Femtosecond Ablation Research Articles

ChemInform Abstract: Laser‐Induced Crystallization and Crystal Growth.

AbstractReview: 59 refs.

Fabrication and characterization of aluminum nanostructures and nanoparticles obtained using femtosecond ablation technique

In this communication we report the fabrication of nanoparticles (NPs) and nanostructures through the interaction of ultrashort laser pulses (∼40fs) with bulk aluminum immersed in chloroform and carbon tetrachloride. The size distribution and surface morphology of substrates were investigated using scanning electron microscope images. Depending on the focal position we observed sub-micron structures and nano-ripples on the irradiated surface. Picosecond nonlinear optical studies of the prepared NPs colloidal solutions using open aperture Z-scan technique demonstrated complex behavior of switching from saturable absorption to reverse saturable absorption (RSA) at lower peak intensities to pure RSA at higher peak intensities.

SURFACE PLASMON PROPERTIES OF HOLLOW AUAG ALLOYED TRIANGULAR NANOBOXES AND ITS APPLICATIONS IN SERS IMAGING AND POTENTIAL DRUG DELIVERY

We successfully synthesized hollow AuAg alloyed triangular nanoboxes (TNBs) with localized surface plasmon resonances (LSPR) spectra position from visible to NIR region. We then study the surface plasmon properties of AuAg alloyed TNBs and explore their application in surface enhanced Raman scattering (SERS) imaging. We also investigated the laser induced near-field ablation of TNBs, which have the potentials of drug delivery for cancer treatment. Finite Difference Time Domain (FDTD) method is used to calculate electromagnetic fields induced by optical excitation of LSPR of AuAg alloyed TNBs for the first time. The calculated results are proved through in-vivo SERS imaging by three types of SERS tags based on TNBs. Furthermore, the unique hollow structure of TNBs may facilitate direct encapsulation of anticancer drugs, without any surface coatings. The femtosecond laser near-field ablation experiment is studied as one possible method to release the drug encapsulated inside the hollow structure. These studies show that the nanostructures are easy to break down and promising as a nanodevice model for controlled drug delivery. Received 19 April 2012, Accepted 15 May 2012, Scheduled 15 May 2012 * Corresponding author: Sailing He (sailing@kth.se).

Study of femtosecond ablation on aluminum film with 3D two-temperature model and experimental verifications

In this paper, a 3D two-temperature model is introduced to investigate femtosecond ablation on aluminum film. 3D temperature evolutions for both electrons and lattice are obtained, which present us a vivid view of the energy transformation process during femtosecond ablation. Simulated 3D ablation craters irradiated by a single pulse with different energy are acquired, from which we can easily and precisely predict crater depth and radius before ablation takes place. In the experiment we measure the radii of the craters ablated by pulses with different energy and numbers delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluence for both single and multi pulses are obtained. Comparisons are made between results of the experiment and relative simulated calculations show the reliability of our proposed calculation model.

Effect of film properties on the material removing characteristics in femtosecond laser rear-side ablation of chromium film

The microfabrication of films with femtosecond lasers has been researched widely for its high spatial resolution and sub-spot-size features. Compared with the common front-side ablation, femtosecond laser rear-side ablation mechanism of films is more complex due to the possible film breaking process. In this paper, the effect of film properties such as adhesion strength and cohesion strength on the material removing characteristics in femtosecond laser rear-side ablation of Cr film is investigated. The possible film breaking process in the rear-side ablation is analyzed firstly, and then some experiments with different films, the vapor deposited Cr film, the sputtered Cr film and the Cr film of photomask are performed. The experimental results indicate that the film properties are key factors influencing material removal characteristics for laser rear-side ablation. By varying film properties and laser fluence, femtosecond laser rear-side ablation technique can be applied in laser cleaning process and fabrication of nanostructures. The unique feature of rear-side ablation will widen the application of femtosecond laser micromachining technique.

Fabrication of binary Fresnel lenses in PMMA by femtosecond laser surface ablation

We report on the fabrication of binary Fresnel lenses by femtosecond laser surface ablation of poly(methyl methacrylate) (PMMA) substrates. Tight focusing of the laser pulses produced a minimum ablated feature size of 600 nm, enabling the creation of lenses with numerical apertures as high as 0.5 and focal lengths ranging from 500 µm to 5 mm. A precise control of the ablation depth allowed the achievement of a 30% focusing efficiency, close to the maximum theoretical value for this kind of lenses.

Nanosecond and femtosecond ablation of La0.6Ca0.4CoO3: a comparison between plume dynamics and composition of the films

Thin films of La0.6Ca0.4CoO3 were grown by pulsed laser ablation with nanosecond and femtosecond pulses. The films deposited with femtosecond pulses (248 nm, 500 fs pulse duration) exhibit a higher surface roughness and deficiency in the cobalt content compared to the films deposited with nanosecond pulses (248 nm, 20 ns pulse duration). The origin of these pronounced differences between the films grown by ns and fs ablation has been studied in detail by time-resolved optical emission spectroscopy and imaging. The plumes generated by nanosecond and femtosecond ablation were analyzed in vacuum and in a background pressure of 60 Pa of oxygen. The ns-induced plume in vacuum exhibits a spherical shape, while for femtosecond ablation the plume is more elongated along the expansion direction, but with similar velocities for ns and fs laser ablation. In the case of ablation in the background gas similar velocities of the plume species are observed for fs and ns laser ablation. The different film compositions are therefore not related to different kinetic energies and different distributions of various species in the plasma plume which has been identified as the origin of the deficiency of species for other materials.

Experimental Study of the Effect of Pulsewidth on Ablation of Silicon Substrate Using Mega Hertz Repetition Rate Femtosecond Laser

A systematic experimental study of influence of pulsewidth on silicon wafer ablation was investigated. Pulsewidth ranging from 214 fs to 3.5 ps was investigated. Crucial parameters such as the ablation threshold, kerf width, and ablation depth are deduced as functions of pulsewidth. The ablation depth is found to decrease and the ablation threshold increases slightly with the increase in the pulsewidth. The spherical recasts were found to depend upon the repetition rate and the scan speed. This study contributes to better control of the machining quality of the femtosecond ablation at repetition rate in MHz.

Fabrication of large-area hole arrays using high-efficiency two-grating interference system and femtosecond laser ablation

We present a novel method to fabricate hole arrays by forming a four-beam interference pattern with two gratings. In this method a femtosecond laser beam is split into four and collected to interfere using two cascaded diffractive gratings. One benefit of this grating pair is that it is achromatic, because of the geometry of the grating pair, and therefore it is suitable for femtosecond ablation. Grating pairs were designed and fabricated for a standard Ti:sapphire femtosecond laser, with 800-nm central wavelength, so that the interference pattern generates holes with less than 1-μm diameter. Holes with this size diffract with a colorful visual appearance in the visible wavelength range and therefore these structures are suitable for security, authentication and decorative marking. We show that this method is suitable for fast ablation of hole arrays in both silicon and steel.

Ablation of aluminum oxide ceramics using femtosecond laser with multiple pulses

The ablation of the aluminum oxide ceramics is theoretically investigated using the femtosecond laser with multiple pulses in this paper. There is now a growing interest in laser micromachining using pulses of femtosecond duration. Among its advantages is the possibility machining hard and brittle materials that can be easily damaged by conventional drilling and cutting procedures. Femtosecond laser micromachining of ceramics can produce nanoscale patterns due to the laser pulse duration shorter than the relaxation time of heat transport among phonons. Laser ceramics ablation possesses another features, which include the phase transition from solid to vapor occurs without melt, the strong dependence of the thermophysical parameters and the volumetric absorption. The thermal process of femtosecond laser ceramics ablation is modeled by the heat conduction equation considering material evaporation and plasma plume absorption. The ablation depth per pulse and squared crater diameter predicted by this work are compared with the available experimental data. The predictions of temperature field and ablation shape in the aluminum oxide ceramics are also shown in this study.

Time-resolved optical diagnosis of intense femtosecond laser ablation of silica glass

The dynamic process of femtosecond laser pulse ablation of silica glass has been experimentally investigated by ultrafast time-resolved optical diagnosis. The digital holograms of material ejections outside the silica glass are recorded, and two-dimensional phase difference maps at different delay times are obtained, from which the composition and the structure of the ejected materials are deduced. Besides, the time-resolved shadowgraphs of the phenomena inside the silica glass are also recorded, and the evolution process of two stress waves inside the silica glass is observed in the shadowgraphs, which are believed to be related to two material ejections outside the target.

Water-Assisted Femtosecond Laser Pulse Ablation of High Aspect Ratio Holes

Two novel techniques to enhance hole drilling in a silicon using a femtosecond laser, is presented. Firstly, the sprayed thin water layer has been used in an ablation region. Sprayed water removes the ablation debris and enables more efficient pulse energy transition into ablated material than in the air ambient. Secondly, the diffractive optical element (DOE) that generates into its far field 5x5 hole matrix at single irradiation, is used. By using the DOE instead of single hole drilling it is possible to ablate several holes simultaneously. Both of these techniques enhance the femtosecond ablation rate significantly.

Modeling of femtosecond ablation of aluminum film with single laser pulses

Detailed investigation of pulsed laser ablation dynamics is performed for aluminum target under action of 100 fs pulses with peak intensity 3.95 × 10 12 W/cm 2 and wavelength 0.8 μm.Non-equilibrium two-temperature model with hydrodynamic Stephan problem was used for modeling. Explicit tracking of moving interphase boundaries permits exact determination of their velocity and amount of removed and evaporated material. Detailed ablation process is analyzed using the study of temperature, pressure and density evolution in the target. High phase front velocities (melting up to 5 km/s and evaporation up to 350 m/s) are caused by strong overheating of solid and liquid phases.

Femtosecond laser machining of electrospun membranes

We demonstrate that a femtosecond laser can be used to machine arbitrary patterns and pattern arrays into free-standing electrospun polycaprolactone (PCL) membranes. We also examine the influence of various laser irradiation settings on the final microstructure of electrospun membranes. A beam fluence of 0.6J/cm2 is used to ablate holes in 100μm thick PCL membranes. The machined holes have an average diameter of 436μm and a center-to-center spacing of 1000μm. Based on these results, the femtosecond ablation of electrospun membranes shows great potential for fabricating a variety of functional tissue scaffolds. This technique will advance scaffold design by providing the ability to rapidly tailor surface morphology, while minimizing and controlling the deformation of the electrospun fibers.

Effects of femtosecond laser-assisted multilayer intrastromal ablation in the midperipheral cornea

We describe the femtosecond laser-assisted midperipheral multilayer intrastromal ablation technique, which combines the benefit of radial keratotomy, ie, not disturbing the optical zone, with currently available technology to correct myopia. The technique was performed in 2 patients, both of whom experienced a decrease in mean corneal power. In 1 patient, the manifest refraction improved, with a 1-line increase in uncorrected visual acuity 1 day postoperatively. The multilayer intrastromal ablation technique, which flattens the cornea and decreases corneal power, can be used to correct moderate myopia without injuring the corneal epithelium.

Near-field local enhancement by ordered arrays of sub-wavelength scattering centers fabricated by femtosecond ablation

We show how the ultrafast laser inscription of nanodimple arrays enables near-field local enhancement in high refractive-index crystals. We first demonstrate both experimentally and numerically how, when two nanodimples are separated by intermediate distances (700–1000 nm), near-field interference can take place resulting in a near-field enhancement between nanodimples. Based on this effect, near-field enhancements close to 60% are then demonstrated at the center of nanodimple arrays, and strong optical contrasts in excess of 100% are produced. The integration of these arrays into crystal substrates provides a versatile tool for new optical trapping and evanescent-field nanosensing devices.

Alignment of liquid crystal molecules in a micro-cell fabricated by femtosecond laser

We show that uniform nanoripples can be produced on the sidewalls of the micro-cell vertically embedded in fused silica glass by femtosecond laser direct ablation. The nanoripples can be used for homogeneous liquid crystal (LC) alignment with different azimuthal angles. The azimuthal anchoring energy at the interface between the LC and nanoripples is obtained by measuring the twist angle of LC director in the fabricated twisted nematic LC micro-cell. This method allows for fabrication of LC micro-cells integrated in glass chip with three-dimensional (3D) configurations, opening a new venue to fabrication of 3D miniaturized LC devices.

Controlling the Hydrophobic Properties of Material Surface Using Femtosecond Ablation

The wetting properties of the material can be altered by changing the surface topography. We have demonstrated the controlling of the wetting properties of the metal surface by texturing it using femtosecond laser ablation. Contact angles well above 150 degrees can be obtained to the steel sample using self-organized structures generated by the laser-matter interaction. The combination of micro- and nano-sized structures, obtained using specific laser parameters, changes the stainless steel surface to water repellant.

Damage mechanism and morphology characteristics of chromium film in femtosecond laser rear-side ablation

In this paper, damage mechanism and morphology characteristics of chromium film in femtosecond laser rear-side ablation are investigated. The film removing process includes two key sub-processes: the laser ablation dynamic process and subsequent breaking and ejecting dynamic process. Film morphology in rear-side ablation is determined by the interrelation between the laser energy and the film strength. When lower laser energy is used, residual out-layer film is relative thick and tends to break into some large fragments, which results in an irregular ablation shape. While when higher pulse energy is used, thinner residual film with weaker break strength breaks into small fragments, the ablation quality improves correspondingly. Besides laser energy and film property, energy distribution of laser beam also affects the ablation quality. In experiments, this kind of effect is researched by changing the focal position. It is found that ripples, which are familiar nano-structures in front-side ablation, also exist in rear-side ablation. These ripples are formed initially at the interface between quartz substrate and film, and their coverage varies with the energy distribution. Additionally, increasing number of scans is an effective method to shorten the period of ripples.

Surface acoustic wave transducers fabricated by femtosecond laser ablation

The fabrication of surface acoustic wave (SAW) devices by femtosecond laser micromachining has been demonstrated. The interdigital SAW transducers of 50 μm wavelength were formed on YZ lithium niobate substrate by femtosecond pulse ablation of an aluminium layer. The two-port SAW device has insertion loss of 11 dB at 68 MHz. The S-parameter measurements revealed no degradation in substrate elastic, piezoelectric, and dielectric properties of the substrate due to laser processing.