| Laser Induced Periodic Surface Structure(LIPSS)is a prevalent physical phenomenon observed in laser ablation,characterized by the formation of closely spaced parallel grooves with a strong dependence on laser polarization.Due to its simple fabrication process(a single step),wide applicability,and low cost,LIPSS has emerged as a versatile surface texturing technique and has gained significant attention in various fields,including optics,electronics,fluid dynamics,mechanical engineering,and medicine.Despite decades of research that have led to significant developments in the theory,technology,and applications of LIPSS,it still faces some challenges.Firstly,the formation mechanism remains unclear.The current consensus among researchers is that the interaction between incident light and the excited surface electromagnetic waves leads to the inhomogeneous redistribution of electric fields,driving the ablation of periodic structures.However,there is still ongoing debate about which surface electromagnetic waves play the dominant role.In particular,the formation mechanism of deep sub-wavelength LIPSS lacks a convincing explanation.Secondly,controlling the periodicity of LIPSS is crucial for their versatility and applicability.Existing research has explored various means of controlling LIPSS periodicity,including pulse number,pulse energy,laser wavelength,pulse duration,and material properties.However,these methods have their limitations.The first two approaches can only achieve limited control over LIPSS periodicity within a small range,while the latter three are challenging to modify in real-time during experiments.Lastly,there is a tradeoff between processing accuracy and efficiency in LIPSS fabrication,which is a common challenge in almost all laser processing techniques.Addressing the aforementioned challenges in the development of LIPSS,this paper focuses on tackling the first two issues.Through systematic experiments,combined with relevant theoretical and numerical models,we investigate the formation mechanism of deep subwavelength ripples.Furthermore,we propose a non-reciprocal nanoripples writing technique that enables real-time and continuous control of the nanoripples periodicity.The main research objectives and achievements are summarized as follows:(1)To address the unclear evolution process of deep subwavelength nanoripples,we designed an experimental scheme that involved point-by-point variations of pulse energy and pulse number under tight focusing conditions.Additionally,we built a femtosecond laser direct writing system tailored to the experimental requirements.The experimental results presented six types of structures induced by femtosecond laser.From these results,we derived the threshold effects and pulse accumulation effects in femtosecond laser direct writing.Building upon the experiments,we constructed a selfconsistent semi-quantitative electromagnetic model based on Maxwell’s equations.This model proposed that the combination of optical far-field(incident laser)and the nearfield redistribution caused by structural scattering drives nanoscale ablation.We categorized the evolution of nanoripples into four stages,including seed structure formation,growth of seed structures into nanogrooves,formation of secondary seed structures,and formation of secondary nanogrooves.By cycling through these four stages,nanoripples progressively form under the driving force of redistributed near field.Furthermore,we investigated the evolution process of other induced structures by combining experiments and simulations.The model of redistributed near field driving nanoscale structure evolution exhibited excellent agreement with all types of structures,demonstrating its accuracy and self-consistency.(2)To address the challenge of distinguishing the influence of optical far-field in nanoripple formation,we employed spatial beam shaping techniques to transform a highly symmetric circular Gaussian beam into a rectangular Gaussian beam with an aspect ratio of 7:3.By rotating the rectangular beam and manipulating laser polarization,we generated four types of focused spots(optical far field)and fabricated induced structure arrays with point-by-point variations of pulse energy and pulse number.From the experimental results,it was determined that the optical far field determines the formation of initial seed structures,while in the subsequent redistribution near-field,the optical far field serves as the energy source,influencing the range and contour of the redistributed near field,thereby affecting the distribution and dimensions of the nanoripples。(3)Based on a comprehensive understanding of the formation mechanism of deep subwavelength nanoripples,we propose a non-reciprocal nanoripple writing technique.By utilizing the naturally triangular-shaped focused spot of a reflective objective,we discovered the dependence of nanoripple periodicity on the scanning direction of an asymmetric focused spot.Through a simple change in the scanning direction,we achieved real-time and continuous control of nanoripple periodicity,ranging from 47nm(13.7% λ)to 112 nm(32.7% λ),with a maximum control of up to 33 nm(close to λ/10).Additionally,using our developed semi-quantitative electromagnetic model,we revealed that the non-reciprocity of nanoripples stems from the asymmetry of the focused spot.Furthermore,we utilized a customized paper mask to further control the symmetry of the focused spot,thereby altering the dependence of nanoripple periodicity on the scanning direction.In conclusion,this thesis provides a systematic study on the formation mechanism of LIPSS,with a particular focus on deep subwavelength nanoripples.From experimental results to theoretical models,it offers a more physically insightful understanding of the nanoripple formation mechanism,which can potentially advance the development of surface texturing techniques with higher precision and better controllability.The proposed non-reciprocal nanoripple writing technique also introduces a new degree of freedom for the control of LIPSS periodicity,enhancing the practicality and versatility of LIPSS technology. |
