Increasing Laser Fluence Research Articles

Ultra-short laser processing of transparent material at the interface to liquid

Similarly to laser-induced backside wet etching (LIBWE) with nanosecond ultraviolet (ns UV) laser pulses, the irradiation of the solid/liquid interface of fused silica with sub-picosecond (sub-ps) UV and femtosecond near infrared (fs NIR) laser pulses results in etching of the fused silica surface and deposition of decomposition products from liquid. Furthermore, the etch threshold is reduced compared with both direct ablation with an fs laser in air and backside etching with UV ns pulses. Using 0.5 M pyrene/toluene as absorbing liquid, the thresholds were determined to be 70 mJ cm−2 (sub-ps UV) and 330 mJ cm−2 (fs NIR). Furthermore, an almost linear increase in the etch rate with increasing laser fluence was found. The roughness of surfaces backside etched with ultra-short pulses is higher in comparison with ns pulses but lower than that obtained using direct fs laser ablation. Hence a combination of processes involved in fs laser ablation and ns backside etching can be expected. The processes at the ultra-short pulse laser irradiated solid/liquid interface are discussed, considering the effects of ultra-fast heating, multi-photon absorption processes, as well as defect generation in the materials.

Transition between nonthermal and thermal ablation of metallic targets under the strike of high-fluence ultrashort laser pulses

The mutual transition dynamics between nonthermal and thermal dominant ablation processes is investigated in the hole-drilling and line-scribing experiments on aluminum samples with intense 50 fs to 24 ps laser pulses. It is found that a critical pulse width that separates the two different ablation regimes monotonically reduces with the increasing laser fluence. Theoretical analyses suggest that the complex interplay between photomechanical stress fragmentation and phase explosion could be responsible for these observations. A semiempirical transition law between the two ablation regimes is introduced, which is consistent with measured experimental data.

Spectroscopic Study of Laser-Induced Phase Transition of Gold Nanoparticles on Nanosecond Time Scales and Longer

The pulsed laser induced phase transition of gold nanoparticles in aqueous solution was observed via a transient absorption on nanosecond time scales and longer. Gold nanoparticles were excited with an intense picosecond laser pulse (355 nm, 30 ps), and the subsequent changes were monitored using two continuous wave laser wavelengths (488 and 635 nm). On the nanosecond time scale, below 6.3 mJ cm(-2), no change was observed; however, in the low fluence region between 6.3 and 17 mJ cm(-2), gold nanoparticles produced a bleach signal (488 nm) attributed to the melting of the gold nanoparticles, which decreased linearly with increasing laser fluence. Laser fluences above 17 mJ cm(-2) resulted in a strong absorption at both wavelengths, which is ascribed to vaporization of gold nanoparticles rather than solvated electrons (ejected from gold nanoparticles) or light scattering. The decay of both signals was faster than the 5 ns time resolution used in our experimental system. On the microsecond time scale, increase in absorbance at 635 nm was observed with a time constant of 1.0 micros, while no change was observed at 488 nm. It is considered that this increase is attributed to the formation of smaller gold nanoparticles resulting from pulsed laser induced size reduction of initial gold nanoparticles.

Characterization of Polycrystalline Si Films Produced by ArF Excimer Laser Irradiation

ArF excimer laser irradiation of amorphous silicon (a-Si) films has great potential on the production of poly-crystalline silicon (poly-Si) thin film transistor liquid crystal displays (TFT-LCDs). The main advantage of using poly-Si films instead of a-Si films to fabricate high performance electronic devices is due to its ability to obtain higher field effect mobility. However, the surface morphology, grain size and defect density of the poly-Si film have profound effect on whether this is to be achieved. The specimens used in this study contain a 100 nm-thick a-Si thin film grown on glass substrate by plasma enhanced chemical vapor deposition (PECVD) technique. The influence of processing parameters such as laser fluence, shot number and repetition rate on the surface morphology and recrystallization behaviour of laser annealed a-Si films was investigated in this study. The results showed that the threshold fluence for partially melting a-Si thin films at 1Hz and single shot was around 150 mJ/cm 2 . Further increasing laser fluence and shot number resulted in the formation of microvoids trapped inside poly-Si film due to the evolution of hydrogen gas during the laser annealing process. The surface morphology induced by laser annealing of a-Si thin films is found dependent upon the laser fluence and shot number. The super-lateral grown (SLG) poly-Si grains can be obtained at the fluence around 190 mJ/cm 2 .

Pulsed laser ablation of borax target in vacuum and hydrogen DC glow discharges

The aim of our experiment was to produce a material with B H bonds for applications in hydrogen storage and generation. By using KrF excimer laser ( λ = 248 nm) ablation of borax (Na 2B 4O 7) target, thin films were deposited on KBr and silicon substrates. Ablation was performed both in vacuum and in hydrogen atmosphere. DC glow discharge technique was utilized to enhance hydrogen gas ionization. Experiments were performed using laser fluence from 5 to 20 J/cm 2. Films were deposited under gas pressure of 1 × 10 −5 to 5 × 10 −2 mbar and substrate temperatures of 130–450 °C. Scanning electron microscopy analysis of films showed presence of circular particulates. Film thickness, roughness and particulates number increased with increase in laser fluence. Energy dispersive X-ray spectroscopy analysis shows that sodium content in the particulates is higher than in the target. This effect is discussed in terms of atomic arrangements (both at surface and bulk) in systems where ionic and covalent bonds are present and by looking at the increased surface/bulk ratio of the particulates with respect to the deposited films. The Fourier transform infrared spectroscopy measurements showed presence of B O stretching and B O B bending bonds. Possible reasons for absence of B H bonds are attributed to binding enthalpy of the competing molecules.

Bias-dependent spin relaxation in a Spin-LED

We have investigated the bias-dependent spin relaxation in Cu–CoFe–AlO x –GaAs/AlGaAs-type of Spin-LEDs using microscopic time-resolved magnetization modulation spectroscopy (TIMMS). We observed a significant dependence of the electron spin relaxation time (effects as large as 40%) as a function of applied bias. The additional spin relaxation at non-zero bias is found to scale almost linearly with the injection current, and thereby with the current-induced hole density in the active region. This observation is indicative for a dominant contribution by Bir–Aronov–Pikus (BAP) electron-hole spin-flip scattering. In agreement with this observation, a similar BAP-enhanced spin relaxation shows up at increased laser fluence. From spatio-temporal imaging of spin relaxation, scanning pump and probe beams across the ≈ 50 μ m outside of optical window, we found a significant position dependence (lateral effects) of the spin dynamics.

Recombination effects during the laser-produced plasma expansion

The precise characterization of plasmas generated by laser irradiation is needed for the development of ion sources. As the characteristic parameters of the expanding plasma vary with both the distance and the time, an experimental study of their evolution is appropriate for a deeper knowledge of the plasma. The purpose of this work is to study the same characteristics of laser plasma produced by ablation of a pure Cu target as a function of the distance from the target along the propagation axis of the plasma plume. We irradiated the target by a KrF laser and a lens of 15 cm focal length. As the diagnostic system, a small Faraday cup array and an axial Faraday cup were utilized to study the spatial variation in the total charge carried by plasma ions. Charge loss during the plasma expansion was observed due to the recombination of charged species, which occurs within a critical distance, relatively close to the target, where the plasma density is high enough. The critical distance was determined for different laser fluences; beyond the critical distance, the collisions among plasma particles are negligible and the ion charge remains frozen. It was observed that the critical distance increases as the laser fluence increases.

Growth of ultrathin Fe films on Cu(1 1 1) by pulsed laser deposition

Ultrathin Fe films have been grown on Cu(1 1 1) by pulsed laser deposition (PLD) and thermal deposition (TD) and analyzed by scanning tunneling microscopy (STM) and low-energy ion scattering (LEIS). PLD was performed using nanosecond pulses of a Nd:YAG laser providing three different wavelengths. Compared to the widely investigated Fe films grown by thermal deposition on Cu(1 1 1), which exhibit bilayer and multi-layer (3D) island growth at low coverage, PLD-grown films show enhanced layer-by-layer growth when using sufficiently high laser fluence. However, by increasing the laser spot size on the Fe target and adjusting the laser power to achieve an unchanged deposition rate, resulting in a lower laser fluence, we observe bilayer growth reminiscent of the TD films. Using STM and LEIS, we observe an increasing number of Fe atoms implanted into the Cu substrate with increasing laser fluence and consequently mixing of Fe and Cu in the layer-by-layer films even at a preparation temperature of 200 K. We therefore suggest that the reason for layer-by-layer growth in this system is not the high instantaneous deposition rate of PLD, but the implantation of Fe atoms due to their higher kinetic energy at higher fluences.

Channels of energy redistribution in short-pulse laser interactions with metal targets

The kinetics and channels of laser energy redistribution in a target irradiated by a short, 1 ps, laser pulse is investigated in computer simulations performed with a model that combines molecular dynamics (MD) simulations with a continuum description of the laser excitation and relaxation of the conduction band electrons, based on the two-temperature model (TTM). The energy transferred from the excited electrons to the lattice splits into several parts, namely the energy of the thermal motion of the atoms, the energy of collective atomic motions associated with the relaxation of laser-induced stresses, the energy carried away from the surface region of the target by a stress wave, the energy of quasi-static anisotropic stresses, and, at laser fluences above the melting threshold, the energy transferred to the latent heat of melting and then released upon recrystallization. The presence of the non-thermal channels of energy redistribution (stress wave and quasi-static stresses), not accounted for in the conventional TTM model, can have important implications for interpretation of experimental results on the kinetics of thermal and mechanical relaxation of a target irradiated by a short laser pulse as well as on the characteristics of laser-induced phase transformations. The fraction of the non-thermal energy in the total laser energy partitioning increases with increasing laser fluence.

Characterization and composition evolution of multiple-phase nanoscaled ceramic powders produced by laser ablation

Nanosized ceramic powder was prepared by laser ablation under different atmospheres using a ceramic target composed of ZrO 2, TiO 2, ZnO and Al 2O 3. Physical characteristics and microstructure of nanoparticles, including particle morphology, phase transformation, powder compositions and far-infrared emissivity, have been investigated. The laser-ablated nanoparticles exhibit two kinds of particle size distribution with 7–15 (70–90%) and 40–100 nm (10–30%). Nanoparticles synthesized at lower laser fluences show poor crystallinity but rich Zn composition. While increasing laser fluence, better crystalline nanoparticles with rich Zr composition were obtained. It was found that both composition and morphology of nanoparticles change with laser fluence. The average far-infrared emissivity of the nanoparticles varies with crystallinity of nanoparticles.

Fabrication of diamond-like carbon thin films by femtosecond laser ablation of frozen acetone

Diamond-like carbon (DLC) thin films were fabricated by the ablation of frozen acetone with a 790 nm, 130 fs Ti:sapphire laser. Compared to a solid carbon target, frozen acetone could significantly reduce the number of fragments mixed into the films. The optical and mechanical properties of the fabricated DLC films were determined when the laser fluence was varied from 3 to 470 J/cm2. With the increase in laser fluence, the films tinged with brown and the optical bandgap of the films decreased from 2.0 to 1.2 eV. Also, the refractive index and hardness of the films increased from 1.75 to 1.99 and from 10 to 16 GPa, respectively. The sp3 content was not changed even if the laser fluence was varied. The change in properties resulted from the hydrogen content of the films.

Bonding configurations in amorphous carbon and nitrogenated carbon films synthesised by femtosecond laser deposition

The effect of nitrogen addition and laser fluence on the atomic structure of amorphous carbon films (a-C) synthesized by femtosecond pulsed laser deposition has been studied. The chemical bonding in the films was investigated by means of X-ray photoelectron (XPS) and Raman spectroscopies. XPS studies revealed a decrease in the sp3 bonded carbon sites and an associated increase in the N-sp2C bonding sites with increasing nitrogen content in the CNx films. An increase in laser fluence from 0.36 to 1.7 J/cm2 led to a rise in sp3C sites. These results were further confirmed by Raman spectroscopy. The ID/IG ratio increased monotonically and G line-width decreased with the increase of nitrogen content in the films indicating a rise in either the number or the size of the sp2 clusters. Furthermore a visible excitation wavelength dependence study established the resonant Raman process in a-C and CNx films.

Photochemical keratodesmos for bonding corneal incisions.

To evaluate the immediate and long-term effectiveness of a dye-plus-laser irradiation treatment (photochemical keratodesmos [PKD]) for sealing corneal incisions. Incisions (3.5 mm) in rabbit corneas were treated on the incision walls with rose bengal dye followed by exposure to 514-nm laser radiation. PKD was evaluated in three groups (n = 3-6) using laser fluences of 115, 153, or 192 J/cm(2) (180-, 240-, and 300- second exposures, respectively) compared with an untreated group (n = 8). The intraocular pressure at which leakage occurred (IOP(L)) during infusion of saline into the anterior chamber was determined. In a long-term study, treated and control corneas were observed weekly for 10 weeks for the appearance of neovascularization, anterior chamber inflammation, iridocorneal adhesion, corneal melting, and scarring. Immediately after treatment, the IOP(L) increased with increasing laser fluence, producing IOPs of 230 +/- 90, 370 +/- 120, and more than 500 mm Hg at 115, 153, and 192 J/cm(2), respectively, compared with 40 +/- 20 mm Hg in control eyes (P < 0.005). No reduction in the IOP(L) was observed up to 14 days after surgery. Corneal melting in PKD-treated or control eyes was not observed in the 10-week healing study. Neovascularization, which peaked at 4 weeks but resolved by 8 weeks, was detected around the incision in both PKD-treated and control eyes. Immediate and lasting sealing of corneal incisions was obtained in eyes treated with PKD, using short irradiation times. These results suggest that PKD has potential for improved corneal tissue bonding.

Room temperature indium tin oxide by XeCl excimer laser annealing for flexible display

A XeCl excimer laser ( λ=308 nm) has been used to anneal Indium Tin Oxide (ITO) films deposited at 25 °C using DC magnetron sputtering. With increasing laser fluence, the film crystallinity was improved while retaining the as-deposited 〈111〉 texture. As a result of laser irradiation, the sheet resistance of 100 nm ITO films decreased from 191 Ω/□ (1.91×10 −3 Ω cm) to 25 Ω/□ (2.5×10 −4 Ω cm), while the optical transmittance in the visible range increased from 70% to more than 85%. Surface roughness and etching properties were also significantly improved following laser annealing.

In vitro laser ablation of laboratory developed biofilms using an Nd:YAG laser of 532 nm wavelength.

We studied the laser ablation of laboratory-developed biofilm on titanium and glass surfaces. Specifically, Pseudoalteromonas carrageenovora, a marine biofilm forming bacterium was used to generate laboratory biofilm. Two fluences, 0.05 and 0.1 J/cm(2) and three durations of irradiation, 30 s, 5 min, and 10 min were tested using an Nd;YAG laser of 532 nm wavelength (in the green light area). Nonirradiated coupons with biofilm served as control. The biofilm removal efficiency increased with the increase in laser fluence and duration of irradiation. The maximum biofilm area cover on control coupons of glass and titanium was 62.5 and 76.0%, respectively. Upon irradiation with fluence 0.1 J/cm(2) for the very short duration of 30 s, this reduced to 5.6 and 12.4% and at 10 min to 2.17 and 0.7% on glass and titanium coupons, respectively, while the controls did not show any reductions (62.5 and 76.0% respectively, for glass and titanium coupons). The biofilm TRC (Total Resuscitated Cells) reduction during this period was even more prominent than the area cover, indicating that the remaining biofilm portions on coupons after irradiation were largely composed of dead bacterial cells. The TRC in the irradiation chamber medium for short durations of irradiation showed a significant increase, indicating that the laser irradiation removed live bacteria from the biofilm. The re-growth of the resuscitated cells showed they could grow like the control cells but with a significant lag. The laser's efficiency in the removal of biofilm was better seen on titanium coupons than on glass. Our results showed that a low-power pulsed laser irradiation could be used to remove biofilm formed on hard surfaces.

Laser-induced cavitation bubbles for cleaning of solid surfaces

When a high-power laser beam is focused into liquid, it results in a shock wave emission and cavitation bubble generation. Upon inserting a rigid substrate into the liquid, the bubbles migrate towards the substrate due to the Bjerknes attractive force. Due to bubble–substrate and/or bubble–free-surface interaction, a high-speed liquid jet is formed during bubble collapse, and a collapse shock wave is generated at the moment of bubble collapse near the substrate. These shock waves and liquid jet induce large forces acting on the substrate to remove particles from it. For a substrate several millimeters away from the laser focus point, the collapse shock wave and liquid jet play key roles in removal of particles. The cleaning efficiency increases with an increase of laser fluence and decreases with an increase of distance between substrate surface and laser beam focus point or depth below liquid surface. In a case of bubbles close to substrate and liquid-surface boundaries, implosion of the bubbles will give rise to shock waves and liquid jets oblique to the substrate surface with the parallel and perpendicular components of the forces onto the particles. These oblique liquid jets and shock waves result in high cleaning efficiency. A liquid, such as alcohol and commercial washing solution, as the surrounding medium, rather than air or vacuum, can reduce adhesion force and enhance cleaning efficiency.

Pulsed laser desorption of alkali atoms from PDMS thin films

The dynamics of short pulsed laser-induced desorption of alkali atoms from poly-dimethylsiloxane (PDMS) coated glass prism surfaces is investigated. Kinetic energies of desorbing sodium atoms of several hundred meV are found, increasing with increasing laser fluence. Evanescent wave measurements suggest that the desorbed atoms stem from a subsurface layer of the PDMS film, resulting in diffusional characteristics for the total yield of the desorption process.

UV laser micromachining of piezoelectric ceramic using a pulsed Nd:YAG laser

UV laser (λ=355 nm) ablation of piezoelectric lead zirconate titanate (PZT) ceramics in air has been investigated under different laser parameters. It has been found that there is a critical pulse number (N=750). When the pulse number is smaller than the critical value, the ablation rate decreases with increasing pulse number. Beyond the critical value, the ablation rate becomes constant. The ablation rate and concentrations of O, Zr and Ti on the ablated surface increase with the laser fluence, while the Pb concentration decreases due to the selective evaporation of PbO. The loss of the Pb results in the formation of a metastable pyrochlore phase. ZrO2 was detected by XPS in the ablated zone. Also, the concentrations of the pyrochlore phase and ZrO2 increase with increasing laser fluence. These results clearly indicate that the chemical composition and phase structure in the ablated zone strongly depend on the laser fluence. The piezoelectric properties of the cut PZT ceramic samples completely disappear due to the loss of the Pb and the existence of the pyrochlore phase. After these samples were annealed at 1150 °C for 1 h in a PbO-controlled atmosphere, their phase structure and piezoelectric properties were recovered again. Finally, 1–3 and concentric-ring 2–2 PZT/epoxy composites were fabricated by UV laser micromachining and their thickness modes were measured by impedance spectrum analysis and a d33 meter. Both composites show high piezoelectric properties.

Formation efficiency of radical cations of stilbene and methoxy-substituted stilbenes during resonant two-photon ionization with a XeCl excimer laser

Formation efficiency of radical cations of trans-stilbene and the methoxy-substituted stilbenes ( S = 1– 6 ) during the resonant two-photon ionization (RTPI) in acetonitrile has been studied by laser flash photolysis with a XeCl excimer laser (308 nm, 25 ns). The transient absorption spectra of S radical cations ( S +) were observed with a peak around 450–540 nm. The formation quantum yield of S + ( Φ ion) was 0.06–2.1%. No relation between Φ ion and E ox of S was observed, although Φ ion increased with the increasing lifetime of S in the lowest excited singlet state (S 1). These results are coincident with a two-step two-photon excitation from the ground state (S 0) to S 1 and from S 1 to higher excited singlet state (S n ), and ionization from S n . Considerably large Φ ion was obtained for the RTPI of 3,5-dimethoxystilbene ( 5) with relatively high oxidation potential and a long S 1 lifetime among S. This is explained by the intermediacy of the internal charge-transfer excited state for the RTPI of 5 based on the fluorescence lifetime measurement, solvatochromic measurement, and the dipole moment calculation. The concentration of S + increased with the increasing laser fluence ( F). The log–log plot of [ S +] versus F gave a linear line with a slope of approximately 2 for the RTPI of 1– 4, while approximately 1 for the RTPI of 5 and 6. The slope of 2 indicates clearly two-photon excitation during the RTPI. The slope of 1 is explained by the photostationary state among S 0, S 1, and S n states attained during the laser pulse, particularly for the RTPI of S with a long S 1 lifetime.

Transient Dynamics of Pulsed-Current-Biased YBa2Cu3O7-δ Superconducting Photoswitches

Studies on time-resolved dynamics of optically-thick YBa 2 Cu 3 O 7-δ microbridges biased with supercritical current pulses I of nanosecond duration and synchronously excited by femtosecond optical pulses have been carried out. The switching process is explained by the transient vortex dynamics in the superconducting stripe. It was found that the amplitude of the photoresponse increased linearly with the increase of laser fluence up to &lt; 30 μ J / cm 2 and with the increase of the bias current up to ~ 1.9Ic. Calculations of quantum responsivity showed that our YBa 2 Cu 3 O 7-δ bridges biased with I &gt; Ic could driven by optical pulses of comparatively weak fluence in the range of few tens of mW.