Extraordinary Control Of Terahertz Beam Reflectance In .

Letterpubs.acs.org/NanoLettExtraordinary Control of Terahertz Beam Reflectance in GrapheneElectro-absorption ModulatorsBerardi Sensale-Rodriguez,*,† Rusen Yan,†,‡ Subrina Rafique,† Mingda Zhu,† Wei Li,‡,§ Xuelei Liang,§David Gundlach,‡ Vladimir Protasenko,† Michelle M. Kelly,† Debdeep Jena,† Lei Liu,†and Huili Grace Xing*,††Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United StatesSemiconductor and Dimensional Metrology Division, National Institute of Standards and Technology, Gaithersburg, Maryland20899, United States§Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing, 100871,China‡ABSTRACT: We demonstrate a graphene-based electroabsorption modulator achieving extraordinary control ofterahertz reflectance. By concentrating the electric fieldintensity in an active layer of graphene, an extraordinarymodulation depth of 64% is achieved while simultaneouslyexhibiting low insertion loss ( 2 dB), which is remarkablesince the active region of the device is atomically thin. Thismodulator performance, among the best reported to date,indicates the enormous potential of graphene for terahertzreconfigurable optoelectronic devices.KEYWORDS: Graphene, terahertz, electro-absorption, modulators, filters, active tuningTarray of gated graphene stripes;10 a higher modulation depthwas reported in GaN plasmonic structures but at cryogenictemperatures.11 Since larger modulation depths are required forvarious applications, more efficient structures such as activeterahertz metamaterials12 are still the most promisingalternatives. In many of these metamaterials, the change ofconductivity, and thus free carrier absorption in a semiconductor active region, located between periodic metallicpatterns (i.e., frequency selective surface), translates into anexceptional control over terahertz wave at frequencies close tothe structure intrinsic resonance. This significant tuning can beunderstood on the basis of an electric field enhancement in thedevice active region. In this letter, we demonstrate an extremelysimple, efficient, and polarization independent terahertzmodulator, which employs atomically thin graphene layers asthe active material and achieves modulation depth comparableto the state of the art while simultaneously exhibiting muchlower losses.Since its discovery, graphene has attracted much attention inthe research community due to its remarkable electrical andmechanical properties.13,14 For instance, graphene high-speedtransistors with an intrinsic cutoff frequency near 300 GHzwere recently demonstrated,15 as well as flexible electronics16and so on. In the optical range, graphene has also shownerahertz (THz) technology, owing to its applications inmany diverse areas such as astronomy, biomedicine,communications, defense, and so forth, has recently becomeone of the most dynamic fields of scientific research.1 But thelimited performance of available terahertz devices restricts thedevelopment of many of these applications. Although the pastdecades have seen increasingly rapid advances in terahertztechnologies, particularly emitters and detectors, there is still adire need for efficient reconfigurable devices such as activefilters, modulators, and switches.2 Terahertz modulators arestructures capable of actively controlling the amplitude (orphase) of the transmitted (or reflected) waves. Applications ofthese devices include: power stabilization of terahertz sources inspectroscopy,3 carrier-wave modulation for terahertz bandcommunication systems,4 switches for coded aperture terahertzimaging,5 and so forth. Device proposals so far rely on directlymodifying the device geometry (e.g., in MEMS6) or effectivelymodifying material properties (primarily optical conductivity)of the tunable elements. For instance, metal-gated semiconductors can provide room-temperature broadband intensitymodulation but with low modulation depth7 ( 6% intensitymodulation was experimentally demonstrated8). By employinggraphene as an active “gate”, a remarkable broadbandmodulation of 20% was recently reported,9 though short ofthe theoretical expectation of unity modulation due to thequality of the large area graphene currently available. Takingadvantage of the plasmonic effects, a modulation depth of 13% was also observed at room temperature employing an 2012 American Chemical SocietyReceived: April 30, 2012Revised: July 25, 2012Published: August 3, 20124518dx.doi.org/10.1021/nl3016329 Nano Lett. 2012, 12, 4518 4522

Nano LettersLetterbroadband transmittance modulators: by applying a voltagebetween the top graphene layer and the back metal, the carrierconcentration and thus the absorption of terahertz waves ingraphene is tuned. Although carriers of the opposite type ingraphene are accumulated near the capacitively coupled SiO2/Si interface, their influence in modulator performance can beneglected9 as the mobility of these carriers in Si is much smallerthan those in graphene. When the Fermi level in graphene istuned at the Dirac point (V VCNP), the terahertz absorptionby the modulator is minimum. On the other hand, when theFermi level shifts into valence (π) or conduction band (π*) ofgraphene, the density of states available for intrabandtransitions, and thus the optical absorption, increases. Sincethe back metal acts as both an electrode and a reflector, theterahertz wave intensity is zero at the back metal; the wavestrength in the active graphene layer on top depends on thesubstrate optical thickness and the terahertz wavelength. Asshown in Figure 1b, when the substrate optical thickness is anodd-multiple of the THz wavelength, the field intensity ingraphene is at maxima. Consequently, very large absorptionswings, that is, extraordinary modulation, can take place whengraphene conductivity is tuned. On the other hand, if thesubstrate optical thickness is an even-multiple of the THzwavelength, graphene does not absorb, the modulatorreflectance becomes unity and independent of its conductivity,since the field-intensity is zero in the active graphene layer. Asimilar idea was adopted recently by Lee et al.24 to demonstratea reflectance modulator using graphene in the IR range.However, the observed modulation depth was only 4% becauseof the small tunability of graphene absorption in the IR range atnormal incidence, which is not the case in the THz band.7Theoretical analysis of the dependence of reflectance ongraphene conductivity and substrate optical thickness is shownin Figure 1b. Given that the optical thickness of the SiO2 ( 100nm) is much smaller than the THz wavelength ( 500 μm inair at 600 GHz), its effect can be neglected.9 For the samegraphene conductivity swing reported in ref 9 (between 0.2 and0.9 mS), a modulation depth of 70% (4 larger) with aninsertion loss of 1.5 dB is expected. These devices canpotentially operate with an insertion loss as low as 1 dB and amodulation depth approaching 95% if the large area grapheneand insulator quality allow its conductivity to be tuned betweenthe commonly observed DC minimum conductance of 4e2/h 0.15 mS25 and a maximum conductivity approaching 1.8 mS.Since the top graphene is not patterned, the modulator isintrinsically polarization-independent.The modulator was fabricated using chemical vapordeposition (CVD) graphene grown on copper.26 The p-Sisubstrate is highly resistive (ρ 1000 Ω·cm) with a 70-nmthick SiO2 grown thermally. Graphene was transferred bymeans of poly(methyl methacrylate) (PMMA) and wet etchmethods. Metal contacts (Ti/Au) were deposited on grapheneand the back side of the Si substrate, as depicted in the deviceschematic (Figure 1a). Finally, the graphene surface wascovered with a thin layer of PMMA. After fabrication, thedevice was characterized using a terahertz imaging andspectroscopy setup27 based on a Virginia Diode, Inc. (VDI)multiplier source, capable of providing continuous wave (CW)radiation in the 570 630 GHz frequency band and abroadband Schottky diode detector.28 In all of the experimentsthe beam was at normal incidence to the samples. A sketch ofthe setup is presented in Figure 2a. Two control samples werealso used: a bare Si substrate and a Si substrate with the metalpromising device performance, for example, in high-speedinfrared (IR) electro-absorption modulators,17 transparentelectrodes,18,19 and photodetectors.20 The optical absorptionof graphene can be described based on the contributions ofinter- and intraband transitions and plasmonic effects, thereforeleading to two distinct regions of operation:21 first, the IR/visible range where interband transitions dominate and opticalconductivity has either a weak dependence on the Fermi levelor is a constant, and second, the lower terahertz range, whereoptical conductivity is mainly due to intraband transitions, wellrepresented by a Drude model,22 and closely follows theelectrical conductivity. In the terahertz range, by electrostatically tuning the density-of-states (DOS) available forintraband transitions, terahertz beam transmittance can beeffectively controlled.7,9,23 Graphene also enables very lowinsertion loss ( 0.5 dB) owing to its small minimumconductivity.9 In addition, taking advantage of plasmoniceffects in patterned graphene, the possibility of activelymanipulating waves in the upper THz/far-IR range wasrecently demonstrated.10The device considered in this work (Figure 1a) consists of asingle-layer of graphene on top of a SiO2/Si substrate with ametal back gate, which also acts as a reflector. The electricalcontrol mechanism is similar to the one previously employed inFigure 1. Structure of the graphene electro-absorption modulator andoperating principle: (a) Schematic of the terahertz modulator. Thesingle-layer graphene was transferred onto a SiO2/p-Si substrate. Thetop and bottom metal contacts were employed to tune the grapheneconductivity. The electric field intensity exhibits a node at the backcontact since it also acts as a reflector. Typical band diagrams forvarious voltages (V) are depicted on the right. When the Fermi level ingraphene is tuned to Dirac point (V VCNP), reflectance is at itsmaxima. (b) Calculated power reflectance as a function of thesubstrate optical thickness normalized to terahertz wavelength fordifferent graphene conductivities. When the substrate optical thicknessis an even-multiple of a quarter-wavelength, the electric field intensityvanishes in the graphene layer, therefore leading to zero absorption.On the other hand, if the substrate optical thickness is an odd-multipleof a quarter-wavelength, absorption can be extraordinarily tuned.4519dx.doi.org/10.1021/nl3016329 Nano Lett. 2012, 12, 4518 4522