Generation of wavelength-dependent, periodic line pattern in metal nanoparticle-containing polymer films by femtosecond laser irradiation

Thin polymer films containing metal nanoparticles were irradiated with ultrashort, linearly polarized laser pulses. As result of irradiation, nanostructural changes occur in a type of periodically arranged, line-like areas with modified particle size and shape distribution. The periodic formation in this nanocomposite material is observed only for a small filling factor range, which can be attributed to the percolation region or nearby. Transmission (also in cross section) and scanning electron microscopy were applied to investigate the obtained structure modifications. A linear dependency between the period Λ of the line structures and the laser wavelength λ (800, 528, 400, and 266 nm ) used with Λ ∕ λ ≈ 0.70 is assumed. The structural changes and the physical mechanism of the periodic formation are discussed.