Carbon nanotubes, recently created cylinders of tightly bonded carbon atoms,
have dazzled scientists and engineers with their seemingly endless list of special
abilities--from incredible tensile strength to revolutionizing computer chips.
In today's issue of Science, two University of Rochester researchers
add another feat to the nanotubes' list: ideal photon emission.
"The emission bandwidth is as narrow as you can get at room temperature,"
says Lukas Novotny, professor of optics at Rochester and co-author of the study.
Such a narrow and steady emission can make such fields as quantum cryptography
and single-molecule sensors a practical reality.
The emission profile came as a surprise to Todd Krauss, assistant professor
of chemistry at the University, and Novotny. They had set out to simply define
the emission, or fluorescence, of a single carbon nanotube. By using
a technique called confocal microscopy, the team illuminated a single nanotube
with a strongly focused laser beam. The tube absorbed the light from the laser
and then re-emitted light at new frequencies that carried information about
the tube's physical characteristics and its surroundings.
The light emitted from the nanotube was in precise, discrete wavelengths, unlike
most objects like molecules that radiate into a broader (i.e. more "fuzzy")
range of wavelengths at room temperature.
But a greater surprise was in store for the team.
"The emission wasn't just perfectly narrow, it was steady as far as we
could measure," says Krauss. In a strange quirk of quantum physics, molecules
usually emit their photons for a certain time and then cease, only to resume
again later, like a telegraph signal. The tubes that Krauss and Novotny measured,
however, remained steady beacons to the limits of their instruments' sensitivity.
"This is very exciting because for any application in quantum optics, you
want a steady and precise photon emitter," says Novotny.
Narrow emissions and a complete absence of blinking have tempting implications
for single photon emitters--devices needed to dependably release a single photon
on command. The U.S. Department of Defense is very interested in developing
quantum cryptography, a theoretically unbreakable method of coding information,
which necessitates a reliable way to deliver single photons on demand.
Other applications come in the form of sensors so sensitive they can detect
a single molecule of a substance. For example, when a biological molecule such
as a protein binds to a nanotube, the nanotube's perfect emission changes, revealing
the presence and characteristics of the molecule. Detecting the change would
be impossible if it weren't for the remarkably steady nature of the nanotube
emission, because a researcher wouldn't know for certain if a sudden change
in the emission was just a blink, or was meant to indicate the presence of the
target molecule.
Until just a few months ago, determining the emission characteristics of a nanotube
was impossible. Carbon nanotubes cannot be made individually-rather they come
as a jumble like a pile of spaghetti. Trying to measure the photon emission
of a tube in the jumble is impossible because the tube will pass the photons
it absorbs to other tubes instead of re-emitting them in its telltale fashion.
What scientists end up with is a sort of average of what the collection of tubes
will emit--not the emission characteristics of a single tube. Only within the
past few months have researchers figured out how to remove a single nanotube
from the pile of spaghetti in order to study its properties as an individual.
Krauss and Novotny are now devising experiments to test the steadiness of the
nanotube fluorescence beyond the range of the initial experiments, and are pursuing
studies aimed at determining the ultimate minimum possible emission bandwidth
at ultracold temperatures.
This work was funded by the National Science Foundation, the U.S. Department
of Energy, the Research Corporation, and the New York State Office of Science
and Academic Research.