L. Hallo

Bioprinting by laser-induced forward transfer for tissue engineering applications: jet formation modeling

In this paper, a nanosecond LIFT process is analyzed both from experimental and modeling points of view. Experimental results are first presented and compared to simple estimates obtained from physical analysis, i.e. energy balance, jump relations and analytical pocket dynamics. Then a self-consistent 2D axisymmetric modeling strategy is presented. It is shown that data accessible from experiments, i.e. jet diameter and velocity, can be reproduced. Moreover, some specific mechanisms involved in the rear-surface deformation and jet formation may be described by some scales of hydrodynamic process, i.e. shock waves propagation and expansion waves, as a consequence of the laser heating. It shows that the LIFT process is essentially driven by hydrodynamics and thermal transfer, and that a coupled approach including self-consistent laser energy deposition, heating by thermal conduction and specific models for matter is required.

Experimental observations and modeling of nanoparticle formation in laser-produced expanding plasma

Interaction of a laser beam with a target may generate a high velocity expanding plasma plume, solid debris, and liquid nano- and microparticles. They can be produced from plasma recombination, vapor condensation or by a direct expulsion of the heated liquid phase. Two distinct sizes of particles are observed depending on the temperature achieved in the plasma plume: Micrometer-size fragments for temperatures lower than the critical temperature, and nanometer-size particles for higher temperatures. The paper presents experimental observations of fragments and nanoparticles in plasma plumes created from gold targets. These results are compared with theoretical models of vapor condensation and microparticle formation.

Self-consistent modeling of jet formation process in the nanosecond laser pulse regime

Laser induced forward transfer (LIFT) is a direct printing technique. Because of its high application potential, interest continues to increase. LIFT is routinely used in printing, spray generation and thermal-spike sputtering. Biological material such as cells and proteins have already been transferred successfully for the creation of biological microarrays. Recently, modeling has been used to explain parts of the ejection transfer process. No global modeling strategy is currently available. In this paper, a hydrodynamic code is utilized to model the jet formation process and estimate the constraints obeyed by the bioelements during the transfer. A self-consistent model that includes laser energy absorption, plasma formation via ablation, and hydrodynamic processes is proposed and confirmed with experimental results. Fundamental physical mechanisms via one-dimensional modeling are presented. Two-dimensional (2D) simplified solutions of the jet formation model equations are proposed. Predicted results of the model are jet existence and its velocity. The 2D simulation results are in good agreement with a simple model presented by a previous investigator.

Surface structuring by ultrashort laser pulses: A review of photoionization models

Photoionization models have been introduced and compared in a three-dimensional code utilizing electromagnetic wave propagation in dielectric materials such as fused silica. Ionization rates are initially compared and matched to linear and circular polarizations. Then, they are implemented in the propagation code followed by some hydrodynamic simulations. Results on the structural and optical modifications have been compared with available experimental data. Experimental damage, ablation threshold, and a theoretical damage threshold criteria have been utilized to discriminate between the different models.

Formation of nanocavities in dielectrics: A self-consistent modeling

Tight focusing of a subpicosecond laser pulse in transparent dielectrics is an efficient way to release laser energy and to produce plasma. A micro-explosion results in a submicrometer cavity formation if the deposited laser energy exceeds a threshold. A self-consistent model is developed that describes this process. The energy deposition is described by a full set of Maxwell’s equations in the three-dimensional geometry and it accounts for nonlinear propagation phenomena in the femtosecond time scale. The calculated energy deposition is transferred to a hydrodynamic code that describes the cavity formation. Numerical simulations show that cavity size in silica depends strongly on the latent heat of sublimation. An equation of state is developed and introduced into the hydrodynamic model that takes into account the influence of such material parameters as the binding energy, the bulk modulus, and the Gruneisen coefficient. The cavity and shock-affected region sizes are compared to experimental data. This ...

A KDP equation of state for laser-induced damage applications

High power lasers such as NIF in the USA or LMJ in France are being developed for inertial confinement fusion applications. However, the performance of the optics is limited by laser-induced damage (LID), which occurs, for instance, in the potassium dihydrogen phosphate (KH2PO4 or KDP) crystals utilized for frequency conversion. An accurate equation of state (EOS) is required to explain the LID process and to predict damage size. For the design of such EOS, a pulsed electron beam was used to generate a quasi-plane stress wave of 0.7 GPa in KDP. The sample response was deduced from photonic Doppler velocimetry. Equations of state and deviatoric stress components are designed and compared to experimental data. They are used in laser-induced bulk damage simulations, showing that strength may play a significant role.

Toward a New Laser Induced Hydrodynamic Forward Transfer Process with Femtosecond Lasers

Theoretical approaches to the photoionization of few-electron atoms are discussed. These include nonequilibrium Greens functions and wave function based approaches. In particular, the Multiconfiguration Time-Dependent Hartree-Fock method is discussed and applied to model a one-dimensional atom with four electrons. We compute ground state energies and the time-dependent photoionization by the field a strong laser pulse with two different frequencies in the ultraviolet (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser-Matter Interaction in Transparent Materials: Confined Micro-explosion and Jet Formation

High intensity laser beam was tightly focussed inside bulk of dielectrics at adjustable distance from the outer boundary (1-15 µm). Laser- matter interaction region is thus confined inside a cold and dense material, with and without boundary effects. In what follows we first describe self-consistently the relevant laser-matter interaction physics. At high intensity of the laser beam in a focal region (> 6 × 10 12 W/cm 2 ) the material is converted into a hot and dense plasma. The shock and rarefaction waves propagation, formation of a void inside the target are all described. Then, a model was developed to predict size of the voids in the bulk of materials, i.e. without boundary effects. Results were compared to experimental observations. The size of a void formed by 800 nm 150 fs laser pulses is ~0.2 µm 3 . Finally we present new results in confined geometries and we show that jets can develop sizes and expansion velocities depending both on energy laser and distance from the rear surface. This jet formation regime, apparently new, can be related to some LIFT process, with submicrometer diameter jets.

Linear and non-linear amplification of high-mode perturbations at the ablation front in HiPER targets

The linear and non-linear sensitivity of the 180 kJ baseline HiPER target to high-mode perturbations, i.e. surface roughness, is addressed using two-dimensional simulations and a complementary analysis by linear and non-linear ablative Rayleigh–Taylor models. Simulations provide an assessment of an early non-linear stage leading to a significant deformation of the ablation surface for modes of maximum linear growth factor. A design using a picket prepulse evidences an improvement in the target stability inducing a delay of the non-linear behavior. Perturbation evolution and shape, evidenced by simulations of the non-linear stage, are analyzed with existing self-consistent non-linear theory.

Stability study of planar targets using standard and adiabat shaping pulses

The hydrodynamic stability of a planar target is considered for the conditions of the direct drive inertial confinement fusion. It has been recently proposed to reduce the ablative Rayleigh-Taylor instability growth by using the adiabat shaping in the ablation zone. In this work, we consider the relaxation adiabat shaping scheme [K. Anderson and R. Betti, Phys. Plasmas 11, 5 (2004); R. Betti, K. Anderson, J. P. Knauer, T. J. B. Collins, R. L. McCrory, P. W. McKenty, and S. Skupsky, Phys. Plasmas 12, 042703 (2005)]. In this scheme, a prepulse (“picket”) is followed by a relaxation period, when the laser is turned off. A parametric study of picket parameters is performed with a code dedicated to the linear stability analysis on the basis of spherical realistic simulations including full physics. The influence of the picket parameters is investigated numerically. Simulations show that the set picket/relaxation time mainly determines the target stability and that the adiabat shaping scheme modifies the pertur...