We have studied plasma formation and relaxation dynamics along with the corresponding topography modifications in fused silica and sapphire induced by single femtosecond laser pulses (800 nm and 120 fs). These materials, representative of high bandgap amorphous and crystalline dielectrics, respectively, require nonlinear mechanisms to absorb the laser light. The study employed a femtosecond time-resolved microscopy technique that allows obtaining reflectivity and transmission images of the material surface at well-defined temporal delays after the arrival of the pump pulse which excites the dielectric material. The transient evolution of the free-electron plasma formed can be followed by combining the time-resolved optical data with a Drude model to estimate transient electron densities and skin depths. The temporal evolution of the optical properties is very similar in both materials within the first few hundred picoseconds, including the formation of a high reflectivity ring at about 7 ps. In contrast, at longer delays (100 ps-20 ns) the behavior of both materials differs significantly, revealing a longer lasting ablation process in sapphire. Moreover, transient images of sapphire show a concentric ring pattern surrounding the ablation crater, which is not observed in fused silica. We attribute this phenomenon to optical diffraction at a transient elevation of the ejected molten material at the crater border. On the other hand, the final topography of the ablation crater is radically different for each material. While in fused silica a relatively smooth crater with two distinct regimes is observed, sapphire shows much steeper crater walls, surrounded by a weak depression along with cracks in the material surface. These differences are explained in terms of the most relevant thermal and mechanical properties of the material. Despite these differences the maximum crater depth is comparable in both material at the highest fluences used ͑16 J/cm 2 ͒. The evolution of the crater depth as a function of fluence can be described taking into account the individual bandgap of each material.
The e − e , e − i, i − i and charge-charge static structure factors have been calculated for Alkali and Be 2+ plasmas using Gregori's method [14]. The dynamic structure factors for Alkali plasmas have been calculated using Adamjans' et al method [52], [53]. In both methods the screened Hellmann-Gurskii-Krasko potential, obtained on a base of Bogoljubow method, has been used taking into account not only the quantum-mechanical effects but also the ion structure [13].
The expansion dynamics of the plasma generated during pulsed laser deposition of gold in vacuum has been investigated at the laser fluences of 2.5, 6.0, and 9.0 J cm −2 . A severe distortion of the expansion is observed in the presence of a substrate that is accompanied at 9.0 J cm −2 by the appearance of a secondary plasma front expanding from the substrate surface. Langmuir probe analysis at 9.0 J cm −2 shows that the substrate surface is bombarded by a high transient flux of energetic Au + ions ͑3.0ϫ 10 19 ions cm −2 s −1 ͒ having very large kinetic energies ͑Ͼ400 eV͒. Analysis of the plasma dynamics shows that these observations are consistent with self-sputtering of Au neutrals from the substrate induced by incident Au ions while a fraction of them are backscattered. Self-sputtering is found to be 2 orders of magnitude larger than backscattering. The comparison with experimental data allows concluding that the apparent recoil of the plasma front is caused by collision with self-sputtered neutrals, while the secondary emission is originated by backscattered ions. INTRODUCTIONPulsed laser deposition ͑PLD͒ has become a well established technique for the production of a broad variety of materials, particularly complex oxides.1,2 However, much less attention has been paid to the production of metals despite early attempts to synthesize them in the 1970s. 3 More recently, the interest in complex thin film metal nanostructures such as metal-dielectric or metal multilayers, 4,5 as well as metal nanoparticles, 6-8 has triggered new efforts to use PLD for metal deposition. These metal structures are characterized by the localization and enhancement of the electric and magnetic fields, leading to effects such as giant magnetoresistance, 9 enhanced nonlinear optical properties, 8 or enhanced Raman response, 10 among others. Layer thickness, bulk and interface structure in the case of the multilayers, or size, shape, and arrangement of nanoparticles determine these properties. Thus, practical applications require excellent control of the structure and morphology of the nanostructures.PLD is characterized by a high instantaneous flux of species reaching the substrate, a fraction of them having high kinetic energies.11-14 Both characteristics are expected to have a strong influence on the features of the metal structures produced. 6,15 In addition, the presence of highly energetic species may induce undesired processes such as selfsputtering, also termed resputtering, of a fraction of the material that is being deposited 14,[16][17][18][19] or an increase of surface roughness, 4 among others. Finally, both self-sputtering and backscattering of a fraction of the incident species may have a significant effect on the morphology of the nanoparticles due to their nanometric dimensions. While self-sputtering is considered to be a common process when PLD is performed in vacuum, backscattering has seldom been observed or discussed in the literature, 20 and little effort has been devoted to determine its influence on the properti...
We have analyzed the influence of the temporal pulse shape on femtosecond (fs) laser-induced surface ablation processes in sapphire. To this end, single transform-limited (TL), stretched, and third-order-dispersion (TOD) shaped fs pulses have been used, while the dynamics of the interaction were analyzed by fs-resolved microscopy and correlated with plasma emission intensity and crater morphology. The modification of the pulse shape enables changing the ablation mechanism from a strong, thermally mediated ablation process to a gentle ablation process mediated by Coulomb explosion (CE), with respective ablation depths of 100-200 nm and 5-10 nm. Analysis of the transient optical response allows direct comparison of the transient plasma carrier densities involved, observing comparable peak values for both processes. For strong ablation induced by TL pulses, a direct relation between plasma density and local ablation depth is found, but this does not hold for the CE-mediated process observed for TOD-shaped pulses. For TOD-shaped pulses at very high fluence, a different ablation mechanism involving explosive boiling is identified. This mechanism leads to the formation of deep craters with reduced lateral extension and steep walls. This amount of control over the ablation mechanisms by a simple selection of the pulse shape should be of interest for new surface structuring approaches.
The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules, quantum dots and plasmonic nanoparticles. Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive, yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics. In ensemble measurements, such control is often achieved by resorting to a closed-loop optimization strategy, where the spectral phase of a broadband laser field is iteratively optimized. This scheme needs long measurement times and strong signals to converge to the optimal solution. This requirement is in conflict with the nature of single emitters whose signals are weak and unstable. Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature, using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser. Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps, even when operating at the few-photon level. Full optimization of the single dot luminescence is obtained within ~100 trials, with a typical integration time of 100 ms per trial. These times are faster than the typical photobleaching times in single molecules at room temperature. Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules, thus extending the available experimental toolbox to the active control of nanoscale coherences.
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