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SAN FRANCISCO, Oct. 3 (Xinhua) -- Researchers with Stanford University have created a two-dimensional superlattice, sent electrons through the sheet, and achieved what theory suggested would be needed to conduct terahertz signals.
The new semiconductor material, made by sandwiching a sheet of atomically thin graphene in between two sheets of electrically insulating boron nitride, may allow for electromagnetic oscillations at terahertz frequency range theorized by the late Stanford professor and Nobel laureate Felix Bloch.
While large portions of the electromagnetic spectrum have been harnessed for diverse technologies, from X-rays to radios, a chunk of that spectrum, known as the terahertz gap, has remained largely out of reach. It is located between radio waves and infrared radiation, two parts of the spectrum now in use in everyday technologies including cell phones, TV remotes and toasters.
Bloch suggested that a specially structured material that allowed electrons to oscillate in a particular way might be able to conduct terahertz signals, and researchers have long thought that materials with repeating spatial patterns on the nanoscale might allow for Bloch's oscillations. Such a material requires that electrons travel long distances without deflection, where even the smallest imperfection in the medium through which the electrons flow can put them off their original path.
In the Stanford study, published in the recent issue of the journal Science, as they are protected from air and contaminants by boron nitride above and below in the 2-D superlattice, electrons in the graphene flow along smooth paths without deflection, exactly as Bloch's theory suggested would be needed to conduct terahertz signals.
The researchers were able to send electrons through the graphene sheet, collect them on the other side and use them to thus infer the activity of the electrons along the way, David Goldhaber-Gordon, a physics professor and co-author of the study, was quoted as saying in a new release. And the electrons can be confined to narrower bands of energy. Combined with very long times between deflections, it should lead the electrons to oscillate in place and emit radiation in the terahertz frequency range, a foundational success on the path toward creating controlled emission and sensing of terahertz frequencies.
In addition, the researchers found a completely surprising change in the electronic structure of their superlattice material. "In semiconductors, like silicon, we can tune how many electrons are packed into this material," said Goldhaber-Gordon. "If we put in extra, they behave as though they are negatively charged. If we take some out, the current that moves through the system behaves as if it' s instead composed of positive charges, even though we know it's all electrons."
The researchers believe future applications of this reversal in the character of the electrons could come in the form of more efficient p-n junctions, which are crucial building blocks to most semiconductor electronic devices such as solar cells, LEDs and transistors. Normally, if one shines light on a p-n junction, sending out one electron for every photon absorbed is considered excellent performance. But these new junctions could emit several electrons per photon, harvesting the energy of the light more effectively.
While the research hasn't yet created a Bloch oscillator, the researchers have achieved the first step by showing that the momentum and velocity of an electron can be preserved over long times and distances within this superlattice, said Menyoung Lee, co-author of the study who conducted the research as a graduate student in the Goldhaber-Gordon Group.
Terahertz oscillations someday may lead to improvements in technologies from solar cells to airport scanners.
Terahertz has similar transmission properties like microwaves but shorter wavelength, potentially revealing even nonmetal concealed objects at high resolution at airport security checkpoints, Goldhaber-Gordon explained, adding that terahertz scanners could also be used to detect defects such as hidden cavities in objects on a manufacturing assembly line.
