One of the hottest areas of the electromagnetic spectrum being explored today is the terahertz (THz) range. Terahertz waves, lying between microwave and optical frequencies, offer improved performance for a variety of applications in everyday life. For instance, THz waves can carry more information than radio/microwaves for communications devices. They also provide medical and biological images with higher resolution than microwaves, while offering much smaller potential harm of exposure than X-rays.
“A major bottleneck in the promise of THz technology has been the lack of efficient materials and devices that manipulate these energy waves,” says Berardi Sensale- Rodriguez, a graduate student in the Department of Electrical Engineering at Notre Dame. “Having a naturally two-dimensional material with strong and tunable response to THz waves, for example, graphene, gives us the opportunity to design THz devices achieving unprecedented performance.
Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.
Terahertz transmittance through single and two-layer graphene. (a) Optical image of a CVD graphene sample on a 200 micron quartz substrate. (b) Intensity transmittance map at 600 GHz of the same sample shown in (a). The blue triangle delineates the edges of the quartz substrate owing to strong edge scattering effects, and low transmission in the center (blue) is due to absorption/reflection in graphene. The substrate attenuation is negligible because of the low dielectric constant of quartz, which reduces Fabry-Perot oscillations. (c) Transmittance as a function of position along the z direction line cut depicted in (b). (d) Measured transmittance as a function of frequency for single and two-layer graphene samples on quartz.
Operating principle and modulator structures.