Efficient surface modification of organic polymer by dual action

We have developed a novel and highly effective method for structural surface modification (J. Appl. Phys. 105, 026105 (2009)) achieved by simultaneous dual action of XUV, obtained from high-order harmonic generation, and visible-near infrared ultrashort laser pulses. Although the fluence of each individual beam was far below the surface modification threshold, very efficient and specific material expansion was observed after irradiation of poly(methyl methacrylate) sample by more than 20 shots of mixed XUV/visible-NIR radiation. The visible-NIR pulse interacts with the free charge carriers produced by the energetic XUV photons, such that its absorption dramatically increases. This results in heating of the sample surface while, simultaneously, XUV induced damage to the polymer chain is enhanced due to the temperature effect. The observed phenomena can be utilized in applications such as surface nanopatterning with widely available, inexpensive table-top radiation sources. We have performed this experiment at KAIST in collaboration with the group of Prof. Chang Hee Nam. This work is a continuation of previous joint experiment on efficient high-harmonic generation in a two-color laser field. Strong HHG was realized in the two-color laser field with a long gas jet, obtaining submicrojoule harmonics in the 20 nm spectral region, by optimizing the medium length and position as well as the relative phase between the two fields and second harmonic conversion. The strongest harmonic at the 38th order (21.6 nm) reached an energy of 0.6 µJ, giving an unprecedented conversion efficiency of 2 x 10⁻⁴.

Fig. 1. Experimental setup.

Fig. 2. AFM images of the PMMA surface simultaneously irradiated with XUV and visible-NIR ultrashort pulses.