The
plasmonic meron spin angular momentum (SAM) texture. The arrows show meron-like
SAM pseudovector texture at the plasmonic vortex core; they are overlaid on a
map of L-line singularity of SPP fields that delineates the quasiparticle
density. Credit: University of Pittsburgh).
Light
travels at a speed of about 300,000,000 meters per second as light particles,
photons, or equivalently as electromagnetic field waves. Experiments led by
Hrvoje Petek, an H.K. Mellon professor in the Department of Physics and
Astronomy examined ideas surrounding the origins of light, taking snapshots of
light, stopping light and using it to change properties of matter.
Petek
worked with students and collaborators Prof. Chen-Bin (Robin) Huang of the
National Tsing Hua University in Taiwan, and Atsushi Kubo of the Tsukuba
University of Japan on the experiments.
Their
findings were reported in Nature ("Plasmonic topological quasiparticle on
the nanometre and femtosecond scales").
Petek
credited graduate student Yanan Dai for his foresight and work in the process.
“The denouement of the research, however, is
that Yanan, who performed the experiments and provided the theoretical
modeling, demonstrated that he was educated far beyond his Professor’s level
and could interpret incisively the nanofemto topological properties and
interactions of optical fields,” he said.
The team
performed an ultrafast microscopy experiment, where they trapped green light
pulses of 20 fs (2x10-14 s) duration as composite light-electron density
fluctuation waves and imaged their propagation on a silver surface at the speed
of light. But they did this with a twist so that the light waves came together
from two sides to form a light vortex where light waves appear to circulate
about a stationary common core as a whirlwind of waves. They could generate a
movie of how light waves churn on their nanometer (10-9 m) wavelength scale by
imaging electrons that two light photons coming together cause to emit from the
surface.
Gathering
all such electrons with an electron microscope forms images where the light had
passed, thus enabling the researchers to take its snapshot. Of course, if
nothing is faster than light, one cannot take its snapshot, but by sending in
two light pulses with their time separation advanced in 10-16 s steps, they
could image how light waves come together causing their joint amplitude to rise
and fall at fixed points in space forming a light vortex on the nano (10-9
m)-femto (10-15 s) scale.
Such light
vortices form when you shine your red or green laser pointer onto a rough
surface and see a speckle reflection, but they also have a cosmological
significance. The light vortex fields can potentially cause transitions in the
quantum mechanical phase order in solid state materials, such that the
transformed material structure and its mirror image cannot be superimposed. In
other words, the sense of the vortex rotation generates two materials that are
topologically distinct.
Petek said
such topological phase transitions are at the vanguard of physics research
because they are thought to be responsible for some aspects of the structure of
the Universe.
“Even the
forces of nature including light, are thought to have emerged as symmetry
breaking transitions of a primordial field. Thus, the ability to record the
optical fields and plasmonic vortices in the experiment opens the way to
perform ultrafast microscopy studies of related light-initiated phase
transitions in condensed matter materials at the laboratory scale,” he said.