Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser
Schematic diagram of regular and deep periodic surface structures induced by temporally shaped femtosecond laser pulse.
Laser-induced periodic surface structures (LIPSS) provide a direct laser writing method for fabricating nano-gratings on sample surfaces. These periodic nano-structures efficiently modify the properties of materials and have many applications in surface coloring, large-area grating, birefringence optical elements, data storage, and surface wettability.
The formation mechanism of low-spatial-frequency LIPSS (LSFL) with a spatial period Λ > λ∕2, where λ is the laser wavelength, was widely accepted as a result of the laser energy periodic distribution induced by surface plasmon polaritons (SPPs).
During LSFL formation on the semiconductor and metal surfaces induced by traditional femtosecond laser pulses (with a repetition frequency of 1 kHz), the deposited debris and the residual heat significantly affected the SPPs excitation, propagation, and light field distribution during the subsequent laser irradiation, resulting in significant distortions and bifurcations on LSFL. Three major challenges exist in the fabrication of regular and uniform LSFL: enhancing the periodic energy deposition, reducing the residual heat, and avoiding the deposited debris.
In order to solve the problems, Prof. Tianqing Jia's group from the State Key Laboratory of Precision Spectroscopy at East China Normal University, China, has proposed and demonstrated extremely regular periodic surface structures efficiently induced by temporally shaped femtosecond laser, which was published in Photonics Research, Vol. 9, Issue 5, 2021 (Yuchan Zhang, Qilin Jiang, Kaiqiang Cao, Tianqi Chen, Ke Cheng, Shian Zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun and Jianrong Qiu. Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser[J]. Photonics Research, 2021, 9(5): 050839).
4f configuration zero-dispersion pulse shaping system is able to generate nearly arbitrarily shaped ultrafast optical wave forms, which means is capable to control the ultrafast process of the interaction between femtosecond laser pulse and materials.
Based on 4f configuration zero-dispersion pulse shaping system, a Fourier transform limit (FTL) pulse is shaped into a pulse train with varying intervals in the range of 0.25–16.2 ps using periodic π-phase step modulation. Under the irradiation of the shaped pulse with an interval of 16.2 ps, extremely regular LSFL are efficiently fabricated on silicon.
The ultrafast imaging results demonstrated that the transient LIPSS began to form on the sample surface after several to tens of picoseconds (ps) after the laser irradiated on the Si surface. Therefore, there are transient LIPSS on Si surfaces after irradiation by the two main sub-pulses of 16.2 ps.
When the subsequent sub-pulse reaches the surface, the surface changes to a metal-like state and effectively supports the excitation of SPPs. The transient LIPSS induced by the previous sub-pulses enhance the excitation of SPPs and the periodic laser field. The substrate in the surface region remains at very high temperature when the subsequent subpulse reaches the Si surface. Part of the material is further excited and ejected from the surface, and it removes the deposited heat (ablation-cooling effect).
The ejected materials by the previous sub-pulse, including plume and debris, will be further excited by the subsequent sub-pulses, and the debris is further ionized and vaporized into aerosol, resulting in less deposited particles. The SPPs and the periodic laser energy deposition are enhanced, and the residual heat is reduced. Therefore, regular and deep LSFL can be induced on a Si surface by the shaped pulse of 16.2 ps.
The fabrication efficiency, LSFL depth, and regularity using a shaped pulse of 16.2 ps are significantly better than those by Fourier transform limit (FTL) femtosecond laser pulse. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is 2 times deeper, and the diffraction efficiency is 3 times higher than those of LSFL using the FTL pulse. The large-area LSFL demonstrate a very bright and pure structural color from blue to red, observed at different angles.
The enhancement of SPPs and the reduced of residual heat are two main factors which affect the formation of LIPSS. According to previous reports, the transient LIPSS became deep and clear at a delay time of 30–300 ps after irradiated by a single femtosecond laser pulse. Kerse et al. predicted and experimental demonstrated that the ablation–cooling effect was very clear at a repetition rate higher than GHz (corresponding to an interval less than 1 ns).
Thus, the group estimate that the optimum interval of sub-pulse is in a range of 30–300 ps. Due to the limitation of the size of optical table, the interval of sub-pulses cannot be increased any longer. The group are trying best to construct an F-P cavity to generate a pulse train with a larger interval, which will be constructed in the near future.
Prof. Jianrong Qiu, an optics expert from the same optics group, says: "Temporally shaped femtosecond pulse has potential to control the ultrafast dynamics during the interaction between femtosecond laser and materials. This work is of great significance to promote LIPSS to industrial application."
时频操控飞秒激光，高效制备周期结构
时域整形飞秒激光脉冲诱导规则周期性表面结构原理示意图
超快激光诱导周期性表面结构（laser-induced periodic surface structures, LIPSS）可以有效地调控材料特性，在表面着色、大面积光栅、双折射光学元件、光存储和表面润湿性等方面具有广阔的应用前景。调控和高效制备规则、均匀、深的LSFL（low-spatial-frequency LIPSS）是实现这些功能的关键。
关于空间周期大于1/2入射光波长的低空间频率LIPSS的形成机制，通常认为是表面等离激元（surface plasmon polaritions, SPP）诱导的激光能量周期性沉积的结果。
传统飞秒激光脉冲（重复频率为1 kHz）在半导体和金属表面诱导形成LSFL的过程中，烧蚀喷出物沉积在表面的碎屑和烧蚀剩余热会严重影响SPP的激发、传播和后续激光照射时光场的分布，导致LSFL产生明显的弯曲和分叉。
目前，在加工规则且均匀的LSFL过程中主要存在以下三个挑战：增强周期性能量沉积、减少烧蚀剩余热、避免碎屑的沉积。
为了解决以上问题，华东师范大学精密光谱科学与技术国家重点实验室的贾天卿教授课题组提出并证明了利用时频域整形飞秒激光高效率地制备非常规则的周期表面纳米结构，文章发表在Photonics Research2021年第5期 (Yuchan Zhang, Qilin Jiang, Kaiqiang Cao, Tianqi Chen, Ke Cheng, Shian Zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun and Jianrong Qiu. Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser[J]. Photonics Research, 2021, 9(5): 050839)。
4f结构零色散脉冲整形系统可以产生任意时域形状的超快激光脉冲，可以控制超快激光与材料相互作用的超快动力学过程。利用周期性π相位阶跃调制，将傅里叶极限（Fourier transform limit, FTL）脉冲整形成为飞秒脉冲序列，脉冲序列内的子脉冲间隔在0.25-16.2 ps范围内可调。利用子脉冲间隔为16.2 ps的脉冲序列，在硅表面上高效率加工出了非常规则的LSFL。
超快成像结果表明，在飞秒激光照射硅表面后的几皮秒到几十皮秒内，样品表面开始形成瞬态LIPSS。因此，当子脉冲间隔为16.2 ps的脉冲序列内的两个最强的子脉冲照射硅表面后，表面会形成瞬态LIPSS。当后续子脉冲到达表面时，表面处于类金属态，能有效支持SPP的激发。前面子脉冲诱导形成的瞬态LIPSS增强了SPP的激发和周期性光场分布。
此外，当后续子脉冲照射硅表面时，表面区域内仍保持一个高温状态，部分材料被进一步激发并喷出表面，带走沉积的热量（烧蚀-冷却效应）。前面子脉冲产生的喷出物，包括羽流和碎屑，会被后续子脉冲进一步激发，碎屑被电离和气化成气溶胶，使得沉积颗粒大大减少。SPP和周期性激光能量沉积得到增强，同时剩余热和沉积的碎屑也大大减少。因此，子脉冲间隔为16.2 ps的脉冲序列在硅表面高效地加工出了规则且较深的LSFL。
该LSFL的效率、深度和规则性都大大优于传统飞秒脉冲。相较于传统飞秒脉冲，加工规则LSFL的扫描速度提高至2.3倍，同时LSFL条纹深度提高至2倍，制备的光栅的衍射效率提高至3倍。在不同观察角度下，大面积LSFL展现出了从蓝色到红色的明亮且纯净的结构色。
增强SPP和减少剩余热是影响LIPSS形成的两个主要因素。根据前期报道，在单个飞秒脉冲照射之后的30-300 ps的延迟时间内，瞬态LIPSS逐渐加深且清晰。Kerse等人预测且实验性地证明了当重复频率高于GHz（对应子脉冲间隔小于1 ns）时，烧蚀-冷却现象效应非常明显。
因此，该课题组估计最优的子脉冲间隔在30-300 ps范围内。由于光学平台尺寸的限制，子脉冲的间隔无法继续增大。该课题组正在搭建基于法布里珀罗干涉仪，用来产生子脉冲间隔的更大的脉冲序列。
浙江大学的邱建荣教授认为：“时频域整形飞秒激光脉冲在控制飞秒激光与物质相互作用的超快动力学上有极大的潜力。这项工作在将LIPSS推向工业应用上有非常重要的意义。”