Metal nanoplasmas as bright sources of hard X-ray pulses

Nanowire implosion under laser amplified spontaneous emission pedestal irradiation

Nanowire array targets exhibit high optical absorption when interacting with short, intense laser pulses. This leads to an increased yield in the production of accelerated particles for a variety of applications. However, these interactions are sensitive to the laser prepulse and could be significantly affected. Here, we show that an array of aligned nanowires is imploded when irradiated by an Amplified Spontaneous Emission pedestal of a [Formula: see text] laser with an intensity on the order of [Formula: see text]. Using radiation hydrodynamics simulations, we demonstrate that the electron density profile is radially compressed at the tip by the rocket-like propulsion of the ablated plasma. The mass density compression increases up to [Formula: see text] when a more dense nanowire array is used. This is due to the ablation pressure from the neighboring nanowires. These findings offer valuable information for selecting an appropriate target design for experiments aimed at enhancing production of accelerated particles.

Luminous, relativistic, directional electron bunches from an intense laser driven grating plasma

Bright, energetic, and directional electron bunches are generated through efficient energy transfer of relativistic intense (~ 10¹⁹ W/cm²), 30 femtosecond, 800 nm high contrast laser pulses to grating targets (500 lines/mm and 1000 lines/mm), under surface plasmon resonance (SPR) conditions. Bi-directional relativistic electron bunches (at 40° and 150°) are observed exiting from the 500 lines/mm grating target at the SPR conditions. The surface plasmon excited grating target enhances the electron flux and temperature by factor of 6.0 and 3.6, respectively, compared to that of the plane substrate. Particle-in-Cell simulations indicate that fast electrons are emitted in different directions at different stages of the laser interaction, which are related to the resultant surface magnetic field evolution. This study suggests that the SPR mechanism can be used to generate multiple, bright, ultrafast relativistic electron bunches for a variety of applications.

Effects of simulation dimensionality on laser-driven electron acceleration and photon emission in hollow microchannel targets

Using two-dimensional (2D) and three-dimensional (3D) kinetic simulations, we examine the impact of simulation dimensionality on the laser-driven electron acceleration and the emission of collimated γ-ray beams from hollow microchannel targets. We demonstrate that the dimensionality of the simulations considerably influences the results of electron acceleration and photon generation owing to the variation of laser phase velocity in different geometries. In a 3D simulation with a cylindrical geometry, the acceleration process of electrons terminates early due to the higher phase velocity of the propagating laser fields; in contrast, 2D simulations with planar geometry tend to have prolonged electron acceleration and thus produce much more energetic electrons. The photon beam generated in the 3D setup is found to be more diverged accompanied with a lower conversion efficiency. Our paper concludes that the 2D simulation can qualitatively reproduce the features in 3D simulation, but for quantitative evaluations and reliable predictions to facilitate experiment designs 3D modeling is strongly recommended.

Efficient second-harmonic generation of a high-energy, femtosecond laser pulse in a lithium triborate crystal

We demonstrate the highest efficiency (∼80%) second harmonic generation of joule level, 27 fs, high-contrast pulses in a type-I lithium triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate gives a maximum efficiency of 26%. LBO thus offers high-intensity (>10^18-19W/cm²), ultra-high contrast femtosecond pulses, which have great potential for high energy density science and applications, particularly with nanostructured targets.

Kα X-ray Emission from Nanowire Cu Targets Driven by Femtosecond Laser Pulses for X-ray Conversion and Backlight Imaging

A high-quality X-ray source was proposed by modifying the target material structure characteristics driven by ultrahigh laser energy. The experiments were performed on the Ti:sapphire femtosecond laser beam device (4.3-6 J, 30 fs), one of the three XG-III lasers in Laser Fusion Research Center of China Academy of Engineering Physics. The femtosecond laser beam drove the nanowire copper material with an average length of 18-50 μm and a diameter of about 260 nm. A single-photon counting charge-coupled device was employed to measure the copper Kα X-ray emission of the nanowire and foil targets. A clear maximum photon yield of the nanowire target was calculated to be 3.6 × 10⁸ photons sr^-1 s^-1, the conversion efficiency was up to 0.0087%, and the average yield was 2.5 times that of the copper foil targets. In addition, by using a pinhole imaging method of φ10 μm, the minimum full width at half maximum spot size of the X-ray source was calculated in the range of 85-240 μm, which was similar to that of the copper foil material with a long radius of 170 μm and a short radius of 63 μm. The experimental data illustrate that the nanowire has the potential to enhance the energy absorption of femtosecond laser for X-ray conversion and backlight imaging.

The emission of γ-Ray beams with orbital angular momentum in laser-driven micro-channel plasma target

We investigated the emission of multi-MeV γ-Ray beams with orbital angular momentum (OAM) from the interaction of an intense circularly polarized (CP) laser with a micro-channel plasma target. The driving laser can generate high energy electrons via direct laser acceleration within the channel. By attaching a plasma foil as the reflecting mirror, the CP laser is reflected and automatically colliding with the electrons. High energy gamma-photons are emitted through inverse Compton scattering (ICS) during collision. Three-dimensional particle-in-cell simulations reveal that the spin angular momentum (SAM) of the CP laser can be transferred to the OAM of accelerated electrons and further to the emitted gamma-ray beam. These results may guide future experiments in laser-driven gamma-ray sources using micro-structures.

Hard X-ray Generation from ZnO Nanowire Targets in a Non-Relativistic Regime of Laser-Solid Interactions

We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5 × 1016 W/cm2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in highenergy density (HED) physics.

A source to deliver mesoscopic particles for laser plasma studies

Intense ultrashort laser produced plasmas are a source for high brightness, short burst of X-rays, electrons, and high energy ions. Laser energy absorption and its disbursement strongly depend on the laser parameters and also on the initial size and shape of the target. The ability to change the shape, size, and material composition of the matter that absorbs light is of paramount importance not only from a fundamental physics point of view but also for potentially developing laser plasma sources tailored for specific applications. The idea of preparing mesoscopic particles of desired size/shape and suspending them in vacuum for laser plasma acceleration is a sparsely explored domain. In the following report we outline the development of a delivery mechanism of microparticles into an effusive jet in vacuum for laser plasma studies. We characterise the device in terms of particle density, particle size distribution, and duration of operation under conditions suitable for laser plasma studies. We also present the first results of x-ray emission from micro crystals of boric acid that extends to 100 keV even under relatively mild intensities of 10¹⁶ W/cm².

Aligned copper nanorod arrays for highly efficient generation of intense ultra-broadband THz pulses

We demonstrate an intense broadband terahertz (THz) source based on the interaction of relativistic-intensity femtosecond lasers with aligned copper nanorod array targets. For copper nanorod targets with a length of 5 μm, a maximum 13.8 times enhancement in the THz pulse energy (in ≤20 THz spectral range) is measured as compared to that with a thick plane copper target under the same laser conditions. A further increase in the nanorod length leads to a decrease in the THz pulse energy at medium frequencies (≤20 THz) and increase of the electromagnetic pulse energy in the high-frequency range (from 20–200 THz). For the latter, we measure a maximum energy enhancement of 28 times for the nanorod targets with a length of 60 μm. Particle-in-cell simulations reveal that THz pulses are mostly generated by coherent transition radiation of laser produced hot electrons, which are efficiently enhanced with the use of nanorod targets. Good agreement is found between the simulation and experimental results.

Energy penetration into arrays of aligned nanowires irradiated with relativistic intensities: Scaling to terabar pressures

Ultrahigh-energy density (UHED) matter, characterized by energy densities >1 × 10⁸ J cm^-3 and pressures greater than a gigabar, is encountered in the center of stars and inertial confinement fusion capsules driven by the world's largest lasers. Similar conditions can be obtained with compact, ultrahigh contrast, femtosecond lasers focused to relativistic intensities onto targets composed of aligned nanowire arrays. We report the measurement of the key physical process in determining the energy density deposited in high-aspect-ratio nanowire array plasmas: the energy penetration. By monitoring the x-ray emission from buried Co tracer segments in Ni nanowire arrays irradiated at an intensity of 4 × 10¹⁹ W cm^-2, we demonstrate energy penetration depths of several micrometers, leading to UHED plasmas of that size. Relativistic three-dimensional particle-in-cell simulations, validated by these measurements, predict that irradiation of nanostructures at intensities of >1 × 10²² W cm^-2 will lead to a virtually unexplored extreme UHED plasma regime characterized by energy densities in excess of 8 × 10¹⁰ J cm^-3, equivalent to a pressure of 0.35 Tbar.

Tuning laser plasma x-ray source for single shot microscopy using nano-porous targets

Detailed calculations show that we can control and enhance x-ray line emission in the so-called water-window wavelength region using laser irradiated nano-porous or foam-like targets. The effects of target porosity on the non-LTE plasma ionization are studied to obtain optimum conditions for maximum narrowband line emission. Results show that for specified irradiation conditions, the population of emitting ions can be significantly improved using a target with optimum initial mass density. In such conditions, an efficient line x ray can be emitted from the created plasma making it a suitable flash point source for high-contrast x-ray imaging of living samples.

Enhanced x-ray emission from nano-particle doped bacteria

Recently, it has been greatly appreciated that intense light matter interaction is modified due to the nano- and microstructures in the target by--surface plasmons, laser energy localization scattering etc. Extreme laser intensities produce dense plasmas and collective mechanisms generate energetic electrons, ions and hard x-rays. Recently, it is postulated that the anharmonic electron motion, driven by ultrashort, high-intensity laser pulses, provides a universal mechanism for the laser absorption. Here, we provide the first demonstration of anharmonic-resonance-aided high laser-absorption in a biological system. At intensities of ∼ 10¹⁶⁻¹⁸ W/cm², 40 fs pulses excite a plasma formed with E. coli bacteria. The density-inhomogeneities due to the micro- and nanostructures in the bacterial target increase anharmonic resonance (AHR) heating and result in a 10⁴-fold enhancement in the hard x-ray yield compared to plain solid targets. These observations lead to novel high-energy x-ray sources that have implications to lithography, imaging and medical applications.

Preferential enhancement of laser-driven carbon ion acceleration from optimized nanostructured surfaces

High-intensity ultrashort laser pulses focused on metal targets readily generate hot dense plasmas which accelerate ions efficiently and can pave way to compact table-top accelerators. Laser-driven ion acceleration studies predominantly focus on protons, which experience the maximum acceleration owing to their highest charge-to-mass ratio. The possibility of tailoring such schemes for the preferential acceleration of a particular ion species is very much desired but has hardly been explored. Here, we present an experimental demonstration of how the nanostructuring of a copper target can be optimized for enhanced carbon ion acceleration over protons or Cu-ions. Specifically, a thin (≈0.25 μm) layer of 25–30 nm diameter Cu nanoparticles, sputter-deposited on a polished Cu-substrate, enhances the carbon ion energy by about 10-fold at a laser intensity of 1.2×10¹⁸  W/cm². However, particles smaller than 20 nm have an adverse effect on the ion acceleration. Particle-in-cell simulations provide definite pointers regarding the size of nanoparticles necessary for maximizing the ion acceleration. The inherent contrast of the laser pulse is found to play an important role in the species selective ion acceleration.

Laser nano- and micro-structuring of silicon using a laser-induced plasma for beam conditioning

A technique based on the interaction between a laser pulse and a laser-induced plasma is proposed as a very simple and potentially powerful method for surface nanostructuring. A laser pulse was focused onto a metallic target in order to generate a plasma, while a second laser pulse was directed to the plasma and crossed it perpendicularly to the first pulse and, subsequently, hit a silicon substrate. In this conditions, the second pulse interacts with the plasma which acted as an optical element whose properties could be modified by varying the energy density of the first pulse or the delay between the two pulses. Microscopic analysis carried out on the silicon surface revealed a wide variety of nanostructured patterns.

Bacterial cells enhance laser driven ion acceleration

Intense laser produced plasmas generate hot electrons which in turn leads to ion acceleration. Ability to generate faster ions or hotter electrons using the same laser parameters is one of the main outstanding paradigms in the intense laser-plasma physics. Here, we present a simple, albeit, unconventional target that succeeds in generating 700 keV carbon ions where conventional targets for the same laser parameters generate at most 40 keV. A few layers of micron sized bacteria coating on a polished surface increases the laser energy coupling and generates a hotter plasma which is more effective for the ion acceleration compared to the conventional polished targets. Particle-in-cell simulations show that micro-particle coated target are much more effective in ion acceleration as seen in the experiment. We envisage that the accelerated, high-energy carbon ions can be used as a source for multiple applications.

Effects of front-surface target structures on properties of relativistic laser-plasma electrons

We report the results of a study of the role of prescribed geometrical structures on the front of a target in determining the energy and spatial distribution of relativistic laser-plasma electrons. Our three-dimensional particle-in-cell simulation studies apply to short-pulse, high-intensity laser pulses, and indicate that a judicious choice of target front-surface geometry provides the realistic possibility of greatly enhancing the yield of high-energy electrons while simultaneously confining the emission to narrow (<5°) angular cones.

Macroscopic transport of mega-ampere electron currents in aligned carbon-nanotube arrays

We demonstrate that aligned carbon-nanotube arrays are efficient transporters of laser-generated mega-ampere electron currents over distances as large as a millimeter. A direct polarimetric measurement of the temporal and the spatial evolution of the megagauss magnetic fields (as high as 120 MG) at the target rear at an intensity of (10(18)-10(19))  W/cm2 was corroborated by the rear-side hot electron spectra. Simulations show that such high magnetic flux densities can only be generated by a very well collimated fast electron bunch.

A bright point source of ultrashort hard x-ray pulses using biological cells

We demonstrate that the interaction of intense femtosecond light on a plain solid substrate can be substantially altered by a few micron layer coating of bacterial cells, live or dead. Using E. Coli cells, we show that at an intensity of 10(16)W cm(-2), the bremsstraahlung hard x-ray emission (up to 300 keV), is increased by more than two orders of magnitude as compared to a plain glass slab. Particle-in-cell simulations carried out by modeling the bacterial cells as ellipsoidal particles show that the hot electron generation is indeed enhanced by the presence of microstructures. This new methodology should pave way for using microbiological systems of varied shapes to control intense laser produced plasmas for EUV/x-ray generation.

Near-complete absorption of intense, ultrashort laser light by sub-lambda gratings

We demonstrate near-100% light absorption and increased x-ray emission from dense plasmas created on solid surfaces with a periodic sub-lambda structure. The efficacy of the structure-induced surface plasmon resonance, responsible for enhanced absorption, is directly tested at the highest intensities to date (3 x 10{15} W cm{-2}) via systematic, correlated measurements of absorption and x-ray emission. An analytical grating model as well as 2D particle-in-cell simulations conclusively explain our observations. Our study offers a definite, quantitative way forward for optimizing and understanding the absorption process.

A broadband laser plasma x-ray source for application in ultrafast chemical structure dynamics

A plasma source free from characteristic emission lines is described, based on laser irradiation of a water jet in a helium atmosphere. Various key aspects of the laser interaction are presented along with practical characterization of the observed isotropic approximately 4-10 keV x-ray emissions, measurements of which indicate subpicosecond duration. Observations are consistent with a vacuum heating plasma mechanism at the helium-water interface and indicate strong potential for in-house ultrafast chemical structure dynamics application when coupled to contemporary detector developments.

Control of strong-laser-field coupling to electrons in solid targets with wavelength-scale spheres

Irradiation of a planar solid by an intense laser pulse leads to fast electron acceleration and hard x-ray production. We have investigated whether this high field production of fast electrons can be controlled by introducing dielectric spheres of well-defined size on the target surface. We find that the presence of spheres with a diameter slightly larger than half the laser wavelength leads to Mie enhancements of the laser field which, accompanied by multipass stochastic heating of the electrons, leads to significantly enhanced hard x-ray yield and temperature.

Optimum hot electron production with low-density foams for laser fusion by fast ignition

We propose a foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition. A thin low-density foam is proposed to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.

Nanostructures, local fields, and enhanced absorption in intense light-matter interaction

Recent literature has reported impressive enhancements in hard-x-ray emission from short-lived solid plasmas by modulation of the interacting surface with nanostructures. We show that the modification of local electric fields near surface structures results in excessive absorption and enhanced x-ray production. A simple model based on local field variations explains the observed x-ray enhancements quantitatively.