Heat accumulation effects in laser processing of diamond-like nanocomposite films with bursts of femtosecond pulses

In this paper we have investigated the burst mode (BM) ablation and surface structuring of diamond-like nanocomposite (DLN) a-C:H:Si:O films with femtosecond laser pulses (wavelength λ=515 nm, pulse duration τ=320 fs, pulse repetition rate f=100 kHz) under different scanning conditions (single spots, linear structures). The pulse separation in the bursts is 25 ns (intra-burst frequency f=40 MHz) and the pulse number is varied from 1 to 8. The ablation depth and specific ablation rates (μm3/μJ) are found to be higher for the burst mode compared to single pulse irradiation, increasing with the pulse number in the burst. The obtained experimental data of the higher ablation efficiency are shown to correlate with computer simulations of the BM ablation. In correlation with the ablation findings, Raman spectra of single spots and microgrooves have evidenced a growing graphitization of amorphous film structure with the pulse number in the bursts (at equal energy deposited into the films). Contact-mode atomic force microscopy (AFM) is applied to reveal an influence of the BM processing on the surface properties (nanoscale relief, friction) of laser-structured films. Based on the ablation and Raman data analysis, AFM examination of ablated/redeposited layers and computer simulations of the burst mode ablation, the heat accumulation is identified as the main factor responsible for the enhanced ablation efficiency during the BM processing of DLN films. In addition, results of high precision surface microstructuring of DLN films in the burst mode are presented

UV Picosecond-Laser Induced Bulk Modifications and Luminescence in Single-Crystal Diamond

Bulk laser-graphitized microstructures have been fabricated in type IIa single-crystal 1.2-mm-thick diamond plates by UV laser irradiation with 10-ps pulses at λ=355 nm wavelength. It is found that the crystallographic-plane-dependent character of structural modifications in the bulk is influenced by the laser wavelength and the direction of the laser beam incidence relative to a given crystallographic direction (<100> or <110>) in the diamond plates. High-order Stokes Raman lasing is observed during UV laser irradiation and bulk modifications of single-crystal diamond. It is shown that the formation of bulk microstructures results in dramatic changes in the behavior of the stimulated Raman scattering in diamond. The formation and migration of the 3H defects (self-interstitial related centers) is also found to take place in the course of bulk microstructuring with UV ps-pulses. Important limitations of the bulk microstructuring caused by high internal stresses in laser-modified regions resulting in ‘uncontrollable’ damage of the diamond single crystals are discussed

Femtosecond laser-induced periodic surface structures on diamond-like nanocomposite films

We study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films and titanium-doped DLN films during femtosecond (fs) laser ablation processing with linearly-polarized beams of IR and visible fs-lasers (wavelengths 1030 nm and 515 nm, pulse duration 320 fs, pulse repetition rates 100 kHz-2 MHz, scanning beam velocity 0.04–0.4 m/s). The studies are focused on (i) comparison of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) formed on DLN and Ti-DLN films by IR fs-laser processing, (ii) effects of the pulse repetition rate on the parameters of LIPSS formed on the DLN and Ti-DLN films, (iii) Raman spectroscopy analysis of the LIPSS-structured films with application for ultrathin surface graphitization, and (iv) relationship between the fs-laser-induced surface graphitization and LIPSS formation on the films. A variety of the HSFL and LSFL have been produced on the surface of DLN and Ti-DLN films, with all the LIPSS being oriented perpendicular to the beam polarization direction. The HSFL periods are varied from ~80 to 240 nm and the LSFL periods are varied from 355 to 840 nm, depending on the fs-laser irradiation conditions (wavelength, fluence, pulse repetition rate) and films properties. Various plasmonic effects such as the superposition of the HSFL and LSFL and emergence of very unusual sinusoid-like structures on the DLN and Ti-DLN films are presented and discussed

Light conversion in thin films of a mixture of mesotetraphenylporphyrin and erbium-doped yttrium vanadate crystallites: 2. Optical properties

The optical properties of two-component films composed of mesotetraphenylporphyrin (TPP) and erbium-doped yttrium vanadate Yt0.95Er0.05VO4 prepared by spincoating have been studied for the first time. A decrease in the TPP content in the films leads to a hypsochromic shift of the Soret band peak by 1–9 nm; this finding suggests that the degree of aggregation of TPP decreases with decreasing TPP content in the film. The fluorescence peak of TPP is located at an emission wavelength of λem = 634 nm and an excitation wavelength of λex = 420 nm. The fluorescence peaks of Y0.95Er0.05VO4 at λem = 526, 546, and 555 nm (λex = 300 nm) correspond to the following transitions of the Er3+ ion: the band at 526 nm, to the 2H11/2 → 4I15/2 transition; the bands at 546 and 555 nm, to the 4S3/2 λ 4I15/2 transition. The fluorescence band peaks preserve their positions with a change in the ratio of components in the film; that is, the fluorescent characteristics of TPP and Y0.95Er0.05VO4 clusters do not depend on their interaction. For both TPP and Y0.95Er0.05VO4, the maximum fluorescence intensity is observed at a TPP content in the film of 40%; the gain with respect to single-component TPP and Y0.95Er0.05VO4 films is 70 and 4–15%, respectively. In this case, a significant effect is exerted not so much by the nature and structure of the components and their interaction as by the topographic features of organization of the photoactive elements in the film, their ratio, and mutual orientation, which determine the energy capture probability. © 2016, Pleiades Publishing, Ltd

Femtosecond-laser surface modification and micropatterning of diamond-like nanocomposite films to control friction on the micro and nanoscale.

Laser surface micropatterning (texturing) of hard materials and coatings is an effective technique to improve tribological systems. In the paper, we have investigated the laser-induced surface modifications and micropatterning of diamond-like nanocomposite (DLN) films (a-C:H,Si:O) using IR and visible femtosecond (fs) lasers, focusing on the improvement of frictional properties of laser-patterned films on the micro and macroscale. The IR and visible fs-lasers, operating at λ = 1030 nm and λ = 515 nm wavelengths (pulse duration 320 fs and pulse repetition rate 101 kHz), are used to fabricate different patterns for subsequent friction tests. The IR fs-laser is applied to produce hill-like micropatterns under conditions of surface graphitization and incipient ablation, and the visible fs-laser is used for making microgroove patterns in DLN films under ablation conditions. Regimes of irradiation with low-energy IR laser pulses are chosen to produce graphitized micropatterns. For these regimes, results of numerical calculations of the temperature and graphitized layer growth are presented to show good correlation with surface relief modifications, and the features of fs-laser graphitization are discussed based on Raman spectroscopy analysis. Using lateral force microscopy, the role of surface modifications (graphitization, nanostructuring) in the improved microfriction properties is investigated. New data of the influence of capillary forces on friction forces, which strongly changes the microscale friction behaviour, are presented for a wide range of loads (from nN to μN) applied to Si tips. In macroscopic ball-on-disk tests, a pair-dependent friction behaviour of laser-patterned films is observed. The first experimental data of the improved friction properties of laser-micropatterned DLN films under boundary lubricated sliding conditions are presented. The obtained results show the DLN films as an interesting coating material suitable for laser patterning applications in tribology. ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation under Project No. 15-12-00039

Femtosecond-laser-ablation induced transformations in the structure and surface properties of diamond-like nanocomposite films

Femtosecond laser ablation processing is applied for surface modification and micropatterning of diamond-like nanocomposite (DLN) films (a-C:H:Si:O films). Using a visible femtosecond laser (wavelength 515 nm, pulse duration 320 fs), microgroove patterns have been fabricated on the DLN films, aimed at further studies of their properties. The studies were focused on (i) structural transformations in the surface layers using Raman spectroscopy and transmission electron microscopy (TEM), (ii) wettability of laser-patterned films, and (iii) nano/ microscale friction properties of laser-patterned DLN films using lateral force microscopy. Raman spectroscopy and TEM data showed characteristic features of the surface graphitization during ultrashort-pulse ablation. High resolution TEM study of the microgrooves revealed the formation of cubic SiC nanocrystals (4–8 nm size) on the laser-ablated surface. The water contact angle measurements showed anisotropic wetting behavior of the grooved surfaces (the contact angle was different in the directions parallel and perpendicular to microgrooves), depending on the groove depth (aspect ratio). Lateral force microscopy examination (with micro-sized Si tips) showed that the laser-patterned regions exhibited low friction properties compared to the original surface. The obtained results demonstrate that femtosecond laser processing is an effective technique to generate new properties of hard DLN coatings at the micro and macroscale

Light conversion in thin films of mixtures of mesotetraphenylporphyrin and yttrium vanadate crystallites doped with erbium. I. Photovoltaic properties and structure

The optical and photovoltaic properties of two-component TPP–Yt0.95Er0.05VO4 films prepared by the spincoating method are for the first time studied. A 30% increase in the photovoltage (PV) of TPP: Yt0.95Er0.05VO4 = 3: 2 films on silicon carbide (SiC) supports as compared to TPP one-component films is observed. In contrast, for films on tin dioxide (SnO2) supports, the PV drops by 35–96% as the TPP content in the film decreases from 80 to 20%. The properties of the films are monitored by atomic force microscopy. The highest roughness of the films corresponds to a 40–60% TPP content. © 2016, Pleiades Publishing, Ltd