Laser Energy Deposition Research Articles

Numerical Analysis of Ultrashort Pulse Laser-Induced Thermomechanical Response of Germanium Thin Films

Ultrashort pulse laser heating of a germanium nanofilm was simulated using a combined continuum–atomistic method that couples the molecular dynamics and a self-consistent energy model for ultrafast laser–semiconductor interaction. Both a single pulse and a pulse burst were considered. To accurately describe laser energy deposition, the transient optical properties were computed based on the Drude formula. It was found that for a single pulse at low fluence (e.g., 0.02 J/cm2), the pulse duration had little impact on the lattice temperature response. In contrast, a higher lattice temperature could be obtained for a longer pulse (e.g., 5 ps) at higher fluences (e.g., 0.06 J/cm2) due to lower surface reflectivity. A strong thermal stress wave could be induced by the laser heating, with its maximum compression and tension occurring in the front and rear film regions, respectively. The investigations of laser burst heating revealed that a laser burst not only can retain the advantages of ultrashort pulse lasers but can enhance the photon efficiency for material melting as well.

Control of Mach Reflection by a Single Laser Pulse

In the supersonic engine inlet, Mach Reflection probably appears when a supersonic flow goes through the symmetric wedges, causing a great total pressure loss. A single pulsed laser energy deposition leads a decrease of the Mach stem height and reduces the total pressure loss. By solving the two-dimensional RANS equations, with the condition of symmetric wedges at 22 degrees, and the free stream Mach number 3.45, influences of the deposition location and the magnitude of pulsed laser energy in Mach Reflection are investigated. The results indicate that when laser energy rises from 70mJ to 270mJ, the height of Mach stem changes and a vale value is obtained. The deposition position is also optimized.

Photothermal bleaching in time‐lapse photoacoustic microscopy

We studied the phenomenon of photothermal bleaching - a gradual reduction of contrast agent particles during repeated scans in photoacoustic microscopy. The dependence of the photothermal bleaching rate on the excitation pulse energy, pulse duration, and the absorber's size was determined while the laser focal diameter was held constant. Our results showed that, the dependence of the photothermal bleaching rate on the excitation pulse energy differed before and after the absorbers were raised to their melting point by the deposited laser energy. Based on this finding, we suggested an optimal excitation pulse energy, which balances the photothermal bleaching rate and signal amplitude, for time-lapse imaging applications. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

A study of the surface erosion of zircaloy material during laser ablation process by thin layer activation technique

Abstract The thin layer activation (TLA) technique was applied to study the surface erosion of zircaloy clad material during laser ablation process by producing a suitable γ-ray emitting radioisotope in the clad surface using 40 MeV α-particles. This paper reports the rate of surface erosion of zircaloy as a function of deposited laser energy in the range of a few hundred of millijoules to a few kilojoule. The minimum detection limit was found to be 80 ± 25 nm in the measurement of surface loss of material due to laser exposure by TLA technique.

A calibration and data assimilation method using the Bayesian MARS emulator

We present a method for calibrating the uncertain inputs to a computer model using available experimental data. The goal of the procedure is to estimate the posterior distribution of the uncertain inputs such that when samples from the posterior are used as inputs to future model runs, the model is more likely to replicate (or predict) the experimental response. The calibration is performed by sampling the space of the uncertain inputs, using the computer model (or, more likely, an emulator for the computer model) to assign weights to the samples, and applying the weights to produce the posterior distributions and generate predictions of new experiments with confidence bounds. The method is similar to Metropolis–Hastings calibration methods with independently sampled updates, except that we generate samples beforehand and replace the candidate acceptance routine with a weighting scheme.We apply our method to the calibration of a Hyades 2D model of laser energy deposition in beryllium. We employ a Bayesian Multivariate Adaptive Regression Splines (BMARS) emulator as a surrogate for Hyades 2D. We treat a range of uncertainties in our application, including uncertainties in the experimental inputs, experimental measurement error, and systematic experimental timing errors. The resulting posterior distributions agree with our existing intuition, and we validate the results by performing a series of leave-one-out predictions. We find that the calibrated predictions are considerably more accurate and less uncertain than blind sampling of the forward model alone.

Non-linear absorption of focused femtosecond laser pulses at 1.3 μm inside silicon: Independence on doping concentration

We investigate experimentally local non-linear absorption when 1.3μm wavelength femtosecond pulses are tightly focused inside n-type doped silicon. We show that 130fs pulses with only few nanojoule energy is enough to initiate free-carrier generation. Our results also demonstrate that the laser energy deposition is independent on the doping concentration for substrates with free-carrier densities up to 1018cm−3. For deep focusing experiments, the energy deposition can remain confined in micron-scale focal regions provided we perform the experiments with focused beam corrected for spherical aberration. The high degree of control observed in the experiments and the independence on doping are major assets for future 3D-micromachining technology developments based on this approach.

Spatio-temporal evolution of laser-induced air plasma in the stage of laser pulse action

Spatially and temporally resolved emission spectra of laser-induced air plasma in the stage of laser pulse action were studied. Due to the expansion of laser supported detonation wave and shielding effect at the critical surface for CO2 laser radiation, a behavior of spatial separation of the radiative plasma along the radial direction was clearly recognized. Based on the Stark broadening, we investigated the spatio-temporal evolution of the electron density and temperature of the plasma, which was evaluated by fitting the measured spectral profile to the summation of Voigt profiles of all spectral lines in a selected spectral range. The electron density was distributed densely around the focal point and thinly near the plasma edge at early times. On the contrary, the electron temperature around the focal point was a minimum at early times but became a maximum at later times. Unlike the spatio-temporal resolution results of post-pulse in previous work, the results in the present work revealed the information related to the processes of laser energy deposition.

Study on the effects of ion motion on laser-induced plasma wakes

A 2D analytical model is presented for the generation of plasma wakes (or bubbles) with an ultra-intense laser pulse by taking into account the response of plasma ions. It is shown that the effect of ion motion becomes significant at the laser intensity exceeding 1021 W/cm2 and plasma background density below 1019 cm−3. In this regime, ion motion tends to suppress the electrostatic field induced by charge separation and makes the electron acceleration less effective. As a result, the assumption of immobile ions overestimates the efficiency of laser wake-field acceleration of electrons. Based on the analytical model, the dynamics of plasma ions in laser-induced wake field is investigated. It is found that only one bubble appears as the plasmas background density exceeds the resonant density and the deposited laser energy is concentrated into the bubble, resulting in the generation of an ion bunch with extremely high energy density.

Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.

Numerical Analysis of Interaction Between Single-Pulse Laser-Induced Plasma and Bow Shock in a Supersonic Flow

The interaction of laser-induced plasma and bow shock over a blunt body is investigated numerically in an M∞ = 6.5 supersonic flow. A ray-tracing method is used for simulating the process of laser focusing. The gas located at the focused zone is ionized and broken down and transformed into plasma. In a supersonic flow the plasma moves downstream and begins to interact with the bow shock when it approaches the surface of the blunt body. The parameters of flowfield and blunt body surface are changed due to the interaction. By analyzing phenomena occurring in the complex unsteady flowfield during the interaction in detail, we can better understand the change of pressure on the blunt body surface and the mechanism of drag reduction by laser energy deposition. The results show that the bow shock is changed into an oblique shock due to the interaction of the laser-induced low-density zone with the bow shock, so the wave drag of the blunt body is reduced.

Effects of laser plasma on damage in optical glass induced by pulsed lasers

Laser-induced plasma can expedite the deposition of incident laser energy and the laser-induced damage in optical glass is considerably affected by the magnitude and distribution of the plasma shock wave. The spatial distribution of energy deposition and expansion pressure of the laser plasma shock wave is analyzed based on the moving breakdown model. Furthermore, damage morphologies are discussed in light of the spatial distribution of pressure and glass properties. It was found that with the increase of laser pulse energy, the shock wave expands rapidly in the direction opposite to the incident laser, resulting in that the damage morphologies transform from sphere to spindle gradually. The laser energy deposits mostly in a narrow plasma channel. The diffusion of the plasma with high temperature and pressure leads to the shock wave; the intensity of which decreases sharply with the axial distance from the centerline. As a consequence, the glass near the centerline fractures and melts, and the refractive index also changes near the end of cracks.

Direct drive implosion experiments on SGIII prototype laser facility: Assessing energy coupling efficiency and implosion symmetry

Direct drive implosion experiments were conducted on SGIII prototype laser facility. From the time resolved x-ray images, the bright ring and the central bright spot are observed. The radial velocity of the convergent bright ring indicates the shell velocity, and the times when the central bright spot is first seen and becomes most intensive indicate the times of shock convergence and later stagnation, respectively. Radiation hydrodynamic simulations were carried out by changing laser energy deposition factors. When the simulated results are brought close to the measured ones, it is found that the energy coupling efficiency is around 70%. The implosion symmetry is indicated by the core x-ray emission pattern which is pancake when viewing from the equator, and splits into several bright spots when viewing close to the pole. A simple model is developed to understand this asymmetry. It is speculated that the observed implosion asymmetry can be attributed to the laser arrangement which is originally designed for indirect drive experiments. Further improvements of energy coupling efficiency and implosion symmetry in future experiments can be achieved by optimizing target design and laser arrangement.

A Review of Control Methods for Cavity Flows and Feasibility of Laser Energy Deposition as an Actuator

Studies related to supersonic cavity flow are very important for aeronautics applications. Understanding the physics of cavity flows is crucial for both fluid mechanics and engineering applications. Supersonic cavity flow and control techniques for this type of flow have been studied for many decades. A review of literature on numerical and experimental cavity flow is provided and the flow mechanism inside the cavity is explained using example studies from literature. It is necessary to understand the complex nature of the flow before developing control methods for the cavity flow oscillations. The literature on the development of methods to control the flow oscillations in cavities which are harmful for aircraft is also reviewed. Feasibility of laser energy deposition as an actuator for flow control is investigated demonstrating examples from literature.

Parametric Study of Laser Energy Deposition in Mach 8 Bow Shock

A numerical study was performed to investigate the effect of laser energy deposition in a Mach 8 bow shock. The computational model was extracted from the experiment in which a nominal two-dimensional Mach 8 flow over a cylinder with diameter of 76 mm was studied. The surface pressure was in good agreement with experiments, and the standoff distance agreed well with the theoretical value. The process of blast wave propagation and bow shock distortion was analyzed for energy deposition. The effect of energy level, deposition location and pulsing frequency was studied. For a single pulse of energy deposition, results showed that the reduction of the time-averaged surface pressure increases beyond certain energy level, while the deposition location had less effect on the time-averaged pressure. For a continuously pulsed deposition, the surface pressure in the stagnation region was reduced with increase of frequency. A 33% pressure reduction was achieved with a frequency of 100 kHz. The envelop of the blast w...

Molecular Dynamics Simulation of the Ablation Process in Ultrashort Pulsed Laser Machining of Polycrystalline Diamond

The mechanism of ultrashort pulsed laser ablation of polycrystalline diamond (PCD) is investigated using molecular dynamics simulation. The simulation model provides a detailed atomic-level description of the laser energy deposition to PCD specimens and is verified by an experiment using 300 fs laser irradiation of a PCD sample. It is found that grain boundaries play an important role in the laser ablation. Melting starts from the grain boundaries since the atoms in these regions have higher potential energy and are melted more easily than the perfect diamond. Non-homogeneous melting then takes place at these places, and the inner crystal grains melt more easily in liquid surroundings presented by the melting grain boundaries. Moreover, the interplay of the two processes, photomechanical spallation and evaporation, are found to account for material removal in ultrashort pulsed laser ablation of PCD.

Simulating radiative shocks in nozzle shock tubes

We use the recently developed Center for Radiative Shock Hydrodynamics (CRASH) code to numerically simulate laser-driven radiative shock experiments. These shocks are launched by an ablated beryllium disk and are driven down xenon-filled plastic tubes. The simulations are initialized by the two-dimensional version of the Lagrangian Hyades code which is used to evaluate the laser energy deposition during the first 1.1 ns. Later times are calculated with the CRASH code. CRASH solves for the multi-material hydrodynamics with separate electron and ion temperatures on an Eulerian block-adaptive-mesh and includes a multi-group flux-limited radiation diffusion and electron thermal heat conduction. The goal of the present paper is to demonstrate the capability to simulate radiative shocks of essentially three-dimensional experimental configurations, such as circular and elliptical nozzles. We show that the compound shock structure of the primary and wall shock is captured and verify that the shock properties are consistent with order-of-magnitude estimates. The synthetic radiographs produced can be used for comparison with future nozzle experiments at high-energy-density laser facilities.

Modeling of ultrafast phase changes in metal films induced by an ultrashort laser pulse using a semi-classical two-temperature model

A semi-classical two-temperature (2T) model, together with the extended Drude model for temperature-dependent optical properties, is proposed to describe ultrafast thermal transport and phase changes in gold films irradiated by an ultrashort laser pulse. This model reduces to a classical 2T model when the electron momentum equation and the two terms related to electron drifting velocity in the electron energy equation are dropped off. It is found that the significant changes in reflectivity and absorption coefficient due to rapid temperature rise could drastically alter laser energy deposition and in turn, the thermal response in metal films. It is also found that the semi-classical 2T model could predict lower electron and lattice temperature and less severe solid–liquid and liquid–vapor phase change than the classical 2T model. The difference could be more distinct with increase of laser fluence and/or increase of laser pulse duration.

Optical properties and thermal response of copper films induced by ultrashort-pulsed lasers

A critical point model with three Lorentzian terms for interband transition was proposed to describe temperature-dependent reflectivity (R) and absorption coefficient (α) for copper irradiated by ultrashort-pulsed lasers of wavelength 200–1000 nm. After validated with experimental data at room temperature, it was incorporated into a two-temperature model to study ultrafast laser-material interactions. The dynamic changes of optical properties R and α, distributions of laser heat density, electron and lattice temperature, and phase changes of a copper film were investigated. Comparing with the experimental data of average absorption showed that the proposed two-temperature model together with the critical point model can simulate satisfying results for temperature-dependent R and α. The drastic changes in R and α could alter laser energy deposition in a heated target, leading to different thermal responses than those predicted with constant R and α at room temperature.

Non-linear absorption of 1.3-μm wavelength femtosecond laser pulses focused inside semiconductors: Finite difference time domain-two temperature model combined computational study

We present a theoretical model, which describes local energy deposition inside IR-transparent silicon and gallium arsenide with focused 1.3-μm wavelength femtosecond laser pulses. Our work relies on the ionization rate equation and two temperature model (TTM), as we simulate the non-linear propagation of focused femtosecond light pulses by using a 3D finite difference time domain method. We find a strong absorption dependence on the initial free electron density (doping concentration) that evidences the role of avalanche ionization. Despite an influence of Kerr-type self-focusing at intensity required for non-linear absorption, we show the laser energy deposition remains confined when the focus position is moved down to 1-mm below the surface. Our simulation results are in agreement with the degree of control observed in a simple model experiment.

MULTI-fs – A computer code for laser–plasma interaction in the femtosecond regime

The code MULTI-fs is a numerical tool devoted to the study of the interaction of ultrashort sub-picosecond laser pulses with matter in the intensity range from 10 11 to 10 17 W cm −2. Hydrodynamics is solved in one-dimensional geometry together with laser energy deposition and transport by thermal conduction and radiation. In contrast to long nanosecond pulses, short pulses generate steep gradient plasmas with typical scale lengths in the order of the laser wavelength and smaller. Under these conditions, Maxwellʼs equations are solved explicitly to obtain the light field. Concerning laser absorption, two different models for the electron–ion collision frequency are implemented to cover the regime of warm dense matter between high-temperature plasma and solid matter and also interaction with short-wave-length (VUV) light. MULTI-fs code is based on the MULTI radiation-hydrodynamic code [R. Ramis, R. Schmalz, J. Meyer-ter-Vehn, Comp. Phys. Comm. 49 (1988) 475] and most of the original features for the treatment of radiation are maintained. Program summary Program title: MULTI-fs Catalogue identifier: AEKT_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKT_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 49 598 No. of bytes in distributed program, including test data, etc.: 443 771 Distribution format: tar.gz Programming language: FORTRAN Computer: PC (32 bits and 64 bits architecture) Operating system: Linux/Unix RAM: 1.6 MiB Classification: 19.13, 21.2 Subprograms used: Cat Id: AECV_v1_0; Title: MULTI2D; Reference: CPC 180 (2009) 977 Nature of problem: One-dimensional interaction of intense ultrashort (sub-picosecond) and ultraintense (up to 10 17 W cm −2) laser beams with matter. Solution method: The hydrodynamic motion coupled to laser propagation and several transport mechanisms is solved in one-dimensional geometry using a fractional step scheme. Fluid motion together with heat diffusion is solved by using an implicit Lagrangian method. Transport by thermal conduction and radiation as well as electron–ion energy transfer are treated in a two-temperature (electron and ion) model covering the wide range from solid state to high temperature plasma. Laser propagation is calculated from the one-dimensional Maxwell equations. Radiation transfer is solved by using the forward-reverse method for a discrete number of frequency groups. Matter properties are interpolated from tables (equations-of-state, ionization, opacities, and emissivities) generated by external codes. An alternative WKB laser deposition package is available to be used for long pulse lasers. Restrictions: The code has been designed for typical conditions prevailing in short pulse (fs–ps time scale) laser–matter interactions at moderate intensities (10 12–10 17 W cm −2). Although a wider range of situations can be treated, extrapolations to regions beyond this design range need special care. Additional comments: A graphical post processor is included in the package. Its use requires the previous installation of code MULTI2D (see “Subprograms used” above). Running time: 4.8 seconds for the example supplied.