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

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 CrO₂ oxide prevails in the composition for the case of TLIPSS formed on thin films which have the minimal period, whereas Cr₂O₃ 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.