Lithographical laser ablation using femtosecond laser

Lithographical laser ablation was demonstrated using a femtosecond laser with a lithographical optical system. In this method, a femtosecond laser beam passes through a mask and the pattern is imaged on a film by a coherent optical system. As a result, the film is lithographically ablated, and a micron-sized pattern can be generated in a single shot. The resolution of generation was 13 m, and the narrowest width of a generated line was about 4 m. Moreover, the system was applied to transmission gratings as masks, and nano-sized periodic structures such as nano-sized hole matrices and nano-meshes were generated in a single shot. PACS 52.38.Mf; 42.25.Hz; 42.82.Cr; 81.16.-c     

Diffraction modelling of laser ablation using transmission masks

We present an analysis of near-field diffraction effects in ablation with transmission masks, based on coupling a simplified form of the Fresnel–Kirchhoff diffraction integral with basic models for material removal. Modelling for square, hexagonal and circular proximity masks is described and compared with previously reported experiments on glass, silicon and polyimide using excimer, femtosecond and CO₂ lasers. The model has general applicability and can provide useful insight into the effect of near-field diffraction in ablation patterning.     

Pulsed laser ablation of GaAs using nano pulse length

Ablation using very short pulses has shown a great promise in facilitating the growth of complex multi-element films with stoichiometries matching those of their parent materials. GaAs is an important material in the electronic and opto-electronic industries and due to its compound structure it is an intriguing candidate for pulsed laser deposition. This work investigates the effect of nanosecond laser pulse lengths on the ablation of GaAs in an inert atmosphere. The number of pulses was varied in order to find the optimal condition for nano particles formation in our setup. The deposited structures were studied by grazing incidence small angle X-ray scattering and atomic force microscopy. It is shown that the GaAs nanoparticle sizes and size distributions can be controlled partly by the number of laser pulses applied in their production.     

Fabrication of tin monosulfide nanosheet arrays using laser ablation

We demonstrate the non-catalytic, low-temperature (∼200°C) synthesis of two-dimensional (2D) tin monosulfide (SnS) nanosheet arrays vertically aligned on the substrate by means of pulsed laser ablation (PLD) process. The prepared nanosheets were characterized using X-ray diffraction, Raman spectroscopy, field-emission electron microscopy, electron probe micro-analysis, and transmission electron microscopy (HRTEM). The growth mechanism of 2D morphological feature, which is widespread but very slim (∼10 nm), is further investigated in detail from the crystallographic point of view by observing highly-resolved lattice images taken from plane-normal and in-plane directions using focused ion-beam manipulation and high-resolution transmission microscopy.     

Metal-on-oxide nanoparticles produced using laser ablation of microparticle aerosols

A continuous aerosol process has been studied for producing nanoparticles of oxides that were decorated with smaller metallic nanoparticles and are free of organic stabilizers. To produce the oxide carrier nanoparticles, an aerosol of 3–6 μm oxide particles was ablated using a pulsed excimer laser. The resulting oxide nanoparticle aerosol was then mixed with 1.5–2.0 μm metallic particles and this mixed aerosol was exposed to the laser for a second time. The metallic micron-sized particles were ablated during this second exposure, and the resulting nanoparticles deposited on the surface of the oxide nanoparticles producing an aerosol of 10–60 nm oxide nanoparticles that were decorated with smaller 1–5 nm metallic nanoparticles. The metal and oxide nanoparticle sizes were varied by changing the laser fluence and gas type in the aerosol. The flexibility of this approach was demonstrated by producing metal-decorated oxide nanoparticles using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.     

Synthesis of nanocrystalline cubic zirconia using femtosecond laser ablation

We report on the synthesis of nanocrystalline zirconia in liquid using femtosecond laser ablation. Nanocrystalline cubic zirconia has been prepared by femtosecond laser ablation of zirconium in ammonia, while nanocrystalline tetragonal and monoclinic zirconia was synthesized in water. The physical and chemical mechanisms of the formation of nanocrystalline metastable zirconia are discussed. The intrinsic properties of femtosecond laser ablation in liquid and OH⁻¹ may be responsible for the synthesis of cubic zirconia. It is suggested that the femtosecond laser pulse can create higher temperature and pressure conditions at a localized area in the liquid than the nanosecond laser pulse and the cooling is also faster in the femtosecond laser ablation process, which determined the difference between the products synthesized with femtosecond and nanosecond-pulsed laser ablation.     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

Tooth ablation using a CPA-free thin disk femtosecond laser system

Caries – the most frequent cause for dental surgery – still is mainly treated with conventional mechanical drills, although lasers have meanwhile been successfully applied to various clinical disciplines. Since ultrashort laser pulses with sufficient pulse energies have only been available at low repetition rates (< 1 kHz) in recent decades, solely continuous wave radiation or pulse durations longer than thermal diffusion processes were applied with the result of severe thermal damage and pain. In this report we present results on dental tissue ablation obtained with a novel thin disk Yb:KYW regenerative amplifier system that does not require chirped pulse amplification (CPA). We show that femtosecond laser pulses provide us with todays optimal tool to treat dental decay in an acceptable time, in an excellent quality, and with unsurpassed caries selectivity. The superior quality is a result of the non-thermal laser-tooth interaction. All our results are based on environmental scanning electron microscopy. PACS 42.62.–b; 06.60.Jn; 82.80.–d     

Preparation of nano-sized functional materials using laser ablation in liquids

We propose a convenient technique applicable for investigations of various functions of nanoparticles produced by laser ablation in liquids. It was demonstrated that nanoparticles of anatase-TiO₂, a electrode material for lithium secondary batteries, produced by laser ablation in acetone could be efficiently deposited on a substrate by using an electrophoresis technique. Analysis of the electrochemical properties of nanoparticles become much more facile with those deposited nanoparticles than with dispersed nanoparticles. In addition, it was demonstrated that comparison of the electrochemical properties between nanoparticles and microparticles were possible by means of this technique.     

Creation of silicon nanocrystals using the laser ablation in liquid

The method for the formation of silicon nanoparticles by picosecond laser pulses is studied upon the surface irradiation of the single-crystal silicon in various liquids. The ablation products are investigated using the atomic-force microscopy and Raman spectroscopy. The experimental results indicate the crystal-line structure of nanoparticles and the dependence of their size on the ablation medium.     

Fabrication of a random convex-lens-shaped microstructure using a laser ablation

A convex-lens-shaped microstructure with a diameter of 50 μm on a metallic mold substrate was fabricated in this paper. A laser ablation process, in which the laser beam was focused and irradiated on the metallic mold substrate in order to remove a part of the substrate, was used for that. The convex-lens-shaped microstructure has not been reported in any studies of microstructure using the laser ablation process. It was proposed that the unbalanced ablation and re-adherence of the melted particles was the processing mechanism of the convex lens shape. The convex-lens-shaped microstructure fabricated in this study is smaller than the focused spot. It was expected that the same convex-lens-shaped microstructure can be fabricated even if the focused spot size is increased, so long as the fluence of the laser can be maintained. Therefore, the method proposed in this paper will improve the low processing speed, which has been the problem of a laser ablation process. The fabricated convex-lens-shaped microstructure on the metallic substrate can be used as the mold for the micro lens.     

Single-shot ablation threshold of chromium using UV femtosecond laser pulses

Single-shot ablation threshold for thin chromium film was studied using 266 nm, femtosecond laser pulses. Chromium is a useful material in the nanotechnology industry and information on ablation threshold using UV femtosecond pulses would help in precise micromachining of the material. The ablation threshold was determined by measuring the ablation crater diameters as a function of incident laser pulse energy. Absorption of 266 nm light on the chromium film was also measured under our experimental conditions, and the absorbed energy single-shot ablation threshold fluence was \(46 \pm 5\)  mJ/cm². The experimental ablation threshold fluence value was compared to time-dependent heat flow calculations based on the two temperature model for ultrafast laser pulses. The model predicts a value of 31.6 mJ/cm² which is qualitatively consistent with the experimentally obtained value, given the simplicity of the model.     

Nanocomposites based on globular photonic crystals grown by laser ablation using femtosecond laser pulses

The prepared samples were characterized by reflection and transmission spectra of broadband visible-range radiation, and the volume fraction of substances introduced into pores of artificial opals is estimated. It is found that the introduction of solid-state nanoparticles with negative real part of the permittivity into opal pores results in a decrease in the effective refractive index and a short-wavelength shift in the spectral position of the stop band in artificial opal.     

Plasma dynamics and determination of ablation parameters using the near-target magnified imaging during pulsed laser ablation

A comparative study of the ultrafast dynamics on ZnO thin films deposited on flat Si substrates and on Si micro-cones following ultrashort laser excitation has been carried out using time-resolved optical spectroscopy. By monitoring the transient band gap renormalization induced by nonlinearly excited carriers it is found that fast electron scattering and trapping occurs more efficiently in the micro-cones as compared to the flat films. This enhanced trapping efficiency is attributed to the defects and imperfections that are introduced by the increased surface roughness due to the conical shape.     

Synthesis of Ag-deionized water nanofluids using multi-beam laser ablation in liquids

Multi-pulse laser ablation of silver in deionized water was studied. The laser beams were arranged in a cross-beam configuration. In our experiments, two single-mode, Q-switched Nd-Yag lasers operating at 1064 nm, pulse duration of 5.5 ns and 10 Hz rep rate were used. The laser fluence of the second beam was 0.265 J/cm² for all tests. Two levels of the laser fluences were used for the ablating beam: 0.09 and 0.265 J/cm² (11,014 and 33,042 J/cm² at the focal point, respectively). The silver target was at 50 mm from the cell window and 10 mm deep. The second beam was aligned parallelly with the silver target and focused at 2 mm in front of the focal point of the ablating beam. For all cases, the delay time between the ablating beam and the cross-beam was 40 μs. In general, the ablated particles were almost all spherical. For fluence of 0.09 J/cm ² and single-beam approach, the mean particle size was about 29 nm. The majority of the particles, however, were in 19–35 nm range and there were some big ones as large as 50–60 nm in size. For double-beam approach, the particles were smaller with the average size of about 18 nm and the majority of the particles were in 9–21 nm range with few big one as large as 40 nm. For the beam fluence of 0.265 J/cm² and single-beam configuration, the particle sizes were smaller, the mean particles size was about 18 nm and the majority of the particles were in the range of 10–22 nm with some big one as large as 40 nm. For double-beam approach, the mean particle size was larger (24.2 nm) and the majority of the particle were distributed from 14 to 35 nm with some big particles can be found with sizes as big as 70 nm. Preliminary measurements of the thermal conductivity and viscosity of the produced samples showed that the thermal conductivity increased about 3–5% and the viscosity increased 3.7% above the base fluid viscosity even with the particle volume concentration as low as 0.01%.     

Morphology of Si nanowires fabricated by laser ablation using gold catalysts

Si nanowires (NWs) were fabricated successfully by laser ablation using Au as catalyst. Si wafers were used as the collector. The diameters of Si NWs ranged from 20 to 150 nm. Different forms of Si NWs were observed at different local sites inside a furnace: Si NWs with a high aspect ratio of length to diameter, Si NWs with defects and Si NWs with Au-containing nanoparticles being embedded. Especially, a nano-particle embedded Si NW is a new nanostructure that is observed for the first time. PACS 81.07.-b; 81.07.Bc; 81.16.-c; 81.20.-n     

Projection ablation of glass-based single and arrayed microstructures using excimer laser

Ablation of single and arrayed microstructures using an excimer laser is studied. The single feature microstructures are fabricated for evaluating the ablation mechanism, threshold fluence, and associated material removing (ablation) rate. The morphology changes during ablation are investigated with the focus on the formation of the ablation defects, debris or recast. The possibility of removing these defects is also evaluated and demonstrated. The present study concentrates on the borosilicate glass, although ablation of polyimide and silicon are performed and discussed for comparison. Polyimide and silicon are the most popular polymer or semiconductor material used in the electronics industry. The arrayed microstructures are ablated to demonstrate the fact that, by repetition of a simple-patterned mask associated with synchronized laser pulses and substrate movement, arrayed and more complex structures can be cost-effectively manufactured. The potential applications of these arrayed microstructures are discussed and illustrated. A low-cost replication technique that uses the arrayed microstructure presently machined as the forming mold for making electroforming nickel microneedles is specifically presented. Finally, the potential areas of using excimer laser in micromachining of glass-based structures for future research are also briefly covered.     

Polarization-dependent ablation of silicon using tightly focused femtosecond laser vortex pulses

We demonstrate experimentally that, in a tight focusing geometry, circularly polarized femtosecond laser vortex pulses ablate material differently depending on the handedness of light. This effect offers an additional degree of freedom to control the shape and size of laser-machined structures on a subwavelength scale.     

A proposed ablation laser

A new type of eximer laser is proposed; it involves the simultaneous sublimation and excitation of a frozen film by means of a relativistic electron beam. Calculated beam requirements for noble gases are ～ 10 kA/cm² of 400 kV electrons for periods of 2 to 12 ns. Predicted advantages of this laser are high gain and the elimination of wavelength limiting optical windows. Preliminary experimental fluorescence spectra have been obtained from xenon films. The emission from the solid phase consists primarily of two 100 Å wide bands centered at 1690 Å and 1730 Å the fluorescence lifetimes are 4 ± 2 ns for both bands.