62 research outputs found
White light generation and anisotropic damage in gold films near percolation threshold
Strongly enhanced and confined electromagnetic fields generated in metal
nanostructures upon illumination are exploited in many emerging technologies by
either fabricating sophisticated nanostructures or synthesizing colloid
nanoparticles. Here we study effects driven by field enhancement in vanishingly
small gaps between gold islands in thin films near the electrically determined
percolation threshold. Optical explorations using two-photon luminescence (TPL)
and near-field microscopies reveals super-cubic TPL power dependencies with
white-light spectra, establishing unequivocally that the strongest TPL signals
are generated with close to the percolation threshold films, and occurrence of
extremely confined (~ 30 nm)and strongly enhanced (~ 100 times) fields at the
illumination wavelength. For linearly polarized and sufficiently powerful
light, we observe pronounced optical damage with TPL images being sensitive to
both wavelength and polarization of illuminating light. We relate these effects
to thermally induced morphological changes observed with scanning electron
microscopy images. Fascinating physics involved in light interaction with
near-percolation metal films along with their straightforward and scalable
one-step fabrication procedure promises a wide range of fascinating
developments and technological applications within diverse areas of modern
nanotechnology, from bio-molecule optical sensing to ultra-dense optical data
storage.Comment: 42 pages in total of the main (27 pages) and supplementary (15 pages)
material with 4 main and 10 supplementary figure
Quantum well confinement and competitive radiative pathways in the luminescence of black phosphorus layers
Black phosphorus (BP) stands out from other 2D materials by the wide
amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from
Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness.
This singularity made the optical and excitonic properties of BP difficult to
map. Specifically, the literature lacks in presenting experimental and
theoretical data on the optical properties of BP on an extended thickness
range. Here we report the study of an ensemble of photoluminescence spectra
from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm
to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation
steps induce additional defects states that compete the radiative recombination
from bound excitons observed in the crystal. We also show that the evolution of
the photoluminescence energy versus thickness follows a quantum well
confinement model appreciable from a thickness predicted and probed at 25 nm.
The BP slabs placed in different 2D heterostructures show that the emission
energy is not significantly modulated by the dielectric environment.
Introduction Confinement effectsComment: 11 pages, 3 figures - Main text 12 pages, 5 figures - Supporting
informatio
Where do the laws of physics come from?
The laws of physics were not handed down from above. Neither are they rules somehow built into the structure of the universe. They are ingredients of the models that physicists invent to describe observations. Rather than being restrictions on the behavior of matter, the laws of physics are restrictions on the behavior of physicists. If the models of physics are to describe observations based on an objective reality, then those models cannot depend on the point of view of the observer. This suggests a principle of point-of-view invariance that is equivalent to the principle of covariance when applied to space-time. As Noether showed, space-time symmetries lead to the principles of energy, linear momentum, and angular momentum conservation--essentially all of classical mechanics. It also leads to Lorentz invariance and special relativity. When generalized to the abstract space of functions such as the quantum state vector, point-of-view invariance is identified with gauge invariance. Quantum mechanics is then just the mathematics of gauge transformations with no additional assumptions needed to obtain its rules, including the superposition and uncertainty principles. The conservation and quantization of electric charge follow from global gauge invariance. The electromagnetic force is introduced to preserve local gauge invariance. Although not discussed here, the other forces in the standard model of elementary particles are also fields introduced to preserve local gauge invariance. Gravity can also be viewed as such a field. Thus practically all of fundamental physics as we know it follows directly from the single principle of point-of-view invariance
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