LIPSS on thin metallic films: New insights from multiplicity of laser-excited electromagnetic modes and efficiency of metal oxidation

Abstract

Thin Cr films 28-nm thick deposited on glass substrates were processed by scanning low-intensity femtosecond laser pulses with energy well below single-pulse damage threshold.Two types of laser-induced periodic surface structures (LIPSS) were produced, depending on the scanning velocity, (1) parallel to laser light polarization with periodicity somewhat smaller than laser wavelength and (2) perpendicular to polarization with spatial period much smaller than wavelength. All structures are formed as protrusions above the initial film surface and exhibit a high degree of oxidation. To explain formation of the LIPSS and their conversion from one to another type, a rigorous numerical approach for modeling surface electromagnetic waves in thin-film geometry has been developed, which takes into account the change of optical properties of material due to laser-induced oxidation and porosity. The approach addresses the multiplicity of electromagnetic modes allowed for thin films. It has been found that the low spatial frequency LIPSS parallel to laser polarization, which are formed at low scanning 2 velocities, are well described by the Sipe theory for surfaces of low roughness. The SEW mode responsible for high spatial frequency LIPSS formation at high scanning velocities has been identified. The mechanisms of optical feedback and transformation between types of LIPSS with scanning velocity have been proposed.3

The Role of the Laser-Induced Oxide Layer in the Formation of Laser-Induced Periodic Surface Structures

“…In this case, the proposed model includes the optical properties of both Cr oxides and ratios between them; however, the obtained periodicities are~1.5 times larger than the ones obtained experimentally. Only by assuming a certain porosity inside the oxide layer, the predicted periodicities are close to the measured values [29]. From all this, it is clear that the incorporation of additional effects, such as superficial oxidation, into the calculations to predict the LSFL periodicity and orientation is not a trivial task.…”

“…This process occurs when the incoming focused laser beam interferes with radiation scattered at existing nanostructures or surface defects leading to the imprint of a periodic fluence pattern of maxima and minima, allowing the incorporation of oxygen in the places where the intensity maxima is located [27]. Dostovalov et al [28,29] very recently published several works on thermochemical formation of elevated parallel and perpendicular surface structures on metallic chromium films produced by local oxidation at the local intensity maxima produced by high-repetition rate (200 kHz) laser pulses in the non-ablative regime. One of them indicates that it is possible to control the periodicity of the LIPSS by choosing the proper Cr film thickness.…”

“…The simulated field spatial distribution demonstrates that changes in the chemistry induced thermally by the laser pulses affect the final intensity pattern imprinted into the material [28]. In another similar system of Cr films, the authors consider different beam scanning speeds to investigate the influence on the transition from HSFL perpendicular to the polarization to LSFL being parallel to it (LSFL ), involving the formation of both Cr oxides and the interaction of a surface electromagnetic wave between two dielectric materials, air and glass, in which the Cr film has been deposited [29]. In this case, the proposed model includes the optical properties of both Cr oxides and ratios between them; however, the obtained periodicities are~1.5 times larger than the ones obtained experimentally.…”

Maxwell Meets Marangoni—A Review of Theories on Laser‐Induced Periodic Surface Structures

“…[39] An extended thin-filmplasmonic model considering transient changes of the dielectric permittivities along with the electromagnetic coupling at the two interfaces was presented by Derrien et al [43] Later it was transferred to oxidic films involved in the formation of a specific type of sub-ablative LIPSS on metals. [44] The experimentally observed alignment of the LSFL relative to the linear beam polarization results from the directional excitation of the electrons of the material by the incident laser radiation. Depending on the electrical properties (metallic, dielectric), this leads to a directional radiation characteristic (radiative and nonradiative scattered fields, propagation) and, thus, to a specific, anisotropic field distribution of the induced SEWs.…”

“…[ 43 ] Later it was transferred to oxidic films involved in the formation of a specific type of sub‐ablative LIPSS on metals. [ 44 ]…”