High-Contrast Terahertz Wave Modulation by Gated Graphene Enhanced by Extraordinary Transmission through Ring Apertures Weilu Gao, † Jie Shu, † Kimberly Reichel, † Daniel V. Nickel, † Xiaowei He, † Gang Shi, ‡ Robert Vajtai, ‡ Pulickel M. Ajayan, ‡ Junichiro Kono, †,‡,§ Daniel M. Mittleman, † and Qianfan Xu* ,† † Department of Electrical and Computer Engineering, ‡ Department of Materials Science and NanoEngineering, and § Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States ABSTRACT: Gate-controllable transmission of terahertz (THz) radiation makes graphene a promising material for making high-speed THz wave modulators. However, to date, graphene-based THz modulators have exhibited only small on/off ratios due to small THz absorption in single-layer graphene. Here we demonstrate a ∼50% amplitude modu- lation of THz waves with gated single-layer graphene by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer. The extraordinary transmission induced ∼7 times near-filed enhancement of THz absorption in graphene. These results promise complementary metal-oxide-semiconductor compatible THz modulators with tailored operation frequencies, large on/ off ratios, and high speeds, ideal for applications in THz communications, imaging, and sensing. KEYWORDS: Graphene photonics, THz modulator, extraordinary optical transmission, near-field enhancement, high on/off ratio T he unique properties of graphene have stimulated world- wide interest in developing novel devices for electronics, photonics, and optoelectronics. 1-3 In particular, gate-control- lable electronic properties of graphene are expected to lead to a diverse range of devices, 4 including ultrafast photodetectors, 5,6 transparent electrodes, 7 optical modulators, 8 active plasmonic devices, 9,10 and ultrafast lasers. 11 In the terahertz (THz) frequency region, electrically controllable Drude-like intraband absorption makes graphene a promising platform for building active, graphene-based optoelectronic devices 12-15 such as THz modulators. Compared to THz modulations demonstrated with free carriers in conventional semiconductor materials 16-21 and two-dimensional electron gases in quantum-well structures, 22,23 graphene-based devices have higher carrier mobilities at room temperature with an electrically tunable carrier density. Despite the broadly tunable carrier density, the extinction ratio that can be obtained for THz wave modulations with single-layer graphene (SLG) is limited due to its one-atomic-layer thickness and the nonresonant nature of the intraband absorption in the THz region. Recently, efforts to enhance the SLG absorption in the THz region have been reported, including exciting plasmonic resonances in graphene, 9 integrating graphene with photonic cavities, 13,14 and integrating graphene with metamaterials. 15,24 However, no devices demonstrated to date have a combination of a large modulation depth, a high speed, and a designable resonance frequency, which we report in this paper. The extraordinary optical transmission (EOT) effect 18-21 of subwavelength apertures in a metallic film has been used to enhance THz absorption in various materials such as vanadium dioxide (VO 2 ). 18,20,21 In particular, we previously showed that ring-shaped apertures have a strong polarization-insensitive EOT effect, which allowed us to achieve THz transmission suppression by 18 dB with a thin layer of carriers in a silicon substrate underneath the apertures. 25 Here, we use ring-shaped apertures in a metallic film to enhance the extinction ratio of a graphene- based THz modulator. We show that apertures resonating at ∼0.44 THz enhance the intraband absorption in SLG under- neath the apertures by ∼675%, which leads to a modulation depth of ∼50% when the carrier density in SLG is tuned using a back-gating scheme. The modulator has a transmission peak with a bandwidth of ∼0.25 THz, which can suppress any off- resonance background signals. By scaling the circumference of the apertures, the operation frequency can be tuned for different applications. In addition, the small gated area and high conductivity of graphene makes high speed and low-energy consumption possible since the aperture-to-area ratio (the ratio of the aperture area to the total metal area) of the EOT structure is only ∼1%, and the graphene layer only needs to be present in the area underneath the apertures. These results suggest that complementary metal-oxide-semiconductor (CMOS) com- patible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds can be built, which will find a diverse range of applications, including THz communications, imaging, and sensing. 26,27 Results. The graphene-based THz modulator structure is schematically shown in Figure 1a and b. The EOT THz Received: November 6, 2013 Revised: February 2, 2014 Published: February 3, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 1242 dx.doi.org/10.1021/nl4041274 | Nano Lett. 2014, 14, 1242-1248