Graphene’s two-dimensional honeycomb lattice is stronger than steel, lightweight, transparent and flexible, leading some to describe it as a ‘wonder material’ that will ‘revolutionise the 21st century
’. It has numerous potential applications in optoelectronics system, such as solar cells, very fast transistors and flexible displays. Theoretical studies and measurements of absorption have previously suggested that it could lase in the terahertz (the very long wavelength region of the spectrum, often used for security imaging), but there were no direct measurements.
The Artemis (link opens in a new window) experiment, carried out by a team from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg and the Max Planck Institute for Solid State Research in Stuttgart, involved using ultra-short flashes of extreme ultraviolet light to capture the electron dynamics of graphene. They found that a population inversion can be produced, when an infrared laser pulse is used as the pump source. The discovery is surprising because graphene lacks a classic semiconductor property, long considered a prerequisite for population inversion: a 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.
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”, lead scientist Isabella Gierz explains. The measurements indicate that graphene could be used to amplify terahertz light which is currently only produced using inefficient nonlinear processes. A terahertz graphene laser would be particularly useful for research in condensed matter physics.
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.
Isabella Gierz, Jesse C. Petersen, Matteo Mitrano, Cephise Cacho, I. C. Edmond Turcu, Emma Springate, Alexander Stöhr, Axel Köhler, Ulrich Starke & Andrea Cavalleri, “Snapshots of non-equilibrium Dirac carrier distributions in graphene”, Nature Materials 12 1119 (2013). doi:10.1038/nmat3757 (link opens in a new window)
Open access: arXiv:1304.1389 (link opens in a new window)
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“Max Planck team demonstrates that graphene can lase”, Laser Focus World (link opens in a new window), Oct 2013.