Laser-solid Interactions Research Articles

Measurements of extended magnetic fields in laser-solid interaction

Magnetic fields generated from a laser-foil interaction are measured with high fidelity using a proton radiography scheme with x-ray fiducials. In contrast to prior findings under similar experimental conditions, this technique reveals the self-generated, Biermann-battery fields extend beyond the edge of the expanding plasma plume to a radius of over 3.5 mm by t=+1.4 ns. An analysis of two monoenergetic proton populations confirms that proton deflection is dominated by magnetic fields far from the interaction (>2 mm) and electric fields are insignificant. The results are not captured in state-of-the-art magnetohydrodynamics simulations and suggest the need to consider additional physics mechanisms for the magnetic field generation and transport in laser-solid interactions. Published by the American Physical Society 2024

Scaling of the emission of low frequency radiation by transient surface currents in laser–solid interactions

Abstract It has been suggested that ‘transient surface currents’ caused by multi-MeV fast electrons can be responsible for the emission of low frequency radiation (e.g. in THz range) from ultra-intense laser–solid interactions. This mechanism has been analyzed, and analytic upper bounds on the intensity, electric field amplitude, and normalized vector potential have been developed and tested against 1D EM Particle-in-Cell simulations. The ‘transient surface current’ mechanism is effective and sufficiently efficient to fully account for all radiation that has been emitted in experiments so far.

Dynamic convergent shock compression initiated by return current in high-intensity laser–solid interactions

We investigate the dynamics of convergent shock compression in solid cylindrical targets irradiated by an ultrafast relativistic laser pulse. Our particle-in-cell simulations and coupled hydrodynamic simulations reveal that the compression process is initiated by both magnetic pressure and surface ablation associated with a strong transient surface return current with density of the order of 1017 A/m2 and lifetime of 100 fs. The results show that the dominant compression mechanism is governed by the plasma β, i.e., the ratio of thermal pressure to magnetic pressure. For targets with small radius and low atomic number Z, the magnetic pressure is the dominant shock compression mechanism. According to a scaling law, as the target radius and Z increase, the surface ablation pressure becomes the main mechanism generating convergent shocks. Furthermore, an indirect experimental indication of shocked hydrogen compression is provided by optical shadowgraphy measurements of the evolution of the plasma expansion diameter. The results presented here provide a novel basis for the generation of extremely high pressures exceeding Gbar (100 TPa) to enable the investigation of high-pressure physics using femtosecond J-level laser pulses, offering an alternative to nanosecond kJ-laser pulse-driven and pulsed power Z-pinch compression methods.

Geant4 simulation of secondary photons generated by laser–solid interaction in the presence of external magnetic field

A parametric study of the average energy and scattering angle distribution of Bremsstrahlung x-ray photons generated in over-dense fuel were investigated using the Geant4 simulation toolkit in fast ignition concept. In our simulation, over-dense fuel with the mass ρc∼(300−1000)g.cm−3, and fast ignitor with the intensity I=(1021,1023)W.cm−2 and wavelength λ=(0.35,0.53)μm were considered in the presence of an external magnetic field with the strength Bext=(0−10)kT. Further, for the produced MeV electrons in pre-plasma, two types of energy distribution functions, i.e., exponential and quasi two-temperature, were considered. The simulation results show that by applying an external magnetic field strength Bext∼(1−2)kT, a smooth increase was observed in the x-ray photon average energy and a considerable increase in scattering angle and photon counts in over-dense plasma obtained, so that the peak values mostly obtained for Bext∼1kT. Meanwhile, between the two energy distribution functions for electrons, the optimal average energy and scattering angle values were obtained by considering the quasi-two-temperature energy distribution function and for the fast igniter intensity and wavelength, I=1021W.cm−2,λ=0.35μm respectively, and the fuel density ρc∼300g.cm−3. Finally, regarding to the obtained optimal average photon energy (i.e., Ebph<0.4MeV), which could be considered as minimum photon energy loss and maximum electron penetration toward the fuel, the minimum thickness of lead and concrete shields required to reduce the flux of photons to its one-tenth value were found to be 1.5 cm and 30.7 cm, respectively.

Review and meta-analysis of electron temperatures from high-intensity laser–solid interactions

The accelerated electron spectrum from high-intensity laser–solid interaction is often conveniently described using a Boltzmann distribution, whose temperature is known within the field as the hot-electron temperature. The importance of the electron temperature is highlighted by the sheer number of experimental and simulation studies on the subject over the past three decades. Recently, multi-kJ, multi-ps pulses have yielded electron spectra with temperatures far beyond the expected ponderomotive result. Expressions that predict the electron temperature considering laser parameters beyond intensity and wavelength have been developed, albeit using small datasets. In this review, we present what is, to the best of our knowledge, the largest dataset of electron temperatures gathered from experimental measurements and particle-in-cell simulations. This dataset allows us to compare existing analytical and empirical hot-electron temperature scaling models over a wide parameter range. We also develop new scaling models that incorporate the laser pulse duration of the laser and the plasma scale length. Three models that include pulse-duration and scale length dependence are especially successful at predicting both simulated and experimental data. The dataset will soon be made publicly available to encourage further investigation.

High harmonic generation in monolayer indium nitride

In our work, we theoretically investigate high harmonic generation (HHG) in monolayer hexagonal indium nitride (h-InN) based on the semiconductor Bloch equation under strong laser fields. Compared with h-BN, there is no multiplateau in h-InN. This is because the intraband mechanism dominates the total HHG, and the harmonic generated by the intraband current is about three orders of magnitude higher than that generated by the interband polarization. We find that the higher order part of the HHG is mainly supplied by the interband current, which can be analyzed on the basis of the transition dipole moments between the energy bands. In addition, we found that the HHG of h-InN is sensitive to the external strains due to the modified band dispersion in the electronic structures. This study provides a useful reference for understanding the microscopic mechanism of laser-solid interaction.

From linear to nonlinear Breit-Wheeler pair production in laser-solid interactions.

During the ultraintense laser interaction with solids (overdense plasmas), the competition between two possible quantum electrodynamics (QED) mechanisms responsible for e^{±} pair production, i.e., linear and nonlinear Breit-Wheeler (BW) processes, remains to be studied. Here, we have implemented the linear BW process via a Monte Carlo algorithm into the QED particle-in-cell (PIC) code yunic, enabling us to self-consistently investigate both pair production mechanisms in the plasma environment. By a series of two-dimensional QED-PIC simulations, the transition from the linear to the nonlinear BW process is observed with the increase of laser intensities in the typical configuration of a linearly polarized laser interaction with solid targets. A critical normalized laser amplitude about a_{0}∼400-500 is found under a large range of preplasma scale lengths, below which the linear BW process dominates over the nonlinear BW process. This work provides a practicable technique to model linear QED processes via integrated QED-PIC simulations. Moreover, it calls for more attention to be paid to linear BW pair production in near future 10-PW-class laser-solid interactions.

Spectral modulation of high-order harmonics in relativistic laser-solid interaction.

Spectral modulation of high-order harmonics generated in relativistic laser-solid interaction is investigated. Numerical simulations show that the modulation depends on surface plasma density profile, resulting in spectral envelope modulation and regular and irregular harmonic splitting. The mathematical and physical connections between the spectral modulation of high-order harmonics and the temporal modification of attosecond pulse train are explained. Based on these understandings, we propose a possible method to produce isolated attosecond pulses by tailoring surface the plasma profile.

Time-resolved optical shadowgraphy of solid hydrogen jets as a testbed to benchmark particle-in-cell simulations

Particle-in-cell (PIC) simulations are a widely-used tool to model kinetics-dominated plasmas in ultrarelativistic laser-solid interactions (dimensionless vectorpotential a0 > 1). However, interactions approaching subrelativistic laser intensities (a0 ≲ 1) are governed by correlated and collisional plasma physics, calling for benchmarks of available modeling capabilities and the establishment of standardized testbeds. Here, we propose such a testbed to experimentally benchmark PIC simulations of laser-solid interactions using a laser-irradiated micron-sized cryogenic hydrogen-jet target. Time-resolved optical shadowgraphy of the expanding plasma density, complemented by hydrodynamics and ray-tracing simulations, is used to determine the bulk-electron-temperature evolution after laser irradiation. We showcase our testbed by studying isochoric heating of solid hydrogen induced by laser pulses with a dimensionless vectorpotential of a0 ≈ 1. Our testbed reveals that the initial surface-density gradient of the target is decisive to reach quantitative agreement at 1 ps after the interaction, demonstrating its suitability to benchmark controlled parameter scans at subrelativistic laser intensities.

Efficient angular dispersed high-order harmonic generation by dual laser pulses interacting with a solid target

High-order harmonics emitted from relativistic laser–solid interaction can be used as a bright extreme ultraviolet source. However, such a source usually has mixed frequencies along the emission direction. Through theoretical and numerical studies, we propose a scheme to realize monochromatic harmonic emission along individual directions by using dual laser pulses with orthogonal polarizations, where a relativistic P-polarized laser is used as a pump laser in combination with a moderate S-polarized laser as a seed laser. We find that harmonics of the S-polarized laser can be efficiently generated with several orders of magnitude enhancement and spontaneous angular dispersion depending on the harmonic order. Our scheme may be beneficial to future applications requiring monochromatic radiation along a specific direction or multi-chromatic radiations in different directions.

Complex diagnostic and numerical study of x-ray and particle emissions under relativistic ultra-short laser-solid interaction

In this report, we present the experimental results on the generation of x-ray emission and particle acceleration using high temporal contrast (∼10–9 at picosecond time scale), ultrashort (), relativistic () near-IR laser pulses interacting with Titanium foils. Complex diagnostics, including the energy spectra of accelerated electrons from both front and rear target sides, electron angular distribution and x-ray spectroscopy of the plasma emission were employed. Analysis of the characteristic radiation produced by highly charged Ti+20 ions led to the conclusion that laser-plasma interaction, which leads to the generation of keV hot plasma, occurs at plasma densities 10x higher than relativistic critical electron density. Numerical simulations, including hydrodynamic calculations to model pre-plasma, generated on nanosecond-picosecond time scale under our experimental conditions and relativistic particle-in-cell simulations for main pulse interaction with the plasma reproduce well electron energy and angular distributions. They show the onset of hole boring effect under near normal incidence that leads to plasma density steepening and enables penetration of high intensity laser radiation into the overcritical plasma to much higher densities (up to 30 ), what is in a good agreement with the results of x-ray spectroscopy.

The femtosecond structure of extreme contrast, multi-terawatt second-harmonic laser pulses at 400 nm

Ultrahigh intensity contrast and short pulse laser–solid interactions offer an attractive platform for investigating high-energy-density matter, particularly in the context of structured and ultra-thin targets that form hot, dense plasma conditions. Harmonic generation can improve the contrast of laser pulses by several orders of magnitude. In this study, we present the characterization of extreme contrast, relativistic intensity second-harmonic pulses at 400 nm, using the self-diffraction frequency-resolved optical gating technique. The 400 nm pulses were generated at various input intensities using potassium dihydrogen phosphate and lithium triborate crystals. Our observations reveal the presence of spectral broadening, pulse compression, and complex structures at higher input intensities. We see that extreme contrast, few tens of femtosecond pulses can have multiple “prepulses” at the 100s femtosecond scale as large as ten percent of the peak value. These can preionize a solid significantly and may influence the interaction. Simulations based on nonlinear pulse propagation equations reinforce our findings.

Observation of proton modulations in laser–solid interaction

We report on an experimental investigation into proton acceleration from the interaction of an intense laser pulse, with an intensity of about 1020 W cm−2, with a thin foil of aluminum, titanium and gold of thickness 2 µm. Protons are accelerated via the TNSA mechanism from the rear surface of the target and, in addition, protons accelerated from the front surface are also detected on the radio chromic films. Hollow proton rings could be seen on the radio chromic films, corresponding to 1–3 MeV protons. The protons from the front surface are driven into the target and directed towards the rear side of the target by the Kilotesla magnetic fields generated from the laser plasma. 2D particle-in-cell simulations predict an increase in the flux of lower energy protons similar to experimental observations and also show strong magnetic field structures in the laser–target interaction region.

Two-color high-harmonic generation from relativistic plasma mirrors.

High-intensity laser solid interactions are capable of generating attosecond light bursts via high-harmonic generation-most work focuses on single beam interactions. In this paper, we perform a numerical investigation on the role of wavelength and polarization in relativistic, high-harmonic generation from normal-incidence, two-beam interactions off plasma mirrors. We find that the two-beam harmonic-generation mechanism is a robust process described by a set of well-defined selection rules. We demonstrate that the emitted harmonics from normal-incidence interactions exhibit an intensity optimization when the incident fields are of equal intensity for two-color circularly polarized fields.

Electron bunch dynamics and emission in particle-in-cell simulations of relativistic laser–solid interactions: On density artifacts, collisions, and numerical dispersion

Sub-optical-cycle dynamics of dense electron bunches in relativistic-intensity laser–solid interactions lead to the emission of high-order harmonics and attosecond light pulses. The capacity of particle-in-cell simulations to accurately model these dynamics is essential for the prediction of emission properties because the attosecond pulse intensity depends on the electron density distribution at the time of emission and on the temporal distribution of individual electron Lorentz-factors in an emitting electron bunch. Here, we show that in one-dimensional collisionless simulations, the peak density of the emitting electron bunch increases with the increase in the spatial resolution of the simulation grid. When collisions are added to the model, the peak electron density becomes independent of the spatial resolution. Collisions are shown to increase the spread of the peaks of Lorentz-factors of emitting electrons in time, especially in the regimes far from optimum generation conditions, thus leading to lower intensities of attosecond pulses as compared to those obtained in collisionless simulations.

Energy coupling in intense laser solid interactions: Material properties of gold

In the double-cone ignition inertial confinement fusion scheme, a high density deuterium–tritium (DT) fuel is rapidly heated with high-flux fast electrons, which are generated by short and intense laser pulses. A gold cone target is usually used to shorten the distance between the critical surface and the compressed high density DT core. The material properties of the solid gold may affect the generation and transport of fast electrons significantly, among which the effects of ionization and collision are the main concerns. In this work, the effects of ionization, collision, and blow-off plasma on the laser energy absorption rate are investigated using the LAPINS code: A three-stage model is adopted to explain the mechanism of fast electron generation and the change in the laser energy absorption rate. With the increase in the charge state of Au ions, the laser–plasma interaction transfers to the later stage, resulting in a decrease in the laser energy absorption rate. Collision has both beneficial and harmful effects. On the one hand, collision provides a thermal pressure, which makes it easier for electrons to escape into the potential well in front of the target and be accelerated in the second stage. On the other hand, collision increases stopping power and suppress electron recirculation within the target in the third stage. The vacuum sheath field behind the target enhances the electron circulation inside the target and, thus, improves the laser energy absorption; however, this effect will be suppressed when the blow-off plasma density behind the target increases or collision is considered.

Characterizing Anomalous High-Harmonic Generation in Solids.

Anomalous high-harmonic generation (HHG) arises in certain solids when irradiated by an intense laser field, originating from a Berry-curvature-induced perpendicular anomalous current. The observation of pure anomalous harmonics is, however, often prohibited by contamination from harmonics stemming from interband coherences. Here, we fully characterize the anomalous HHG mechanism, via development of an abinitio methodology for strong-field laser-solid interaction that allows a rigorous decomposition of the total current. We identify two unique properties of the anomalous harmonic yields: an overall yield increase with laser wavelength; and pronounced minima at certain laser wavelengths and laser intensities around which the spectral phases drastically change. Such signatures can be exploited to disentangle the anomalous harmonics from competing HHG mechanisms, and thus pave the way for the experimental identification and time-domain control of pure anomalous harmonics, as well as reconstruction of Berry curvatures.

Maximizing MeV x-ray dose in relativistic laser-solid interactions

Simulations of intense laser-solid interactions reveal the optimal configuration of laser intensity and duration at constant energy to maximize 1--5 MeV x-ray yield. A phenomenological model predicts the hot-electron temperature as a function of the laser parameters and density scale length.

Intense multicycle THz pulse generation from laser-produced nanoplasmas

We present a novel scheme to obtain robust, narrowband, and tunable THz emission using a nano-dimensional overdense plasma target, irradiated by two counter-propagating detuned laser pulses. So far, no narrowband THz sources with a field strength of GV/m-level have been reported from laser-solid interaction (mostly half-or single-cycle THz pulses with only broadband frequency spectrum). From two- and three-dimensional particle-in-cell simulations, we find that the strong plasma current generated by the beat ponderomotive force in the colliding region, produces beat-frequency radiation in the THz range. Here we report intense THz pulses (fsimeq 30 ;THz) with an unprecedentedly high peak field strength of 11.9 GV/m and spectral width (Delta f/fsimeq 5.3%), which leads to a regime of an extremely bright narrowband THz source of TW/cm^{2}, suitable for various ambitious applications.

Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy.

Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultra-high-speed shadowgraph imaging, hydrophone measurements, three-dimensional passive cavitation mapping (3D-PCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance (SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated "pear-shaped" bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laser-induced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A non-circular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and self-intensified collapse of an inverted "triangle-shaped" or "horseshoe-shaped" bubble. Third, high-speed shadowgraph imaging and 3D-PCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a "smiling-face" shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pear-shaped bubble are decisive for the solid damage.