飞秒激光烧蚀硅材料表面形成周期波纹形貌研究

基于Sipe-Drude模型和表面等离子体激元(SPP)的干涉理论分别对单脉冲飞秒激光诱导硅表面形成低频率周期性波纹进行分析研究.探究了波长800 nm、脉宽150 fs的单个飞秒激光烧蚀硅造成不同激发水平下波纹形貌的变化,考虑到材料的光学性质变化(由Drude模型得到的介电常数变化),引入包含双温方程的电子数密度模型.计算结果表明,Sipe-Drude和SPP理论都适用于分析和解释高激发态下周期性波纹,但Sipe-Drude理论更适合分析更为广泛的周期性波纹结构.同时,波纹延伸方向总是垂直于入射激光偏振方向,其空间周期略小于激光波长,并受到入射激光通量的影响.在激光通量为0.38 J/cm~2时,波纹周期达到最小值.另外,还得到了不同入射角度的波纹周期变化情况,并在不同偏振态下随入射角度增大时波纹周期呈现相反的变化趋势.该研究对于理解飞秒激光造成硅表面形成周期结构及其在加工硅材料领域具有重要参考意义.

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 τ=150 fs and central wavelength λ=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 η(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 η(k) first increases until reaching a maximum at an electron density of 6×10²¹ 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 η(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.