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9ALT'22
LM-O-16
LASER-MATTER INTERACTION
Nanostructuring of polymer surfaces mediated by colloidal microparticle lens array by pair of femtosecond laser pulses of different colors
Institute of Applied Physics of Russian Academy of Sciences, 46 Ul'yanov Street, 603950 Nizhny Novgorod, Russia
Polymer surfaces covered by the colloidal microparticle lens arrays (polystyrene microspheres, 1mm in diameter) were irradiated by two femtosecond pulses separated in time. The one was at the fundamental frequency of a Ti:sapphire laser and the other was at the second harmonic. Here, about 5 percent of the energy of the fundamental frequency pulse was converted to the second harmonic. Laser irradiation resulted in the elimination of the microspheres and formation of ablation craters of about 100 nanometer size. In our experiments we study the effect of the delay time between the pulses on the nanostructuring results.
Two different polymers were explored. PMMA has a strong linear absorption at the wavelength of the third harmonic of the laser, whereas TOPAS is almost transparent at this wavelength, but it strongly absorbs at the fourth harmonic. We investigated both the dependence of the ablation threshold and the morphology of the craters on the time delay between the pulses. The study of the structures was performed by means of AFM.
We have found that the zero delay time is the most effective for PMMA, as it has been done in our previous paper [1]. On the contrary, the irradiation scheme where the blue pulse comes first proves to be more beneficial for TOPAS.
The explanation relies on the role of the two-photon absorption in the generation of seed electrons in the conduction band. In the case of PMMA, the ©+2© process dominates, whereas for TOPAS the 2©+2© process is preferable. In the most favorable configuration for TOPAS, the seed electrons are generated by the pure second harmonic, whereas the role of the following fundamental frequency pulse is to provide its energy to 'heat' those seed electrons. The simple model of this process is reported.
The laser irradiation through the array of microspheres makes many similar nanostructures at the polymer surface with a single laser shot. What is important is that we obtain a huge amount of realizations of the same laser interaction process simultaneously. This provides a possibility for the statistical filtering of the structure parameters. One can clearly distinguish the observed dependences from artifacts associated with surface inhomogeneities.
When analyzing the ablation nanocraters emerging just beneath the microparticles, one should be aware that the nanostructuring result depends on the environment of the sphere. The recent study shows that the spheres located at the edge of the close-packed array have to be treated in a separate way [2].
This work was financially supported by the Russian Science Foundation grant 22-19-00322.
[1] A. Afanasiev, V. Bredikhin, A. Pikulin, I. Ilyakov, B. Shishkin, R. Akhmedzhanov, N. Bityurin, Two-color beam improvement of colloidal particle lens array assisted surface nanostructuring, Applied Physics Letters, 106, 183102 (2015).
[2] A. Afanasiev, A. Pikulin, I. Ilyakov, B. Shishkin, N. Bityurin, Edge effect at the microsphere colloidal array in near-field particle lithography on polymer surfaces, Surfaces and Interfaces, 29, 101735A (2022).
A. Afanasiev, I. Ilyakov, B. Shishkin, A. Pikulin, N. Bityurin
