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Graphene has caused a lot of excitement among scientists since theextremely strong and thin carbon material was discovered in 2004.Just one atom thick, the honeycomb-shaped material has severalremarkable properties combining mechanical toughness with superiorelectrical and thermal conductivity. Now a group of scientists at Iowa State University, led byphysicist Jigang Wang, has shown that graphene has two otherproperties that could have applications in high-speedtelecommunications devices and laser technology - populationinversion of electrons and broadband optical gain. Wang is an assistant professor in the Department of Physics andAstronomy in the College of Liberal Arts and Sciences at Iowa StateUniversity. He also is an associate scientist with the Departmentof Energy's Ames Laboratory. Wang's team flashed extremely short laser pulses on graphene. Theresearchers immediately discovered a new photo-excited graphenestate characterized by a broadband population inversion ofelectrons. Under normal conditions, most electrons would occupylow-energy states and just a few would populate higher-energystates. In population-inverted states, this situation is reversed: moreelectrons populate higher, rather than lower, energy states. Suchpopulation inversions are very rare in nature and can have highlyunusual properties. In graphene, the new state produces an opticalgain from the infrared to the visible. Simply stated, optical gain means more visible light comes out thangoes in. This can only happen when the gain medium is externallypumped and then stimulated with light (stimulated emission). Wang'sdiscovery could open doors for efficient amplifiers in thetelecommunication industry and extremely fast opto-electronicsdevices. Graphene as a gain medium for light amplification "It's very exciting," Wang said. "It opens the possibility ofusing graphene as a gain medium for light amplification. It couldbe used in making broadband optical amplifiers or high-speedmodulators for telecommunications. It even provides implicationsfor development of graphene-based lasers." Wang's team unveiled its findings in the journal Physical ReviewLetters on April 16. In addition to Wang, the paper's other authorsare Tianq Li, Liang Luo and Junhua Zhang, Iowa State physicsgraduate students; Miron Hupalo, Ames Laboratory scientist; andMichael Tringides and Jorg Schmalian, Iowa State physics professorsand Ames Laboratory scientists. Wang is a member of the Condensed Matter Physics program at IowaState and the Ames Laboratory. He and his team conduct opticalexperiments using laser spectroscopy techniques, from the visibleto the mid-infrared and far-infrared spectrum. They use ultrashortlaser pulses down to 10 quadrillionths of a second to study theworld of nanoscience and correlated electron materials. In 2004 United Kingdom researchers Andre Geim and KonstantinNovoselov discovered graphene, which led to their winning the 2010Nobel Prize in Physics. Graphene is a two-dimensional (height andwidth) material with a growing list of known unique properties. It is a single layer of carbon only one atom thick. The carbonatoms are connected in a hexagonal lattice that looks like ahoneycomb. Despite a lack of bulk, graphene is stronger than steel,it conducts electricity as well as copper and conducts heat evenbetter. It is also flexible and nearly transparent. An understanding gap existed, Wang explained, between the twoscientific communities that studied the electronic and photonicproperties of graphene. He believed his group could help bridge thegap by elaborating the non-linear optical properties of grapheneand understanding the non-equilibrium electronic state. Wang explained that linear optical properties only transmit light -one light signal comes into a material and one comes out. "Thenon-linear property can change and modulate the signal, not justtransmit it, producing functionality for novel deviceapplications." Graphene in a highly non-linear state Wang said other scientists have studied graphene's opticalproperties, but primarily in the linear regime. His teamhypothesized they could generate a new "very unconventional state"of graphene resulting in population inversion and optical gain. "We were the first group to break new ground, to start looking atit in a highly excited state consisting of extremely denseelectrons - a highly non-linear state. In such a state, graphenehas unique properties." Wang's group started with high-quality graphene monolayers grown byHupalo and Tringides in the Ames Laboratory. The researchers usedan ultrafast laser to "excite" the material's electrons with shortpulses of light just 35 femtoseconds long (35 quadrillionths of asecond). Through measurements of the photo-induced electronic states, Wang'steam found that optical conductivity (or absorption) of thegraphene layers changed from positive to negative - resulting inthe optical gain - when the pump pulse energy was increased above athreshold. The results indicated that the population inverted state inphotoexcited graphene emitted more light than it absorbed. "Theabsorption was negative. It meant that population inversion isindeed established in the excited graphene and more light came outof the inverted medium than what entered, which is optical gain,"Wang said. "The light emitted shows gain of about one percent for a layer amere one atom thick, a figure on the same order to what's seen inconventional semiconductor optical amplifiers hundreds of timesthicker." The key to the experiments, of course, was creating the highlynon-linear state, something "that does not normally exist inthermal equilibrium," Wang said. "You cannot simply put grapheneunder the light and study it. You have to really excite theelectrons with the ultrafast laser pulse and have the knowledge onthe threshold behaviors to arrive at such a state." Wang said a great deal more engineering and materials perfectionlies ahead before graphene's full potential for lasers and opticaltelecommunications is ever realized. "The research clearly shows,though, that lighting up graphenes may produce brighter emissionsas well as a bright future," he said. The e-commerce company in China offers quality products such as Led Diode Resistor Manufacturer , Tool Cabinet on Wheels, and more. For more , please visit High Power White Leds today!
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