Nanofoaming in the surface of biopolymers by femtosecond pulsed laser irradiation

In this work, the nanostructuring induced in femtosecond (fs) laser irradiation of biopolymers is examined in self-standing films of collagen and gelatine. Irradiation by single 90 fs pulses at 800, 400 and 266 nm is shown to result in the formation of a modified layer with submicrometric size structures. The size and uniformity of the observed features are strongly dependent on irradiation wavelength and on the characteristics of the biopolymer (water content and mechanical strength). Examination of the films by laser induced fluorescence serves to assess the chemical modifications induced by laser irradiation, revealing changes in the emission bands assigned to the aromatic amino acid tyrosine and its degradation products. The results are discussed in the framework of a mechanism involving the generation of large free-electron densities, through multiphoton and avalanche ionization, which determine the temperature and stress distribution in the irradiated volume.     

Synthesis of nanosized particles during laser ablation of gold in water

In this paper we report the formation of gold nanoparticles during laser ablation of gold target in water in the absence of any additives. The experiments were carried out by using the radiation of the pulsed Nd:YAG laser, operating at the second (532 nm, 10 ns, 10 Hz), or the fourth harmonic (266 nm) wavelengths. The properties of the nanoparticles were found to be susceptible to the additional 532 and 266 nm laser irradiation. It has been established that both the mean size of the nanoparticles and their stability could be varied by proper selection of the parameters of laser ablation and postirradiation such as laser fluence and wavelength combinations.     

Femtosecond pulsed laser deposition of nanostructured TiO2 films

Nanostructured deposits of TiO₂ were grown on Si (1 0 0) substrates by laser ablating a TiO₂ sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 80 fs pulses. The effect of the laser irradiation wavelength on the obtained nanostructures, was investigated using 800, 400 and 266 nm at different substrate temperatures and pressures of oxygen. The composition of the deposits was characterized using X-ray photoelectron spectroscopy (XPS) and the surface morphology was studied by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Deposits are absent of microscopic droplets in all conditions explored. The best deposits, constituted by nanoparticles of an average diameter of 30 nm with a narrow size distribution, were obtained at the shorter laser wavelength of 266 nm under vacuum at substrate room temperature.     

Emission properties of femtosecond (fs) laser fabricated microstructures in Polystyrene (PS)

We fabricated microstructures and micro craters in thin films and bulk of PS. To the best of our knowledge, this is the first report of the emission from fs laser modified regions of PS when excited at 337, 400, 458, 488 and 514 nm wavelengths with different emission peaks. We systematically studied the emission in context of formation of optical centers and analyzed the spectra of irradiated PS. Change in the excitation wavelength leads to a shift in the emission peak, whereby, we infer that the emission should be due to a myriad of optical centers. Interestingly these optical centers have a similar excitation spectrum. Diphenylbutadiene (DPBD) is probably the main optical center among other optical centers thus formed in the process of fs laser irradiation of PS.     

Two-color two-photon excitation of indole using a femtosecond laser regenerative amplifier

The fluorescence emission from indole resulting from two-color two-photon (2C2P) excitation with 400 and 800 nm wavelengths is observed, using the second harmonic and fundamental wavelength of a 800 nm 40 fs pulsed Ti:Sapphire femtosecond (fs) regenerative amplifier operating at a repetition rate of 1 kHz. By delaying one fs laser pulse relative to the other, the cross correlation of fluorescence is observed, which indicates the generation of 2C2P fluorescence signal in the experiment. The strongest 2C2P fluorescence emission characterized by the peak of cross correlation curve suggests optimal temporal overlap of the two fs laser pulses. The 2C2P fluorescence signal is linearly dependent on the total excitation intensity. The fluorescence signals with 400 nm and 800 nm irradiation alone are also demonstrated and discussed in this paper.     

Nanofoaming dynamics in biopolymers by femtosecond laser irradiation

We have recently shown that irradiation of self-standing films of the biopolymers collagen and gelatine with single femtosecond laser pulses produces a nanofoaming layer with regular bubble size which can be controlled by wavelength selection. Following these initial studies, here we report on the temporal evolution of the foaming effect by measurements in situ and in real time of the change in the transmittance of a cw probe HeNe laser through the irradiated films. Self standing films of the biopolymers were irradiated with 90 fs laser pulses at 800, 400, and 266 nm. For fluences below and above the modification threshold a permanent attenuation of the transmission occurs (increasing with fluence). The initial decay of the transmission is fast (around few tens of ns), and is followed by dynamics in the longer timescale (micro and milliseconds). The temporal evolution of the transmission measured upon fs laser irradiation is similar with that determined in the irradiation of the biopolymer films at 248 nm with 25 ns laser pulses. The method allows separating in time the different processes occurring after irradiation that lead to a permanent nanofoaming structure, while the results allow us to understand the mechanisms of femtosecond laser processing of the biopolymers and their interest in biomedical applications.     

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200 nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental (λ = 800 nm, 100 fs) and the second harmonic (λ = 400 nm, 250 fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200 nm to below 40 nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.     

Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses

The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for structuring transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF₂, MgF₂), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond (t_p = 10 ps) lasers at UV wavelengths (λ₁ = 355 nm and λ₂ = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with different slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a λ₁ = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths (λ₁ = 266 nm and λ₂ = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm², respectively.     

Patterning of indium-tin oxide on glass with picosecond lasers

The results of patterning of the indium-tin oxide (ITO) film on the glass substrate with high repetition rate picosecond lasers at various wavelengths are presented. Laser radiation initiated the ablation of the material, forming grooves in ITO. Profile of the grooves was analyzed with a phase contrast optical microscope, a stylus type profiler, scanning electron microscope (SEM) and atomic force microscope (AFM). Clean removal of the ITO film was achieved with the 266 nm radiation when laser fluence was above the threshold at 0.20 J/cm², while for the 355 nm radiation, the threshold was higher, above 0.46 J/cm². The glass substrate was damaged in the area where the fluence was higher than 1.55 J/cm². The 532 nm radiation allowed getting well defined grooves, but a lot of residues in the form of dust were generated on the surface. UV radiation with the 266 nm wavelength provided the widest working window for ITO ablation without damage of the substrate. Use of UV laser radiation with fluences close to the ablation threshold made it possible to minimize surface contamination and the recast ridge formation during the process.     

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr₅(PO₄)₃F laser. The broad gain bandwidth can completely support ∼6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm.     

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (M_w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M_w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M_w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.     

Femtosecond laser-induced fragmentation and cluster formation studies of solid phase trinitrotoluene using time-of-flight mass spectrometry

We use surface-femtosecond laser mass spectrometry to study the fragments/products formed when trinitrotoluene (TNT) is subjected to femtosecond laser pulse irradiation and to study the conditions under which TNT is removed from a solid surface. In surface-femtosecond laser mass spectrometry a compound is deposited on a solid substrate and is desorbed into vacuum by femtosecond irradiation forming a plume of ionized and neutral species. The positive or negative ions are then accelerated by an electric potential and allowed to drift in the field-free region of a time-of-flight mass spectrometer. The mass-to-charge ratio of each ion is obtained using the value of the accelerating field and the ion flight time. In this paper we report femtosecond laser mass spectra for the positive ions formed by desorbing TNT with 130 fs pulses centered at 800 nm for fluences ranging from 7 to 1.4 × 10⁵ J/m². The conditions under which TNT removal and ionization occur are also discussed.     

Mechanism of ablation of CdS at laser wavelengths in the visible and in the UV

The mechanisms of laser ablation of CdS targets at different laser wavelengths have been investigated. (CdS)_n⁺ cluster formation is only observed upon 532 nm ablation. The time and energy distributions of neutral S, S₂, Cd and CdS show significant dependence on laser wavelength. Bimodal distributions are observed at 266 and 308 nm. For the former, the average kinetic energy increases significantly with mass, taking values in the range of 0.3-1.7 eV. At 308 nm the slow component of the time distribution disappears at distances above the target larger than 1 cm. At this wavelength, the observed time distribution appears to reflect mainly the dynamics of the expansion. At 532 nm the time distribution is monomodal and the average kinetic energies are below 0.2 eV. Clear indications of the participation of thermal (at 532 nm) and non-thermal mechanisms (at 266 nm) have been found. It is tentatively concluded that the cluster formation observed upon ablation at 532 nm can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and their similar velocities favor the aggregation processes.     

Femtosecond and nanosecond laser damage thresholds of doped and undoped triazenepolymer thin films

The influence of pulse duration on the laser-induced damage in undoped or infrared-absorbing-dye doped thin triazenepolymer films on glass substrates has been investigated for single, near-infrared (800 nm) Ti:sapphire laser pulses with durations ranging from 130 fs up to 540 fs and complementarily for infrared (1064 nm) Nd:YAG ns-laser single-pulse irradiation. The triazenepolymer material has been developed for high resolution ablation with irradiation at 308 nm. Post-irradiation optical microscopy observations have been used to determine quantitatively the threshold fluence for permanent laser damage. In contrast to our previous studies on a triazenepolymer with different composition [J. Bonse, S.M. Wiggins, J. Solis, T. Lippert, Appl. Surf. Sci. 247 (2005) 440], a significant dependence of the damage threshold on the pulse duration is found in the sub-picosecond regime with values ranging from ∼500 mJ/cm² (130 fs) up to ∼1500 mJ/cm² (540 fs). Other parameters such as the film thickness (50 nm and 1.1 μm samples) or the doping level show no significant influence on the material behavior upon irradiation. The results for fs- and ns-laser pulse irradiation are compared and analyzed in terms of existent ablation models.     

The potential of UV femtosecond laser ablation for varnish removal in the restoration of painted works of art

Despite significant advances, laser ablation with nanosecond pulses presents limitations in dealing with the restoration of classes of painted works of art, such as paintings with a very thin layer of varnish. Femtosecond laser processing promises the means for overcoming such limitations. To this end, femtosecond ablation of two typical varnishes, dammar and mastic, is examined. For these varnishes, processing by Ti:Sapphire irradiation (800 nm) turns out to be ineffective. In contrast, irradiation with 248 nm ∼500 fs laser pulses results in a higher etching resolution (etching rates of ∼1 μm/pulse or less). For irradiation with few laser pulses at moderate laser fluences, etched morphology is far smoother than in the processing with nanosecond laser pulses. Furthermore, chemical modifications are considerably reduced (by nearly an order of magnitude), and exhibit a number of additional novel differences. Both etching rates and extent of chemical modifications are largely independent of varnish absorptivity. In all, femtosecond UV laser irradiation is indicated to hold a high potential, offering new perspectives for the restoration of painted works of art. Finally, a tentative model is advanced accounting in a consistent way for the observations.     

Single- and multi-pulse formation of surface structures under static femtosecond irradiation

Femtosecond surface structure modifications are investigated under irradiation with laser pulses of 150 fs at 800 nm, on copper and silicon. We report sub-wavelength periodic structures formation (ripples) with a periodicity of 500 nm for both materials. These ripples are perpendicular to the laser polarization and can be obtained with only one pulse. The formation of these ripples corresponds to a fluence threshold of 1 J/cm² for copper and 0.15 J/cm² for silicon. We find several morphologies when more pulses are applied: larger ripples parallel to the polarization are formed with a periodicity of 1 μm and degenerate into a worm-like morphology with a higher number of pulses. In addition, walls of deep holes also show sub-wavelength and large ripples.     

Ultraviolet laser microstructuring of silicon and the effect of laser pulse duration on the surface morphology

The study of the laser pulse duration effect on the silicon micro-spikes morphology is presented. The microcones were produced by ultraviolet (248 nm) laser irradiation of doped Si wafers in SF₆ environment. The laser pulse duration was adjusted at 450 fs, 5 ps and 15 ns. We have analyzed the statistical nature of the spikes’ morphological characteristics, such as periodicity and apex angle by exploiting image processing techniques, on SEM images of the irradiated samples. The correlation of the quantitative morphological characteristics with the laser parameters (pulse duration, laser fluence and number of pulses) provides new insight on the physical mechanisms, which are involved on the formation of Si microcones.     

Passively Q-switched Nd:YAG pumped UV lasers at 280 and 374 nm

Pulsed UV lasers at the wavelengths of 374 and 280 nm are realized by cascaded second harmonic generation (SHG) and sum frequency generation (SFG) processes using a Nd:YAG laser at 1123 nm. The Nd:YAG laser is longitudinally pumped and passively Q-switched, and it has a high peak power of 3.2 kW. The UV peak powers at 280 and 374 nm are 100 and 310 W, with pulse lengths of 6 and 8 ns, respectively. Spectral broadening of 374 nm laser by stimulated Raman scattering is studied in single mode pure silica core UV fiber. Realizations of UV lasers enabling compact design at 280 and 374 nm wavelengths are demonstrated.     

Structure formation on the surface of alloys irradiated by femtosecond laser pulses

Laser-induced periodic surface structures with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (120 fs, 800 nm, 1 Hz to 1 kHz pulse repetition frequency) irradiation on alloys. With the increasing number of pulses, nanoripples, classical ripples and modulation ripples with a period close to half of classical ripples have all been induced. The generation of second-harmonic has been supposed to be the main mechanism in the formation of modulation ripples.     

Femtosecond laser micromilling of Si wafers

Femtosecond laser micromilling of silicon is investigated using a regeneratively amplified 775 nm Ti:Sapphire laser with a pulse duration of 150 fs operating at 1 kHz repetition rate. The morphological observation and topological analysis of craters fabricated by single-shot laser irradiation indicated that the material removal is thermal in nature and there are two distinct ablation regimes of low fluence and higher fluence with logarithmical relations between the ablation depth and the laser fluence. Crater patterns were categorized into four characteristic groups and their formation mechanisms were investigated. Femtosecond laser micromilling of pockets in silicon was performed. The effect of process parameters such as pulse energy, translation speed, and the number of passes on the material removal rate and the formation of cone-shaped microstructures were investigated. The results indicate that the microstructuring mechanism has a strong dependence on the polarization, the number of passes and laser fluence. The optimal laser fluence range for Si micromilling was found to be 2-8 J/cm² and the milling efficiency attains its maximum between 10 and 20 J/cm².