ИСТИНА

Herein, we describe our experiments with thin amorphous silicon films (1000 nm), including the films covered with a 10 nm aluminum layer, irradiated with femtosecond laser pulses (515 and 1030 nm, 300 fs). As a result, laser-induced periodic surface structures (LIPSS) were fabricated, and they covered areas up to 2х2 mm^2. Structural features of irradiated surfaces were examined by optical and scanning electron microscopy. All LIPPS are gratings oriented perpendicular to the laser beam polarization and have periods close to the wavelength. The observed LIPSS formation is well explained with the plasmon-polariton mechanism and the so-called Sipe-Drude theory. Surface periodicity may lead to artificial anisotropy according to the generalized Bruggeman model. To examine the anisotropy, we studied our samples by Fourier-transform infrared spectroscopy in the range of 500–6500 cm^-1 where silicon possesses relatively low absorption. The measured reflectance spectra are characterized by interference in the thin silicon films. The spectra are different for various polarization of the incident light. An analysis of the values of interference extremums and their spectral positions allowed to estimate the dichroism and birefringence values, respectively. The best results were obtained in the spectral region of 1900–2700 nm: the dichroism is 120 mm^-1 (the difference between the absorption coefficients) and the birefringence is 0.2 (the difference between the refractive indices for the ordinary and extraordinary waves). These values look sufficient to further designing polarization-sensitive planar devices for the near infrared region.