Birth and decay of coherent optical phonons in femtosecond-laser-excited bismuth

The transient reflectivity of bismuth crystal excited by a 45 fs laser pulse in the near-infrared range has been recovered with an accuracy of 10-5, at initial sample temperatures ranging from 50 to 510 K, and at pump fluences from 2 mJ/ cm2 to 21 mJ/ cm2. The coherent phonon excitation and decay processes were imprinted into the time-dependent reflectivity and this allows us to uncover the temporal phonon history preceding the structural transformation of solid Bi. Analysis showed that the first coherent atomic displacement was produced by the polarization force and the electron pressure force during the laser pulse, and that manifests itself by a negative change in the reflectivity. The frequency of the subsequent reflectivity oscillations was chirped, redshifted from the initial value due to the lattice heating. The amplitude decreased gradually while electrons transferred their energy to the lattice. Heating and thermal expansion of the lattice transformed the initially coherent harmonic vibrations of atoms into strongly nonlinear chaotic motion that signifies the onset of disordering of the solid. This process was identified through measurement of the damping rate of the reflectivity oscillations and interpretation of this rate as the decay rate of an optical phonon into two acoustic phonons. The analysis of the reflectivity oscillations provides evidence that the overheated solid experiences only the onset of the solid-liquid phase transition but did not proceed into the liquid phase. General relations between the laser-exerted forces, the atomic motion, and the optical parameters were established. The proposed theory reproduces well the measured transient reflectivity across a wide range of crystal temperatures and laser excitation fluences

Dynamics of the dielectric function in fs-laser excited bismuth

Time-resolved study of the dielectric function of femtosecond laser excited bismuth demonstrates that excitation of coherent phonons leads to a solid-plasma phase transition, and into a quasi-stable excited state lasting up to 4 ns

Small Atomic displacements Recorded in Bismuth by the Optical Reflectivity of Femtosecond Laser-Pulse Excitations

Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10-5. Analysis shows that a novel effect reported here-an initial negative drop in reflectivity-relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling

Three-dimensional Maxwell-Bloch calculation of the temporal profile of a seeded soft x-ray laser pulse

International audienceWe present three-dimensional modeling of amplification of a high-order harmonic seed by a soft x-ray laser plasma. The time-dependent evolution of the x-ray signal is determined from a fully dynamic Maxwell-Bloch calculation. At high seed intensities , a simplified one-dimensional calculation leads to strong Rabi-like temporal oscillations of the output signal. However, such oscillations have not been observed experimentally. Our three-dimensional calculations demonstrate that this is due to spatial non-uniformities in the plasma gain that cause the Rabi oscillations to dampen dramatically. Large amplitude Rabi-like oscillations are expected to appear only in long and uniform plasma. Such targets require optimized guiding techniques

Laser based synchrotron radiation

Beams of x rays in the kiloelectronvolt energy range have been produced from laser-matter interaction. Here, energetic electrons are accelerated by a laser wakefield, and experience betatron oscillations in an ion channel formed in the wake of the intense femtosecond laser pulse. Experiments using a 50 TW laser (30 fs duration) are described, as well as comparisons with numerical simulations. These results pave the way of a new generation of radiation in the x-ray spectral range, with a high collimation and an ultrafast pulse duration, produced by the use of compact laser system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87767/2/023101_1.pd

Demonstration of relativistic electron beam focusing by a laser-plasma lens

Laser-plasma technology promises a drastic reduction of the size of high energy electron accelerators. It could make free electron lasers available to a broad scientific community, and push further the limits of electron accelerators for high energy physics. Furthermore the unique femtosecond nature of the source makes it a promising tool for the study of ultra-fast phenomena. However, applications are hindered by the lack of suitable lens to transport this kind of high-current electron beams, mainly due to their divergence. Here we show that this issue can be solved by using a laser-plasma lens, in which the field gradients are five order of magnitude larger than in conventional optics. We demonstrate a reduction of the divergence by nearly a factor of three, which should allow for an efficient coupling of the beam with a conventional beam transport line

Quasi-monoenergetic electron beams production in a sharp density transition

International audienceUsing a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters