Excitation of Coherent Phonons in Crystalline B: theory for driving atomic vibrations by femtosecond pulses

In this paper we present experimental and theoretical studies of reflectivity oscillations of an optical probe beam reflected from a single-crystal of bismuth excited by 35 fs laser pulses at deposited energy density above the melting temperature. Coherent and incoherent lattice dynamics as well as electrons dynamics were investigated starting from the reflectivity changes, measured with high accuracy ΔR/R < 10. The complex behaviour of the reflectivity could not be explained in the light of the existing theories. Therefore, we developed a new theory, starting from the very basic principles of lasermatter interaction, which shows good agreement with experimental results. We establish a direct dependence of the transient reflectivity on atomic motions driven by electron temperature gradient through electron-phonon coupling

Unconventional magnetism in all-carbon nanofoam

We report production of nanostructured carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in Ar atmosphere. A combination of characterization techniques revealed that the system contains both sp2 and sp3 bonded carbon atoms. The material is a novel form of carbon in which graphite-like sheets fill space at very low density due to strong hyperbolic curvature, as proposed for ?schwarzite?. The foam exhibits ferromagnetic-like behaviour up to 90 K, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.Comment: 14 pages, including 2 tables and 7 figs. Submitted to Phys Rev B 10 September 200

Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics

The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulae for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.Comment: 27 pages incl.3 figs; presented at CLEO-Europe'2000 11-15 Sept.2000; papers QMD6 and CTuK11

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

Laser matter interaction in the bulk of transparent dielectrics: Confined micro-explosion

We present here the experimental and theoretical studies of drastic transformations induced by a single powerful femtosecond laser pulse tightly focused inside a transparent dielectric, that lead to void formation in the bulk. We show that the laser pulse energy absorbed within a volume of less than 1μm3 creates the conditions with pressure and temperature range comparable to that formed by an exploding nuclear bomb. At the laser intensity above 6 × 1012 W/cm2 the material within this volume is rapidly atomized, ionized, and converted into a tiny super-hot cloud of expanding plasma. The expanding plasma generates strong shock and rarefaction waves which result in the formation of a void. Our modelling indicates that unique states of matter can be created using a standard table-top laser in well-controlled laboratory conditions. This state of matter has temperatures 105 K, heating rate up to the 1018 K/s, and pressure more than 100 times the strength of any solid. The laser-affected sites in the bulk were detected ("read") by generation of white continuum using probe femtosecond pulses at much lower laser intensity of 1010 W/cm 2 - 1011 W/cm2. Post-examination of voids with an electron microscope revealed a typical size of the void ranges from 200 to 500 nm. These studies will find application for the design of 3D optical memory devices and for formation of photonic band-gap crystals

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

Subpicosecond laser ablation of dental enamel

Laser ablation of dental enamel with subpicosecond laser pulses has been studied over the intensity range of (0.1–1.4)×10¹⁴ W/cm² using 95 and 150 fs pulses at a pulse repetition rate of 1 kHz. The experimentally determined ablation threshold of 2.2±0.1 J/cm² was in good agreement with theoretical predictions based on an electrostaticablation model. The ablation rate increased linearly with the laser fluence for up to 15 times the ablation threshold. The absence of collateral damage was observed using optical and scanning electron microscopy. Pulpal temperature measurements showed an increase of about 10 °C during the 200 s course of ablation. However, air cooling at a rate of 5 l/min resulted in the intrapulpal temperature being maintained below the pulpal damage threshhold of 5.5 °C. The material removal rates for subpicosecond precision laser ablation of dental enamel are compared with other techniques

Graphene as a quantum surface with curvature-strain preserving dynamics

We discuss how the curvature and the strain density of the atomic lattice generate the quantization of graphene sheets as well as the dynamics of geometric quasiparticles propagating along the constant curvature/strain levels. The internal kinetic momentum of Riemannian oriented surface (a vector field preserving the Gaussian curvature and the area) is determined.Comment: 13p, minor correction