Researchers at Chalmers University of Technology, in Sweden
have reported that a single laser pulse
can create complex, ordered nanostructure systems. This previously
unobserved phenomenon has just beeen described in an article in the
scientific journal Nature Photonics.
"We have discovered a method for controlling the pattern into
which the nanoparticles organize themselves", says physicist Dinko
Chakarov, one of the authors of the article . The complex
nanostructures that are created may find applications in fibre optics,
optical sensors and advanced light emitting diodes and lasers.
The researchers started with a layer of disordered
nanoparticles of gold or silver on a membrane of nanometre thickness.
The patterning is a consequence of several transformations of the
light, which finally results in partial melting and moving of the
nanoparticles.
First, the light is caught by the particles, resulting in
resonant swinging back and forth of the particle electrons (so called
localized plasmon resonances). This specific excitation gives rise to
scattering and coupling of electromagnetic energy into trapped,
waveguided modes of the thin membrane. The edges of the membrane cause
a standing wave pattern to be formed.
The end result is hot and cold zones of a specific periodicity
on the membrane surface, and if the laser light energy is high enough,
the field energy in the hot zones is high enough to melt and move the
gold particles. All of this occurs within a few nanoseconds or even
faster, and the resulting patterns have dimensions that can be both
smaller and larger than the laser wavelength.
The results demonstrate that complex nanostructured systems
can be fabricated and manipulated by a single laser pulse. In addition,
the study shows in a very concrete manner that assemblies of optically
active nanoparticles can be used to trap light in a waveguide (membrane
or fibre) with nanometer dimensions.
The researchers have shown that the pattern can be controlled
by varying several parameters: the laser light angle, wavelength and
polarization, as well as the membrane thickness and the type of
particles on the membrane.
The discovery contributes to the understanding of the
fundamental interaction between light and matter. The study also shows
how plasmon resonance can be used to enhance light absorption, which
may be of use for the production of better solar cells, see previous
article: "Energetic nanoparticles swing sunlight into electricity"
Posted 29th May 2008
