Search Results (21 total)

Recently achieved high intensities of short laser pulses open new prospects in their application to hole boring in inhomogeneous overdense plasmas and for ignition in precompressed DT fusion targets. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10²²  W/cm² may penetrate deeply into the plasma as a result of efficient ponderomotive acceleration of ions in the forward direction. The penetration depth as big as hundreds of microns depends on the laser fluence, which has to exceed a few tens of GJ/cm². The fast ions, accelerated at the bottom of the channel with an efficiency of more than 20%, show a high directionality and may heat the precompressed target core to fusion conditions.

The advent of ultraintense laser pulses generated by the technique of chirped pulse amplification (CPA) along with the development of high-fluence laser materials has opened up an entirely new field of optics. The electromagnetic field intensities produced by these techniques, in excess of 10¹⁸  W∕cm², lead to relativistic electron motion in the laser field. The CPA method is reviewed and the future growth of laser technique is discussed, including the prospect of generating the ultimate power of a zettawatt. A number of consequences of relativistic-strength optical fields are surveyed. In contrast to the nonrelativistic regime, these laser fields are capable of moving matter more effectively, including motion in the direction of laser propagation. One of the consequences of this is wakefield generation, a relativistic version of optical rectification, in which longitudinal field effects could be as large as the transverse ones. In addition to this, other effects may occur, including relativistic focusing, relativistic transparency, nonlinear modulation and multiple harmonic generation, and strong coupling to matter and other fields (such as high-frequency radiation). A proper utilization of these phenomena and effects leads to the new technology of relativistic engineering, in which light-matter interactions in the relativistic regime drives the development of laser-driven accelerator science. A number of significant applications are reviewed, including the fast ignition of an inertially confined fusion target by short-pulsed laser energy and potential sources of energetic particles (electrons, protons, other ions, positrons, pions, etc.). The coupling of an intense laser field to matter also has implications for the study of the highest energies in astrophysics, such as ultrahigh-energy cosmic rays, with energies in excess of 10²⁰  eV. The laser fields can be so intense as to make the accelerating field large enough for general relativistic effects (via the equivalence principle) to be examined in the laboratory. It will also enable one to access the nonlinear regime of quantum electrodynamics, where the effects of radiative damping are no longer negligible. Furthermore, when the fields are close to the Schwinger value, the vacuum can behave like a nonlinear medium in much the same way as ordinary dielectric matter expanded to laser radiation in the early days of laser research.

Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.

An intense laser-plasma interaction regime of the generation of high density ultrashort relativistic ion beams is suggested. When the radiation pressure is dominant, the laser energy is transformed efficiently into the energy of fast ions.

Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (λ³) can create relativistic intensity with maximal gradients, using minimal energy. With particle-in-cell simulations we found, that single 200 attosecond pulses could be produced efficiently in a λ³ laser pulse reflection, via deflection and compression from the relativistic plasma mirror created by the pulse itself. An analytical model of coherent radiation from a charged layer confirms the pulse compression and is in good agreement with simulations. The novel technique is efficient (∼10%) and can produce single attosecond pulses from the millijoule to the joule level.

Generation of relativistic electrons from the interaction of a laser pulse with a high density plasma foil, accompanied by an underdense preplasma in front of it, has been studied with two-dimensional particle-in-cell (PIC) simulations for pulse durations comparable to a single cycle and for single-wavelength spot size. The electrons are accelerated predominantly in forward direction for a preplasma longer than the pulse length. Otherwise, both forward and backward electron accelerations occur. The primary mechanism responsible for electron acceleration is identified. Simulations show that the energy of the accelerated electrons has a maximum versus the pulse duration for relativistic laser intensities. The most effective electron acceleration takes place when the preplasma scale length is comparable to the pulse duration. Electron distribution functions have been found from PIC simulations. Their tails are well approximated by Maxwellian distributions with a hot temperature in the MeV range.

Since its birth, the laser has been extraordinarily effective in the study and applications of laser-matter interaction at the atomic and molecular level and in the nonlinear optics of the bound electron. In its early life, the laser was associated with the physics of electron volts and of the chemical bond. Over the past fifteen years, however, we have seen a surge in our ability to produce high intensities, 5 to 6 orders of magnitude higher than was possible before. At these intensities, particles, electrons, and protons acquire kinetic energy in the megaelectron-volt range through interaction with intense laser fields. This opens a new age for the laser, the age of nonlinear relativistic optics coupling even with nuclear physics. We suggest a path to reach an extremely high-intensity level 10^26–28 W/cm² in the coming decade, much beyond the current and near future intensity regime 10²³ W/cm², taking advantage of the megajoule laser facilities. Such a laser at extreme high intensity could accelerate particles to frontiers of high energy, teraelectron volt, and petaelectron volt, and would become a tool of fundamental physics encompassing particle physics, gravitational physics, nonlinear field theory, ultrahigh-pressure physics, astrophysics, and cosmology. We focus our attention on high-energy applications, in particular, and the possibility of merged reinforcement of high-energy physics and ultraintense laser.

Using interferometry, we investigate the dynamics of interaction of a relativistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 μm in diameter, with a radial gradient of electron density ∼5×10²² cm^-4 and with an on-axis electron density approximately ten times less than its maximum value of 8×10¹⁹ cm^-3. A high radial velocity of the surrounding gas ionization of ∼3.8×10⁸ cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas excitation and ionization. Comparing the model predictions with the interferometric data, we reconstructed the axial profile of laser channel and on-axis laser intensity. The estimated maximum energy of accelerated ions is about 500 keV, and the total energy of the fast ions is 5% of the laser pulse energy.

We present a single-shot damage threshold measurement and modeling for fused silica at 800 nm as a function of pulse duration down to 20 fs. We examine the respective roles of multiphoton ionization, tunnel ionization, and impact ionization in laser damage. We find that avalanche predominates even in the case of sub-100-fs pulses.

Experimental evidence is presented demonstrating avalanche ionization as the dominant mechanism for dielectric breakdown in silicon with ultrafast laser pulses at above-gap photon energies. Data are presented for pulses between 80 fs and 9 ns at 786 nm and 1.06 μm. Associated electric fields range from 0.3 to 40 MV/cm. Avalanche ionization coefficients range from 10¹⁰ to 10¹⁴ s^-1 and are discussed in relation to semiempirical dc ionization theory and recent ac Monte Carlo calculations. Correlation is obtained between electron collision times and associated ionization rates.

We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For τ<100 fs, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than 10¹⁴ W/cm² slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials.

The temporal envelope of plasma density oscillations in the wake of an intense ( I∼4×10¹⁸ W/cm², λ  =  1 μm) laser pulse (400 fs) is measured using forward Thomson scattering from a copropagating, frequency-doubled probe pulse. The wakefield oscillations in a fully ionized helium plasma ( n_e  =  3×10¹⁹ cm^-3) are observed to reach maximum amplitude ( δn_e/n_e∼0.1) 300 fs after the pump pulse. The wakefield growth ( 3.5 ps^-1) and decay ( 1.9 ps^-1) rates are consistent with the forward Raman scattering instability and Landau damping, respectively.

In this work, we show that x-ray-line polarization spectroscopy can be a powerful diagnostic to study laser-produced plasmas. Kinetic calculations are compared to experiments designed to probe the low-density plasma region (with a 1-ps laser pulse) and the overdense plasma (with a 400-fs laser pulse). We observe the transition from a ‘‘pancakelike’’ electron distribution function at low density to a ‘‘beamlike’’ electron distribution function in the overdense plasma. The results are in agreement with the calculations which indicate non-Maxwellian behavior and strong anisotropy due to nonlocal electron heat flow.

We report the first measurements of significant polarization of the He-like emission in a laser-produced plasma. From these measurements we infer a negative second-order (i.e., oblate) anisotropy of the electron velocity distribution function in the energy deposition region. This result is in agreement with our kinetic Fokker-Planck calculations which indicate non-Maxwell behavior and strong anisotropy due to nonlocal electron heat flow.

We have performed continuous and time-resolved photoluminescence experiments on novel double-quantum-well structures in Schottky diodes. We have directly observed the buildup of a charge-transfer (CT) state in which the electrons and holes are in separate wells because of the fact that they tunnel in opposite directions. We have studied the effect of an electric field on the CT state formation, and have observed a strong, linear Stark shift of the CT luminescence.

We have measured the absorption of 1-ps laser pulses interacting with matter at intensities from 10¹⁰ to 10¹⁶ W/cm². The variations of absorption with incidence angle and polarization have been used to infer submicron plasma-density-gradient scale lengths. The results show a transition between a regime of laser interaction with sharply bounded dense cold matter (I≤5×10¹² W/cm²), where absorption is by the usual skin depth effect, to a regime of interaction with a plasma of very steep density gradient (L / λ≤0.2) (5×10¹² W/cm²≤I≤10¹⁵ W/cm²).

Amplified 150–300-fs laser pulses are applied to monitor the thermal modulation of the transmissivity of thin copper films. Non- equilibrium electron-lattice temperatures are observed. The process of electron-phonon energy transfer was time resolved and was observed to be 1–4 ps increasing with the laser fluence.

We report experimental measurements of the voltage response of current-biased superconducting thin films excited by 30-ps-FWHM (full width at half-maximum) pulsed laser radiation. Experiments performed with Pb and Sn films irradiated by a full train of laser pulses provide direct evidence for the existence of a nonstationary intermediate state. The state was observed during the superconducting-to-normal transition, as well as during the recovery of the film from the nonequilibrium normal state into the superconducting state. Single-pulse laser experiments enabled us to study the superconducting-to-normal transition in detail. Depending on the optical power, both triangular and quadruangular voltage pulses were observed. The triangular pulses had amplitudes dependent on the laser power and reflected the transition to the intermediate state, where superconducting and normal domains coexisted. The quadrangular pulses were associated with the full transition into the normal state. Very short triangular voltage pulses with full widths of the order of hundreds of picoseconds and rise times as short as about 100 ps were observed for thin Pb films illuminated by a low, near-threshold, laser power. The pulse rise time was almost constant and it only depended weakly on the film thickness, while the pulse width and the fall time were very sensitive to the optical input power as well as other experimental conditions (e.g., film thickness, Pb-substrate interface). The pulse decay was determined to be linear. The linear decay reflects a geometric relaxation of the intermediate state. An experiment performed with a stretched laser pulse (about 470 ps FWHM) demonstrated an initial thresholding effect and indicated that the subsequent voltage is proportional to the time integral of the laser pulse (optical energy). This confirms that the transition from the superconducting state to the normal state is a two-step transition. First, a sudden jump leads to the nonstationary, intermediate state followed by a gradual increase of the volume of the normal phase with a rate proportional to the rate of increase of the laser pulse energy. All of our observations are in good agreement with numerical simulations based on Elesin’s theory of the intermediate state.

The technique of picosecond electron diffraction is used to resolve in time the laser-induced melting of thin aluminum films. It is observed that under rapid heating conditions, the long-range order of the lattice subsists for lattice temperatures well above the equilibrium point, indicative of superheating. The melting time is found to vary according to the degree of superheating. The initial density of nuclei is determined under the assumption of a two-dimensional expansion model. These results show for the first time the relationship between superheating and the rate of transformation.

Photoluminescence (PL) in As₂S₃ glass was measured with ∼7-psec resolution using a streak camera. We find that the Stokes shift occurs in <20 psec, and that the maximum radiative rate is ν₁=(4±1)×10⁸ Hz. Nonradiative decay dominates at high T, but persists at low T, probably due to tunneling. For T>100 K the observed decay time varies as exp(-T/T₀) like the cw PL efficiency, indicating that similar nonradiative mechanisms may apply for slow and fast PL. We suggest that transient absorption and PL probe different carrier subsets. A localized exciton model is discussed in relation to the fastest PL processes.