47 research outputs found
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
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
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
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