High-contrast Femtosecond Laser Pulse Research Articles

High efficiency laser-driven proton sources using 3D-printed micro-structure

Fine structured targets are promising in enhancing laser-driven proton acceleration for various applications. Here, we apply 3D-printed microwire-array (MWA) structure to boost the energy conversion efficiency from laser to proton beam. Under irradiation of high contrast femtosecond laser pulse, the MWA target generates over 1.2 × 1012 protons (>1 MeV) with cut-off energies extending to 25 MeV, corresponding to top-end of 8.7% energy conversion efficiency. When comparing to flat foils the efficiency is enhanced by three times, while the cut-off energy is increased by 32%. We find the dependence of proton energy/conversion-efficiency on the spacing of the MWA. The experimental trend is well reproduced by hydrodynamic and Particle-In-Cell simulations, which reveal the modulation of pre-plasma profile induced by laser diffraction within the fine structures. Our work validates the use of 3D-printed micro-structures to produce high efficiency laser-driven particle sources and pointed out the effect in optimizing the experimental conditions.

Absorption of high-intensity, high-contrast femtosecond laser pulses by a dielectric solid

Basic understanding of high-intensity femtosecond laser absorption by a solid is crucial for high-energy-density science. This multidimensional problem has many variables like laser parameters, solid target material, and geometry of the excitation. This study is important for a basic understanding of intense laser-matter interaction as well for applications such as ‘plasma mirror’. Here, we experimentally observe high-contrast femtosecond laser absorption by an optically polished fused silica target at near-relativistic laser intensities (∼1018 W cm−2). Laser absorption as a function of angle of incidence and incident energy is investigated for both p- and s-polarized pulses in detail, providing a strong indication of the presence of collisionless processes. At an optimum angle of incidence, almost 80% of the p-polarized laser energy gets absorbed by the target. We also observed high reflectivity (∼60%) at lower angle of incidence (30∘) for both the polarization of laser, indicating this study is fundamentally important for ‘plasma mirrors’ at near-relativistic intensities.

X-ray production and charged-particle acceleration in the irradiation of micro- and nanorod arrays by high-power femtosecond laser pulses

The effect of the size of the rods at the structured silicon target surface on the parameters of the plasma produced when the target is exposed to a high-contrast femtosecond laser pulse with an intensity of 2 × 1018 W cm−2 is investigated. It is shown that irradiation of a target with subwavelength rods (∼ 100 nm in diameter) provides a significant (up to 250 keV) increase in the temperature of hot plasma electrons and their number in comparison with those obtained in the case of a plane target. It was found that in this case the yield of hard X-ray radiation increases several-fold. The observed effect can be attributed to increased absorption and particle motion in a complex field near plasma nonuniformities. It is shown that the presence of rods with a characteristic size of ∼ 1 μm promotes an increase in the production efficiency of multiply charged silicon ions. The result obtained is apparently associated with the radiation penetration between the rods and with collisional ionisation of particles as they fly into vacuum from deeper target layers.

A Soft X-Ray Radiation Source Formed in Supersonic Argon Gas Jets under the Action of High-Contrast Femtosecond Laser Pulses of Relativistic Intensity

The possibility of designing a compact source of soft X-ray radiation without the accompanying generation of fast electrons is under study. The source is formed by the interaction of high-contrast femtosecond laser pulses of relativistic intensity with supersonic argon gas jets. The optimal conditions are found under which the conversion coefficient to soft X-ray radiation in the range of 2.9–3.3 keV reaches 8.57 × 10–5.

Clean source of soft X-ray radiation formed in supersonic Ar gas jets by high-contrast femtosecond laser pulses of relativistic intensity

In this work, we optimized a clean, versatile, compact source of soft X-ray radiation $(E_{\text{x}\text{-}\text{ray}}\sim 3~\text{keV})$ with an yield per shot up to $7\times 10^{11}~\text{photons}/\text{shot}$ in a plasma generated by the interaction of high-contrast femtosecond laser pulses of relativistic intensity $(I_{\text{las}}\sim 10^{18}{-}10^{19}~\text{W}/\text{cm}^{2})$ with supersonic argon gas jets. Using high-resolution X-ray spectroscopy approaches, the dependence of main characteristics (temperature, density and ionization composition) and the emission efficiency of the X-ray source on laser pulse parameters and properties of the gas medium was studied. The optimal conditions, when the X-ray photon yield reached a maximum value, have been found when the argon plasma has an electron temperature of $T_{\text{e}}\sim 185~\text{eV}$ , an electron density of $N_{\text{e}}\sim 7\times 10^{20}~\text{cm}^{-3}$ and an average charge of $Z\sim 14$ . In such a plasma, a coefficient of conversion to soft X-ray radiation with energies $E_{\text{x}\text{-}\text{ray}}\sim 3.1\;(\pm 0.2)~\text{keV}$ reaches $8.57\times 10^{-5}$ , and no processes leading to the acceleration of electrons to MeV energies occur. It was found that the efficiency of the X-ray emission of this plasma source is mainly determined by the focusing geometry. We confirmed experimentally that the angular distribution of the X-ray radiation is isotropic, and its intensity linearly depends on the energy of the laser pulse, which was varied in the range of 50–280 mJ. We also found that the yield of X-ray photons can be notably increased by, for example, choosing the optimal laser pulse duration and the inlet pressure of the gas jet.

Acceleration of multiply charged ions by a high-contrast femtosecond laser pulse of relativistic intensity from the front surface of a solid target

It is shown that the acceleration efficiency of protons and multiply charged ions (and also the charge composition of the latter) accelerated backwards under irradiation of the front surface of thick solid targets by high-power femtosecond laser radiation with an intensity of is determined by the contrast of this radiation. Thus, highly ionised ions up to and are recorded on polyethylene, silicon and molybdenum targets at a contrast of , the ions with charges up to and possessing an energy of more than per unit charge. In the case of a metal target, the acceleration efficiency of protons is significantly reduced, which indicates cleaning of the target surface by a pre-pulse. The measurements performed at a contrast increased by two-to-three orders of magnitude show the presence of fast protons (up to ) on all targets, and also a decrease in the energy and maximum charge of multiply charged ions.

Bright X-Ray Source from a Laser-Driven Microplasma Waveguide.

Owing to the rapid progress in laser technology, very high-contrast femtosecond laser pulses of relativistic intensities have become available. These pulses allow for interaction with microstructured solid-density plasma without destroying the structure by parasitic prepulses. This opens a new realm of possibilities for laser interaction with micro- and nanoscale photonic materials at relativistic intensities. Here we demonstrate, for the first time, that when coupled with a readily available 1.8J laser, a microplasma waveguide (MPW) may serve as a novel compact x-ray source. Electrons are extracted from the walls and form a dense helical bunch inside the channel. These electrons are efficiently accelerated and wiggled by the waveguide modes in the MPW, which results in a bright, well-collimated emission of hard x rays in the range of 1∼100 keV.

Ionization effects in the generation of wake-fields by ultra-high contrast femtosecond laser pulses in argon gas

Difference in mechanisms of wake-field generation and electron self-injection by high contrast femtosecond laser pulses in an initially neutral Argon gas and in pre-ionized plasma without ionization is studied via 2D particle-in-cell simulations including optical ionization of the media. For shorter laser pulses, 40 fs, ionization results only in an increase of the charge of accelerated electrons by factor of ∼3 with qualitatively the same energy distribution. For longer pulses, 80 fs, a more stable wake field structure is observed in the neutral gas with the maximal energy of the accelerated electrons exceeding that in the fixed density plasma. In higher density Argon, an ionizing laser pulse converts itself to a complex system of solitons at a self-induced, critical density ramp.

High-contrast 116 PW Ti:sapphire laser system combined with a doubled chirped-pulse amplification scheme and a femtosecond optical-parametric amplifier

Based on a combined scheme of doubled chirped-pulse amplification and a femtosecond noncollinear optical-parametric amplifier, a high-contrast femtosecond laser pulse with energy of up to 32.3 J has been generated by improving the gain efficiency and boosting the pump energy to 120 J in the final amplifier. Our measurements show that the contrast ratio of the main laser pulse is around 10(10) within the time scale of -400 ps and the duration of compressed pulse is 27.9 fs, corresponding to a peak power of 1.16 PW.

Carrier-envelope phase stabilization of high-contrast femtosecond laser pulses with a relativistic intensity

We report on the generation of carrier-envelope phase (CEP)-stabilized pulses with a relativistic intensity and a high-contrast ratio. The CEP stabilization is achieved with a jitter of 0.95rad from a 0.5kHz femtosecond laser pulses with a focal intensity of 2.6×1018W∕cm2 and a picosecond contrast of 2.5×10−9. CEP noise analysis shows that the beam pointing at the pulse compressor is a dominant factor of the CEP fluctuation with our laser system.

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×1016 W/cm2, which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm2 energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.

Ultra-short efficient laser-driven hard X-ray source operated at a kHz repetition rate

Ultra-short bursts of hard X-ray radiation are generated by interaction of high-contrast femtosecond laser pulses with a jet of liquid Ga. The X-ray emission shows a strong dependence on the angle of incidence and polarization of the laser beam, consistent with the processes of resonant absorption and vacuum heating. As much as 60% of the total X-ray emission consists of [Ga]Kα radiation (9.22–9.25 keV) with a photon flux of 6×109 photons/(s srad). Using this novel X-ray source, static diffraction patterns from a GaAs(111) crystal were recorded with an acquisition time of 2 s.

High magnification imaging with a laser-based hard X-ray source

The usefulness of laser-based X-ray sources for imaging is assessed. A very bright point source of 20-keV X-rays, with a diameter of 10 /spl mu/m, has been created by the interaction of a high contrast femtosecond laser pulse with solid targets. This X-ray source has been used for a feasibility study of high magnification imaging. Images of mammography phantoms have been obtained and spatial resolution of 35 /spl mu/m has been demonstrated with object magnification up to ten. A new laser technology recently introduced allows the construction of very intense, compact and cost effective short pulse laser systems and makes now the ultrafast laser based hard X-ray sources very attractive for medical application.