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.

We report production of nanostructured magnetic 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 sp 2 and sp 3 bonded carbon atoms. The material is a form of carbon containing graphitelike sheets with hyperbolic curvature, as proposed for "schwarzite." The foam exhibits ferromagnetic-like behavior 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.

We demonstrate a new principle of optical trapping and manipulation increasing more than 1000 times the manipulation distance by harnessing strong thermal forces while suppressing their stochastic nature with optical vortex beams. Our approach expands optical manipulation of particles into a gas media and provides a full control over trapped particles, including the optical transport and pinpoint positioning of ∼100  μm objects over a meter-scale distance with ±10  μm accuracy.

Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties.

Two effects of the light-matter interaction are widely used for micromanipulation of particles with laser beams [1]: radiation pressure [2], originating from a direct transfer of momentum from photons, and electric dipole (gradient) force acting on polarized particles [3]. In gaseous media, however, the heating of absorbing particles by light leads to much stronger radiometric forces [4] considered until now only as an obstacle which prevents a stable trapping [5]. In contrast, here we show that photophoretic force [6,7] can be specifically tailored to trap and manipulate absorbing particles in open air. We demonstrate experimentally the optical guiding of clusters of carbon nanoparticles [8,9] in the form of nanofoam fragments of arbitrary shape, with the size in the range 0.1-10 micrometers, and for laser powers lower than one milli Watt. The optical trap is created by two counter-propagating "doughnut" vortex beams [10,11], and it allows simultaneous trapping of several particles as well as their stable positioning and controlled guiding along the optical axis. Only a small fraction of operating power is actually absorbed because the particles are trapped in the region of vanishing intensity at the vortex core. Thus the alteration of physical and chemical properties of airborne particles is minimal in the trap, this feature is important for experiments with aerosols [12]. Furthermore, the non-contact and remote optical trapping of absorbing aerosol particles can be applied to detect, monitor, and possibly reduce the exposure to engineered nanomaterials in air [13], a necessary tool [14] for the studies of impact of nanotechnology on health [15] and environment [16]. The photophoretic trapping can be also employed to simulate, on laboratory scales, the processes studied in atmospheric [17,18] and planetary [19,20] sciences.When a photon is absorbed by a small particle its momentum contributes towards radiation pressure while its energy dissipates in heat. The latter leads to thermal forces [4] deliberately avoided in optical tweezers [1]: an optical trap utilizing a gradient force produced by a single strongly focused laser beam [3]. The applications of optical tweezers range from trapping of colloidal particles [21,22] and living cells [23] to manipulation of single molecules [24] and atoms [25]. Yet these applications are limited by the condition of vanishing radiometric forces and exclude, for example, trapping of strongly absorbing particles in air [21,26]. Indeed, if the surface of an aerosol particle is nonuniformly heated by an incident light, the gas molecules rebound off the surface with different velocities creating an integrated force on the particle, this effect was discovered by Ehrenhaft [6] and termed photophoresis (PP) [4,7]. A rough comparison [27] of the radiation pressure force, F a = P/c, exerted by a beam with power P , and the PP force, F pp = P/3v, for particles with zero thermal conductivity [17], shows that for air at room temperature the later dominates by several orders of magnitude,...