Generation of deterministic nanostructures with ultrashort UV pulses under predefined interface boundary conditions: theory and experiment

强烈的飞秒激光脉冲可以产生剧烈的结构变化，并导致固体中的材料去除。特定选择的辐照条件可用于制造纳米结构表面。在这个项目中，我们计划创造特定的界面边界条件，这将使飞秒激光纳米结构的材料使用空间调制辐射具有前所未有的精度。为此，我们将从实验和理论上研究以下界面类型：(I)材料顶部的透明薄膜，（II）周期性预结构材料表面，（III） (I)和（II）的组合。(I)材料顶部的透明薄膜将限制激光诱导的材料膨胀（限制）。此外，界面附近材料的介电性能也会受到影响。我们预计这将影响激光能量进入电子系统的耦合。（二）通过对材料表面进行预构造，形成空间调制的介电函数。我们将研究表面等离子激元的激发和相应的场增强是否会影响结构的形成。（III）我们计划结合(I)和（II）条件，实现透明薄膜下预结构表面的激光加工。在（II）和（III）中引起的介电函数的空间调制可以导致材料中的调制场增强。这种放大由表面等离子体介导，并影响电子系统吸收的能量。仔细控制这些影响可以改善加工效果。在实验中，将使用紫外激光（248 nm）。要研究的材料是金和硅。此外，我们将使用薄水和SiO2薄膜在(I)和（III）中产生约束。然而，理论描述将只处理薄的水膜（由数百万个H2O分子描述）。本方案理论部分的一个重要目标是实现时空相关介电函数（DF）与双温模型（TTM）分子动力学（MD）模拟之间的耦合。这种耦合应该涉及对TTM中的源项的修改。Au的DF-TTM-MD模拟将基于已经实现的TTM-MD方法。对于Si，我们还将使用激光诱导原子间势来研究非热激光诱导效应的影响，该势是在上一个项目中通过拟合ab-initio MD模拟得出的。首次进行了预结构表面的模拟。在相同时空尺度下的实验结果与理论结果的比较，将有助于更好地理解激光诱导结构的形成，并为在亚100 nm水平上控制纳米结构提供基础。

Intense femtosecond laser pulses can create dramatic structural changes and lead to material removal in solids. Specifically chosen irradiation conditions can be utilized to fabricate nanostructured surfaces.In this project, we plan to create specific interface boundary conditions, which will allow femtosecond laser nanostructuring of materials using spatially modulated irradiation with an unprecedented accuracy. For this purpose we will investigate, both experimentally and theoretically, the following interface types: (I) transparent film on top of the material, (II) periodically pre-structured material surface, (III) a combination of (I) and (II).(I) A transparent thin film on top of the material will restrict laser induced material expansion (confinement). Furthermore, the dielectric properties of the material near the interface will also be affected. We expect that this will influence the coupling of the laser energy into the electronic system.(II) A spatially modulated dielectric function will be created by pre-structuring of the material surface. We will investigate whether the excitation of surface plasmons and the corresponding field enhancement can influence the structure formation.(III) We plan to combine the conditions (I) and (II) to achieve laser processing of a pre-structured surface under a transparent thin film.The spatial modulation of the dielectric function induced in (II) and (III) can lead to a modulated field enhancement in the material. This amplification is mediated by the surface plasmons and influences the energy absorbed by the electronic system. Careful control of these effects can enable improving the processing results. In the experiments a UV-fs-Laser (248 nm) will be used. The materials to be studied are Gold and Silicon. Moreover, we will use thin water and SiO2 films to produce confinement in (I) and (III). However, the theoretical description will only deal with thin water films (described by millions of H2O molecules). One important goal of the theory part of the present proposal is to achieve a coupling between the time- and space dependent dielectric function (DF) and the two-temperature model (TTM) molecular dynamics (MD) simulation. This coupling should involve the modification of the source term in the TTM. The DF-TTM-MD simulations for Au will be based on an already implemented TTM-MD method. For Si we will also investigate the influence of non-thermal laser induced effects by using a laser induced interatomic potential, which was derived by fitting to ab-initio MD simulations during the previous project. For the first time, simulations with a pre-structured surface are performed.The comparison between the experimental and theoretical results at the same spatial and temporal scales should allow for a better understanding of the laser induced structure formation and serve as a basis for the control of nanostructuring on the sub-100 nm level.