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.