Production of nanostructures on bulk metal samples by laser ablation for fabrication of low-reflective surfaces

Abstract

Nanostructure formation on bulk noble metals (copper, gold and silver) by a femtosecond laser was studied aiming at the production of low-reflectivity surfaces. The target surface was irradiated with the beam of a 775 nm wavelength and 150 fs pulse duration Ti:sapphire laser. The fluence was in the 16-2000 mJ/cm 2 range, while the average pulse number was varied between 10 and 1000 depending on the scanning speed of the sample stage. The reflectivity of the treated surfaces was measured with a visible-nearinfrared microspectrometer in the 450-800 nm range, while the morphology was studied with a scanning electron microscope. A strong correlation was found between the decreasing reflectivity and the nanostructure formation on the irradiated surface; however, the morphology of silver significantly differed from those of copper and gold. For the two latter metals a dense coral-like structure was found probably as a result of cluster condensation in the ablation plume followed by diffusion-limited aggregation. In the case of silver the surface was covered by nanodroplets, which formation was probably influenced by the 'spitting' caused by ambient oxygen absorption in the molten silver followed by its fast release during the resolidification.

Atomistic simulation of laser-pulse surface modification: Predictions of models with various length and time scales

“…Thus, the material removal occurs at central absorbed fluence F 0 ¼ F abl ¼ 60 mJ/cm 2 . The simulation results agree with the experiments [27][28][29] where producing of nanoparticles and nanojets takes place at the absorbed fluences about 40 6 15 mJ/cm 2 . In our simulation, the nanojets height may reach 50-70 nm.…”

“…This fact agrees with the experiments where the changes of surface morphology were observed as nanoscale ripples without ablation. 5,10,27 For both studied metals, swelling is obtained as initial stage of the long laser ablation. At the same time, the short type of ablation is revealed for gold.…”

“…One of the features of the surface modification of gold is strong discrepancy between the experimental threshold fluence of surface modification and the calculated one. In the experiments, the threshold value of absorbed fluence F is about 10 6 5 mJ/cm 2 for creation of nanoscale ripples, 10,27 and producing nanoparticles and nanojets [27][28][29] requires F about 40 6 15 mJ/cm 2 . At the same time, the simulation gives threshold absorbed fluence for ablation about 150 mJ/ cm 2 .…”

“…At low energy of laser pulse, the nanoscale ripples may be induced on the surface by the melting without laser ablation. 5,10,27 The nanoscale changes of the surface are due to the splash of molten metal under fluence/temperature gradient of the laser beam. The depth of this modification is no more than several nanometers.…”

Derivation of effective penetration depth of femtosecond laser pulses in metal from ablation rate dependence on laser fluence, incidence angle, and polarization

“…The reflectance is strongly influenced by surface morphology. 36,37 The craters produced by ablation are covered in many nano-and micro-structures because multi-pulse irradiation is used to create a crater deep enough to be measureable. The difference between R h and R m suggests that the surface morphology created by high fluence irradiation absorbs more than that created by middle fluence irradiation, as has been reported previously.…”