Oxide composition and period variation of thermochemical LIPSS on chromium films with different thickness

Abstract

In this paper, we present the results of thermochemical LIPSS formation on a chromium film with a thickness in the range of 28-350 nm induced by femtosecond laser radiation (λ = 1026 nm, ν = 200 kHz, τ = 232 fs). The period, height, morphology and chemical composition of TLIPSS as a function of the metal film thickness and focusing configuration are investigated. The growth of TLIPSS period from 678 nm to 950 nm with increasing thickness of the film has been explained by a formation of oxides with different stoichiometry composition. So, the CrO oxide prevails in the composition for the case of TLIPSS formed on thin films which have the minimal period, whereas CrO oxide is dominant in the case of TLIPSS formed on thick chromium films which have the maximal period value. The results obtained are in agreement with numerical modeling of a period defined by the interference between an incident radiation and a scattered one from a single oxide ridge with a different chemical composition.

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

“…In our cases, even inter-ridge regions remain at or above the initial surface level (see Fig. 3 in [17]).…”

“…They can be classified as low spatial frequency LIPSS (LSFL) [9,14]. It should be underlined that the LSFL produced in this work grow up above the initial surface of the film 6 [17], contrary to those formed at the regimes of strong ablation when the peaks of the LIPSS ridges occur below the initial surface [27]. In our cases, even inter-ridge regions remain at or above the initial surface level (see Fig.…”

“…Shortly, at low scanning velocities, the LSFL parallel to laser polarization are formed on the Cr films with the thickness in the range 28-350 nm. The height of the LSFL ridges can be comparable or even larger than the film thickness while between the LSFL ridges the film remains at almost its initial level for the films up to 125 nm thick [17]. Hence, here we focus on only one film thickness, 28 nm, anticipating that our theoretical analysis presented below is valid for thicker films.…”

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

“…A simulation allowed determining the influence of different spatial concentrations of two different Cr oxides, i.e., CrO 2 and Cr 2 O 3 , along one of the produced ridges. 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].…”

“…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.…”