Laser Pulse Fluence Research Articles

List of abstracts

. Results obtained in the analysis of single layer and multilayered carbon stacks produced by PLD at low temperature will be presented. The mechanical properties of carbon single layers deposited at various laser pulse fluences, like hardness and elasticity, were determined by means of nano indentation measurements. It will be shown that a wide variety of mechanical properties can easily be adjusted by adapting the process parameters.

Femtosecond laser ablation of dentin and enamel for fast and more precise dental cavity preparation

PurposeThe purpose of the present work was to achieve fast and more precise ablation in dentin and enamel by using a commercial femtosecond laser system with high repetition rate, whilst avoiding any collateral irreversible damages in the hard tissue and pulp area. MethodsWe used fluence of the incident laser pulses which was marginally higher than the ablation threshold for dentin and enamel. The study was based on the hypothesis that femtosecond laser operating with a repetition rate in the range of 100–500 kHz can controllably ablate dental tissue obtaining sufficiently high removal rate whilst avoiding any collateral irreversible damages in the hard tissue and pulp area. ResultsThe ablation yielded the formation of 1 mm3 craters with well-defined precise vertical cavity sides and edges. Advantageous high porosity and numerous interconnected pores were introduced in the ablated zones. Thermal load and hence collateral thermo-mechanical damages were avoided and the crystalline structure of the tooth constituent hydroxyapatite was preserved. ConclusionThe ultrafast femtosecond laser used in our work hold the promise of a significant drilling ability without collateral thermomechanical effects. It achieves high processing efficiency, overcomes disadvantages of other laser systems reported and can be used to develop an instrument for cavity preparation based on fast and precise ablation. Our further aim is to exceed the speed of traditional drilling instruments and thus to reduce the treatment time which in turn will bring comfort to the patient.

Fabrication of Through via Holes in Ultra-Thin Fused Silica Wafers for Microwave and Millimeter-Wave Applications.

Through via holes in fused silica are a key infrastructure element of microwave and millimeter-wave circuits and 3D integration. In this work, etching through via holes in ultra-thin fused silica wafers using deep reactive-ion etching (DRIE) and laser ablation was developed and analyzed. The experimental setup and process parameters for both methods are presented and compared. For DRIE, three types of mask materials including KMPR 1035 (Nippon Kayaku, Tokyo, Japan) photoresist, amorphous silicon and chromium—with their corresponding optimized processing recipes—were tested, aiming at etching through a 100 μm fused silica wafer. From the experiments, we concluded that using chromium as the masking material is the best choice when using DRIE. However, we found that the laser ablation method with a laser pulse fluence of 2.89 J/cm2 and a pulse overlap of 91% has advantages over DRIE. The laser ablation method has a simpler process complexity, while offering a fair etching result. In particular, the sidewall profile angle is measured to be 75° to the bottom surface of the wafer, which is ideal for the subsequent metallization process. As a demonstration, a two-inch wafer with 624 via holes was processed using both technologies, and the laser ablation method showed better efficiency compared to DRIE.

Soft Picosecond Infrared Laser Extraction of Highly Charged Proteins and Peptides from Bulk Liquid Water for Mass Spectrometry.

We report the soft laser extraction and production of highly charged peptide and protein ions for mass spectrometry directly from bulk liquid water at atmospheric pressure and room temperature, using picosecond infrared laser ablation. Stable ion signal from singly charged small molecules, as well as highly charged biomolecular ions, from aqueous solutions at low laser pulse fluence (∼0.3 J cm-2) is demonstrated. Sampling via single picosecond laser pulses is shown to extract less than 27 pL of volume from the sample, producing highly charged peptide and protein ions for mass spectrometry detection. The ablation and ion generation is demonstrated to be soft in nature, producing natively folded proteins ions under sample conditions described for native mass spectrometry. The method provides laser-based sampling flexibility, precision and control with highly charged ion production directly from water at low and near neutral pH. This approach does not require an additional ionization device or high voltage applied directly to the sample.

Optoacoustic Effects in Pentaerythritol Tetranitrate with Ultrafine Aluminum-Particle Inclusions under Pulsed-Laser Action

Results of studying optoacoustic characteristics of pentaerythritol tetranitrate (PETN) specimens with inclusions of ultrafine aluminum particles (100 nm) are presented. Regularities of the increase of extinction coefficient kef and signal amplitude U on a piezodetector by increasing laser-pulse fluence H have been established. Estimates have been made, and it has been concluded that, during a laser pulse, heating of aluminum inclusions and a shell surrounding it occurs up to a gasification temperature and appear craters on a specimen’s surface appear at H > 0.1 J/cm2.

ZnO synthesized in air by fs laser irradiation on metallic Zn thin films

We present results on rapid femtosecond laser synthesis of nanostructured ZnO. We used metallic Zn thin films to laser scan along straight tracks, until forming nanostructured ZnO. The synthesis dependence on laser irradiation parameters such as the per pulse fluence, integrated fluence, laser scan speed, and number of scans were explored carefully. SEM characterization showed that the morphology of the obtained ZnO is dictated by the integrated fluence and the laser scan speed; micro Raman and XRD results allowed to identify optimal laser processing conditions for getting good quality ZnO; and cathodoluminescence measurements demonstrated that a single laser scan at high per pulse laser fluence, but a medium integrated laser fluence and a medium laser scan speed favors a low density of point-defects in the lattice. Electrical measurements showed a correlation between resistivity of the laser produced ZnO and point-defects created during the synthesis. Transmittance measurements showed that, the synthesized ZnO can reach down to the supporting fused silica substrate under the right laser irradiation conditions. The physical mechanism for the formation of ZnO, under ultrashort pulse laser irradiation, is discussed in view of the distinct times scales given by the laser pulse duration and the laser pulse repetition rate.

Impact of air, laser pulse width and fluence on U–Pb dating of zircons by LA-ICPMS

The accuracy of zircon U–Pb dating by LA-ICPMS is limited by matrix effects related to differences in U–Pb fractionation between an unknown and the calibration standard.

Laser pulse number dependent nanostructure evolution by illuminating self-assembled microsphere array

Pulse number dependent evolution from nanodents to nanobumps has been studied on a bearing steel substrate, which was coated with a self-assembled monolayer of silica microspheres and repeatedly irradiated by an 800 nm femtosecond laser. Scanning electron microscope and atomic force microscope were employed to characterize nanopatterns, the dimensions of which were related to the laser pulse number and pulse fluences. The transformation depending on the number of laser pulses could be attributed to the changes of electric field distribution and material property after the impacts of multiple laser pulses, the process of which could be divided into three steps. First, the bottoms of silica microspheres were ablated because of the incubation effects from repeated irradiation. Second, strong plasmonic localization at the edges of the deep nanodents resulted in plasma-chemical reactions between ablated materials, which was confirmed by electromagnetic simulations. Third, recrystallized solid matter from ablated materials deposited in nanodents and then formed nanobumps, which was confirmed by transmission electron microscope and energy dispersive X-ray spectrometer analyses on their longitudinal sections.

Wavelength and fluence-dependent third-order optical nonlinearity of mono- and multi-layer graphene

The objective of this study is to investigate third-order optical nonlinearities of monolayer and randomly stacked bi- and four-layer graphene samples, depending on wavelength and fluence of femtosecond laser pulses in the near-infrared spectral region. Nonlinear refractive indices and absorption coefficients, and in consequence, third-order susceptibilities of graphene samples, are determined by z-scan measurements at four different wavelengths and three fluence regimes categorized by nonlinear transmission measurements. In addition, nonlinear refractive indices are independently investigated by optical Kerr gate experiments, showing good agreement with z-scan results. Our study on third-order nonlinearities of mono- and multi-layer graphene samples is significant for understanding nonlinear optical characteristics of graphene and developing graphene-based nonlinear photonic devices.

Effects of wavelength, fluence, and dose on fragmentation pathways and photoproduct ion yield in 213 nm and 266 nm ultraviolet photodissociation experiments.

Ultraviolet photodissociation tandem mass spectrometry is a powerful tool to investigate the structure of biomolecules, due to its ability to generate rich fragmentation patterns or bond selective cleavage, as a function of used laser wavelength, laser fluence, dose (number of accumulated laser pulses), and available chromophores. Herein, we report first results obtained with a newly developed two-wavelength (266 nm and 213 nm) ultraviolet photodissociation setup coupled to a Fourier-transform ion cyclotron resonance mass spectrometer. Photoproduct yields for protonated 3-iodo-l-tyrosine were up to ∼75%. Dose and fluence dependent measurements for protonated 3-iodo-l-tyrosine, doubly charged protonated bradykinin and Fe(II) attached to 1,2-dioleoyl-sn-glycero-3-phosphocholine reveal that the ultraviolet photodissociation mechanism for photoproduct formation qualitatively differs between these model systems. Three derived photodissociation models were used to interpret the experimental results and show that while protonated 3-iodo-l-tyrosine and Fe(II) attached to 1,2-dioleoyl-sn-glycero-3-phosphocholine most likely dissociates via a single-photon process, fragmentation of doubly charged bradykinin ions was found to be most consistent with sequential two-photon dissociation (213 nm). The introduced dissociation models present an easy means to study the mechanism of ultraviolet photodissociation processes for a variety of analytes without prior knowledge of their photochemistry or to optimize experimental conditions by adjusting laser fluence or number of laser pulses.

Second-harmonic generation of ZnO nanoparticles synthesized by laser ablation of solids in liquids

We report the synthesis of small zinc oxide nanoparticles (ZnO NPs) based colloidal suspensions and the study of second-harmonic generation from aggregated ZnO NPs deposited on glass substrates. The colloidal suspensions were obtained using the laser ablation of solids in liquids technique, ablating a Zn solid target immersed in acetone as the liquid medium, with ns-laser pulses (1064nm) of a Nd-YAG laser. The per pulse laser fluence, the laser repetition rate frequency and the ablation time were kept constant. The absorption evolution of the obtained suspensions was optically characterized through absorption spectroscopy until stabilization. Raman spectroscopy, SEM and HRTEM were used to provide evidence of the ZnO NPs structure. HRTEM results showed that 5–8nm spheroids ZnO NPs were obtained. Strong second-harmonic signal is obtained from random ZnO monocrystalline NPs and from aggregated ZnO NPs, suggesting that the high efficiency of the nonlinear process may not depend on the NPs size or aggregation state.

In situ probing of pulsed laser melting and laser-induced periodic surface structures formation by dynamic reflectivity

The melting process and nanostructure formation induced by nanosecond and picosecond laser pulses on bulk silicon and copper thin film were studied by ex situ analysis and in situ real time reflectivity. Three different probing wavelengths (633, 473 and 326 nm) were used during the pump laser processing and were correlated to the beam parameters (pulse duration, laser fluence and number of laser shots) and copper thin film thickness. On a silicon surface using a KrF laser beam (27 ns, 1 Hz, 248 nm), the melting threshold was determined close to 700 mJ cm−2 and the melting duration increased from 10 to 130 ns as the fluence increased from 700 to 1750 mJ cm−2. Nanostructures with a spatial period close to the laser wavelength were formed on both copper thin film and silicon substrate after nanosecond Nd:YAG laser (10 ns, 266 nm, 1 Hz) irradiation. In the picosecond regime, using an Nd:YAG laser (40 ps, 266 nm, 1 Hz), different nanostructures, from spikes to laser-induced periodic surface structures, were formed on 500 nm copper thin film and were analyzed with respect to the drop in dynamic reflectivity changes versus the number of laser shots.

Conditions for thermally induced all-optical switching in ferrimagnetic alloys: Modeling of TbCo

We present atomistic spin dynamics modeling of thermally induced magnetization switching (TIMS) of disordered ferrimagnetic TbCo alloys varying the Tb concentration, laser pulse fluence, and its duration. Our results indicate that deterministic TIMS occurs in a wide range of Tb concentrations and at large laser fluences with a pulse duration of 50 fs. We furthermore demonstrate that the occurrence of the transient ferromagneticlike state is necessary, but after first reversal, the system may switch back. The presence of a magnetization compensation point or going through it is shown not to be required. With the increase of the laser pulse duration TIMS becomes stochastic so that for a 1 ps laser pulse width and beyond the deterministic heat-assisted AOS does not exist.

Critical switching fluence of L10-FePt nanoparticles with practical size to ultrafast all-optical polarization switching

The magnetic storage industry will be pushed into a new era by emerging magnetic materials and ingenious read/write techniques. Recently, L10-FePt nanoparticles with exceptionally high uniaxial anisotropy, combined with the technique of all-optical magnetic switching, have increasingly drawn the attention of researchers. Focusing on L10-FePt nanoparticles, we report the superparamagnetic size limits and three elaborate stages of magnetic orientation switching, which demonstrate that in terms of the opto-magnetic responses of L10-FePt nanoparticles with the size of 6 nm, the fluence of an ultrashort laser pulse should not be larger than the level of 40 mJ/cm2 in order to achieve the helicity-dependent all-optical switching.

Droplet distribution during sub-picosecond laser deposition of gold nanoparticles

Even if it has been recently assessed that sub-picosecond ablation is not a “melting-free” process, a systematic investigation of the issue of droplets is still lacking.In this paper we discuss the effects of fluence (F=0.3, 1, 3J/cm2), target-substrate distance (dTS=5, 6, 7, 8, 9cm) and number of laser pulses per site (Np/s=2, 12, 22, 36, 96) on the morphology in terms of droplets and plasmonic response of gold nanoparticles deposited by sub-picosecond pulsed laser ablation. Among metals, gold is an ideal candidate to study the occurrence of droplets due to its low electron-phonon coupling that favors the appearance of heat affected zones. Our findings demonstrate that Np/s affects the production of large droplets more intensely than F and dTS. In fact, increasing Np/s produced much larger droplets than the ones obtained by tuning the fluence and an increment in the droplet density of 10 times. On the other hand, while the maximum density of small droplets (average diameter≤0.15μm) occurs at F=1J/cm2 (with respect to 0.3J/cm2 and 3J/cm2), the total droplet density exhibits a minimum at such fluence.

Controllable Si (100) micro/nanostructures by chemical-etching-assisted femtosecond laser single-pulse irradiation

In this study, silicon micro/nanostructures of controlled size and shape are fabricated by chemical-etching-assisted femtosecond laser single-pulse irradiation, which is a flexible, high-throughput method. The pulse fluence is altered to create various laser printing patterns for the etching mask, resulting in the sequential evolution of three distinct surface micro/nanostructures, namely, ring-like microstructures, flat-top pillar microstructures, and spike nanostructures. The characterized diameter of micro/nanostructures reveals that they can be flexibly tuned from the micrometer (∼2 μm) to nanometer (∼313 nm) scales by varying the laser pulse fluence in a wide range. Micro-Raman spectroscopy and transmission electron microscopy are utilized to demonstrate that the phase state changes from single-crystalline silicon (c-Si) to amorphous silicon (a-Si) after single-pulse femtosecond laser irradiation. This amorphous layer with a lower etching rate then acts as a mask in the wet etching process. Meanwhile, the on-the-fly punching technique enables the efficient fabrication of large-area patterned surfaces on the centimeter scale. This study presents a highly efficient method of controllably manufacturing silicon micro/nanostructures with different single-pulse patterns, which has promising applications in the photonic, solar cell, and sensors fields.

High speed pulsed laser cutting of [formula omitted] Li-ion battery electrodes

Laser cutting of Li-ion battery electrodes represents an alternative to mechanical blanking that avoids complications associated with tool wear and allows assembly of different cell geometries with a single device. In this study, laser cutting of LiCoO2 Li-ion battery electrodes is performed at up to 5m/s with a 1064nm wavelength nanosecond pulsed fiber laser with a maximum average power of 500W and a repetition rate of up to 2MHz. Minimum average cutting power for cathode and anode multi-layer films is established for 12 parameter groups with velocities over the range 1–5m/s, varying laser pulse fluence and overlap. Within the tested parameter range, minimum energy per unit cut length is found to decrease with increasing repetition rate and velocity. SEM analysis of the resulting cut edges reveals visible clearance widths in the range 20–50μm, with cut quality found to improve with velocity due to a reduction in lateral heat conduction losses. Raman line map spectra reveal changes in the cathode at 60μm from the cut edge, where bands at 486cm-1 and 595cm-1, corresponding to the Eg and A1g modes of LiCoO2, are replaced with a single wide band centered at 544cm-1, and evidence of carbon black is no longer present. No changes in Raman spectra are observed in the anode. The obtained results suggest that further improvements in cutting efficiency and quality could be achieved by increasing the repetition rate above 2MHz, thereby improving ablation efficiency of the metallic conductor layers. The laser source utilized in the present study nonetheless represents an immediately available solution for repeatability and throughput that are superior to mechanical blanking.

Influence of Charge Transport and Defects on the Performance of Planar and Mesostructured Perovskite Solar Cells

Photoinduced charge selective carrier extraction by linearly increasing voltage technique allows straightforward assessment of charge transport properties within planar and mesostructured perovskite solar cells with respect to light intensity and signal delay time. Charge sensitive device architecture is realized through implementation of insulating layer between the anode or cathode to prevent extraction of unwanted type of carriers. Resulting behavior of comparatively efficient mesoporous and planar solar cells exhibits well balanced charge transport with slight dependence of charge mobility on applied laser pulse fluence, for given pulse delay times. Very similar charge carrier mobilities are present within mesoporous devices, whereas holes trail approximately half an order of magnitude behind electrons in planar structured specimens. Moreover, dispersive transport is identified in the electron selective devices with titanium oxide electron transporter, suggesting considerable presence of trapping states at the perovskite interface, whereas no such behavior characterizes planar samples. Variation in delay time between laser pulse and extraction ramp only affects initial charge concentration present within the device, while transient outlay remains unchanged, indicating absence of film charging effect.

Patterned perforation of polyester films by excimer laser ablation

The effects of 248 nm KrF excimer laser irradiation on combinations of different thickness polyethylene terephtalate (PET) films and different stainless steel mask mesh opening sizes were considered as a possible method to achieve different size perforation patterns on different thickness PET films. The findings of this work will provide an effective and controllable perforation technology. Therefore, ablation behavior of PET films was investigated for different laser energy fluence and number of laser pulses. The morphology of irradiated samples was observed by optical microscopy, and atomic forced microscopy was employed to study the topography and microstructures after laser ablation. An attempt has been made to correlate these findings with the orientation, strength, and mechanical properties of the polymer. The experimental results reveal that the percentage of perforation and the average perforated area increase with increasing number of pulses for all film thicknesses. The affected area, defined as...

Assessments of the relation between the degree of order of the ultrafast laser deposited carbon film and the features of the Raman spectrum's D band

Highly oriented pyrolytic graphite (HOPG) is ablated by ultrashort laser pulses with different pulse durations (50fs and 4ps) and laser fluences (68J/cm2 and 34J/cm2) in vacuum (4×10−4Pa). Diamond like carbon (DLC) films are deposited by collecting the ejected material generated by laser pulses with intensities ranging from 0.85×1013W/cm2 to 1.36×1015W/cm2. The material compositions of the films are investigated using micro-Raman spectroscopy and the features of the obtained Raman spectra are analyzed by employing three different sets of peak fitting functions, including Breit-Wigner-Fano (BWF) and Lorentzian functions, two Gaussian functions, and two damped harmonic oscillator functions. It is found that when peak fitting functions with proper symmetry are employed, both D band's position and full width at half maximum (FWHM) can be used as the evaluation parameters for the degree of order of the deposited carbon films. Peak fitting results show that for DLC films, D band's position and FWHM increase as the increase of the degree of order of the films, which can be interpreted by the double-resonant Raman scattering model on account of the crystalline size confinement effect.