ИСТИНА

Femtosecond laser-induced periodic surface structures (LIPSS) on chalcogenide vitreous semiconductor (ChVS) films are of great interest owing to a possibility of optical and electrophysical anisotropy in them caused both by surface relief modulation and phase transformations [1, 2]. Currently, laser-induced surface relief formation accompanied by phase transformations has been well studied for GST225 (Ge2Sb2Te5) films [3]. However, from the point of view of near and mid-infrared optics, arsenic sulfide (As2S3) and arsenic selenide (As2Se3) seem to be more suitable materials [2]. Therefore, this work was aimed to produce LIPSS of various types on the surface of abovementioned materials and analyze the possibility of simultaneous phase transformations in them. To produce LIPSS, thin ChVS films (As2S3 – 576 ± 5 nm; As2Se3 – 842 ± 5 nm) on single-crystal silicon substrates with a chromium sublayer (100 nm) or on quartz glasses were irradiated with femtosecond laser pulses at the frequency of the second optical harmonic (515 nm, 300 fs, pulses energy and number E = 0.1–0.4 μJ, N = 10–1200) using a Satsuma laser system (Amplitude Systems) at normal incidence. The LIPSS with wave- (~515 nm) and subwavelength (150–185 nm) periods, oriented, respectively, orthogonally or along the polarization vector, were formed on the surfaces of the films as a result of irradiation. The type of LIPSS formed varied depending on the number and energy of femtosecond laser pulses. The relief modulation reached 115 and 40 nm for wave- and subwavelength LIPSS, respectively. The numerical simulation within the framework of the plasmon-polariton mechanism for such LIPSS formation was consistent with experimental results: with an increase in the number of laser pulses or their energy, an evolution is observed from subwavelength to wavelength LIPSS. This effect is explained by a change in the mode of surface plasmon-polariton excited, caused by increased concentration of nonequilibrium charge carriers generated by femtosecond laser pulse. Additionally, during such evolution, the structures of both types can be observed simultaneously within the same crater formed by a femtosecond laser pulse impact, in the form of a hierarchical structure consisting of two orthogonal gratings with different periods. Raman spectra analysis (Horiba HR800, 488 nm excitation) demonstrated no structural changes in As2S3 films. However, femtosecond laser-induced phase transformations are possible for As2Se3 films according to Raman data. The obtained LIPSS on ChVS surfaces, possessing form anisotropy, can be used to create polarization-sensitive elements of infrared optics. The work was supported by the Russian Science Foundation (grant 22-19-00035). https://rscf.ru/project/22-19-00035/ [1] A.Kolchin, D. Shuleiko, M. Martyshov, A. Efimova, L. Golovan, D. Presnov, T. Kunkel, V. Glukhenkaya, P. Lazarenko, P. Kashkarov, S. Zabotnov, S. Kozyukhin. Artificial anisotropy in Ge2Sb2Te5 thin films after femtosecond laser irradiation. Materials, vol. 15, art. 3499 (2022) [2] W. Ma, L. Wang, P. Zhang, W. Zhang, B. Song, S. Dai. Femtosecond laser direct writing of dif-fraction grating and its refractive index change in chalcogenide As2Se3 film. Optics Express, vol. 27, pp. 30090–30101 (2019) [3] S. Zabotnov, A. Kolchin, D. Shuleiko, D. Presnov, T. Kaminskaya, P. Lazarenko, V. Glukhenkaya, T. Kunkel, S. Kozyukhin, P. Kashkarov, Periodic relief fabrication and reversible phase transitions in amorphous Ge2Sb2Te5 thin films upon mul-ti-pulse femtosecond irradiation. Micro, vol. 2, pp. 88–99 (2022)