjusted to excite a coherent resonance-enhanced (CRE) surface plasmon polariton (SPP). The CRE SPP efficiently induced homogeneous and extremely straight periodic ripples through gentle ablation, which avoids substantial debris and reduces the roughness of the periodic ripples. At the two edges of the ablation band after single scanning, the ripples were shallow, spaced, and exceptionally straight, which is greatly advantageous in the preparation of large-area gratings line by line. The subwavelength periodic ripples were fabricated on a silicon wafer with a diameter of 100 mm by scanning at a speed of 2 mm/s and a spacing of 1.55 mm. Compared with the alternating amorphous-crystalline ripples induced by a single laser beam, the fabrication efficiency was enhanced by 2–3 orders. Measurements with a confocal optical microscope and white light diffraction demonstrate that the periodic ripples were extremely straight, smooth, and homogeneous, similar to the gold gratings. The diffraction resolution of the nanogratings of subwavelength periodic ripples was nearly as narrow as that of commercial gratings.

Ultrafast dynamics of subwavelength periodic ripples induced by single femtosecond pulse: from noble to common metals

The electron energy band structures and optical properties of silver and nickel are notably different, and they are identified as representative of noble and common metals, respectively. The ultrafast dynamics of the formation of periodic ripples on the surface of silver and nickel films induced by a single 800 nm, 50 fs laser pulse was investigated using the collinear pump-probe imaging method. The dependence of the transient ripple periods on the laser fluences was also studied.A surface plasmon polariton (SPP) model was developed to explain the formation of periodic ripples on noble and common metals induced by a femtosecond laser pulse. The evolution of the electron and the lattice temperature was investigated by numerically solving the equations of the two-temperature model. In addition, the effects of electron collision, localization of hot electrons, and inter-band transition on the dielectric constant in the excited states are discussed in detail as functions of laser fluence. The ripple periods obtained using the developed SPP model agree well with the experimental results, which supports the view that the SPP excitation plays an important role in the formation of periodic ripples. This model is expected to be a general approach for understanding the formation of subwavelength periodic ripples on metals induced by a femtosecond laser pulse.

Hierarchical anti-reflective laser-induced periodic surface structures (LIPSSs) on amorphous Si films for sensing applications.

Here, we applied direct laser-induced periodic surface structuring to drive the phase transition of amorphous silicon (a-Si) into nanocrystalline (nc) Si imprinted as regular arrangement of Si nanopillars passivated with a SiO2 layer. By varying the laser beam scanning speed at a fixed pulse energy, we successfully tailored the resulting unique surface morphology of the formed LIPSSs that change from ordered arrangement of conical protrusions to highly uniform surface gratings, where sub-wavelength scale ripples decorate the valleys between near-wavelength scale ridges. Along with the surface morphology, the nc-Si/SiO2 volume ratio can also be controlled via laser processing parameters allowing the tailoring of the optical properties of the produced textured surfaces to achieve anti-reflection performance or partial transmission in the visible spectral range. Diverse hierarchical LIPSSs can be fabricated and replicated over large-scale areas opening a pathway for various applications including optical sensors, nanoscale temperature management, and solar light harvesting. By taking advantage of good wettability, enlarged surface area and remarkable light-trapping characteristics of the produced hierarchical morphologies, we demonstrated the first LIPSS-based surface enhanced fluorescent sensor that allowed the identification of metal cations providing a sub-nM detection limit unachievable by conventional fluorescence measurements in solutions.

Picosecond Laser-Induced Hierarchical Periodic Near- and Deep-Subwavelength Ripples on Stainless-Steel Surfaces.

Ultrafast laser-induced periodic surface subwavelength ripples, categorized based on the ripple period into near-subwavelength ripples (NSRs) and deep-subwavelength ripples (DSRs), are increasingly found in the variety of materials such as metals, semiconductors and dielectrics. The fabrication of hierarchical periodic NSRs and DSRs on the same laser-irradiated area is still a challenge since the connection between the two remains a puzzle. Here we present an experimental study of linearly polarized picosecond laser-induced hierarchical periodic NSRs and DSRs on stainless-steel surfaces. While experiencing peak power density higher than a threshold value of 91.9 GW/cm2, in the laser-scanned area appear the hierarchical periodic NSRs and DSRs (in particular, the DSRs are vertically located in the valley of parallel NSRs). A large area of the uniformly hierarchical periodic NSRs and DSRs, with the spatial periods 356 ± 17 nm and 58 ± 15 nm, respectively, is fabricated by a set of optimized laser-scanning parameters. A qualitative explanation based on the surface plasmon polariton (SPP) modulated periodic coulomb explosion is proposed for unified interpretation of the formation mechanism of hierarchical periodic NSRs and DSRs, which includes lattice orientation of grains as a factor at low peak power density, so that the initial DSRs formed have a clear conformance with the metallic grains.

Formation of Slantwise Surface Ripples by Femtosecond Laser Irradiation.

We report on the formation of slantwise-oriented periodic subwavelength ripple structures on chromium surfaces irradiated by single-beam femtosecond laser pulses at normal incidence. Unexpectedly, the ripples slanted in opposite directions on each side the laser-scanned area, neither perpendicular nor parallel to the laser polarization. The modulation depth was also found to change from one ripple to the next ripple. A theoretical model is provided to explain our observations, and excellent agreement is shown between the simulations and the experimental results. Moreover, the validity of our theory is also confirmed on bulk chromium surfaces. Our study provides insights for better understanding and control of femtosecond laser nanostructuring.

Modelling periodic structure formation on 100Cr6 steel after irradiation with femtosecond-pulsed laser beams

We investigate the periodic structure formation upon intense femtosecond pulsed irradiation of chrome steel (100Cr6) for linearly polarised laser beams. The underlying physical mechanism of the laser-induced periodic structures is explored, their spatial frequency is calculated and theoretical results are compared with experimental observations. The proposed theoretical model comprises estimations of electron excitation, heat transfer, relaxation processes, and hydrodynamics-related mass transport. Simulations describe the sequential formation of sub-wavelength ripples and supra-wavelength grooves. In addition, the influence of the laser wavelength on the periodicity of the structures is discussed. The proposed theoretical investigation offers a systematic methodology towards laser processing of steel surfaces with important applications.

Vortex-controlled morphology conversion of microstructures on silicon induced by femtosecond vector vortex beams

We demonstrate the morphology conversion of surface microstructures on silicon induced by femtosecond vector vortex beams. By tuning the topological charge of the vortex phase carried by the vector beam, we achieve the transition of ablation crater between subwavelength ripples and hole, and the switching of ripple orientation. It is shown that the vortex phases give rise to the converting of the two polarization components of the focused vector beams, and produce dramatically different polarization and intensity distributions in the focal fields. Such vortex-dependent focal fields of femtosecond vector beams are experimentally generated to realize the morphology conversion of surface microstructures on silicon. Our results not only enable the realization of manipulating the laser-induced morphology but also support the visualized mapping of the polarization state of the focused vector beams.

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.

The influences of surface plasmons and thermal effects on femtosecond laser-induced subwavelength periodic ripples on Au film by pump-probe imaging

In this paper, the influences of surface plasmons and thermal effects on the formation of subwavelength periodic ripples on Au films irradiated by 800 nm femtosecond laser pulses were studied by collinear pump-probe imaging. The spatial and temporal resolutions of the experiment were 300 nm and 1 ps, respectively. No periodic ripples were observed on the Au film during ablation by the first pump pulse, but during ablation by the second pump pulse the appearance of transient ripples was evident from a delay time of hundreds of picoseconds to several nanoseconds. These ripples, however, were not retained after solidification. When the sample was immersed in water during ablation, however, the ripples were retained. It is proposed that, during the second laser pulse irradiation, the surface defects produced by the first pulse induced surface plasmon polaritons on the Au film, which caused a modulated energy deposition and the formation of transient ripples. The weak electron–phonon coupling and significant residual heat erase these ripples after the molten surface was solidified.

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.

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.