Thermoplastic deformation of silicon surfaces induced by ultrashort pulsed lasers in submelting conditions

A hybrid 2D theoretical model is presented to describe thermoplastic deformation effects on silicon surfaces induced by single and multiple ultrashort pulsed laser irradiation in submelting conditions. An approximation of the Boltzmann transport equation is adopted to describe the laser irradiation process. The evolution of the induced deformation field is described initially by adopting the differential equations of dynamic thermoelasticity while the onset of plastic yielding is described by the von Mise's stress. Details of the resulting picometre sized crater, produced by irradiation with a single pulse, are then discussed as a function of the imposed conditions and thresholds for the onset of plasticity are computed. Irradiation with multiple pulses leads to ripple formation of nanometre size that originates from the interference of the incident and a surface scattered wave. It is suggested that ultrafast laser induced surface modification in semiconductors is feasible in submelting conditions, and it may act as a precursor of the incubation effects observed at multiple pulse irradiation of materials surfaces.Comment: To appear in the Journal of Applied Physic

Ultrafast dynamics and sub-wavelength periodic structure formation following irradiation of GaAs with femtosecond laser pulses

A theoretical investigation of the ultrafast processes and dynamics of the excited carriers upon irradiation of GaAs with femtosecond (fs) pulsed lasers is performed in conditions that induce material damage and eventually surface modification of the heated solid. A parametric study is followed to correlate the produced transient carrier density with the damage threshold for various pulse duration values {\tau}p (it increases as ~ at relatively small values of {\tau}p while it drops for pulse durations of the order of some picoseconds) based on the investigation of the fundamental multiscale physical processes following fs-laser irradiation. Moreover, fluence values for which the originally semiconducting material demonstrates a metallic behaviour are estimated. It is shown that a sufficient number of carriers in the conduction band are produced to excite Surface Plasmon (SP) waves that upon coupling with the incident beam and a fluid-based surface modification mechanism lead to the formation of sub-wavelength periodic structures orientated perpendicularly to the laser beam polarization. Experimental results for the damage threshold and the frequencies of induced periodic structures show a good agreement with the theoretical predictions.Comment: 11 color pages To appear in the Physical Review

Fermionic Bound States and Pseudoscalar Exchange

We discuss the possibility that fermions bind due to Higgs or pseudoscalar exchange. It is reasonable to believe on qualitative grounds that this can occur for fermions with a mass larger than 800-900 GeV. An exchange of a pseudoscalar boson leads in the non-relativistic limit to an unacceptable potential which behaves like 1/r^3 at the origin. We show that this singular behaviour is smeared out when relativistic effects are included

Investigations on the Bragg grating recording in all-silica, standard and microstructured optical fibers using 248 nm 5 ps, laser radiation

oai:ojs.pkp.sfu.ca:article/309The fabrication of Bragg reflectors in hydrogenated, all-silica, fluorine cladding depressed and microstructured optical fibers using 248 nm, 5 ps laser radiation, is investigated here. Comparative Bragg grating recordings are performed in both optical fibers, for investigating effects related to the scattering induced by the capillary micro-structure, to the photosensitivity and index engineering yield. Further, finite difference time domain method is employed for simulating the scattering from the above capillary structure and the nominal intensity reaching the fiber core for side-illumination. The maximum modulated refractive index changes inscribed in the standard, step-index fiber were of the order of 8.3x10-5, while the maximum refractive index changes inscribed in one of the microstructured optical fibers was 32% lower and 5.7x10-5, for nominal pulse intensities of 20 GW/cm2 and modest accumulated energy densities

Hysteresis and metastability of Bose-Einstein condensed clouds of atoms confined in ring potentials

We consider a Bose-Einstein condensed cloud of atoms which rotate in a toroidal/annular potential. Assuming one-dimensional motion, we evaluate the critical frequencies associated with the effect of hysteresis and the critical coupling for stability of the persistent currents. We perform these calculations using both the mean-field approximation and the method of numerical diagonalization of the many-body Hamiltonian which includes corrections due to the finiteness of the atom number.Comment: 7 pages, 5 figures, section on experimental relevance added, final versio

Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in sub-ablation conditions

An investigation of ultrashort pulsed laser induced surface modification due to conditions that result in a superheated melted liquid layer and material evaporation are considered. To describe the surface modification occurring after cooling and resolidification of the melted layer and understand the underlying physical fundamental mechanisms, a unified model is presented to account for crater and subwavelength ripple formation based on a synergy of electron excitation and capillary waves solidification. The proposed theoretical framework aims to address the laser-material interaction in sub-ablation conditions and thus minimal mass removal in combination with a hydrodynamics-based scenario of the crater creation and ripple formation following surface irradiation with single and multiple pulses, respectively. The development of the periodic structures is attributed to the interference of the incident wave with a surface plasmon wave. Details of the surface morphology attained are elaborated as a function of the imposed conditions and results are tested against experimental data