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

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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.