Work
on graphene continues to be endlessly fascinating. It also continues to set new rules for what
is possible. Beyond graphene, similar
methods are also applied to the other elements and molecules and we are truly
attacking the true possibilities of material physics.
Expectations
regarding accessing the Nano scale are working out wonderfully.
This
also sets the stage for manipulating photons generally and that will be even
more amazing.
Graphene can emit laser flashes
Individual layers of carbon atoms are suitable
as active material for terahertz lasers, as they permit population inversion
October
08, 2013
Graphene is considered the jack-of-all-trades
of materials science: The two-dimensional honeycomb-shaped lattice made up of
carbon atoms is stronger than steel and exhibits extremely high charge carrier
mobilities. It is also transparent, lightweight and flexible. No wonder that
there are plenty of applications for it – for example, in very fast transistors
and flexible displays. A team headed by scientists from the Max Planck
Institute for the Structure and Dynamics of Matter in Hamburg have demonstrated
that it also meets an important condition for use in novel lasers for terahertz
pulses with long wavelengths. The direct emission of terahertz radiation would
be useful in science, but no laser has yet been developed which can provide it.
Theoretical studies have previously suggested that it could be possible with
graphene. However, there were well-founded doubts - which the team in Hamburg
has now dispelled. At the same time, the scientists discovered that the scope
of application for graphene has its limitations though: in further
measurements, they showed that the material cannot be used for efficient light
harvesting in solar cells.
A
laser amplifies light by generating many identical copies of photons – cloning
the photons, as it were. The process for doing so is called stimulated emission
of radiation. A photon already produced by the laser makes electrons in the
laser material (a gas or solid) jump from a higher energy state to a lower
energy state, emitting a second completely identical photon. This new photon
can, in turn, generate more identical photons. The result is a virtual
avalanche of cloned photons. A condition for this process is that more
electrons are in the higher state of energy than in the lower state of energy.
In principle, every semiconductor can meet this criterion.
The
state which is referred to as population inversion was produced and
demonstrated in graphene by Isabella Gierz and her colleagues at the Max Planck
Institute for the Structure and Dynamics of Matter, together with the Central
Laser Facility in Harwell (England) and the Max Planck Institute for Solid
State Research in Stuttgart. The discovery is surprising because graphene lacks
a classic semiconductor property, which was long considered a prerequisite for
population inversion: a so-called bandgap. The bandgap is a region of forbidden
states of energy, which separates the ground state of the electrons from an
excited state with higher energy. Without excess energy, the excited state
above the bandgap will be nearly empty and the ground state below the bandgap
almost completely populated. A population inversion can be achieved by
adding excitation energy to electrons to alter their energy state to the one
above the bandgap. This is how the avalanche effect described above is
produced.
Until now, terahertz
pulses have only been generated via inefficient non-linear optical processes
However,
the forbidden band in graphene is
infinitesimal. “Nevertheless, the electrons in graphene behave similarly to
those of a classic semiconductor”, Isabella Gierz says. To a certain extent,
graphene could be thought of as a zero-bandgap semiconductor. Because of the
absence of a bandgap, the population inversion in graphene only lasts for
around 100 femtoseconds, less than a trillionth of a second. “That is why
graphene cannot be used for continuous lasers, but potentially for ultrashort
laser pulses”, Gierz explains.
Such
a graphene laser would be particularly useful for research purposes. It
could be used to amplify laser light with very long wavelengths; so-called
terahertz radiation. This type of laser light could be employed in basic
research to study, for example, high-temperature superconductors. To date,
terahertz radiation has been produced using comparatively inefficient,
so-called non-linear optical processes. In addition, the available wavelength
range is often limited by the non-linear material used. The recent findings
indicate that graphene could be used for broad bandwidth amplification of
arbitrarily long wavelengths.
However,
the Hamburg-based team also dashed the hopes of some materials scientists – as
it turns out, graphene is probably not suited for converting solar radiation
into electricity in solar cells. “According to our measurements, a single
photon in graphene cannot release several electrons, as previously expected”, Gierz
says. This is a prerequisite for efficient conversion of radiation into
electricity.
Silicon carbide can be
used to produce graphene for lasers
The
scientists in Hamburg studied the graphene using a method called time-resolved
photoemission spectroscopy. This involved illuminating the material with
ultrashort ultraviolet (UV) light pulses. As a consequence the electrons are
forced out of the sample and the physicists measure their energy and angle of
exit. The resulting data is used to establish the energy distribution of
electrons in the material. Time resolution is achieved by delaying the arrival
time of the UV probe pulse with respect to an arbitrary excitation pulse.
In
the present experiment, the electrons in the graphene were excited using
infrared laser light. Then the scientists employed photoemission spectroscopy
to demonstrate the occurrence of population inversion. In a similar way, they
established that carrier multiplication could not be achieved by radiation.
The
graphene was produced by the scientists through thermal decomposition of
silicon carbide. According to Gierz, this procedure can also be used to make a
graphene laser, since silicon carbide is transparent and will not interfere
with terahertz radiation. However, the physicist admits that a lot of
development work remains to produce a graphene laser.
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