Pulsed laser photodeposition of a-Se nanofilms by ArF laser

Pulsed laser photodeposition from amorphous selenium aqueous colloid solutions using ArF laser radiation at a wavelength of λ = 193 nm has been investigated. Nanometer thick layers were obtained on UV transparent silica substrates in contact with the solution for various photodeposition parameters. Amorphous Se layers, 20 nm thick, were obtained typically by 40 laser pulses of 30 ns duration with a fluence of 50 mJ/cm². Deposition thresholds for depositing 1 nm thick layers were as low as 5 pulses. The deposited nanometer thin surface morphology was analyzed by Evanescent Field Optical Microscopy, Scanning Electron Microscopy and Atomic Force Microscopy. The nanometer thicknesses were evaluated by utilizing the differential evanescent light pattern emanating from the substrates.     

Flexible gratings fabricated in polymeric plate using femtosecond laser irradiation

Flexible gratings embedded in poly-dimethlysiloxane (PDMS) were fabricated using femtosecond laser pulses. Photo-induced gratings in a flexible PDMS plate were directly written by a high-intensity femtosecond (130 fs) Ti: Sapphire laser (λ_p=800 nm). Refractive index modifications with 4 μm diameters were photo-induced after irradiation of the femtosecond pulses with peak intensities of more than 1×10¹¹ W/cm². The graded refractive index profile was fabricated to be symmetric around the center of the focal point. The diffraction efficiency of the grating samples is measured by an He-Ne laser. The maximum value of refractive index change (Δn) in the laser-modified regions was estimated to be approximately 3.17×10⁻³.     

Femtosecond pulsed laser ablation of diamond-like carbon films on silicon

Femtosecond pulsed laser ablation (τ = 120 fs, λ = 800 nm, repetition rate = 1 kHz) of thin diamond-like carbon (DLC) films on silicon was conducted in air using a direct focusing technique for estimating ablation threshold and investigating the influence of ablation parameter on the morphological features of ablated regions. The single-pulse ablation threshold estimated by two different methods were ?_th(1) = 2.43 and 2.51 J/cm². The morphological changes were evaluated by means of scanning electron microscopy. A comparison with picosecond pulsed laser ablation shows lower threshold and reduced collateral thermal damage.     

Pulsed laser deposition of polyhydroxybutyrate biodegradable polymer thin films using ArF excimer laser

We demonstrated the pulsed laser deposition (PLD) of high quality films of a biodegradable polymer, the polyhydroxybutyrate (PHB). Thin films of PHB were deposited on KBr substrates and fused silica plates using an ArF (λ = 193 nm, FWHM = 30 ns) excimer laser with fluences between 0.05 and 1.5 J cm⁻². FTIR spectroscopic measurements proved that at the appropriate fluence (0.05, 0.09 and 0.12 J cm⁻²), the films exhibited similar functional groups with no significant laser-produced modifications present. Optical microscopic images showed that the layers were contiguous with embedded micrometer-sized grains. Ellipsometric results determined the wavelength dependence (λ ∼ 245-1000 nm) of the refractive index and absorption coefficient which were new information about the material and were not published in the scientific literature. We believe that our deposited PHB thin films would have more possible applications. For example to our supposal the thin layers would be applicable in laser induced forward transfer (LIFT) of biological materials using them as absorbing thin films.     

Subwavelength ripple formation induced by tightly focused femtosecond laser radiation

Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, t_p = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO₃. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented.     

Pulsed laser deposition of fluoride glass thin films

The development of integrated waveguide lasers for different applications such as marking, illumination or medical technology has become highly desirable. Diode pumped planar waveguide lasers emitting in the green visible spectral range, e.g. thin films from praseodymium doped fluorozirconate glass matrix (called ZBLAN, owing to the main components ZrF₄, BaF₂, LaF₃, AlF₃ and NaF) as the active material pumped by a blue laser diode, have aroused great interest. In this work we have investigated the deposition of Pr:ZBLAN thin films using pulsed laser radiation of λ = 193 and λ = 248 nm. The deposition has been carried out on MgF₂ single crystal substrates in a vacuum chamber by varying both processing gas pressure and energy fluence. The existence of an absorption line at 210 nm in Pr:ZBLAN leads to absorption and radiative relaxation of the absorbed laser energy of λ = 193 nm preventing the evaporation of target material. The deposited thin films consist of solidified and molten droplets and irregular particulates only. Furthermore, X-ray radiation has been applied to fluoride glass targets to enhance the absorption in the UV spectral region and to investigate the deposition of X-ray treated targets applying laser radiation of λ = 248 nm. It has been shown that induced F-centres near the target surface are not thermally stable and can be easily ablated. Therefore, λ = 248 nm is not suitable for evaporation of Pr:ZBLAN.     

Study of porous carbon thin films produced by pulsed laser deposition

Amorphous carbon is an interesting material and its properties can be varied by tuning its diamond-like (sp³) fractions. The diamond-like fractions in an amorphous carbon films depends on the kinetic energy of the deposited carbon ions. Porous amorphous carbon thin films were deposited onto silicon substrates at room temperature in a vacuum chamber by Glancing Angle Pulsed Laser Deposition (GAPLD). Krypton fluoride (248 nm) laser pulses with duration of 15 ns and intensities of 1-20 GW/cm² were used. In GAPLD, the angles between the substrate normal and the trajectory of the incident deposition flux are set to be almost 90°. Porous thin films consisting of carbon nanowires with diameters less than 100 nm were formed due to a self-shadowing effect. The kinetic energies of the deposited ions, the deposition rate of the films and the size of the nanowires were investigated. The sp³ fraction of the porous carbon films produced at intensity around 20 GW/cm² were estimated from their Raman spectra.     

Nanosecond and femtosecond laser spectroscopy of molecules of biological interest

This paper mainly concerns on nanosecond and femtosecond laser spectroscopy of aromatic organic compounds as neurotransmitters, and plume diagnostics of the ablated species, in order to characterize the plasma dynamics, i.e. the temporal and spatial evolution of the plume. Optical emission spectroscopy has been applied to characterize the transient species produced in the femtosecond (fs) and nanosecond (ns) regimes. The laser sources employed for optical emission spectroscopy are a frequency-doubled Nd:YAG Handy (λ = 532 nm, τ = 5 ns) and a frequency-doubled Nd:glass (λ = 527 nm, τ = 250 fs). These studies aim to detect and give information on the photoexcitation and photodissociation of these biological molecules and to compare the plasma characteristics in the two ablation regimes.     

Particulate assisted growth of ZnO nanorods and microrods by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on the gallium nitride (GaN) and sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) without using any metal catalyst. The experiment was carried out at three different laser wavelengths of Nd:YAG laser (λ = 1064 nm, λ = 532 nm) and KrF excimer laser (λ = 248 nm). The ZnO films grown at λ = 532 nm revealed the presence of ZnO nanorods and microrods. The diameter of the rods varies from 250 nm to 2 μm and the length varies between 9 and 22 μm. The scanning electron microscopy (SEM) images of the rods revealed the absence of frozen balls at the tip of the ZnO rods. The growth of ZnO rods has been explained by vapor-solid (V-S) mechanism. The origin of growth of ZnO rods has been attributed to the ejection of micrometric and sub-micrometric sized particulates from the ZnO target. The ZnO films grown at λ = 1064 nm and λ = 248 nm do not show the rod like morphology. X-ray photoelectron spectroscopy (XPS) has not shown the presence of any impurity except zinc and oxygen.     

Bioactive glass and hydroxyapatite thin films obtained by pulsed laser deposition

Bioactive glass (BG), calcium hydroxyapatite (HA), and ZrO₂ doped HA thin films were grown by pulsed laser deposition on Ti substrates. An UV KrF^* (λ = 248 nm, τ ≥ 7 ns) excimer laser was used for the multi-pulse irradiation of the targets. The substrates were kept at room temperature or heated during the film deposition at values within the (400-550 °C) range. The depositions were performed in oxygen and water vapor atmospheres, at pressure values in the range (5-40 Pa). The HA coatings were heat post-treated for 6 h in a flux of hot water vapors at the same temperature as applied during deposition. The surface morphology, chemical composition, and crystalline quality of the obtained thin films were studied by scanning electron microscopy, atomic force microscopy, and X-ray diffractometry. The films were seeded for in vitro tests with Hek293 (human embryonic kidney) cells that revealed a good adherence on the deposited layers. Biocompatibility tests showed that cell growth was better on HA than on BG thin films.     

Polycaprolactone biopolymer thin films obtained by matrix assisted pulsed laser evaporation

We report the successful deposition of polycaprolactone polymer by MAPLE using a KrF* excimer laser (λ = 248 nm, τ = 7 ns). According to FTIR spectra the deposited films have similar chemical structure to the dropcast material. The fluence plays a key role in optimizing the performances of MAPLE-synthesized polycaprolactone structures. We demonstrated that MAPLE allows for controlling the morphology of films to the level required in targeted drug delivery of pharmacologic agents.     

Au cluster growth on ZnO thin films by pulsed laser deposition

Nanostructures formed by Au nanoparticles on ZnO thin film surface are of interest for applications which include medical implants, gas-sensors, and catalytic systems. A frequency tripled Nd:YAG laser (λ = 355 nm, τ_FWHM ∼ 10 ns) was used for the successive irradiation of the Zn and Au targets. The ZnO films were synthesized in 20 Pa oxygen pressure while the subsequent Au coverage was grown in vacuum. The obtained structures surface morphology, crystalline quality, and chemical composition depth profile were investigated by acoustic (dynamic) mode atomic force microscopy, X-ray diffraction, and wavelength dispersive X-ray spectroscopy. The surface is characterized by a granular morphology, with average grain diameters of a few tens of nanometers. The surface roughness decreases with the increase of the number of laser pulses applied for the irradiation of the Au target. The Au coverage reveals a predominant (1 1 1) texture, whereas the underlying ZnO films are c-axis oriented. A linear dependence was established between the thickness of the Au coverage and the number of laser pulses applied for the irradiation of the Au target.     

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.     

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.     

Parametric study on femtosecond laser pulse ablation of Au films

Ablation process of 1 kHz rate femtosecond lasers (pulse duration 148 fs, wavelength 775 nm) with Au films on silica substrates has been systemically studied. The single-pulse threshold can be obtained directly. For the multiple pulses the ablation threshold varies with the number of pulses applied to the surface due to the incubation effect. From the plot of accumulated laser fluence N × ?_th(N) and the number of laser pulses N, incubation coefficient of Au film can be obtained (s = 0.765). As the pulse energy is increased, the single pulse ablation rate is increasing following two ablation logarithmic regimes, which can be explained by previous research.     

Matrix assisted pulsed laser evaporation processing of triacetate-pullulan polysaccharide thin films for drug delivery systems

We report the first successful deposition of triacetate-pullulan polysaccharide thin films by matrix assisted pulsed laser evaporation. We used a KrF* excimer laser source (λ = 248 nm, τ ≈ 20 ns) operated at a repetition rate of 10 Hz. We demonstrated by FTIR that our thin films are composed of triacetate-pullulan maintaining its chemical structure and functionality. The dependence on incident laser fluence of the induced surface morphology is analysed.     

Ultrafast laser pulses generated from the chromium-doped cunyite laser

Femtosecond pulses were generated from a Cr⁴⁺: Cunyite laser using a combination of a broadband semiconductor saturable absorber mirror (SESAM), chirped mirrors, and passive mode locking. The astigmatically compensated asymmetric X-cavity with a 4.5-mm-long Cr⁴⁺:Ca₂GeO₄ sample was operated with a 2.5% output coupler. Dispersion compensation was achieved using chirped mirrors. During self-starting mode-locked operation, pulses as short as 365 fs were generated at a pulse repetition rate of 100 MHz with output power of 70 mW and a spectral bandwidth of 5.2 nm at the center wavelength of 1432 nm.     

Laser ablation of metals by femtosecond pulses: Theoretical and experimental study

We have investigated ultrashort laser micromachining of metals, both from the point of view of the basic physical processes, and the technological implications. The process of hole drilling of Ni with ≈300 fs SHG (λ = 527 nm) Nd-glass and Al samples with 100 fs Ti:sapphire (λ = 800 nm) laser pulses, respectively, has been experimentally addressed by using time-gated optical emission spectroscopy of the ablated material and SEM analysis of the targets. The ablation process has also been analyzed by classical, molecular dynamics (MD) simulations, by using a Morse potential to describe the interaction between the atoms, and taking into account the electron heat diffusion contribution. The dependence of the ablation depth on laser fluence, as measured by SEM analysis, is in good agreement with the numerical simulations and is also well correlated with the optical emission yield of the expanding plume.     

KrF laser ablation of a polyethersulfone film: Effect of pulse duration on structure formation

Polyethersulfone (PES) films were processed with KrF laser irradiation of different pulse durations (τ). Scanning electron microscopy (SEM) and Raman spectroscopy were employed for the examination of the morphology and chemical composition of the irradiated surfaces, respectively. During ablation with 500 fs and 5 ps pulses, localized deformations (beads), micro-ripple and conical structures were observed on the surface depending on the irradiation fluence (F) and the number of pulses (N). In addition, the number density of the structures is affected by the irradiation parameters (τ, F, N). Furthermore, at longer pulse durations (τ = 30 ns), conical structures appear at lower laser fluence values, which are converted into columnar structures upon irradiation at higher fluences. The Raman spectra collected from the top of the structures following irradiation at different pulse durations revealed graphitization of the ns laser treated areas, in contrast to those processed with ultra-short laser pulses.