Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

飞秒激光在6H SiC晶体表面制备纳米微结构

激光诱导周期性纳米微结构在多种材料包括电介质、半导体、金属和聚合物中观察到。研究了800nm和400nm飞秒激光垂直聚焦于6H SiC晶体表面制备纳米微结构。实验观察到800nm和400nm线偏光照射样品表面分别得到周期为150nm和80nm的干涉条纹,800nm圆偏振激光单独照射样品表面得到粒径约100nm的纳米颗粒。偏振相互垂直的800nm和400nm激光同时照射晶体得到粒径约100nm的纳米颗粒阵列,该纳米阵列的方向随400nm激光强度增加而向400nm偏振方向偏转。利用二次谐波的观点对以上纳米结构的形成给出了解释。     

Sub-wavelength surface structures on silicon irradiated by femtosecond laser pulses at 1300 and 2100 nm wavelengths

We present periodic ripples and arrays of protrusions formed on the surface of silicon after irradiation by low-fluence linearly polarized femtosecond laser pulses. Laser-induced periodic surface structures (LIPSS) are observed for irradiation at center wavelengths of 800, ∼ 1300, and ∼ 2100 nm, with the structure periods somewhat less than the incident wavelengths in air. Additionally, we observe structures with spatial periods substantially less than the incident laser wavelengths. These sub-wavelength periodic structures form only when the photon energy is less than the silicon bandgap energy. We discuss a number of factors which may contribute to the generation of this surface morphology.     

The laser polarization as control parameter in the formation of laser-induced periodic surface structures: Comparison of numerical and experimental results

Considering self-organized surface pattering upon multi-pulse femtosecond laser irradiation, in particularly the strong dependence of ripples orientation on the laser polarization, we present numerical simulations from an adopted surface erosion model and compare the result to our experimental data on laser-induced nanostructures formation. We present the surface morphologies obtained by this model for different polarizations of the incident laser electric field and show good agreement with ripple formation produced by laser ablation experiments. The correlation of ripples orientation with laser polarization can be described within a model where the polarization causes a breaking of symmetry at the surface. Further we discuss a time evolution of pattern formation. Our results support the non-linear self-organization mechanism of pattern formation on the surface of solids.     

Femtosecond laser-induced blazed periodic grooves on metals

In this Letter, we generate laser-induced periodic surface structures (LIPSSs) on platinum following femtosecond laser pulse irradiation. For the first time to our knowledge, we study the morphological profile of LIPSSs over a broad incident angular range, and find that the morphological profile of LIPSSs depends significantly on the incident angle of the laser beam. We show that LIPSS grooves become more asymmetric at a larger incident angle, and the morphological profile of LIPSSs formed at an incident angle over 55° eventually resembles that of a blazed grating. Our study suggests that the formation of the blazed groove structures is attributed to the selective ablation of grooves through the asymmetric periodic surface heating following femtosecond pulse irradiation. The blazed grooves are useful for controlling the diffraction efficiency of LIPSSs.     

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.     

Formation of periodic surface nanostructures on Ti:Al2O3 crystals using femtosecond laser pulses

Periodic surface nanostructures are observed on Ti³⁺:Al₂O₃ 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 2λ/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.     

Ripple formation in various metals and super-hard tetrahedral amorphous carbon films in consequence of femtosecond laser irradiation

Ripple formation in consequence of ultrashort laser pulse irradiation of materials is a well-known phenomenon. We have investigated the formation of ripples in various metals, i.e. steel, tungsten carbide hard metal, as well as in superhard ta-C films, where we used femtosecond laser pulses of 775 nm and 387 nm mean wavelength and 150 fs pulse duration. The aim was to investigate how the ripple parameters depend on irradiation parameters, and if such ripples have a potentiality for applications. In the paper, we will show that on smooth surfaces the ripple orientation is perpendicular to the electric field vector of the linearly polarized laser beam, as is well-known. Moreover, it will be shown that the ripple period decreases with decreasing laser wavelength and/or increasing angle of incidence of the laser beam on the substrate. By using optimum parameters large areas of the materials and films can be rippled swiftly, which would be important for applications. For instance, the improvement of frictional and wear behavior of tribologically stressed surfaces by ripples was investigated on ta-C coated steel surfaces.     

Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses

The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is investigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of the ripple structure turned left or right by 45 degrees with respect to the incident plane when using right and left circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential applications in large-area structuring of metals.     

Ultrafast time-resolved imaging of femtosecond laser-induced periodic surface structures on GaAs

We report the formation dynamics of periodic ripples on Ga As induced by femtosecond laser pulses（800 nm, 50 fs） via a collinear time-resolved imaging technique with a temporal resolution of 1 ps and a spatial resolution of 440 nm. The onset of periodic ripples emerges in the initial tens of picoseconds in the timescale of material ejection. The periodic ripples appear after irradiation of at least two pump pulses at surface defects produced by the first pulse and the ripple positions kept stable until the formation processes complete. The formation mechanisms of laser-induced periodic ripples are also discussed.     

Gold nanosphere propulsion by using femtosecond laser-excited enhanced near field

We propose nanosphere propulsion by using femtosecond laser-excited enhanced near field based on the theoretical calculations and experimental study. The optical intensity distribution and enhancement around a gold nanosphere on a silicon substrate was simulated by a 3D finite-difference time-domain method. The sphere velocities and propelled angles were calculated based on the optical intensity distribution. In our simulation, we calculated the optical intensity for the gold nanospheres with a diameter ranging from 100 to 600 nm. Calculation results show that the sphere velocity was fairly constant for the diameters ranging from 100 to 250 nm, while the velocity decreased for diameters larger than 250 nm. The propelled angle could be controlled up to only 4.6° by varying the incident angles of p-polarized waves. We have demonstrated the gold nanosphere propulsion in experiment. The gold nanospheres with a diameter of 200 nm were used in our experiments. The propelled gold particles have been melted by laser irradiation and deposited on the receiver substrate. The size and spatial distributions of gold particles have been investigated. The decrease in the laser spot size and the gap distance between the donor and receiver substrate would realize the reduction in the existence region of gold particles on the receiver substrate.     

Surface roughness assisted 100 kHz femtosecond laser induced nanostructure formation on silicon surface

Spontaneous nanostructure formations on roughened and smooth silicon surface by the femtosecond laser irradiation with the repetition rate of 100 kHz were systematically studied. In addition to the widely accepted so-called coarse ripple, which has the period analogous to the wavelength of the laser beam and aligns perpendicularly to the electric field of the incident laser beam, the ripple which has the period similar to the wavelength of the incident laser beam but aligns parallel to the electric field of the laser beam was observed on the roughened surface for the lower fluence and the higher number of pulse irradiation. Furthermore, the ensemble of dots formed by the enhancement of the local electric field was found on the roughened surface. This structure is preferentially formed around the scratches aligned perpendicularly to the electric field of the laser beam. These novel nanostructures are considered to be peculiar to the femtosecond laser irradiation and open the possibilities for precise control of the spontaneous nanostructure formation by femtosecond laser irradiation.     

On the physics of self-organized nanostructure formation upon femtosecond laser ablation

We present new results on femtosecond LIPSS on silicon, fostering the dynamic model of self-organized structure formation. The first set of experiments demonstrates LIPSS formation by irradiation with a femtosecond white light continuum. The ripples are, as usual, perpendicular to the light polarization with a fluence-dependent wavelength between 500 and 700 nm. At higher dose (fluence × number of shots), the LIPSS turn to much coarser structures. The second set of experiments displays the dose dependence of pattern evolution at about threshold fluence. In contrast to the general case of multi-pulse LIPSS, where a strong dependence of the structures on the laser polarization is observed, single-shot exposition of silicon at about the ablation threshold results in a concentric pattern of very regular sub-wavelength ripples following the oval shape of the irradiated spot, without any reference to the laser polarization. When increasing the number of pulses, the usual, typical ripples develop and then coalesce into broader perpendicular structures, interlaced with remnants of the first, finer ripples.     

Laser damage studies of silicon surfaces using ultra-short laser pulses

A comparative study of damage morphology in GaAs induced by s- , p- and linearly polarized laser light (1.064 μm, 35 ps) is presented. For linearly polarized light damage initiates in the form of pits. This material damage occurs below the surface. For s- or p-polarized light material damage involves only the surface layer. For larger fluences or number of pulses the differences are less marked and the damage morphology occurs in a similar manner either for linearly polarized or s- or p-polarized light. Ripples are formed when multiple irradiation is used due to interference between the front and back faces of the test sample. The spacing of these ripples is 3 μm, which is in good accordance with the reported work of Guosheng et al. (Phys. Rev. B 26 (1982) 5366).     

Femtosecond laser induced nanostructure formation: self-organization control parameters

In self-organized nanostructure formation upon femtosecond laser ablation, the laser polarization is an important control parameter. Experiments on fluoride crystals, using circular and elliptical polarization, study this influence in more detail. For circular polarization, spherical nanoparticles of about 100 nm diameter are formed. With increasing ellipticity, longer and longer ordered chains and linear structures are generated, oriented perpendicular to the long axis of the polarization ellipse. A similar effect occurs when, for circular polarization, the angle of incidence is varied from normal to 45°: the s-component of the incident field, not attenuated by the projection, determines length and orientation of the ordered ripples. However, surface defects like scratches exert an even stronger influence on the ripples orientation than the polarization, resulting in curved structures bending from polarization-controlled to defect-controlled orientation. Since the structure formation takes place only long after the end of the laser pulse, a certain electrical field memorizer is required to account for this polarization dependence. A promising approach assumes directional atomic surface diffusion anisotropies, arising, e.g. from plasmon-coupled metal–colloid arrays.     

Forming mechanisms and wettability of double-scale structures fabricated by femtosecond laser

Constructing double-scale structures on material surface is helpful for achieving its superhydrophobicity. Therefore, it is quite necessary to investigate the forming mechanisms of double-scale structures on material surface. In the study, the forming mechanisms of various structures induced by femtosecond laser on the K9 glass surface are discussed, such as crater, ripples, and cones. The surface with double-scale structures is fabricated by femtosecond laser. On the surface, the apparent contact angle and sliding angle of a droplet are 152.3±1.5° and 4.6±0.8°, respectively. Lastly, through theoretical and experimental analyses, we demonstrate that the double-scale structures are effective for realizing superhydrophobicity of material surface and restricting the wetting transition from Cassie mode to Wenzel mode.     

节瘤缺陷对中红外高反射膜电场增强影响的数值分析

运用时域有限差分(FDTD)方法建立了薄膜中节瘤缺陷在高斯激光照射下电磁场响应模型，分析了节瘤的深度、起始颗粒大小和入射光角度对薄膜中电磁场的影响。结果表明：节瘤缺馅对薄膜中电场强度有显著的加强作用，其内部峰值场强是入射光的6倍，大而浅节瘤缺陷对倾斜入射p偏振态的激光具有最高的加强效应.     

Structure formation on the surface of alloys irradiated by femtosecond laser pulses

Laser-induced periodic surface structures with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (120 fs, 800 nm, 1 Hz to 1 kHz pulse repetition frequency) irradiation on alloys. With the increasing number of pulses, nanoripples, classical ripples and modulation ripples with a period close to half of classical ripples have all been induced. The generation of second-harmonic has been supposed to be the main mechanism in the formation of modulation ripples.     

Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC

Raman spectroscopy was performed to investigate microscopic structural changes associated with a ripple structure formation initiated by femtosecond laser irradiation on the surface of single-crystal silicon carbide. The amorphous phases of silicon carbide, silicon, and carbon were observed. The intensity ratio between amorphous silicon carbide and silicon changed discretely at the boundary between fine and coarse ripples. The physical processes responsible for the formation of the ripple structure are discussed.     

Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser

Fabrication of superhydrophobic surfaces induced by femtosecond laser is a research hotspot of superhydrophobic surface studies nowadays. We present a simple and easily-controlled method for fabricating stainless steel-based superhydrophobic surfaces. The method consists of microstructuring stainless steel surfaces by irradiating samples with femtosecond laser pulses and silanizing the surfaces. By low laser fluence, we fabricated typical laser-induced periodic surface structures (LIPSS) on the submicron level. The apparent contact angle (CA) on the surface is 150.3°. With laser fluence increasing, we fabricated periodic ripples and periodic cone-shaped spikes on the micron scale, both covered with LIPSS. The stainless steel-based surfaces with micro- and submicron double-scale structure have higher apparent CAs. On the surface of double-scale structure, the maximal apparent CA is 166.3° and at the same time, the sliding angle (SA) is 4.2°.