Boltasseva, Alexandra

Chen, Yong P.

Chung, Ting-Fung

Emani, Naresh K.

Kildishev, Alexander

Ni, Xingjie

Real time switching of a plasmonic resonance may find numerous applications in subwavelength optoelectronics, spectroscopy and sensing. We take advantage of electrically tunable interband transitions in graphene to control the strength of the plasmonic resonance

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Electrically Tunable Plasmonic Resonances with GrapheneEmani, Naresh K.; Chung, Ting-Fung; Ni, Xingjie; Kildishev, Alexander; Chen, Yong P.; Boltasseva,AlexandraPublished in:CLEO Technical DigestPublication date:2012Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Emani, N. K., Chung, T-F., Ni, X., Kildishev, A., Chen, Y. P., & Boltasseva, A. (2012). Electrically TunablePlasmonic Resonances with Graphene. In CLEO Technical Digest (pp. JTu1M.2). Optical Society of America.JTu1M.2.pdf CLEO Technical Digest © OSA 2012Electrically Tunable Plasmonic Resonances with Graphene Naresh K. Emani1, Ting-Fung Chung2, Xingjie Ni1, Alexander Kildishev1, Yong P. Chen2,  and Alexandra Boltasseva*,1,3,4 1School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA 2Department of Physics and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA 3DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, Lyngby, DK-2800, Denmark  4Erlangen Graduate School of Advanced Optical Technologies (SAOT), Universität Erlangen-Nürnberg, Paul-Gordan-Str. 6, 91052 Erlangen, Germany Email: aeb@purdue.edu*  Abstract: Real time switching of a plasmonic resonance may find numerous applications in subwavelength optoelectronics, spectroscopy and sensing. We take advantage of electrically tunable interband transitions in graphene to control the strength of the plasmonic resonance. OCIS codes: (250.5403) Plasmonics; (160.4670) Optical materials; (160.3918) Metamaterials 1. Introduction Plasmonics offers an exciting route to subwavelength optoelectronics by confining the optical fields below the diffraction limit [1]. This is achieved by employing metal nanostructures resonant at a particular optical wavelength. Dynamic control of these plasmonic resonances is critical for many applications like integrated modulators and switches. Active electrical control of resonances has been demonstrated at THz frequencies [2]. Real time switching of plasmonic resonances at optical wavelengths could open up many exciting possibilities. In this paper we demonstrate voltage controlled plasmonic resonance using graphene. Graphene is a hexagonal lattice of sp2 carbon atoms in a single sheet with extraordinary electrical and optical properties [3, 4], that was proposed as a platform for integrated optoelectronics and transformation optics [5-7]. Strong graphene-based modulation has recently been demonstrated for on-chip applications [8].   2. Tunable Plasmonic Resonances Graphene exhibits a unique linear dispersion relation shown in Fig. 1a. The carrier density (n2D) can be electrically controlled by an applied gate voltage, leading hereby to voltage controlled Fermi energy EF. A direct consequence of that is the gate tunable interband threshold, wherein an incident photon can generate an electron-hole (e-h) pair only if its energy is higher than 2EF. For photons of lower energy there will not be interband losses at 0K. At higher temperatures the interband threshold is broadened but shows qualitatively similar behavior. For realistic carrier densities in the range 1011 -1013 cm-2 the interband threshold can be continuously tuned up to near-infrared (NIR) frequencies. When metal nanostructures with resonant frequency in the NIR are fabricated on top of graphene sheet their optical properties can be tuned by the applied gate voltage. The resonance is also accompanied by a local field enhancement which causes interaction with the graphene sheet. Hence, if the resonant frequency is above the interband threshold then strong interband losses are present and the resonance is considerably weakened. Fig. 2b shows a schematic of the experimental setup for demonstrating plasmon resonance tuning. The source-drain contacts were deposited by e-beam evaporation of 10 nm Ti and 50 nm Au on doped Si substrate with 100 nm silicon oxide. A large area graphene sheet grown by chemical vapor deposition (CVD) was transferred onto the substrate ensuring electrical contact with both source and drain. Then, bow-tie like plasmonic structures were fabricated by electron beam lithography, metallization (2 nm Ti and 30 nm Au) and subsequent lift-off processes. The gate voltage applied to the Si substrate controls the carrier concentration in graphene sheet because of the field effect. The resistance between source and drain contacts was monitored to verify the field effect. A Fourier Transform Infrared (FTIR) spectrometer was used to optically characterize the plasmonic resonance behavior. A scanning electron micrograph of the fabricated sample with plasmonic structures on top of the graphene sheet is shown in Fig. 1c. It is clearly seen that CVD graphene is mostly single layered (verified separately by Raman spectroscopy) over the area of ~16 mm2. The optical transmission data shows a resonance at ~ 3µm that is in agreement with the simulated data for the chosen resonator design. The variation of the source-drain resistance with gate voltage is shown in Fig. 1d. Graphene shows minimum carrier concentration at a voltage of 8V that is referred as the Dirac point in literature. At voltages lower than the Dirac point, the hole concentration increases progressively moving the interband threshold towards NIR frequencies. Fig. 1e shows the initial results demonstrating weakening of the resonance with increasing carrier concentration. The percentage change in the resonance strength with applied JTu1M.2.pdf CLEO Technical Digest © OSA 2012voltage is up to 6% in the current sample as shown in Fig. 1f. We note that the maximum change in resonance occurs at 3.6 µm and can be further improved with higher carrier concentration. Further, the observed change of 6% is seen off-resonance and we believe that the change would be much higher with plasmon resonance at 4.5 μm and improved fabrication processes. In summary, we have demonstrated electrically controlled plasmonic resonance using graphene, and efforts are underway to achieve even higher tunability.   Fig 1. a) Band structure of graphene showing threshold for interband transitions which can be controlled by changing the carrier concentration; b) Schematic of the experimental setup for voltage controlled optical transmission measurements; c) SEM image of the fabricated sample showing the metal nanostructures on top of large area CVD grown graphene; d) In-situ electrical measurements showing carrier concentration change under applied voltage bias; e) Optical transmission measurements of bow-tie structures showing significant change in amplitude with applied voltage.; f) Percentage change in transmission showing peak change at 3.6 µm at 40 V bias.   Acknowledgements: This work was supported in part by ONR MURI on “Large-Area 3D Optical Metamaterials with Tunability and Low Loss” and ARO grant W911NF-11-1-0359.  3. References 1. S. Lal, S. Link, and N. J. Halas, "Nano-optics from sensing to waveguiding," Nature Photonics 1, 641-648 (2007). 2. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, "Active terahertz metamaterial devices," Nature 444, 597-600 (2006). 3. A. K. Geim and K. S. Novoselov, "The rise of graphene," Nature materials 6, 183-191 (2007). 4. F. Bonaccorso, Z. Sun, T. Hasan, and A. Ferrari, "Graphene photonics and optoelectronics," Nature Photonics 4, 611-622 (2010). 5. M. Jablan, H. Buljan, and M. Soljačić, "Plasmonics in graphene at infrared frequencies," Physical Review B 80, 245435 (2009). 6. F. Koppens, D. Chang, and J. Garcia de Abajo, "Graphene plasmonics: A platform for strong light-matter interactions," Nano letters (2011). 7. A. Vakil and N. Engheta, "Transformation Optics Using Graphene," Science 332, 1291 (2011). 8. M. Liu, X. Yin, E. Ulin-Avila, B. Geng, T. Zentgraf, L. Ju, F. Wang, and X. Zhang, "A graphene-based broadband optical modulator," Nature 474, 64-67 (2011). 

Electrically Tunable Plasmonic Resonances with Graphene

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Electrically Tunable Plasmonic Resonances with Graphene

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