A team headed by scientists from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg have demonstrated that graphene 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.
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.
Emitting flashes of light: Graphene, a honeycomb lattice made of carbon atoms, is a suitable material for lasers emitting ultrashort terahertz pulses. © Joerg M. Harms
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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