Influence Of Pulse Repetition Rate Research Articles

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. • LIPSS fabrication on DLN and Ti-DLN films with IR and visible femtosecond lasers • Effect of surface graphitization on HSFL and LSFL formation thresholds • Raman analysis of ultrathin surface graphitization on LIPSS-structured DLN films • Influence of pulse repetition rate on fs-laser ablation and LIPSS parameters • Plasmonic effects in the fs-LIPSS formation on diamond-like nanocomposite films

Nucleation and growth of Si nanoparticles under different pulse repetition rates without the baffle for nanosecond pulsed laser-ablated deposition

AbstractIn this article, Si nanoparticle (NP) films were prepared by pulsed laser ablation (PLA) in the argon atmosphere of 10 Pa at room temperature under different pulse repetition rates from 1 to 40 Hz without the baffle. Different from the conventional PLA method, the substrates were placed below and parallel to the ablated plume axis. The obtained films containing NPs were characterized by scanning electron microscopy and Raman spectrometer. The experimental results under constant laser fluence demonstrate the strong dependence of the mean size and the area number density of NPs on the repetition rate. Specifically, with the increase of pulse repetition rate, the mean size of the NPs in the film first decreases and reaches its minimum at 20 Hz, and then increases after 20 Hz, and decreases again till 40 Hz. The area number density shows the contrary trend versus mean size. The in situ diagnostic results of Langmuir probe denote the ablated Si ion density increases monotonously with the increase of repetition rate, while the temperature is almost constant. Combining with the nucleation probability, the growth/aggregation duration of NPs in the “nucleation region” and the effect of the baffle, the influence of pulse repetition rate on the formation of NPs is addressed. It is found that the repetition rate impacts the growth modes of NPs (i.e., growth and aggregation). 1–20, 20–30, and 30–40 Hz, respectively, correspond to growth-, aggregation-, and growth-controlled rate ranges without the baffle; however, 1–10, 10–20, and 20–40 Hz, respectively, correspond to growth-controlled, aggregation/growth-coexisted, and aggregation-controlled rate ranges with the baffle.

Influence of pulse repetition rate on morphology and material removal rate of ultrafast laser ablated metallic surfaces

Ultrafast laser ablation is an efficient method for precise micro-machining. Thanks to the recent development of high repetition rate ultrafast lasers, high speed laser scanning of surfaces is more and more employed to generate micro-/nano- surface structures on metals for a vast variety of applications. Issues associated with these lasers are also identified in micro-machining practice. It is commonly believed that, due to possible shielding effects, the conditions of high fluences and high repetition rates compromise an efficient material removal. However, in this study, based on topography and differential weighing evaluations, we report that the material removal rate holds constant even in sub-MHz regime, up to about 20 J/cm2. The morphology of the post-irradiated surface is found to be determined not only by laser processing conditions but also by the material properties on the other hand. Two trends are experimentally identified in surface laser ablation of Ni, Cu, titanium alloy TA6V and stainless steel 316L: while the former two show a low roughness (Ra below 0.5 µm) at all irradiation conditions, the machining quality of the two later ones degrades rapidly with increasing fluences and repetition rates. In such a scenario, a rugged surface layer of 10–20 µm thickness is formed with the presence of numerous subsurface voids. Microstructural analysis is carried out in order to infer physical transition involved in the micro-machining process. Possible mechanisms accounting for the observation are discussed, especially those related to the electron-phonon coupling, plasma dwelling, and capillary waves. These insights pave the way for tailored, material dependent optimizations of ultrafast laser micro-machining processes.

Influence of pulse repetition rate on the growth of cobalt-doped ZnO thin films by pulsed electron beam ablation

Abstract

Influence of pulse repetition rate and pulse energy on the heat accumulation between subsequent laser pulses during laser processing of CFRP with ps pulses

Heat accumulation between subsequent laser pulses (HAP) is one of the major reasons for the formation of a heat-affected zone during laser processing of carbon fiber-reinforced plastics (CFRP) with picosecond (ps) laser pulses. In the case of CFRP, the formation of a so-called matrix evaporation zone (MEZ) can be observed. Based on a theoretically derived equation that describes the heat accumulation effect and presuming one-dimensional heat flow, an expression for the critical feed rate at which the HAP effect sets in was derived as a function of the pulse repetition rate and the pulse energy. This expression provides a simple dependency of the critical feed rate on the pulse energy and the repetition rate that is a useful tool for process development. To verify this relation, we used dedicated experimental conditions that ensured one-dimensional heat flow and irradiated the material with fluences below the ablation threshold to ensure that the absorbed energy completely remains in the material and is not removed by an ablation process. By doing so, we were able to confirm the validity of the theoretical model. During the actual cutting of CFRP, however, hence, when material is removed, the theoretically predicted dependency of the critical feed rate on the pulse energy and the repetition rate deviates from experimental results when one assumes the fraction of pulse energy that is left in the workpiece as heat (hence not removed with the ablated material) to be constant. The difference, therefore, is most likely attributed to the fact that the fraction of heat remaining in the material after each ablation processes, the so-called residual heat, depends on both the pulse repetition rate and the pulse energy. These dependencies are reported for the first time for pulsed laser processing of CFRP within the presented study.

Influence of pulse repetition rate on percussion drilling of Ti-based alloy by picosecond laser pulses

Percussion drilling of a Ti-based alloy with 8 ps laser pulses was investigated with emphasis on the influence of the pulse repetition rate (1–300 kHz) on the drilling process. Heat accumulation is found to be responsible for simultaneous significant rise of the average drilling rate, the occurrence of a rim around the hole entrance, as well as noticeable surface oxidation. This can be observed as soon as the repetition rate exceeds a certain critical value, which depends on the pulse energy applied. Dramatic rise of the recast layer thickness inside the growing hole was revealed for the limited range of the repetition rates around the lower boundary of the heat accumulation regime. Possible origins of this phenomenon are discussed.

The effects of pulse repetition rate on the structural, surface morphological and UV photodetection properties of pulsed laser deposited Mg-doped ZnO nanorods

The Mg0.05Zn0.95O (MZO) nanorod array (NRA) films have been successfully grown onto SiO2/ n-Si substrates by pulsed laser deposition (PLD) without any template or seed layer and the influence of pulse repetition rate (3 to 15Hz) of a 248nm KrF excimer laser on their crystallinity, surface morphology and UV photodetection properties were systematically investigated. All the samples show the hexagonal wurtzite phase with a preferential c-axis orientation and the optimum crystallization of the MZO NRAs occurs at 5Hz. FE-SEM analysis revealed that the growth of MZO NRAs is strongly influenced by the pulse repetition rate. It was observed that the average film thickness increases almost linearly with the pulse repetition rate and the MZO nanorod arrays grown at 5Hz exhibits best surface area. Moreover, the room temperature UV photodetection properties of the samples were investigated in metal–semiconductor–metal (MSM) planar configurations and are found to be strongly driven by the pulse repetition rate dependent crystalline and surface morphological features. The device current–voltage (I–V) characteristics were measured under dark and UV light conditions. Then, the photocurrent and responsivity were measured with the variation of optical power density and applied voltage, respectively. Transient photoresponse studies show an exceedingly stable and fast switching UV photoresponse for the photodetector having MZO nanorods grown at 5Hz, which demonstrates highest responsivity of 17mA/W upon 2mW/cm2 UV illumination (365nm), at 5V bias.

The influence of laser pulse repetition rate on the intensity of spectral lines in femtosecond laser-induced breakdown spectroscopy of a liquid

The influence of pulse repetition rate of a femtosecond Ti:Sa laser (800 nm, 45 fs, 0.82 mJ) on intensity of spectral lines at optical breakdown on the surface of CaCl2 water solution is experimentally determined. It is demonstrated that, with increasing pulse repetition rate from 20 to 1000 Hz, the intensity of Ca II (393.3 nm) emission line reduces by 44%, while the plasma continuum remains unchanged within measurement error.

Development of CZTS thin films solar cells by pulsed laser deposition: Influence of pulse repetition rate

► Development of CZTS thin film solar cell by PLD . ► Structural optical compositional and morphology properties of CZTS thin films. ► Influence of pulse repetition rate on physicochemical properties CZTS thin films. High-quality Cu 2 ZnSnS 4 (CZTS) thin films were synthesized by pulsed laser deposition as a function of pulse repetition rate onto the SLG substrates. Influence of pulse repetition rate onto the structural, morphological, compositional and optical properties have been investigated for as-deposited and annealed thin films. X-ray diffraction study shows transformation of amorphous to crystalline phase after tuning pulse repetition rate and annealing of samples. FESEM images of thin films show increase in grain size upon annealing. Films are nearly stoichiometric deposited at 10 Hz repetition rate has been confirmed with the help of EDAX and XPS analysis. The direct band gap energy of the deposited CZTS thin films are in the solar energy range. The performance of solar cell based on CZTS absorber layer has been tested and the efficiency is about 2%.

Composite filling removal with erbium:yttrium–aluminum–garnet laser: morphological analyses

Considering the increase in esthetic restorative materials and need for improvement in unsatisfactory restoration substitution with minimal inadvertent removal of healthy tissues, this study assessed the efficacy of erbium:yttrium-aluminum-garnet (Er:YAG) laser for composite resin removal and the influence of pulse repetition rate on the morphological analyses of the cavity by scanning electron microscope. Composite resin fillings were placed in cavities (1.0 mm deep) prepared in bovine teeth, and the 75 specimens were randomly assigned to five groups according to the technique used for composite filling removal (high-speed diamond bur, group I, as a control, and Er:YAG laser, 250 mJ output energy and 80 J/cm(2) energy density, using different pulse repetition rates: group II, 2 Hz; group III, 4 Hz; group IV, 6 Hz; group V, 10 Hz). After the removal, the specimens were split in the middle, and we analyzed the surrounding and deep walls to check for the presence of restorative material. The estimation was qualitative. The surfaces were examined with a scanning electron microscope. The results revealed that the experimental groups presented bigger amounts of remaining restorative material. The scanning electron microscopy (SEM) analyses showed irregularities of the resultant cavities of the experimental groups that increased proportionally with increase in repetition rate.

Influence of Pulse Repetition Rate on Temperature Rise and Working Time During Composite Filling Removal with the Er:YAG Laser

The purpose of this study was to assess the efficacy of Er:YAG laser energy for composite resin removal and the influence of pulse repetition rate on the thermal alterations occurring during laser ablation. Composite resin filling was placed in cavities (1.0 mm deep) prepared in bovine teeth and the specimens were randomly assigned to five groups according to the technique used for composite filling removal. In group I (controls), the restorations were removed using a high-speed diamond bur. In the other groups, the composite fillings were removed using an Er:YAG laser with different pulse repetition rates: group 2-2 Hz; group 3-4 Hz; group 4-6 Hz; and group 5-10 Hz. The time required for complete removal of the restorative material and the temperature changes were recorded. Temperature rise during composite resin removal with the Er:YAG laser occurred in the substrate underneath the restoration and was directly proportional to the increase in pulse repetition rate. None of the groups had a temperature increase during composite filling removal of more than 5.6 degrees C, which is considered the critical point above which irreversible thermal damage to the pulp may result. Regarding the time for composite filling removal, all the laser-ablated groups (except for group 5 [10 Hz]) required more time than the control group for complete elimination of the material from the cavity walls. Under the tested conditions, Er:YAG laser irradiation was efficient for composite resin ablation and did not cause a temperature increase above the limit considered safe for the pulp. Among the tested pulse repetition rates, 6 Hz produced minimal temperature change compared to the control group (high-speed bur), and allowed composite filling removal within a time period that is acceptable for clinical conditions.

Influence of pulse repetition rate of Er:YAG laser and dentin depth on tensile bond strength of dentin–resin interface

The study evaluated the in vitro influence of pulse-repetition rate of Er:YAG laser and dentin depth on tensile bond strength of dentin-resin interface. Dentin surfaces of buccal or lingual surfaces from human third molars were submitted to tensile test in different depths (superficial, 1.0 and 1.5 mm) of the same dental area, using the same sample. Surface treatments were acid conditioning solely (control) and Er:YAG laser irradiation (80 mJ) followed by acid conditioning, with different pulse-repetition rates (1, 2, 3, or 4 Hz). Single bond/Z-250 system was used. The samples were stored in distilled water at 37 degrees C for 24 h, and then the first test (superficial dentine) was performed. The bond failures were analyzed. Following, the specimens were identified, grounded until 1.0- and 1.5-mm depths, submitted again to the treatments and to the second and, after that, to third-bond tests on a similar procedure and failure analysis. ANOVA and Tukey test demonstrated a significant difference (p < 0.001) for treatment and treatment x depth interaction (p < 0.05). The tested depths did not show influence (p > 0.05) on the bond strength of dentin-resin interface. It may be concluded that Er:YAG laser with 1, 2, 3, or 4 Hz combined with acid conditioning did not increase the resin tensile bond strength to dentin, regardless of dentin depth.

Influence of pulse repetition rate on the average size of silicon nanoparticles deposited by laser ablation

To investigate the influence of pulse repetition rate on the average size of the nanoparticles, nanocrystalline Si films were prepared by pulsed laser ablation in high-purity Ar gas with a pressure of 10 Pa at room temperature, under the pulse repetition rates between 1 and 40 Hz, using a nanosecond laser. Raman, X-ray diffraction spectra, and scanning electron microscopy images show that with increasing pulse repetition rate, the average size of the nanoparticles in the film first decreases and reach its minimum at 20 Hz, and then increases, which may be attributed to the nonlinear dynamics of the laser-ablative deposition. In our experiment conditions, the duration of the ambient restoration, a characteristic parameter being used to distinguish nonlinear or linear region, is about a few seconds from the order of magnitude, which is consistent with the previous experimental observation. More detailed model to explain quantitively the observed effect is under investigation.