  Graphene is a single layer of covalently bonded carbon atoms, which was
discovered only 8 years ago and yet has already attracted intense research and
commercial interest. Initial research focused on its remarkable electronic
properties, such as the observation of massless Dirac fermions and the
half-integer quantum Hall effect. Now graphene is finding application in
touch-screen displays, as channels in high-frequency transistors and in
graphene-based integrated circuits. The potential for using the unique
properties of graphene in terahertz-frequency electronics is particularly
exciting; however, initial experiments probing the terahertz-frequency response
of graphene are only just emerging. Here we show that the photoconductivity of
graphene at terahertz frequencies is dramatically altered by the adsorption of
atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the
signature of terahertz stimulated emission from gas-adsorbed graphene. Our
findings highlight the importance of environmental conditions on the design and
fabrication of high-speed, graphene-based devices.
