T. Donnelly

Dynamics of the plumes produced by ultrafast laser ablation of metals

We have analyzed ultrafast laser ablation of a metallic target (Nickel) in high vacuum addressing both expansion dynamics of the various plume components (ionic and nanoparticle) and basic properties of the ultrafast laser ablation process. While the ion temporal profile and ion angular distribution were analyzed by means of Langmuir ion probe technique, the angular distribution of the nanoparticulate component was characterized by measuring the thickness map of deposition on a transparent substrate. The amount of ablated material per pulse was found by applying scanning white light interferometry to craters produced on a stationary target. We have also compared the angular distribution of both the ionic and nanoparticle components with the Anisimov model. While the agreement for the ion angular distribution is very good at any laser fluence (from ablation threshold up to ≈1 J/cm2), some discrepancies of nanoparticle plume angular distribution at fluencies above ≈0.4 J/cm2 are interpreted in terms of the ...

Double pulse ultrafast laser ablation of nickel in vacuum

We have studied ultrafast laser ablation of nickel using a pair of identical ≈250 fs 527 nm laser pulses separated by ≈1 to ≈1000 ps. Scanning white light interferometry was used to measure the ablated volume, and an ion probe was used to measure the angular distribution of the ablation plasma plume and the total ion emission. As the delay of the second pulse increased from ≈10 to 100 ps the ablated volume decreased by more than a factor of 2; indeed it falls to a value below the single pulse case. Conversely, it is found that the ion yield is sharply increased in this delay regime. It seems that both these features can be explained by the interaction of the second laser pulse with the ablated material produced by the first pulse.

CO2 laser pulse shortening by laser ablation of a metal target

A repeatable and flexible technique for pulse shortening of laser pulses has been applied to transversely excited atmospheric (TEA) CO(2) laser pulses. The technique involves focusing the laser output onto a highly reflective metal target so that plasma is formed, which then operates as a shutter due to strong laser absorption and scattering. Precise control of the focused laser intensity allows for timing of the shutter so that different temporal portions of the pulse can be reflected from the target surface before plasma formation occurs. This type of shutter enables one to reduce the pulse duration down to ~2 ns and to remove the low power, long duration tails that are present in TEA CO(2) pulses. The transmitted energy is reduced as the pulse duration is decreased but the reflected power is ~10 MW for all pulse durations. A simple laser heating model verifies that the pulse shortening depends directly on the plasma formation time, which in turn is dependent on the applied laser intensity. It is envisaged that this plasma shutter will be used as a tool for pulse shaping in the search for laser pulse conditions to optimize conversion efficiency from laser energy to useable extreme ultraviolet (EUV) radiation for EUV source development.

Heating and compression of a laser produced plasma in a pulsed magnetic field

A pulsed 0.3 T magnetic field was used to heat and compress a low-temperature laser produced copper plasma. The magnetic field was generated using a planar 3-turn coil positioned 10 mm above the ablation spot. The plasma flowing through a central aperture in the coil was strongly focused. Inductive heating of the plasma caused a large enhancement of the overall visible light emission and the appearance of Cu II line emission. The plasma focusing is also evident in the constriction of the spatial distribution of deposited copper. The plasma heating and focusing can be explained in the framework of resistive magnetohydrodynamics.

Laser produced plasma for efficient extreme ultraviolet light sources

Extreme ultraviolet emission from laser produced plasma and their relevance to EUV source development is discussed. The current state of the field for Sn LPP sources operating at 13.5 nm is described and initial results are given for EUV emission from CO2 laser irradiation of a bulk Sn target. A maximum conversion efficiency of 1.7% has been measured and the influence of the CO2 laser temporal profile on the CE is discussed. A double pulse irradiation scheme is shown to increase CE up to a maximum value of 2.1% for an optimum prepulse - pulse delay of 150 ns. The emergence of a new EUVL source wavelength at 6.7 nm based on Gd and Tb LPPs has been outlined. An initial experiment investigating picosecond laser irradiation as a means to produce strong 6.7 nm emission from a Gd2O3 target has been performed and verified.

Measurement of electron density in a laser produced plasma using a hairpin resonance probe

A floating hairpin resonance probe has been used for the first time to measure the spatial and time evolution of local electron density in a laser produced plasma expanding in vacuum. The measured variation in electron density agrees closely with the variation of ion charge density as measured with a time-of-flight planar Langmuir ion probe confirming the reliability of Langmuir probe in the laser produced plasma.

Interaction of surface plasmons with CdTe quantum dot excitons

A 5-fold enhancement in the luminescence of CdTe nanocrystal quantum dots (QDs) is observed when they are placed in proximity to a nanostructured Au film deposited by pulsed laser deposition technique. No enhancement is observed with a nanostructured Ag film. The enhancement is due to the interaction of the QDs excitons with the localized surface plasmons (LSP). The Au surface plasmon (SP) frequency is closer to the QDs emission frequency than Ag LSP frequency and this accounts for the differences in observed behavior. As the SP-QD interaction strongly depends on the geometric structure and shape of the metal nanoparticles, a comparison with QDs deposited on a film of Au colloidal nanoparticles is presented. In the case of QDs placed directly on the Au colloids the luminescence quenching is much stronger and with a spacer layer a 3.5-fold enhancement over the bare QDs luminescence is observed.

Plume dynamics in femtosecond laser ablation of metals

In femtosecond laser ablation the plume has two components: a faster‐moving plasma part and a slower nanoparticle plume which contains most of the ablated material. This paper describes the results of experiments to comprehensively characterize the plume in laser ablation of Ni with ≈300 fs pulses at 527 nm. Both single‐pulse and double‐pulse irradiation was used. The laser ablation depth was measured using white light interferometry. The dynamics of the plasma part of the ablation plume was measured using Langmuir ion probes. The shape of the overall ablation plume was recorded by depositing a thin film on a transparent substrate and measuring the thickness distribution. The expansion of the plasma plume is well described by the Anismov isentropic model of plume expansion. Just above the ablation threshold, the nanoparticle plume is also well described by the Anisimov expansion model. However, at higher fluence a wider plume is formed, perhaps due to the pressure exerted by plasma. For double‐pulse ablation it is observed that as the second pulse is delayed beyond ≈20 ps the ablation depth is reduced and the ion yield is increased. This behaviour is due to reheating of the nascent plasma plume produced by the first pulse. This generates a pressure pulse that acts as a tamper which impedes the fragmentation and ablation of deeper layers of material.

Atmospheric pulsed laser deposition and thermal annealing of plasmonic silver nanoparticle films

A new method for pulsed laser deposition of plasmonic silver nanoparticle (NP) films in flowing gas at atmospheric pressure is described. The ablation was done using an excimer laser at 248 nm. Fast optical imaging shows that the ablation plume is captured by the flowing gas, and is expected to form a NP aerosol, which is carried 5-20 mm to the substrate. The dependence of the deposition rate on laser fluence, gas flow velocity, and target-substrate distance was investigated using electron microscopy and absorption spectroscopy of the deposited films. The NP films were annealed in argon and hydrogen at 400 °C, and in air for temperatures in the range 200 °C-900 °C, leading to strong enhancement, and narrowing of the surface plasmon resonance. The films were used for surface enhanced Raman spectroscopy of a 10-5 molar solution of Rhodamine 6G; films annealed in air at 400 °C were five times more sensitive than the as-deposited films.

Laser-driven rapid functionalization of carbon surfaces and its application to the fabrication of fluorinated adsorbers

The use of laser sources can expand the range of applications of photochemical surface functionalization strategies, increasing reaction rate and sample throughput. However, high irradiances can result in thermal effects and/or changes in the mechanism of photoinduced reactions. In this work we report on the use of a pulsed UV laser source for the modification of carbon surfaces using fluorinated terminal alkenes. A perfluorinated alkene, 1H,1H,2H-perfluoro-dec-1-ene (PFD), was used to modify amorphous carbon surfaces using a pulsed excimer laser (248 nm). The rate and yield of photoinduced PFD chemisorption was measured using Infrared Reflectance Absorption Spectroscopy (IRRAS) and compared to that obtained using a continuous lamp source. Quartz Crystal Microbalance (QCM) measurements were also used to obtain quantitative estimates of surface coverage and quantum yields. We found that, under the experimental conditions investigated, PFD chemisorption rates at bare carbon are proportional to the rate of incident photons. Simulations indicated that thermal effects of laser irradiation are expected to be minor, thus supporting the conclusion that the pulsed source can be used to accelerate the reaction rate without leading to changes in reaction mechanism. However, we observed that the limiting chemisorption yield was ∼30% higher for the laser source. We propose that this difference is due to photochemical formation of multilayers, a reaction that is slower than chemisorption at bare carbon, but that becomes evident when very high total fluence is applied via pulsed sources. Finally, we investigated the influence of reaction conditions on the ability of fluorinated carbon surfaces obtained via laser- and lamp-driven reactions to adsorb and capture fluorinated ligands via non-covalent fluorous–fluorous interactions.