26,876 research outputs found
Universal threshold for femtosecond laser ablation with oblique illumination
We quantify the dependence of the single-shot ablation threshold on the angle
of incidence and polarization of a femtosecond laser beam, for three dissimilar
solid-state materials: a metal, a dielectric and a semiconductor. Using the
constant, linear value of the index of refraction, we calculate the laser
fluence transmitted through the air-material interface at the point of ablation
threshold. We show that, in spite of the highly nonlinear ionization dynamics
involved in the ablation process, the so defined transmitted threshold fluence
is universally independent of the angle of incidence and polarization of the
laser beam for all three material types. We suggest that angular dependence of
ablation threshold can be utilized for profiling fluence distributions in
ultra-intense femtosecond laser beams.Comment: 4 pages, 5 figure
Microscopic Realization of 2-Dimensional Bosonic Topological Insulators
It is well known that a Bosonic Mott insulator can be realized by condensing
vortices of a bo- son condensate. Usually, a vortex becomes an anti-vortex (and
vice-versa) under time reversal symmetry, and the condensation of vortices
results in a trivial Mott insulator. However, if each vortex or anti-vortex
interacts with a spin trapped at its core, the time reversal transformation of
the composite vortex operator will contain an extra minus sign. It turns out
that such a composite vortex condensed state is a bosonic topological insulator
(BTI) with gapless boundary excitations protected by
symmetry. We point out that in BTI, an external flux monodromy defect
carries a Kramers doublet. We propose lattice model Hamiltonians to realize the
BTI phase, which might be implemented in cold atom systems or spin-1 solid
state systems.Comment: 5 pages + supplementary materia
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