ИСТИНА

Femtosecond laser modification allows formation of laser-induced periodic surface structures (LIPSS) on the amorphous hydrogenated silicon (a-Si:H) films due to plasmon-polariton excitation [1]. Such anisotropically modified a-Si:H films are of interest as a material for polarization-sensitive optoelectronic devices, since they possess dichroism [2]. To better understand the possibility to control experimentally observed LIPSS-related anisotropy of the modified a-Si:H surface, it is necessary to provide both experimental and theoretical description of various LIPSS types formation at different laser pulse exposure. In this work two main types of LIPSS were revealed on the a-Si:H surface after femtosecond laser pulse treatment (1250 nm, 125 fs, 0.15 J/cm2, 10 Hz) in the scanning mode with different speeds. The mentioned LIPSS have orientation orthogonal or parallel to the laser polarization, and the same period of 1.20 ± 0.02 μm. Such structure formation is caused by nonequilibrium carriers photoexcitation and subsequent surface plasmon-polaritons generation [3]. The LIPSS orientation is turning from perpendicular to parallel relatively to laser polarization when the laser beam scanning speed decreases (the exposure time of the same area increases) due to the feedback emerging between irradiated film optical properties and excitation of certain plasmon-polariton modes at the sample surface [1]. The LIPSS formation was theoretically described using the model proposed by John E. Sipe et al. [4]. According to this model, the LIPSS orientation is turning when the dielectric permittivity real part, achieved on the a-Si:H surface during femtosecond laser pulse action, varies from Reε < –1 to Reε > 1, which is in accordance with data of [3] also. The Reε depends on the photoexcited carrier concentration N, and corresponding threshold value of N required for LIPSS orientation turning was estimated as 8.2 ∙ 1021 cm–3