This is one more very neat trick
to winkle a three orders of magnitude speed out of the appropriate electronics.
What we are now able to
accomplish at almost atomic levels has become astounding and quite enjoyable to
track. It is along way from the radio enthusiasts
of my youth when folks easily understood what they were working on.
This is neat stuff. Again take your time on this item
Nanoplasmonic Breaks Emission Time Record in Semiconductors
by Staff Writers
A rendering of the triple-layered nanowire and 'whispering
gallery'
Renaissance architects demonstrated their understanding of geometry and
physics when they built whispering galleries into their cathedrals. These
circular chambers were designed to amplify and direct sound waves so that, when
standing in the right spot, a whisper could be heard from across the room.
Now, scientists at the University
of Pennsylvania
"When you excite a semiconductor, then it takes a few nanoseconds
to get back to the ground state accompanied by emission of light," Agarwal
said. "That's the emission lifetime. It's roughly the amount of time the
light is on, and hence is the amount of time it takes for it to be ready to be
turned on again.
"If you're making a modulator, something that switches back and
forth, you're limited by this time constant. What we've done is reduced it to
less than a picosecond. It's more than a thousand times faster than anything
currently available."
In semiconductors, the excited state is when energy is present in the
system, and the ground state is when there is none. Normally, the semiconductor
must first "cool down" in the excited state, releasing energy as
heat, before "jumping" back to the ground state, releasing the
remaining energy as light.
The Penn team's semiconductor nanowires, however, can jump directly
from a high-energy excited state to the ground, all but eliminating the
cool-down period.
The advancement in emission lifetime is due to the unique construction
of the team's nanowires. At their core, they are cadmium sulfide, a common
nanowire material. But they are also wrapped in a buffer layer of silicon
dioxide, and, critically, an outer layer of silver.
The silver coating supports what are known as surface plasmons, unique
waves that are a combination of oscillating metal electrons and
of light. These surface plasmons are highly confined to the surface the silicon
dioxide and silver layers meet.
"The previous state of the art was taking a nanowire, just like
ours, and laying it on a metal surface," Agarwal said. "We curved
the metal surface around the wire, making a complete nanoscale plasmonic cavity
and the whispering gallery effect."
For certain nanowire sizes, the silver coating creates pockets of
resonance and hence highly confined electromagnetic fields within the
nanostructure. Emission lifetime can then be engineered by precisely
controlling high intensity electromagnetic fields inside the light-emitting
medium, which is the cadmium sulfide core.
To reach an emission lifetime measured in femtoseconds, the researchers
needed to optimally balance this high-confinement electromagnetic field with an
appropriate "quality factor," the measurement of how good a cavity is
at storing energy.
To complicate matters, quality factor and confinement have an inverse
relationship; the higher the quality-factor a cavity has the bigger it is and
the smaller its confinement.
However, by opting for a reasonable quality factor, the researchers
could vastly increase the confinement of the electric field inside
the nanowire by using resonant surface plasmons and get the record-breaking
emission lifetime.
This many-orders-of-magnitude improvement could find a home in a
variety of applications such as LEDs, detectors and other nanophotonic devices
with novel properties.
"Plasmonic computers could make good use of these nanowires,"
Cho said. "We could increase modulation speed into the terahertz range
whereas electronic computers are limited to a few gigahertz range."
"The same physics governs emission and absorption, so these
nanowires could also be used for increasing efficiency of absorption in
solar cells," Agarwal said.
The research was conducted by associate professor Ritesh Agarwal,
postdoctoral fellows Chang-Hee Cho and Sung-Wook Nam and graduate student Carlos
O. Aspetti, all of the Department of Materials Science and Engineering in
Penn's School of Engineering and Applied Science. Michael E. Turk and James M.
Kikkawa of the Department of Physics and
Astronomy in the School
of Arts
No comments:
Post a Comment