Ultrashort-pulse laser-induced nanostructuring of nonlinear materials (UP-LINN)

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Nanostructuring of optical materials is of increasing interest for various new applications ranging from biomedical and chemical fiber sensors to data storage, bio-mimetic surfaces or molecular imaging. Presently, selected fabrication methods like holographic lithography or light-field supported self-assembling of nanospheres are world-wide investigated in extenso. An alternative, very promising method is the spontaneous formation of laser-induced periodic surface structures (LIPSS) which are known for a few decades and can be observed in metal surfaces as well as dielectric materials. For special cases, the responsible physical mechanisms are well understood, e.g. the initial pattern formation by the interference of the incident wave and a wave scattered in the material, electron-phonon coupling and electron diffusion (like in the case of copper). Recent investigations, however, indicated significantly different basic mechanisms of self-organized ripple generation under highly intense ultrashort-pulse laser irradiation. In particular, the formation of a special type of LIPSS with feature sizes far below the optical wavelength, so-called nanoripples , is only poorly understood up to now. A systematic study of the mechanisms and dynamics of nanoripple generation, in particular with respect to initial stage and nonlinear excitation paths, is still missing. For transparent materials with large nonlinear coefficients like ZnO, only few reliable data were reported. Therefore, one main target of the proposed project is to generalize the model of LIPSS generation by including these new aspects and comparing to reference data from well-studied materials (like, e.g., copper). Typically, LIPSS structures appear as 2D gratings with a certain degree of random distortions such as deviations from periodicity. It is expected that an improved knowledge about the elementary mechanisms opens the road to a better spatio-temporal control of femtosecond LIPSS. We propose a systematic investigation of the LIPSS formation in selected transparent materials, in particular undoped and doped ZnO nanolayers, with high spatial and temporal resolution. The results will be compared to the case of metallic layers like copper. By combining specific know-how of both institutes in shaping and diagnostics of ultrashort pulses, layer fabrication and high-resolution analytics, the following main objectives of the project will be tackled: 1. Study of basic mechanisms of ultrashort-pulse LIPSS formation in dielectric and metallic materials with particular emphasis on the dynamics, polarization dependence and nonlinear excitation channels to explore the nature of nanoripple formation in detail and to further generalize the LIPSS model by including these particular mechanisms. 2. Tailoring of nonlinear thin films (in particular ZnO) with respect to crystalline structure, doping and initial scattering characteristics to enable the generation of defined functional nanostructures via a spatial, spectral and polarization control of LIPSS. 3. Identification and development of appropriate characterization techniques and figures of merit to properly describe the surface topography including the degree of order. Following this course, this study is aimed to extend the knowledge basis concerning lasermaterial interaction, significant correlations between composition-structure-properties of solids, as well as to explain, generalize and control the LIPSS-effect. Thus, first steps towards nonlinear materials with functional nanostructures for sensor applications will be enabled.