Influence of surface effects on the performance of lead-niobium-germanate optical waveguides

Lead-niobium-germanate planar waveguides have been produced by pulsed laser deposition. The composition of the waveguides is found to be relatively weakly dependent on the laser fluence, while their surface morphology is affected dramatically. Smooth surfaces are obtained for a narrow fluence range centered at 2.0 J/cm², while particulates having typical diameters of <0.5 μm or droplets with typical diameters of <10 μm are observed at lower and higher fluences, respectively. The refractive index of the waveguides increases with fluence up to 2.1 at 2.0 J/cm², which is close to the value of the bulk glass, and remains constant at higher fluences. Propagation losses show instead a minimum (≈6.5 dB/cm) at 2.0 J/cm². The characteristics of the ablation process that leads to the ejection of solid particulates or molten droplets as well as the increase of the waveguides density on increasing the fluence are discussed to be responsible for the observed optical behavior.     

Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses

Interaction of a nanosecond transversely excited atmospheric (TEA) CO₂ laser, operating at 10.6 μm, with tungsten-titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24-49 J/cm². The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses.     

Ablation process of silica glass induced by laser plasma soft X-ray irradiation

Silica glass can be machined by irradiation with laser plasma soft X-rays on nano- and micrometer scale. We have investigated the ablation process of silica glass induced by laser plasma soft X-ray irradiation. We observed ionic and neutral species emitted from silica surfaces after irradiation. Dominant ions and neutrals are O⁺ and Si⁺ ions and Si, O, SiO and Si₂ neutrals, respectively. The ions have kinetic energies of 13 and 25 eV, which are much higher than those of particles emitted by evaporation. The energy of laser plasma soft X-rays absorbed to silica glass at a fluence of 1.4 J/cm² is estimated to be 380 kJ/cm³, which is higher than the binding energy of SiO₂ of 76 kJ/cm³. These results suggest that the most of the bonds in silica glass are broken by absorption of laser plasma soft X-rays, that several percent of the atoms are ionized, and that neutral atoms are emitted together with repulsive ions. The process possibly enables us to fabricate nano structures.     

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.     

Ultra fast melting process in femtosecond laser crystallization of thin a-Si layer

In this paper, we investigated the mechanism of crystallization induced by femtosecond laser irradiation for an amorphous Si (a-Si) thin layer on a crystalline Si (c-Si) substrate. The fundamental, SHG, THG wavelength of a Ti:Sapphire laser was used for the crystallization process. To investigate the processed areas we performed Laser Scanning Microscopy (LSM), Transmission Electron Microscopy (TEM) and Imaging Pump-Probe measurements. Except for 267 nm femtosecond laser irradiation, the crystallization occurred well. The threshold fluences for the crystallization using 800 nm and 400 nm femtosecond laser irradiations were 100 mJ/cm² and 30 mJ/cm², respectively. TEM observation revealed that the crystallization occurred by epitaxial growth from the boundary surface between the a-Si layer and c-Si substrate. The melting depths estimated by Imaging Pump-Probe measurements became shallower when the shorter wavelength was used.     

Pulsed Nd:YAG laser depositions of ITO and DLC films for OLED applications

Indium-tin oxide (ITO) films deposited on heated and non-heated glass substrates by a pulsed Nd:YAG laser at 355 nm and ∼2.5 J/cm² were used in the fabrication of simple organic light-emitting diodes (OLEDs), ITO/(PVK + Alq₃ + TPD)/Al. The ITO was deposited on heated glass substrates which possessed resistivity as low as ∼3 × 10⁻⁴ Ω cm, optical transmission as high as ∼92% and carrier concentration of about ∼5 × 10²⁰ cm⁻³, were comparable to the commercial ITO. Substrate heating transformed the ITO microstructure from amorphous to polycrystalline, as revealed by the XRD spectrum. While the polycrystalline ITO produced higher OLED brightness, it was still lower than that on the commercial ITO due to surface roughness. A DLC layer of ∼1.5 nm deposited on this ITO at laser fluence of >12.5 J/cm² improved its device brightness by suppressing the surface roughness effect.     

Application of the laser ablation for conservation of historical paper documents

Laser ablation was applied for surface cleaning and spectroscopic diagnostics of historical paper documents and model samples in the framework of the conservation projects. During cleaning the spectra of ablation products were recorded by means of the LIBS technique which allowed for nearly non-destructive identification of surface layers such as contaminants, substrate and pigments. For consecutive laser pulses a strong decrease of band intensities of the emission lines of Ca, Na, K, Al and Fe ascribed to contaminants were observed. The effect was used for monitoring of the cleaning progress of stained paper. For surface cleaning and spectra excitation the Q-switched Nd:YAG laser of 6 ns pulsewidth operating at wavelengths of 266, 355, 532, and 1064 nm and of fluence selected from the range 0.3-0.9 J/cm² was applied. The ablation parameters were optimized in agreement with the literature and the results were confirmed by surface studies and testing of the mechanical and chemical properties, and also by the response to the ageing process of the paper substrate. In case of the model paper irradiated in the UV range at 266 and 355 nm a visual inspection revealed local damages of the cellulose fibers accompanied by a decrease of the mechanical strength of the substrate. The effect was more pronounced after artificial ageing. The best results were obtained for samples irradiated at 532 nm and at laser fluence below the damage threshold of 0.6 J/cm², which is in agreement with literature.     

Fiber laser annealing of nanocrystalline PZT thick film prepared by aerosol deposition

Nanocrystalline PZT thick films (1 mm square and over 10 μm thick) directly deposited onto stainless-steel substrates (PZT/SUS) by aerosol deposition (AD) technique and then annealed using focused laser beam with a fiber laser to suppress thermal damage to the back sides of the PZT/SUS and substrate near the film edge and to retain the dielectric and/or ferroelectric properties of the PZT/SUS. Compared with CO₂ laser annealing, fiber laser annealing suppressed thermal damage to the substrate. Compared with PZT/SUS annealed at 600 °C using an electric furnace, PZT/SUS annealed at 600 °C using a fiber laser showed superior properties, namely, dielectric constant ? > 1200 at a frequency of 100 Hz, remanent polarization P_r > 30 μC/cm², and coercive field strength E_c < 50 kV/cm at a frequency of 10 Hz. Furthermore, the grain growth for the PZT/SUS formed by AD technique and annealed by fiber laser irradiation was occurred within the laser spot size.     

Laser-assisted decontamination—A wavelength dependent study

We present here the experimental results on cleaning of radioactive dielectric particulates, loosely deposited on stainless steel, by coherent light of 1064 nm wavelength and its three harmonics occurring at 532 nm, 355 nm and 266 nm, derived from an Nd-YAG laser. For the initial few exposures, the decontamination factor has been found to be highest when exposed to 1064 nm radiation. With increasing number of exposures, however, the radiation with reducing wavelength assumes a more important role as a cleaning agent. The observation of almost no cleaning with 1064 nm and much reduced cleaning with its harmonics when the contamination is deposited on a transparent substrate confirms the dominant role played by metal substrate towards expelling the loose particulates from its surface.     

Homogeneous focusing with a transient soft X-ray laser for irradiation experiments

We report the work done on a transient soft X-ray laser (SXRL) beam to deliver a proper extreme UV irradiation source for applications. The same optical tool was first demonstrated on a quasi stationnary state (QSS) soft X-Ray laser at the PALS Institute in Prague. The problem set by the transient soft X-Ray laser developed by the LIXAM at the LULI installation in Palaiseau is more crucial, first because the beam spatial profile is more irregular secondly because high repetition rate soft X-ray laser facilities in the future are based on this SXRL type. The spots obtained show a 20 micron average diameter and a rather homogeneous and smooth profile that make them a realistic irradiation source to interact with targets requiring relatively high fluence (near 1 J/cm²) or intensity (near 10¹¹ W/cm²) in the extreme UV domain.     

Laser backwriting process on glass via ablation of metal targets

Ablation of metal targets onto pyrex glass substrates, using a Q-switched Nd:YAG laser working at 355 nm, was used to study the potential of a laser backwriting process for the fabrication of optical waveguides via an index of refraction change. Metal foils of stainless steel, aluminum, copper, brass and gold have been used as blanks and irradiated by focusing the laser beam through a cylindrical lens under continuous movement in a direction perpendicular to the irradiation. An horizontal setup was found suitable to improve the effect of the plume in the sample. Results were obtained for two different configurations. Transversal profiles were analysed using a contact profilometer, comparing results obtained for the different configurations, traverse speeds and metal targets used. Two ablation regimes were identified, which are related to a critical laser fluence value of 2.7 J/cm². Surface micrographs obtained by scanning electron microscopy are discussed, together with the characteristics of the structures attained, taking into account the optical and thermal properties of the ablated metal blanks.     

Nanostructures on SiC surface created by laser microablation

Silicon carbide (SiC), as it is well-known, is inaccessible to usual methods of technological processing. Consequently, it is important to search for alternative technologies of processing SiC, including laser processing, and to study the accompanying physical processes. The work deals with the investigation of pulsed laser radiation influence on the surface of 6H-SiC crystal. The calculated temperature profile of SiC under laser irradiation is shown. Structural changes in surface and near-surface layers of SiC were studied by atomic force microscopy images, photoluminescence, Raman spectra and field emission current-voltage characteristics of initial and irradiated surfaces. It is shown that the cone-shaped nanostructures with typical dimension of 100-200 nm height and 5-10 nm width at the edge are formed on SiC surface under nitrogen laser exposure (λ = 0.337 μm, t_p = 7 ns, E_p = 1.5 mJ). The average values of threshold energy density 〈W_thn〉 at which formation of nanostructures starts on the 0 0 0 1 and surfaces of n-type 6H-SiC(N), nitrogen concentration n_N ≅ 2 × 10¹⁸ cm⁻³, are determined to be 3.5 J/cm² and 3.0 J/cm², respectively. The field emission appeared only after laser irradiation of the surface at threshold voltage of 1000 V at currents from 0.7 μA to 0.7 mA. The main role of the thermogradient effect in the processes of mass transfer in prior to ablation stages of nanostructure formation under UV laser irradiation (LI) was determined. We ascertained that the residual tensile stresses appear on SiC surface as a result of laser microablation. The nanostructures obtained could be applied in the field of sensor and emitting extreme electronic devices.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

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.     

Surface modifications of a titanium implant by a picosecond Nd:YAG laser operating at 1064 and 532 nm

Interaction of an Nd:YAG laser, operating at 1064 or 532 nm wavelength and pulse duration of 40 ps, with titanium implant was studied. Surface damage thresholds were estimated to 0.9 and 0.6 J/cm² at wavelengths 1064 and 532 nm, respectively. The titanium implant surface modification was studied by the laser beam of energy density of 4.0 and 23.8 J/cm² (at 1064 nm) and 13.6 J/cm² (at 532 nm). The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following titanium/implant surface morphological changes were observed: (i) both laser wavelengths cause damage of the titanium in the central zone of the irradiated area, (ii) appearance of a hydrodynamic feature in the form of resolidified droplets of the material in the surrounding outer zone with the 1064 nm laser wavelength and (iii) appearance of wave-like microstructures with the 532 nm wavelength. Generally, both laser wavelengths and the corresponding laser energy densities can efficiently enhance the titanium/implant roughness. This implant roughness is expected to improve its bio-integration. The process of the laser interaction with titanium implant was accompanied by formation of plasma.     

Influence of substoichiometer on the laser-induced damage characters of HfO2 thin films

HfO₂ is one of the most important high refractive index materials for depositing high power optical mirrors. In this research, HfO₂ thin films were prepared by dual-ion beam reactive sputtering method, and the laser-induced damage thresholds (LIDT) of the sample were measured in 1-on-1 mode for laser with 1064 nm wavelength. The results indicate that the LIDT of the as-grown sample is only 3.96 J/cm², but it is increased to 8.98 J/cm² after annealing under temperature of 200 °C in atmosphere. By measuring the laser weak absorption and SIMS of the samples, we deduced that substoichiometer is the main reason for the low LIDT of the as-grown sample, and the experiment results were well explained with the theory of electronic-avalanche ionization.     

Laser processing of micro-optical components in quartz

Laser induced backside wet etching combined with the diffractive gray tone phase mask has been used for the fabrication of a micro-lens array with a single lens diameter of 1 mm and a micro-prism in quartz. The micro-lens array was tested as beam homogenizer for high power XeCl excimer laser yielding a clear improvement in the quality of the laser beam.The optimum fluence range for fabrication of micro-lenses by laser induced backside wet etching using 1.4 M pyrene in THF solution and 308 nm irradiation wavelength is 1-1.6 J/cm². The etching mechanisms of LIBWE are based on a combination of pressure and temperature jumps at quartz-liquid interface.     

Production of defects in ZBLAN, ZBLAN:Tm and ZBLAN:Cr glasses by ultra-short pulses laser interaction

In this work we have studied pure and thulium- and chromium-doped ZBLAN glasses irradiated by ultra-short laser pulses. A Ti:sapphire CPA system was used, producing a 500 Hz train of pulses, centered at 830 nm, with 375 μJ of energy and 50 fs of duration (FWHM). The beam was focused by a 20 mm lens, producing a converging beam with a waist of 12 μm. The absorption spectra before and after laser irradiation were obtained showing production of color centers in pure, thulium-doped and chromium-doped ZBLAN glasses. A damage threshold of 9.56 T W/cm² was determined for ZBLAN.     

Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation

In this paper the surface topography of titanium samples irradiated by femtosecond laser pulses is described. When the fluence is about 0.5 J/cm² periodic ripples with a period of about 700 nm are formed. For fluences between 0.5 and 2 J/cm², a microcolumnar surface texture develops in the center of the irradiated spots and ripples are formed in the periphery of the spots. When experiments are performed with a non-stationary sample, the microcolumns exhibit ripples similar to those observed when the radiation fluence is about 0.5 J/cm² and in the outer regions of the irradiated areas for fluences between 0.5 and 2 J/cm². Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam.     

Modelling laser cleaning of low-absorbing substrates: the effect of near-field focussing

A three-dimensional model for laser cleaning of spherical, transparent particles on low-absorbing substrates has been developed. It takes into account near-field focussing of the laser radiation by the particles. The intensity distribution under a particle was found using Mie theory together with the geometrical optics approximation. This permits the estimation of the beam width at the substrate surface and the focal distance of the radiation coming from the spherical particle. These parameters are used to find the distribution of intensity within the low-absorbing substrate from the formula for a focussed Gaussian beam. This is in contrast with most other models of laser cleaning, which assume that all absorption occurs at the surface of the substrate. The energy criterion was used to calculate the threshold fluence. The model predicts threshold fluences of the order of 10³ J/cm² for silica spheres having a diameter of the order of a micron on silica substrates, assuming adhesion by van der Waals force. As this is well above the damage threshold for silica, it effectively predicts that laser cleaning of silica spheres from silica will be impossible. For glass slides the threshold fluence is predicted to be a factor of 10^-4 times smaller than that for silica slides (about 0.1 J/cm²). This is due to the much higher absorption of glass compared to that of silica at 248 nm. PACS 42.62.Cf; 81.65.Cf