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Broadband Impedance Match to Monolayer Graphene in the Terahertz Domain


The coupling of an electromagnetic plane wave to a thin conductor depends on the sheet conductance of the material. A poor conductor interacts weakly with the incoming light, allowing the majority of the radiation to pass. A good conductor does not absorb light and reflects the wave almost entirely. For suspended films, the transition from transmitter to reflector occurs when the sheet resistance is approximately the characteristic impedance of free-space (Z0 = 377 Ω). Near this point, the interaction is maximized, and the conductor absorbs strongly. We show that monolayer graphene, a tunable conductor, can be electrically modified to reach this transition, thereby achieving the maximum absorptive coupling across a broad range of frequencies in terahertz (THz) band. This interaction with an electromagnetic wave, to be a transmitter or absorber, is based on tunable electronic properties (rather than geometric structure), and is realized by bottom-up engineering of large-area monolayer graphene devices.

Chemical vapor deposition is used to increase the graphene domain size and decrease grain boundaries. By using a fast two-step oxidized copper growth, individual graphene domains of 5-mm can be synthesized in less than 5-hours total growth duration. Following CVD synthesis, graphene transfer is optimized by removing bubbles that adhere to the graphene surface, and are transferred onto substrates modified by self-assembled monolayers (SAM), leading to improvement of the device on/off ratio. The high mobility graphene devices fabricated demonstrate the largest transmittance depth-of-modulation of THz waves to date, and by impedance matching devices using chemical doping, near maximum absorption is achieved.

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