Aleksander G. Kovačević

Femtosecond laser induced periodic surface structures on multi-layer graphene

In this work, we present an observation of laser induced periodic surface structures (LIPSS) on graphene. LIPSS on other materials have been observed for nearly 50 years, but until now, not on graphene. Our findings for LIPSS on multi-layer graphene were consistent with previous reports of LIPSS on other materials, thus classifying them as high spatial frequency LIPSS. LIPSS on multi-layer graphene were generated in an air environment by a linearly polarized femtosecond laser with excitation wavelength λ of 840 nm, pulse duration τ of ∼150 fs, and a fluence F of ∼4.3–4.4 mJ/cm2. The observed LIPSS were perpendicular to the laser polarization and had dimensions of width w of ∼30–40 nm and length l of ∼0.5–1.5 μm, and spatial periods Λ of ∼70–100 nm (∼λ/8–λ/12), amongst the smallest of spatial periods reported for LIPSS on other materials. The spatial period and width of the LIPSS were shown to decrease for an increased number of laser shots. The experimental results support the leading theory behind high s...

High angular and spectral selectivity of purple emperor (Lepidoptera: Apatura iris and A. ilia) butterfly wings

The iridescent features of the butterfly species Apatura iris (Linnaeus, 1758) and A. ilia (Denis & Schiffermüller, 1775) were studied. We recognized the structural color of scales only on the dorsal side of both the fore and hind wings of males of both of the aforementioned butterfly species. The scale dimensions and microstructure were analyzed by a scanning electron microscope (SEM) and transmission electron microscope (TEM). The optical properties were measured and it was found that the peak reflectivity is around 380 nm, with a spectral width (full width at half maximum) of approximately 50 nm in both species. The angular selectivity is high and a purple iridescent color is observed within the angular range of only 18 degrees in both species.

Microstructural changes of Nimonic-263 superalloy caused by laser beam action

Pulsed laser treatments induce changes in the microstructure of materials. We have exposed the Nimonic-263 superalloy to pulsed laser beams of various repetition rates and wavelengths and investigated the changes in microstructure and microhardness. Two types of laser treatment of the material surface occurred: mechanical and thermomechanical. The microstructure changes have been comparatively analyzed by scanning electron microscopy and energo-dispersive spectrometry. The grain size was measured before and after the treatment and microhardness tests were performed. These investigations aim to contribute to the study of the level of improvement of microstructure and mechanical properties because of the interaction with laser beams.

Damage effects on multi-layer graphene from femtosecond laser interaction

We present a study on the damage effects of femtosecond laser interaction on exfoliated multi-layer graphene using the techniques of optical microscopy, atomic force microscopy, and Raman spectroscopy. Various effects of the interaction were observed. The ablation threshold was found to be ?4 mJ cm?2, and was slightly higher in transmission mode than in reflection mode. This work also demonstrates the feasibility of ultrafast laser patterning of exfoliated multi-layer graphene.

Characterization of laser beam interaction with carbon materials

This paper presents simulation and experimental results for the exposure of some carbon-based materials to alexandrite and Nd3+:YAG (yttrium aluminum garnet) laser radiation. Simulation of the heating effects was carried out using the COMSOL Multiphysics 3.5 package for samples of carbon-based P7295-2 fiber irradiated using an alexandrite laser and carbon-based P4396-2 fiber irradiated using an Nd3+:YAG laser, as well as by applying finite element modeling for P7295-2 samples irradiated using an Nd3+:YAG laser. In the experimental part, P7295-2 samples were exposed to alexandrite laser radiation while samples of carbon-based composite 3D C/C were exposed to Nd3+:YAG laser radiation. Micrographs of the laser induced craters were obtained by light and scanning electron microscopy, and the images analyzed using the ImageJ software. The results obtained enable identification of the laser–material interaction spots, and characterization of the laser induced changes in the materials investigated.

Design of co-existence parallel periodic surface structure induced by picosecond laser pulses on the Al/Ti multilayers

Formation of periodic nanostructures on the Ti/5x(Al/Ti)/Si multilayers induced by picosecond laser pulses is studied in order to better understand the formation of a laser-induced periodic surface structure (LIPSS). At fluence slightly below the ablation threshold, the formation of low spatial frequency-LIPSS (LSFL) oriented perpendicular to the direction of the laser polarization is observed on the irradiated area. Prolonged irradiation while scanning results in the formation of a high spatial frequency-LIPSS (HSFL), on top of the LSFLs, creating a co-existence parallel periodic structure. HSFL was oriented parallel to the incident laser polarization. Intermixing between the Al and Ti layers with the formation of Al-Ti intermetallic compounds was achieved during the irradiation. The intermetallic region was formed mostly within the heat affected zone of the sample. Surface segregation of aluminium with partial ablation of the top layer of titanium was followed by the formation of an ultra-thin Al2O3 fil...

Interaction of laser beams with carbon textile materials

Purpose – The purpose of this paper is to present the results of interaction occurring during the exposition of some specific carbon textile materials obtained in laboratory conditions to beams of various laser types. Design/methodology/approach – Carbon fabric materials – fiber, felt and cloth – obtained from different precursor materials and prepared at various process conditions (oxidized, partially carbonized, carbonized, graphitized), were exposed to pulses of various lasers (Nd3+: YAG, alexandrite, ruby). Findings – Depending on the laser power, plasma and destructive phenomena occurred. In the case of an interaction between a Nd3+: YAG laser beam and specimens of thickness in millimeter range, the authors have estimated the threshold of the energy density for drilling and discussed the possible models of the interaction. Research limitations/implications – The results have implications in the estimations of quality as well as in the improvement of material processing, giving some new light to the c...

Using coherence properties for frequency stabilizing He-Ne laser

We have demonstrated that the frequency of the laser radiation can be stabilized by controlling its coherence properties. Theoretical analysis of the laser radiation visibility function was performed and connection with the longitudinal mode structure was established. It was established that laser emission frequency could be stabilized by controlling the visibility of interference pattern. Calculations have shown that, in theory, stability around 4 kHz can be expected. As a proof-of-the-principle, a HeNe laser, stabilized by measurement of the interference pattern visibility in a modified Mechelson interferometer, was built. Stabilization was performed by temperature control of the laser tube in a digital feedback loop and total frequency variation of 3 MHz was obtained. Possible improvements of the experimental setup, together with some applications of multimode stabilized lasers are discussed.