Nanostrukturierung mit Femtosekundenlaserpulsen (English shop)

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The femtosecond laser has proven itself as an ideal and efficient tool for precise micro-
machining. Due to the short interaction time, thermal and mechanical damages such as
melting and the formation of burrs and cracks can be suppressed using certain laser pa-
rameters. Therefore, the femtosecond laser allows high quality micro-structuring between
one and a few hundred micrometers, and is universally applicable for nearly all solid mate-
rials. In comparison, the potential of femtosecond lasers for high precise nano-structuring
was not clear at the beginning of this work.
Thus, the goal of this work has been to study the fundamentals of femtosecond laser
ablation of solid materials on the nanometer scale, and to determine the potential of this
technology for nano-structuring.
An important physical parameter for the interaction process, and therefore for the ener-
gy transport from the electron to the ion sub-system, is the electron-phonon-coupling-
constant. The influence of this parameter is investigated in detail and it is shown that
metals with weak coupling can be molten by a single laser pulse, whereas metals with
strong coupling can be ablated without melting. Using a two-temperature-model, which
describes the thermal coupling between the electron and ion sub-systems, the experimen-
tal results are numerically modeled, and a deeper understanding of the interaction process
and the temporal and spatial heat transfer is obtained.
By taking advantage of the well-defined ablation threshold, one can overcome the diffrac-
tion limit by choosing the peak laser fluence slightly above the threshold value. In this
case, only the central part of the beam can modify the material, and it becomes possible to
produce sub-wavelength structures. It is shown that by using laser light with wavelength
of 800 nm, structure sizes down to 100 nm can be fabricated in chromium.
Close to the ablation threshold, thermal accumulation effects are studied, which can influ-
ence the structure quality. Investigations on thin metal films show that contrary to micro-
machining, no heat accumulation between the laser pulses arises for nano-structuring.
This is explained by better heat loss from the processing volume for nano-structuring.
However, in both cases structural modifications are induced below the ablation threshold.
In contrast to metals, the absorption of femtosecond laser pulses in transparent materials
takes place using multi-photon processes. In the course of the investigations, it is observed
that for nano-structuring the realized structure size depends directly on the energy band
gap. It is shown for the first time, that the typical universal character for femtosecond
micro-machining is not valid for nano-structuring.
In conclusion, femtosecond lasers have proven to be successful for precise nano-structuring.
However, in contrast to micro-machining, the nano-processing technology and laser pa-
rameters have to be carefully adjusted for certain materials. Using this, non-linear, high
resolution femtosecond material processing can be developed in the near future.