PerspectiveApplied Physics

Graphene Nanophotonics

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Science  22 Feb 2013:
Vol. 339, Issue 6122, pp. 917-918
DOI: 10.1126/science.1231119

Nanophotonics—the control of light at nanometer scales—has produced some amazing feats during the past two decades, using light to trap and manipulate small objects such as living cells, to propagate and process information faster than conventional electronic computers, and to reveal the presence of minute amounts of hazardous substances and pathogenic agents. Advances in precise nanofabrication and self-assembly have enabled the routine production of engineered nanostructures that can confine light to regions much smaller than the optical wavelength, thus enhancing the light intensity by several orders of magnitude at designated locations. To complete the picture, fast ways of modulating the optical response of these nanostructures have been pursued, but with mixed success only. The availability of new materials with novel photonic properties is a welcome stimulus to this challenging task, with graphene—a one-atom-thick layer of carbon atoms—offering great prospects for meeting these challenges.

Although high-quality graphene has been available for less than a decade, it has already made an impressive debut in nanophotonics. In addition to its ability to confine light to small volumes, its optical response can be modulated by subjecting it to electrical potentials, thereby allowing the manipulation of light at fast microelectronics speeds. Graphene can be integrated into devices operating under ambient conditions, it is structurally robust, it is electrically and chemically tunable, it exhibits an unusually strong nonlinear response, it can act both as a highly transparent thin conductor and as a perfect light absorber (depending on how it is prepared), and it is widely available in high quality at moderate cost. For these reasons, graphene has raised high expectations for its exploitation in a vast range of nanophotonic devices (1) and for the development of a new paradigm of tunable photonic structures that can control and process optical signals (2). Funding at an unprecedented level is being committed to support and extend these efforts (3).

The optical potential of graphene is perhaps best exemplified by the exotic properties of its collective electronic excitations, also known as plasmons. The existence of graphene plasmons has been established by the spectral signatures that they produce in the scattering of light from nanopatterned graphene ribbons (4). Direct spatial mapping of plasmons has also been achieved through the use of near-field optical probes (5, 6). In contrast to conventional plasmon-supporting materials, such as gold, graphene must be electrically charged (that is, doped with positive or negative charge carriers) if it is to sustain low-energy plasmons. But rather than a limitation, this allows tuning of the plasmon frequencies, which increase with the density of doping charge carriers. Using microelectronic gates to supply those carriers, we can manipulate light scattered or trapped by plasmons faster than ever before (5, 6) (see the figure, panel A). We thus have at our disposal a powerful tool for designing optical modulators, which currently rely on alternatives that are too slow (liquid crystals) or too bulky and expensive (nonlinear devices).

The graphene nanophotonics landscape.

(A) Light modulation of the optical response of graphene is realized by applying voltages through electrical gates (46). (B) Molecular detection is accomplished through the modifications in the optical response associated with changes in the concentration of charge carriers (9). (C) Measurement of the electrical current modulated by photon absorption leads to efficient light detection (11, 12). (D) Light harvesting occurs when the energy of absorbed photons is converted into charge carriers that are separated by doped gates to generate a net current.

In addition to its electrical tunability, which provides a suitable tool for ultrafast electro-optics, graphene offers a remarkable versatility with respect to control of its optical properties. For example, application of strong magnetic fields leads to the formation of terahertz magnetoplasmons (7). Chemical doping is another option that has been successfully used to tune plasmons in graphene microstructures (8). In a related development, changes in the electronic properties of the carbon layer have been used to monitor single molecules (9), which could also be observed optically (see the figure, panel B).

A promising approach toward light modulation consists of decorating plasmonic nanostructures with graphene (10). The small changes produced in the optical response of the carbon layer upon electrical doping can be amplified by the resonant plasmonic structure. Combined with electrical detection, nanoparticle plasmons have been used to achieve nanoscale spectral photodetectors (11); efficient terahertz detection has also been demonstrated by combining graphene with suitably designed antennas (12) (see the figure, panels C and D).

The unconventional electronic band structure of graphene, characterized by a constant product of electron energy and wavelength, makes it strongly nonlinear (13) and provides extra functionality—the ability to control light by light. A quantum mechanical approach to this goal could exploit the extremely high optical confinement and enhancement associated with graphene plasmons (14), possibly leading to a robust and viable solid-state implementation of devices that can process information encoded as quantum states of light.

Advances in graphene nanophotonics are rapidly configuring a new landscape of unsuspected achievements and possibilities. The extraordinary optical nonlinearity and fast modulation of graphene are triggering novel applications in a field that is replete with challenges and opportunities. A remaining goal is to extend the spectral region of tunable graphene optical response from the infrared toward the visible and near-infrared, where it can find a larger range of applications to optical modulation, spectral light detection, and sensing. These advances will benefit from the outstanding quality of currently available graphene, although new fabrication methods need to be devised in order to retain such quality after nanostructuring and manipulation of the carbon layer. The availability of new atomically thin structures (15) may catalyze the extension of graphene-optics phenomena to other platforms, while opening new areas of research into the symbiotic relation between the electronic and photonic behavior in these materials.

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