Graphene materials and devices in terahertz science and technology
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Introduction “Terahertz” is an unexplored frequency band in the sense that no commercially available microelectronic device can generate, detect, or manipulate electromagnetic waves over the entire terahertz frequency band.1 In the past decade, therefore, development of compact, tunable, and coherent terahertz sources that operate at room temperature has been one of the hottest issues in modern terahertz electronics.1 Graphene, a one-atom-thick planar sheet of sp2-hybridized honeycomb carbon crystals, has attracted considerable attention because of its unique carrier-transport and optical properties.1–4 The conduction and valence bands of graphene have a symmetrical conical shape around the Brillouin zone corners, which are called the K and K′ points, and contact each other at “Dirac points”. Electrons and holes in graphene have a linear dispersion relation with zero bandgap, resulting in peculiar features such as massless relativistic fermion behavior with backscatteringfree ultrafast transport.2–8
In addition, optical or electrical pumping of graphene can dramatically alter its electromagnetic response at frequencies in the technologically underexploited terahertz range,9–11 as a result of nonequilibrium dynamics of carrier relaxation and recombination. Specifically, the very fast energy relaxation of photoexcited electrons and holes through optical-phonon emission in graphene and their relatively slow recombination lead to population inversion over a wide energy range under sufficiently high pumping intensity. This population inversion, in which some higher-energy states contain more electrons than states below them, makes it possible to obtain negative dynamic conductivity in the terahertz spectral range.9–11 Such negative conductivity can produce amplification, or gain, of electromagnetic signals in the appropriate frequency range and, in a well-designed cavity structure, could enable graphene-based coherent laser sources of terahertz radiation.11–14 These unique features of graphene materials can be exploited to develop many high-performance terahertz devices including
Taiichi Otsuji, Tohoku University and JST-CREST, Japan; [email protected] Stephane Albon Boubanga Tombet, Tohoku University, Japan; [email protected] Akira Satou, Tohoku University and JST-CREST, Japan; [email protected] Hirokazu Fukidome, Tohoku University and JST-CREST, Japan; [email protected] Maki Suemitsu, Tohoku University and JST-CREST, Japan; [email protected] Eiichi Sano, Hokkaido University and JST-CREST, Japan; [email protected] Vyacheslav Popov, Russian Academy of Science, Russia; [email protected] Maxim Ryzhii, University of Aizu and JST-CREST, Japan; [email protected] Victor Ryzhii, Tohoku University and JST-CREST, Japan; [email protected] DOI: 10.1557/mrs.2012.241
• VOLUME • DECEMBER MRS 2012 • www.mrs.org/bulletin 2012 Materials Research Society Downloaded© from https://www.cambridge.org/core. Iowa State University Library, on 28 Jan 2019 at 12:46:00, subject to B
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