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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Sunday, July 22, 2012

Capturing, Tuning and Controlling Light with a Single Sheet of Carbon Atoms

Group Leaders: (From Left to Right) Javier Garcia de Abajo, Rainer Hillenbrand, Frank Koppens  

Authors: Jianing Chen1,2, Michela Badioli3, Pablo Alonso-González1, Susokin Thongrattanasiri4, Florian Huth1,5, Johann Osmond3, Marko Spasenović3, Alba Centeno6, Amaia Pesquera6, Philippe Godignon7, Amaia Zurutuza6, Nicolas Camara8, Javier García de Abajo4, Rainer Hillenbrand1,9, Frank Koppens3

1CIC nanoGUNE Consolider, 20018 Donostia-San Sebastián, Spain
2Centro de Fisica de Materiales (CSIC-UPV/EHU) and Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain
3ICFO-Institut de Ciéncies Fotoniques, Barcelona, Spain
4IQFR-CSIC, Madrid, Spain
5Neaspec GmbH, Munich, Germany
6Graphenea SA, 20018 Donostia-San Sebastián, Spain
7CNM-IMB-CSIC–Campus UAB, Barcelona, Spain
8GREMAN, UMR 7347, Université de Tours/CNRS, France
9IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain

Graphene, a remarkable one-atom-thick material consisting of a lattice of carbon atoms possesses extraordinary and gate-tunable optical properties. Interestingly, graphene can also carry strongly confined optical fields that travel along the surface of the sheet. These surface waves, based on the coupling between optical fields and charge carrier oscillations, are also called plasmons. For graphene, these plasmons have unique properties, as they can be tuned by electric fields, they propagate with speeds more than 100 times below the velocity of light, and they have a wavelength that is more than 100 times below the wavelength of light in free space. This makes it possible to confine light to extremely small volumes and to guide light along nanometer scale waveguides.

Since graphene was discovered, many theoretical physicists predicted the existence of graphene plasmons, but no experimental observations of propagating plasmons in graphene were reported so far. Due to the large mismatch in momentum between photons and graphene plasmons, it is not trivial to excite graphene plasmons by just shining light on a graphene sheet. This work has overcome this problem by focusing light on a sharp tip which is placed close to the graphene sheet. Because the tip acts as a nanoantenna, it can provide the extra momentum needed for the plasmons to be created (also called scattering near-field microscopy, s-SNOM). Moreover, the same tip can be used to probe the plasmons, which are reflected at the edges, and propagate back to the tip.

Interestingly, due to the interference between the plasmon waves that propagate away from the tip and towards the tip, it was possible to make real-space images of the plasmon waves with nanometer scale resolution (see Figure). For this experiment, a tapered graphene sheet was used where the variable width allowed for the observation of plasmon resonances defined by the standing plasmon wave between the edges. Similar to what happens with the standing waves on strings, only waves with appropriate characteristics can appear for a certain width.

Tuning the plasmon properties is a novel and unique aspect of graphene. This work, along with the work by Fei et al [2] (see 2Physics article of last week), shows not only the plasmon wavelength can be tuned over a wide range, it’s also possible to completely switch on and off the existence of the plasmons. In this way, it’s possible to electrically control light in a similar fashion as is traditionally achieved with electrons in a transistor. These capabilities, which until now were impossible with other existing plasmonic materials, enable new highly efficient nano-scale optical switches, which can perform calculations using light instead of electricity. In addition, the capability of trapping light in very small volumes could give rise to a new generation of nano-sensors, with applications in diverse areas such as medicine and bio-molecules, solar cells and light detectors, as well as quantum information processing.

[1]  Jianing Chen, Michela Badioli, Pablo Alonso-González, Susokin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza, Nicolas Camara, Javier García de Abajo, Rainer Hillenbrand, Frank Koppens, "Optical nano-imaging of gate-tunable graphene plasmons", Nature, DOI: 10.1038/nature1125 (Published online June 20, 2012). Abstract
[2] Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, D. N. Basov, "Gate-tuning of graphene plasmons revealed by infrared nano-imaging", Nature, DOI:10.1038/nature11253 (published online June 20, 2012). Abstract. 2Physics Article.

Contributions and institutes:
• Optical nano-imaging: CIC nanoGUNE Consolider (San Sebastian, Spain), CFM-CSIC-UPV/EHU (San Sebastian, Spain), Neaspec GmbH (Martinsried, Germany), Ikerbasque (Bilbao, Spain)
• Graphene nano-photonics and optoelectronics: ICFO (Barcelona, Spain)
• Theory: IQFR-CSIC (Madrid, Spain)
• Graphene synthesis: Graphenea (San Sebastian, Spain) University of Tours (Tours, France), and CNM-IMB-CSIC (Barcelona, Spain)

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