|Physicists at Ames Laboratory and Iowa State University have discovered
another useful property of graphene - it can function much like a laser
when excited with very short light pulses. The team has shown that the
material has two technologically important properties - population
inversion of electrons and optical gain. The findings suggest that
graphene could be used to make a variety of optoelectronics devices,
including broadband optical amplifiers, high-speed modulators, and
absorbers for telecommunications and ultrafast lasers.
In their experiments, the team excited high-quality, epitaxially grown
graphene monolayers with pump laser pulses just 35 fs long and photon
energy of around 1.55 eV. They then measured how much light was
reflected by the samples. Because graphene is just one atom thick and
has a zero-energy electronic bandgap, this measurement provides
information on the amount of light absorbed by the material. This in
turn depends on the optical conductivity of graphene.
The researchers found that the optical conductivity changes from being
positive to negative as the intensity of the pump pulses increases. The
intense external pump laser pulses excite electrons in graphene so that
more of these charge carriers exist in the upper 'Dirac cone' - the
conduction band of the material - than in the lower cone. Once such a
population inversion has occurred, a probe photon then stimulates these
excited states to emit infrared light in a coherent cascade.
This optical gain could be observed over a wide range of energies - up
to hundreds of millielectronvolts below the pump photon energy. Such a
broad optical gain might be unique to graphene and related to the fact
that photoexcited electrons in the material scatter extremely fast among
themselves. What is more, an ultrashort pulse just 35 fs long is
sufficient to produce this broadband gain - something that has never
been seen before in any material.