Terahertz Science and Technology of Carbon Nanomaterials

Junichiro Kono Departments of Electrical and Computer Engineering, Physics and Astronomy, and Materials

Science and NanoEngineering, Rice University

The diverse applications of terahertz (THz) radiation and its importance to fundamental condensed matter science makes finding ways to generate, manipulate, and detect THz radiation one of the key areas of modern applied physics. However, despite decades of worldwide efforts, the THz region of the electromagnetic spectrum still continues to be elusive for solid-state technology.

Recently, there has been a growing recognition that carbon nanomaterials – i.e., graphene and carbon nanotubes (CNTs) – have some outstanding electronic and photonic properties that are ideally suited for THz devices [1].

In this talk, after reviewing the past, current, and future of the THz science and technology of graphene and carbon nanotubes, we will present some of our latest results on THz dynamic conductivity and ultrafast carrier dynamic as well as THz devices including polarizers, modulators, and detectors.

CNT THz polarizers CNT THz detector THz plasmons in CNTs          Nano  Lett.  9,  2610  (2009);  12,  787  (2012)          Nano  Lett.  14,  3953  (2014)                        Nano  Lett.  13,  5991  (2013)    

THz dynamics in THz surface plasmon-polaritons THz modulation gated graphene in gated graphene with graphene/EOT                      Nano  Lett.  12,  3711  (2012)                                              Nano  Lett.  13,  3698  (2013)                              Nano  Lett.  14,  1242  (2014)  

1. R. R. Hartmann, J. Kono, and M. E. Portnoi, “Terahertz Science and Technology of Carbon Nanomaterials,” Nanotechnology 25, 322001 (2014).

Broadband Terahertz Polarizers with Ideal Performance Based onAligned Carbon Nanotube StacksLei Ren,†,‡,# Cary L. Pint,‡,§,# Takashi Arikawa,†,‡ Kei Takeya,∥ Iwao Kawayama,∥ Masayoshi Tonouchi,∥

Robert H. Hauge,§ and Junichiro Kono*,†,‡

†Department of Electrical and Computer Engineering, ‡Department of Physics and Astronomy, and §Department of Chemistry, RiceUniversity, Houston, Texas 77005, United States∥Institute of Laser Engineering, Osaka University, Yamadaoka 2-6, Suita, Osaka 565-0871, Japan

*S Supporting Information

ABSTRACT: We demonstrate a terahertz polarizer built with stacks of aligned single-walled carbon nanotubes (SWCNTs)exhibiting ideal broadband terahertz properties: 99.9% degree of polarization and extinction ratios of 10−3 (or 30 dB) from ∼0.4to 2.2 THz. Compared to structurally tuned and fragile wire-grid systems, the performance in these polarizers is driven by theinherent anistropic absorption of SWCNTs that enables a physically robust structure. Supported by a scalable dry contact-transfer approach, these SWCNT-based polarizers are ideal for emerging terahertz applications.KEYWORDS: Terahertz polarizer, aligned carbon nanotubes, chemical vapor deposition, contact transfer

Applications in terahertz (THz) technology are progressingat a rapid rate in concert with new compact techniques to

produce THz radiation.1−5 The potential applications of THztechnology are diverse, as THz radiation provides a uniquemedium for noninvasive imaging, communications, and sensingdevices that are currently being developed on both a researchand industrial scale.1,2 Complementary to the development ofthis technology, it is not only important to devise ways toproduce THz radiation but also to have robust approaches tomanipulate it and extract the detailed information containedwithin a coherent THz pulse. Currently, a host of wire-gridstructures composed of uniformly spaced metal wires areemployed as polarizers and filters for THz applications.Although these exhibit high extinction coefficients at THzwavelengths (>25 dB), they have drawbacks of fragility and astructurally tuned architecture that is not extendable tobroadband THz operation. Conventional THz polarizers aremade by mechanically winding thin metallic strings, such astungsten wires, on rigid frames under physical tension. Suchwidely used and commercially available THz polarizers aretypically free-standing, with function efficiencies highly relianton wire spacing constants.6,7

Here, we introduce a thin-film (<10 μm) homogeneousmaterial composed of single-walled carbon nanotubes(SWCNTs) that gives comparable performance to wire-gridtechnology but has added benefits of (i) broadband THzabsorption driven by the inherent one-dimensional (1-D)character of the SWCNTs and (ii) mechanical robustness indiverse operation conditions. In comparison to wire-gridtechnology, the THz performance of our material is drivennot by the precise structure of the conductive wires but ratherthe inherent anisotropic THz absorption properties of alignedSWCNTs. Although carbon nanomaterials (SWCNTs andgraphene) are predicted to be excellent THz materials,8−10

previous THz measurements of SWCNTs were performedeither on individual SWCNTs or on collective materials notideally suited for this application. Recently, we demonstrated acollective SWCNT material that behaves as a THz polarizer,with an 80% degree of polarization (DOP) and extinction ratio(ER) of 10 dBproperties below industrial standards due tothe nonideal extinction characteristics of the films.11 More

Received: October 26, 2011Revised: January 4, 2012Published: January 23, 2012

Letter

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© 2012 American Chemical Society 787 dx.doi.org/10.1021/nl203783q | Nano Lett. 2012, 12, 787−790

Carbon Nanotube Terahertz DetectorXiaowei He,†,‡ Naoki Fujimura,§ J. Meagan Lloyd,∥,¶ Kristopher J. Erickson,⊥ A. Alec Talin,⊥ Qi Zhang,†,‡

Weilu Gao,†,‡ Qijia Jiang,†,‡ Yukio Kawano,§ Robert H. Hauge,‡,#,∇ Francois Leonard,*,⊥and Junichiro Kono*,†,‡,●,□

†Department of Electrical and Computer Engineering, ‡The Richard E. Smalley Institute for Nanoscale Science and Technology,¶NanoJapan Program, #Department of Chemistry, ●Department of Physics and Astronomy, and □Department of Materials Scienceand NanoEngineering, Rice University, Houston, Texas 77005, United States§Quantum Nano-electronics Research Center, Department of Physical Electronics, Tokyo Institute of Technology, Meguro-ku,Tokyo 152-8552, Japan∥Electrical and Computer Engineering Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States⊥Sandia National Laboratories, Livermore, California 94551, United States∇Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

*S Supporting Information

ABSTRACT: Terahertz (THz) technologies are promisingfor diverse areas such as medicine, bioengineering, astronomy,environmental monitoring, and communications. However,despite decades of worldwide efforts, the THz region of theelectromagnetic spectrum still continues to be elusive for solidstate technology. Here, we report on the development of apowerless, compact, broadband, flexible, large-area, andpolarization-sensitive carbon nanotube THz detector thatworks at room temperature. The detector is sensitivethroughout the entire range of the THz technology gap,with responsivities as high as ∼2.5 V/W and polarization ratiosas high as ∼5:1. Complete thermoelectric and opto-thermalcharacterization together unambiguously reveal the photothermoelectric origin of the THz photosignal, triggered by plasmonicabsorption and collective antenna effects, and suggest that judicious design of thermal management and quantum engineering ofSeebeck coefficients will lead to further enhancement of device performance.KEYWORDS: Carbon nanotubes, THz photodetector, broadband, polarization sensitive

Recently, carbon-based nanomaterialscarbon nanotubes(CNTs) and graphenehave emerged as extraordinary

low-dimensional systems with a variety of outstandingelectronic and photonic properties,1−7 including those ideallysuited for terahertz (THz) devices.8−12 Carbon nanotubes(CNTs) have an extraordinary ability to absorb electromagneticwaves in an ultrawide spectral range, from nearly DC to theultraviolet, through both intraband (free carrier) absorptionand interband (excitonic) absorption processes.7,10,13,14 Anensemble of single-wall CNTs with mixed chiralities can thusabsorb electromagnetic radiation essentially at any frequency inthe entire electromagnetic spectrum, a property also shared bygraphene.15−17 This ultrabroadband property of these materials,combined with high-mobility carriers, promise high-speed andbroadband photodetectors as well as high-efficiency solarcells.4−6

THz detectors are required for a wide range of applicationsin astronomy, sensing, spectroscopy, imaging, defense, andcommunications.18−20 Current THz detectors are mostlycryogenic, narrow-band, or bulky, and thus, entirely novel

approaches or materials systems are being sought for detectingTHz radiation. THz detection has been reported by usingantenna-coupled, bundled21 and individual22 metallic single-wall CNTs at low temperatures, while THz-frequencyelectronic transport phenomena in single-tube devices havealso been investigated.23,24 In parallel, graphene THz detectorshave recently been fabricated and shown to possess promisingproperties,25,26 but much like the above CNT devices, thesesmall-area devices require coupling of the THz radiation withantennas. Furthermore, none of the existing approaches havedemonstrated intrinsic polarization sensitivity due to theabsorbing material. As described below, we have developed apowerless, compact, broadband, flexible, large-area, and polar-ization-sensitive CNT THz detector, which works at roomtemperature.

Received: April 5, 2014Revised: May 18, 2014Published: May 29, 2014

Letter

pubs.acs.org/NanoLett

© 2014 American Chemical Society 3953 dx.doi.org/10.1021/nl5012678 | Nano Lett. 2014, 14, 3953−3958

Plasmonic Nature of the Terahertz Conductivity Peak in Single-WallCarbon NanotubesQi Zhang,† Erik H. Haroz,† Zehua Jin,† Lei Ren,† Xuan Wang,† Rolf S. Arvidson,‡ Andreas Luttge,‡,§

and Junichiro Kono*,†,∥

†Department of Electrical and Computer Engineering, ‡Department of Earth Science, §Department of Chemistry, and ∥Departmentof Physics and Astronomy, Rice University, Houston, Texas 77005, United States

ABSTRACT: Plasmon resonance is expected to occur in metallic anddoped semiconducting carbon nanotubes in the terahertz frequencyrange, but its convincing identification has so far been elusive. Theorigin of the terahertz conductivity peak commonly observed forcarbon nanotube ensembles remains controversial. Here we presentresults of optical, terahertz, and direct current (DC) transportmeasurements on highly enriched metallic and semiconductingnanotube films. A broad and strong terahertz conductivity peakappears in both types of films, whose behaviors are consistent with theplasmon resonance explanation, firmly ruling out other alternativeexplanations such as absorption due to curvature-induced gaps.KEYWORDS: Single-wall carbon nanotubes, terahertz, plasmon resonance, density gradient ultracentrifugation

Understanding the dynamic and plasmonic properties ofcharge carriers in single-wall carbon nanotubes

(SWCNTs) is crucial for emerging applications of SWCNT-based ultrafast electronics and optoelectronics devices,1,2

especially in the terahertz (THz) range.3,4 SWCNTs withdifferent chiralities exhibit either semiconducting or metallicproperties, providing great flexibility for a variety of THz andplasmonic applications, including sources,5−7 detectors,3,5

antennas,8,9 and polarizers.10−12 A pronounced, finite-frequencypeak in THz conductivity spectra has been universally observedin diverse types of SWCNT samples, containing bothsemiconducting and metallic nanotubes.13−26 Two interpreta-tions have emerged regarding the THz peak, but there is noconsensus about its origin. One of the possible mechanismsproposed by many authors14,20−22,24 is based on interbandabsorption across the curvature-induced bandgap27,28 innonarmchair metallic SWCNTs, while the other is the plasmonresonance in metallic and doped semiconducting SWCNTs dueto their finite lengths.16,19,23,25,29 Hence, spectroscopic studiesof type-sorted SWCNT samples are crucial for determiningwhich of the two working hypotheses is correct.In the first scenario, direct interband absorption occurs

across the narrow bandgap27,28,30 induced by lattice distortionin the rolled-up graphene sheet in nonarmchair metallicSWCNTs. The magnitude of the induced bandgap is givenby28 Eg

ind = (3γ0aC−C2 )/(4dt

2) cos 3α, where aC−C = 0.142 nm isthe interatomic distance in graphene, dt is the nanotubediameter, γ0 ∼ 3.2 eV is the tight-binding transfer integral, andα is the chiral angle. For a SWCNT with dt ∼ 1.5 nm and α ∼0, Eg

ind ∼ 20 meV.31 If this scenario is correct, the THz peakshould (1) appear only in nonarmchair (α ≠ 30°) metallicSWCNTs, (2) show a sensitive dependence on dt,

24 (3) be

suppressed by doping or optical pumping,22 have a strong (4)temperature dependence and (5) polarization dependence. It isalso important to note that no detailed theory exists forpredicting the line shape for interband absorption in nonarm-chair metallic SWCNTs including excitonic effects.32,33

In the second scenario, THz radiation incident onto a finite-length metallic or doped semiconducting SWCNT launchescollective charge oscillations (plasmons) along the nanotubeaxis with a frequency given by the charge density and nanotubelength. The expected experimental signatures of this processare: (1) the THz peak should be observable both in metallicand doped semiconducting nanotubes; (2) the frequency of theTHz peak should systematically depend on the nanotube lengthin a predictable manner;25,29 (3) the THz peak intensity shouldbe enhanced by doping in semiconducting SWCNTs; (4) thereshould be weak temperature dependence;14 and (5) thereshould be strong polarization dependence, with no resonanceexpected for polarization perpendicular to the nanotube axis.To systematically prove or disprove these scenarios,

broadband spectroscopic studies on well-separated semi-conducting and metallic SWCNT samples are necessary.34 Inparticular, in order to correctly interpret THz spectra, it isimportant to monitor interband transitions in the near-infrared(NIR), visible (vis), and ultraviolet (UV). Hence, we performedabsorption spectroscopy studies from the THz to the UV aswell as DC transport measurements on highly enrichedsemiconducting and metallic SWCNT films. We clearlyobserved a broad and pronounced THz peak in both types of

Received: August 23, 2013Revised: October 23, 2013Published: November 13, 2013

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© 2013 American Chemical Society 5991 dx.doi.org/10.1021/nl403175g | Nano Lett. 2013, 13, 5991−5996

Terahertz and Infrared Spectroscopy of Gated Large-Area GrapheneLei Ren,† Qi Zhang,† Jun Yao,# Zhengzong Sun,‡ Ryosuke Kaneko,§ Zheng Yan,‡ Sebastien Nanot,†

Zhong Jin,‡ Iwao Kawayama,§ Masayoshi Tonouchi,§ James M. Tour,‡,∥,⊥ and Junichiro Kono*,†,¶

†Department of Electrical and Computer Engineering and ‡Department of Chemistry, Rice University, Houston, Texas 77005, UnitedStates§Institute of Laser Engineering, Osaka University, Yamadaoka 2-6, Suita, Osaka 565-0871, Japan∥Department of Computer Science, ⊥Department of Mechanical Engineering and Materials Science, and ¶Department of Physics andAstronomy, Rice University, Houston, Texas 77005, United States#Applied Physics Program through the Department of Bioengineering, Rice University, Houston, Texas 77005, United States

ABSTRACT: We have fabricated a centimeter-size single-layer graphene device with a gate electrode, which canmodulate the transmission of terahertz and infrared waves.Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range(10−10 000 cm−1), we measured the dynamic conductivitychange induced by electrical gating and thermal annealing.Both methods were able to effectively tune the Fermi energy,EF, which in turn modified the Drude-like intrabandabsorption in the terahertz as well as the “2EF onset” for interband absorption in the mid-infrared. These results not onlyprovide fundamental insight into the electromagnetic response of Dirac fermions in graphene but also demonstrate the keyfunctionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.KEYWORDS: Graphene, Fermi level, terahertz dynamics, infrared spectroscopy

The ac dynamics of Dirac fermions in graphene haveattracted much recent attention. The influence of linear

dispersions, two-dimensionality, electron−electron interactions,and disorder on the dynamic conductivity, σ(ω), has beentheoretically investigated,1−11 whereas unique terahertz (THz)and mid-infrared (MIR) properties have been identified fornovel optoelectronic applications.12−17 For example, it has beenpredicted that the response of Dirac fermions to an applied acelectric field of frequency ω would automatically contain all oddharmonics of (2n + 1)ω, where n is an integer, implyingextremely high nonlinearity.13,14 Furthermore, creation ofelectrons and holes through interband optical pumping isexpected to lead to population inversion near the Dirac point,resulting in negative σ(ω), or gain, in the THz to MIRrange;12,17 photoinduced femtosecond population inversion hasrecently been observed in the near-infrared range.18 Whileinitial experimental investigations on graphene have concen-trated on dc characteristics, these recent theoretical studies haveinstigated a flurry of new experimental activities to uncoverunusual ac properties. A number of experiments have alreadyconfirmed the so-called universal optical conductivity σ0 = e2/4ℏ (e, electronic charge and ℏ, reduced Planck constant) forinterband transitions in a wide spectral range.19−22 On theother hand, experimental studies of the intraband conductivityhave been limited,22−27 except for successful cyclotronresonance measurements to probe Landau levels in magneticfields (for a review, see, e.g., ref 28.).

Intraband absorption is expected to increase as the Fermienergy, EF, moves away from the Dirac point in either direction(p-type or n-type). On the other hand, interband absorption ispossible only when the photon energy is larger than 2EF (if thetemperature, T, is zero).1 Thus, spectroscopic studies with atunable carrier concentration should provide a precisedetermination of the location of the Fermi level, while at thesame time the capability of tuning the type and concentrationsof charge carriers in graphene is desired for many electronic andoptoelectronic applications. Substitution of carbon atoms ingraphene by nitrogen and boron has been attempted, but thisdramatically decreases the mobility by breaking its latticestructure; physically adsorbed molecules can also dopegraphene, but this is not a very controllable method. Therefore,applying a controllable gate voltage to graphene to transfercarriers from a doped silicon substrate is the most commonlyemployed method for tuning EF. By utilizing applied gatevoltages, different groups have observed tunable interbandoptical transitions,21,29 tunable intraband far-IR conductiv-ity,23,26,27 and a systematic G-band change with gate voltage inRaman spectra.30,31

Here, we describe our THz and MIR spectroscopy study oflarge-area (centimeter scale), single-layer graphene with anelectrically tunable Fermi level. In a field-effect transistor

Received: April 20, 2012Revised: June 1, 2012Published: June 4, 2012

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© 2012 American Chemical Society 3711 dx.doi.org/10.1021/nl301496r | Nano Lett. 2012, 12, 3711−3715

Excitation and Active Control of Propagating Surface PlasmonPolaritons in GrapheneWeilu Gao,† Gang Shi,‡ Zehua Jin,†,§ Jie Shu,† Qi Zhang,† Robert Vajtai,‡ Pulickel M. Ajayan,‡

Junichiro Kono,†,§ and Qianfan Xu*,†

†Department of Electrical and Computer Engineering, ‡Department of Mechanical Engineering & Materials Science, and§Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States

ABSTRACT: We demonstrate the excitation and gate controlof highly confined surface plasmon polaritons propagatingthrough monolayer graphene using a silicon diffractive grating.The normal-incidence infrared transmission spectra exhibitpronounced dips due to guided-wave resonances, whosefrequencies can be tuned over a range of ∼80 cm−1 byapplying a gate voltage. This novel structure provides a way toexcite and actively control plasmonic waves in graphene and isthus an important building block of graphene plasmonicsystems.KEYWORDS: Active plasmonics, graphene surface plasmon polaritons, infrared optoelectronics, nanophotonics

The unique electronic properties of graphene1−3 make it apromising platform to build highly integrated active

plasmonic devices4−7 and systems for a wide wavelength rangefrom near-infrared to terahertz (THz),8−17 which enablemanipulation and control of light confined in deeplysubwavelength structures. Existing metal-based active plas-monic devices have either slow speeds18 or very limitedtunability,4,19 and plasmonic devices based on a 2D electron gasin semiconductors20 have been demonstrated only at cryogenictemperatures. In contrast, graphene has been shown to supportsurface plasmon polaritons (SPPs) with stronger modeconfinement and lower propagation loss in the mid-infraredregion due to its large carrier mobility at room temper-ature.9,21,22 The carrier density in graphene can be electricallyadjusted dramatically with a small bias voltage applied to a field-effect transistor (FET), which can achieve tuning times below ananosecond.23 This unique combination makes graphene apromising material for electrically tunable active plasmonicdevices.The key challenge is to efficiently excite SPPs in graphene

with an incident electromagnetic wave, given the largewavevector mismatch between the two waves. Recent studiesdemonstrated near-field excitations and observation ofpropagating SPPs in graphene using near-field microscopywith nanotips.21,22 This type of excitation has a low efficiency asonly a very small percentage of incident photons can beconverted to SPPs. In this paper, we experimentallydemonstrate the excitation of SPPs in graphene using a silicongrating, where SPP is excited by a normal-incident free-spaceinfrared wave through the guided-wave resonance (GWR).24−26

Besides assisting the optical excitations, the silicon grating alsoacts as a gate electrode to tune the resonance frequency of thedevice over a broad spectral range.

Results. To excite SPPs in graphene with a free-spaceinfrared beam, their large difference in wavevector has to beovercome. Optical gratings are widely used to compensate forwavevector mismatches.20,27,28 Here we use a silicon diffractivegrating underneath the graphene layer, as shown in Figure 1a,to facilitate the excitation. By compensating for the wavevectormismatch, a highly confined propagating SPP in graphene layeris excited by a normal-incidence free-space light beam throughGWR.24,25

Assuming that the conductivity of graphene follows theDrude model,9,29,30 the dispersion relationship of the trans-verse-magnetic (TM) mode SPP31 in a continuous monolayerof graphene is approximately given by9

β ω π ε ε εωτ ω≈ ℏ + +⎜ ⎟⎛

⎝⎞⎠e E

i( )( )

12

0 r1 r22

f

2

(1)

where β(ω) is the in-plane wavevector of the SPP, ℏ is thereduced Planck constant, ε0 is the vacuum permittivity, εr1 andεr2 are the dielectric constants of the materials above and belowthe graphene film, τ is the carrier scattering time, Ef =ℏvf(πn)

1/2 is the Fermi energy measured from the Dirac point,n is the sheet carrier density, and vf ≈ 106 m/s is the Fermivelocity in graphene. The carrier scattering time τ determinesthe carrier mobility μ in graphene as τ = μEf/evf

2. The majorcomponent of the electric field of the SPP aligns with the wavepropagation direction.To compensate for the wavevector difference between the

graphene SPPs and a free-space wave incident at an angle θ, the

Received: May 1, 2013Revised: July 3, 2013Published: July 29, 2013

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© 2013 American Chemical Society 3698 dx.doi.org/10.1021/nl401591k | Nano Lett. 2013, 13, 3698−3702

High-Contrast Terahertz Wave Modulation by Gated GrapheneEnhanced by Extraordinary Transmission through Ring AperturesWeilu 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 ofPhysics and Astronomy, Rice University, Houston, Texas 77005, United States

ABSTRACT: Gate-controllable transmission of terahertz(THz) radiation makes graphene a promising material formaking high-speed THz wave modulators. However, to date,graphene-based THz modulators have exhibited only smallon/off ratios due to small THz absorption in single-layergraphene. Here we demonstrate a ∼50% amplitude modu-lation of THz waves with gated single-layer graphene by theuse of extraordinary transmission through metallic ringapertures placed right above the graphene layer. Theextraordinary transmission induced ∼7 times near-filedenhancement of THz absorption in graphene. These resultspromise 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

The 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 adiverse range of devices,4 including ultrafast photodetectors,5,6

transparent electrodes,7 optical modulators,8 active plasmonicdevices,9,10 and ultrafast lasers.11 In the terahertz (THz)frequency region, electrically controllable Drude-like intrabandabsorption makes graphene a promising platform for buildingactive, graphene-based optoelectronic devices12−15 such as THzmodulators. Compared to THz modulations demonstrated withfree carriers in conventional semiconductor materials16−21 andtwo-dimensional electron gases in quantum-well structures,22,23

graphene-based devices have higher carrier mobilities at roomtemperature 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-layergraphene (SLG) is limited due to its one-atomic-layer thicknessand the nonresonant nature of the intraband absorption in theTHz region. Recently, efforts to enhance the SLG absorption inthe THz region have been reported, including exciting plasmonicresonances in graphene,9 integrating graphene with photoniccavities,13,14 and integrating graphene with metamaterials.15,24

However, no devices demonstrated to date have a combinationof a large modulation depth, a high speed, and a designableresonance frequency, which we report in this paper.The extraordinary optical transmission (EOT) effect18−21 of

subwavelength apertures in a metallic film has been used toenhance THz absorption in various materials such as vanadiumdioxide (VO2).

18,20,21 In particular, we previously showed that

ring-shaped apertures have a strong polarization-insensitive EOTeffect, which allowed us to achieve THz transmission suppressionby 18 dB with a thin layer of carriers in a silicon substrateunderneath the apertures.25 Here, we use ring-shaped aperturesin 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 modulationdepth of ∼50% when the carrier density in SLG is tuned using aback-gating scheme. The modulator has a transmission peak witha bandwidth of ∼0.25 THz, which can suppress any off-resonance background signals. By scaling the circumference ofthe apertures, the operation frequency can be tuned for differentapplications. In addition, the small gated area and highconductivity of graphene makes high speed and low-energyconsumption possible since the aperture-to-area ratio (the ratioof the aperture area to the total metal area) of the EOT structureis only ∼1%, and the graphene layer only needs to be present inthe area underneath the apertures. These results suggest thatcomplementary 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 adiverse range of applications, including THz communications,imaging, and sensing.26,27

Results. The graphene-based THz modulator structure isschematically shown in Figure 1a and b. The EOT THz

Received: November 6, 2013Revised: February 2, 2014Published: February 3, 2014

Letter

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© 2014 American Chemical Society 1242 dx.doi.org/10.1021/nl4041274 | Nano Lett. 2014, 14, 1242−1248

Speaker: Junichiro KonoSession: Optics of graphene and 2D materials beyond grapheneSee program for placement.

PQE-2015 Abstract Processed 13 December 2014 0