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中国物理学会期刊

用于飞秒激光纳米加工的TiO2粒子阵列诱导多种基底表面近场增强

CSTR: 32037.14.aps.66.144203

Near field enhancement of TiO2 nanoparticle array on different substrates for femtosecond laser processing

CSTR: 32037.14.aps.66.144203
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  • 利用纳米粒子辅助对飞秒激光能量进行空间局域化,使其在基底表面诱导产生纳米尺度的近场增强,这对超衍射极限微结构加工具有重要意义.目前对于粒子阵列诱导飞秒激光纳米孔加工的研究仅限于金属Au粒子及低折射率聚苯乙烯介电粒子等,本文提出并开展了应用高折射率TiO2介电粒子阵列作为辅助诱导激光近场增强从而进行飞秒激光超衍射纳米孔加工的研究.对TiO2介电粒子阵列在Si,Pt及SiO2表面的近场强度分布进行了数值模拟,研究其基底表面近场增强的规律及物理过程.研究结果发现,使用硅基底时,阵列与单一TiO2球形粒子相比其近场增强仅下降约30%;相对于入射激光强度而言,在直径约为100 nm的空间范围内获得140倍的近场增强,这一现象可用于百纳米孔的激光加工.同时在其他典型基底的理论计算结果中也表明,几乎在所有金属及介电材料表面均可以实现良好的百纳米空间范围内的近场增强,并且具有近场随着基底折射率变大而增强的规律.这些现象的产生归因于TiO2粒子中磁四极振荡产生的激光前向场增强及粒子与基底的耦合作用.进一步引入镜像电荷模型对基底光学参数对其表面近场增强的影响规律进行了分析和解释.本文的模拟结果对飞秒激光近场超衍射极限纳米加工的应用有着重要的意义.

     

    Optical near field enhancement on substrate can be achieved by localizing femtosecond laser energy with nanoparticles. The enhanced field is located in the region between nanoparticles and the substrate. The localized femtosecond optical field is of great significance for fabricating the micro/nano structure with characteristic size beyond the diffraction limit. Up to now, femtosecond processing nanohole assisted by particle array is only possible for metal particle (Au) and low-refractive-index dielectric polystyrene particle. However, previous research results show that it cannot be realized for metal particle arrays (Au) to form periodic nanohole arrays, and it is limited for polystyrene particle to choose the corresponding substrate. In this paper, a novel method is proposed, in which high refractive index TiO2 arrayed particles are placed on the substrate to achieve laser induced near field enhancement. This makes feasible the nanoscale processing beyond the diffraction limit. In this paper, near field distributions of TiO2 particle array on Si, Pt and SiO2 substrates are simulated by the finite-difference time-domain (FDTD) method. The results show that TiO2 particles concentrate the laser energy to a region with a diameter of 100 nm around the particle and the near field enhancement is 140 times higher than the incident laser intensity, which is beneficial to fabricating the nanostructure of super diffraction limit, such as sub-hundred nanometer nanohole ablation by femtosecond laser. For Si substrate, the near field enhancement is only about 30% lower for TiO2 particle array than that for single TiO2 particle. In order to explore the influence mechanism of the substrate material parameters on the near field enhancement of TiO2 nanoparticle array, we further simulate the enhancement factor for the substrates of different refractive indices. It is found that the near field is enhanced with the increase of substrate refractive index, and this is attributed to an increased interaction of the particle with the near field of substrate and the scattering effect in which the TiO2 particle supports forward near field intensity pattern. Moreover, the image charge model is introduced to analyze the effect of substrate optical parameters on local field enhancement. Results in this paper can be applied to most metals as well as dielectric substrate surfaces, and they open a new way for femtosecond laser near field nano-processing with characteristic size beyond the diffraction limit.

     

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