Nonlinear photoionization of transparent solids: a nonperturbative theory obeying selection rules

We provide a nonperturbative theory for photoionization of transparent solids. By applying a particular steepest-descent method, we derive analytical expressions for the photoionization rate within the two-band structure model, which consistently account for the $selection$ $rules$ related to the parity of the number of absorbed photons ($odd$ or $even$). We demonstrate the crucial role of the interference of the transition amplitudes (saddle-points), which in the semi-classical limit, can be interpreted in terms of interfering quantum trajectories. Keldysh's foundational work of laser physics [Sov. Phys. JETP 20, 1307 (1965)] disregarded this interference, resulting in the violation of $selection$ $rules$. We provide an improved Keldysh photoionization theory and show its excellent agreement with measurements for the frequency dependence of the two-photon absorption and nonlinear refractive index coefficients in dielectrics

Suppression of ablation in femtosecond double pulse experiments

We report the physical reasons of a curious decrease in the crater depth observed for long delays in experiments with femtosecond double pulses. Detailed hydrodynamic modeling demonstrates that the ablation mechanism is dumped when the delay between the pulses exceeds the electron-ion relaxation time. In this case, the interaction of the second laser pulse with the expanding target material leads to the formation of the second shock wave suppressing the rarefaction wave created by the first pulse. The evidence of this effect follows from the pressure and density profiles obtained at different delays after the first laser pulse.Comment: Submitted to one of the APS Journal

Vibrational and structural properties of P2O5P_2O_5 glass: Advances from a combined modeling approach

We present experimental measurements and ab initio simulations of the crystalline and amorphous phases of $P_2O_5$. The calculated Raman, infrared, and vibrational density of states (VDOS) spectra are in excellent agreement with experimental measurements and contain the signatures of all the peculiar local structures of the amorphous phase, namely, bridging and nonbridging (double-bonded or terminal) oxygens and tetrahedral $PO_4$ units associated with $Q^2$, $Q^3$, and $Q^4$ species ($Q^n$ denotes the various types of $PO_4$ tetrahedra, with $n$ being the number of bridging oxygen atoms that connect the tetrahedra to the rest of the network). In order to reveal the internal structure of the vibrational spectrum, the characteristics of vibrational modes in different frequency ranges are investigated using a mode-projection approach at different symmetries based on the $T_d$ symmetry group. In particular, the VDOS spectrum in the range from $∼ 600$ to $870$ $cm^-$$^1$ is dominated by bending ($F_2$$_b$) motions related to bridging oxygen and phosphorus ($∼ 800$ $cm^-$$^1$ band) atoms, while the high-frequency doublet zone ($∼ 870 – 1250$ $cm^-$$^1$ is associated mostly with the asymmetric (($F_2$$_s$) and symmetric ($A_1$) stretching modes, and most prominent peak around $1400$ $cm^-$$^1$ (exp. $1380$ $cm^-$$^1$) is mainly due to asymmetric stretching vibrations supported by double-bonded oxygen atoms. The lower-frequency range below $600$ $cm^-$$^1$ is shown to arise from a mixture of bending ($E$ and ($F_2$$_b$) and rotation ($F_1$) modes. The scissors bending ($E$) and rotation ($F_1$) modes are well localized below $600$ $cm^-$$^1$, whereas the ($F_2$$_b$ bending modes spread further into the range $∼ 600 – 870$ $cm^-$$^1$. The projections of the eigenmodes onto $Q^2$, $Q^3$, and $Q^4$ species yield well-defined contributions at frequencies in striking correspondence with the positions of the Raman and infrared bands

Nanofabrication with Pulsed Lasers

An overview of pulsed laser-assisted methods for nanofabrication, which are currently developed in our Institute (LP3), is presented. The methods compass a variety of possibilities for material nanostructuring offered by laser–matter interactions and imply either the nanostructuring of the laser-illuminated surface itself, as in cases of direct laser ablation or laser plasma-assisted treatment of semiconductors to form light-absorbing and light-emitting nano-architectures, as well as periodic nanoarrays, or laser-assisted production of nanoclusters and their controlled growth in gaseous or liquid medium to form nanostructured films or colloidal nanoparticles. Nanomaterials synthesized by laser-assisted methods have a variety of unique properties, not reproducible by any other route, and are of importance for photovoltaics, optoelectronics, biological sensing, imaging and therapeutics

Mechanisms of nanoparticle formation by ultra-short laser ablation of metals in liquid environment

International audienceLaser ablation in liquids is now commonly used to produce colloidal nanoparticles (NPs) that have found numerous applications in different areas. In the experiments, NPs of different materials can be rather easily produced by using laser systems with various pulse durations, shape, wavelengths, and fluence. Here, we focus our attention on metal (gold) nanoparticles produced by ultra-short laser pulses due to their unique plasmonic properties. To better understand the mechanisms of the NPs formation, we perform modeling of ultra-short laser interactions with gold target in the presence of a liquid (water). The model is similar to that presented in Ref. [1]. In the model, we vary laser fluence. Simulation results show that for smaller laser fluence, gold target is heated to moderate temperatures, so that metastable melted layer is then fragmented into particles (Fig. 1a). In addition, a shock wave, a void/bubble is formed in front of the target [2], so that the particles are ejected into the bubble. They rapidly reach the bubble's front, and all the ablated material can be found near the border of the bubble by ~ 3-4 ns. For larger laser fluences (Fig. 1b), meta-stable liquid decomposes in the vicinity of the critical point forming liquid-gas mixture. The enhanced ablation creates much higher pressure. The bubble is also formed but is rapidly stopped by the hot liquid-gas phase (at ~3 ns after the pulse). At that time, the ablated material is composed of two zone, liquid-gas and liquid, which are then compressed and pushed back. The bubble collapses as soon as at ~7.5 ns due to strong backpressure from the liquid. As a result of the interaction with the ablated material, the target is re-heated and is melted down to as deep as ~0.5 µm under initial target's surface. Thus, the obtained results demonstrate that in the presence of a liquid, the fragmentation of metastable material leads to the NPs ejection. At moderate fluences, liquid just limits material expansion. At larger fluences, liquid pushes the ablated material back enhancing target's temperature, so that more NPs can later originate from this region

Ultra-short double pulse laser ablation of metals: what can we know from numerical hydrodynamics and from molecular dynamics simulations?

International audienceThe past decade has witnessed a considerable development in the field of ultra-short, or femtosecond laser systems and of their applications. At the same time, many theoretical and numerical investigations have been proposed underlying the main physical processes involved the interactions of these laser pulses with various materials. In particular, classical two-temperature one fluid hydrodynamics (Hydro-TTM) was used to account for the target material motion and for the phase transitions.1,2 This model allowed us to elucidate the role of fragmentation of the metastable liquid phase as a result of fast heating and shock wave propagation inside metal targets. Hydrodynamics, however, does not describe non-equilibrium state of matter. To better explain such effects, two-temperature molecular dynamics simulations (MD-TTM) were furthermore performed to provide even more detailed insights into the laser ablation mechanisms, such as phase explosion, fragmentation, evaporation, and mechanical spallation. Nevertheless, many effects, such as a curious effect of ablation suppression observed for metals in the case of double laser pulses, required more investigations.A careful application of our models to the case of double pulse laser ablation required a self-consistent simulation of laser energy propagation and absorption, in particular by plasma formed by the 1st pulse. As a result of the performed MD-TTM calculations, two main mechanisms could be identified to be responsible for the suppression of ablation in the double pulse ablation experiments.3 The first one is associated with the suppression of the rarefaction wave,2 which leads to homogeneous nucleation in the liquid layer of the target under tensile stress. For the delays longer than 50 ps, the second pulse generates a novel high-pressured plasma region ahead of the ablated liquid layers. This region pushes a large fraction of the ablated layers back to the target. In this case, the ablation depth can be even smaller than the one in the case of a single pulse regime. These findings bring more light into the recent double-pulse LIBS experiments.References: 1 M.E. Povarnitsyn, T.E. Itina, M. Sentis, K.V. Khishchenko, P.R. Levashov, Phys. Rev. B, 75(23), (2007) 235414.2 M.E. Povarnitsyn, T.E. Itina, K.V. Khishchenko, P.R. Levashov, Phys. Rev. Lett., 103(19), (2009) 195002.3 M.E. Povarnitsyn, V. Fokin, P.R. Levashov, T.E. Itina, Phys. Rev. B, 92(17), (2015) 174104

Hydrodynamic modeling of femtosecond laser ablation of metallic targets in vacuum and in liquid

International audienceWe numerically examine the mechanisms involved in nanoparticle formation by laser ablation of metallic targets in vacuum and in liquid. We consider the very early ablation stage providing initial conditions for much longer plume expansion processes. In the case of ultrashort laser ablation, the initial population of primary nanoparticles is formed at this stage. When a liquid is present, the dynamics of the laser plume expansion differs from that in vacuum. Low compressibility of the ambient liquid results in strong confinement conditions. As a result, ablation threshold rises drastically, the ablated material is compressed, part of it becomes supersaturated and the backscattered material additionally heats the target. The extension of a molten layer leads to the additional ablation at a later stage also favoring nanoparticle formation. The obtained results thus explain recent experimental findings and help to predict the role of the experimental parameters. The performed analysis indicates ways of a control over nanoparticle synthesis

Numerical Analysis of Ultrashort Laser Ablation: Application for Fabrication of Nanoparticles and Nanostructures

International audienceIn this study, first we examine nanoparticle formation by femtosecond laser ablation under different experimental conditions. The dynamics of the laser plume expansion is investigated and the possibility of primary nanoparticle formation is analyzed. Then, we consider thermalization process in a background environment, diffusion-driven nucleation and longer scale nanoparticle aggregation/sintering. In addition, laser-assisted fragmentation of nanoparticles is examined, which can play a particular role in the multi-pulse regime. In this later case, nanoparticle size distribution results from an ensemble of processes thus revealing different particle populations. Calculations are performed for metals under different background conditions. The calculation results explain recent experimental findings and help to predict the role of the experimental parameters. The obtained nanoparticles are also used to form nanostructures. The performed analysis thus indicates ways of a control over the involved laser-assisted techniques