ИСТИНА

Advanced technologies of photonics and microelectronics allow designing compact planar elements for integrated optics and integrated photonic circuits based on silicon. Femtosecond laser pulses irradiation of thin silicon films might change their phase states (amorphous or crystalline) and provide surface texturing via fabrication of laser-induced periodic surface structures (LIPSSs) with micron and even submicron periods. Such structures can be considered as metasurfaces with artificial anisotropy. In our work, we fabricated large area LIPSSs (up to 5x5 mm2) in the amorphous silicon thin films and studied reflectance spectra in the infrared range at various incident light polarizations for the irradiated films as well as carried out conductivity measurements in plane of the samples along and orthogonally LIPSSs. Three types of samples were irradiated and examined: an amorphous silicon layer only, an amorphous silicon layer with a flexible polyimide underlayer, and an amorphous silicon layer with a 10 nm aluminum coating. The amorphous silicon layers have thickness of 600–1000 nm. The samples were irradiated in the raster mode with femtosecond laser pulses generated by Satsuma Amplitude Systems (300 fs, 515 or 1030 nm) and Avesta (125 fs, 1250 nm) lasers. LIPSSs were fabricated in all samples. The periods of the surface gratings obtained are close to the wavelengths used. The appearance of the LIPSSs is caused by photoinduced surface plasmon-polaritons generation and is confirmed by calculations with the Sipe–Drude theory. Reflectance spectra for all samples were measured in the range of 1.5–18 μm at an incident angle of 13° and s- and p-polarizations. The measured spectra indicate the presence of dichroism in the irradiated samples. Its value reaches the maximum value of 0.12 μm-1 in the samples without the aluminum coating. All the spectra are characterized by thin film interference. Analysis of interference maxima positions made it possible to find the refractive indices for the ordinary and extraordinary waves. The maximum birefringence value is 0.2 in the range of 1.9–2.7 μm for the sample with the aluminum coating. The results obtained are in a good agreement with calculations in the framework of the generalized Bruggeman model [4] for alternating components from amorphous and crystalline silicon. Raman spectroscopy data show partial nanocrystallization of the surface (up to 70%). The silicon nanocrystal presence leads to growth of the specific conductivity up to 3 orders for the irradiated samples in comparison with non-irradiated ones. Additionally, the in-plane conductivity anisotropy was revealed. This result is in a good agreement with the Bruggeman model too and absorption spectra for the surface possessing artificial anisotropy. Thus, the silicon films with LIPSSs can be considered as a promising base to design planar devices which are sensitive to the incident light polarization and the applied current direction.