Abstract
Periodic surface structures with periods as small as about one-tenth of the irradiating femtosecond (fs) laser light wavelength were created on the surface of a titanium (Ti) foil by exploiting laser-induced oxidation and third harmonic generation (THG). They were achieved by using 100-fs laser pulses with a repetition rate of 1 kHz and a wavelength ranging from 1.4 to 2.2 μm. It was revealed that an extremely thin TixOy layer was formed on the surface of the Ti foil after irradiating fs laser light with a fluence smaller than the ablation threshold of Ti, leading to a significant enhancement in THG which may exceed the ablation threshold of TixOy. As compared with Ti, the maximum efficacy factor for TixOy appears at a larger normalized wavevector in the direction perpendicular to the polarization of the fs laser light. As a result, the THG-dominated laser ablation of TixOy induces 100-nm periodic structures parallel to the polarization of the fs laser light. The depth of the periodic structures was found to be ~10 nm by atomic force microscopy and the formation of the thin TixOy layer was verified by energy dispersive X-ray spectroscopy.
Highlights
Laser-induced periodic surface structures (LIPSSs) created on the surfaces of different materials have been studied for a long time since the appearance of high-power lasers [1,2,3,4,5]
It is suggested that the surface plasmon polaritons (SPPs) excited by femtosecond laser irradiation play a crucial role in the formation of LIPSSs [6,7,8]
The depth of the LIPSSs was measured by atomic force microscopy (AFM) while the formation of the thin TixOy layer was verified by energy dispersive X-ray spectroscopy (EDS) in the scanning electron microscopy (SEM) measurements
Summary
Laser-induced periodic surface structures (LIPSSs) created on the surfaces of different materials have been studied for a long time since the appearance of high-power lasers [1,2,3,4,5]. Apart from laser-induced oxidation, it seems necessary to consider nonlinear optical processes occurring in the ablation of Ti by using fs laser pulses Another physical mechanism responsible for the sub-100-nm HSFLs formed on Ti surfaces was proposed by Ionin et al They observed sub-100-nm transverse nanoripples on Ti surfaces under multi-shot sub-threshold fs laser irradiation and suggested that such structures were related to frozen patterns of coherent sub-surface nanoscale cavitation [27]. Golosov et al studied the ultrafast changes in the optical properties of a Ti surface during the laser ablation process and found that the dependence of the first-harmonic nanograting spacing on the laser fluence was determined by the change in the instantaneous optical characteristics of the material and the saturation of the interband absorption along with the increasing role of intraband transitions [28] They demonstrated that the periods of LIPSSs could be reduced by carrying out the ablation in water with a larger dielectric constant than air [28,29]. The depth of the LIPSSs was measured by atomic force microscopy (AFM) while the formation of the thin TixOy layer was verified by energy dispersive X-ray spectroscopy (EDS) in the scanning electron microscopy (SEM) measurements
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