Session M7: Focus Session: Graphene Devices VII

Negative refractive index electron `optics', pseudospintronics and chiral tunneling in graphene pn junction -- beating the Landauer switching limit?

We use atomistic quantum kinetic calculations to demonstrate how graphene PN junctions can switch with high ON currents, low OFF currents, steep gate transfer characteristics and unipolar rectification. The physics of such unconventional switching relies on (a) field-engineering with patterned gates to create a \textit{transmission gap,} by sequential filtering of all propagating modes, and (b) using tilted junctions to suppress Klein tunneling under appropriate gate biasing, making that transmission gap \textit{gate tunable}. The doping ratio of the junction dictates the energy range over which the tilt angle exceeds the critical angle for transmission, generating thereby a gate tunable transmission gap that enables switching at voltages less than the Landauer-Shannon thermal limit. The underlying physics involves a combination of `electron optics' driven by Snell's law, negative index metamaterial with a PN junction, and pseudospin driven chiral tunneling, for which we also present experimental verification. [Sajjad et al, APL 99, 123101 (2011); Sajjad et al, PRB 86, 155412 (2012)].   

All-carbon optical diode

Optical diodes that allow unidirectional transport of light, similar to an electronic p-n junction diode, are vital to manipulate and control light for information processing. These ``optical diodes'' have already been realized using photonic crystals (PC) with engineered periodicity. However, an important criterion for the functioning of a PC-based optical diode is that the periodicity of the PC should be on the same length scale as half the wavelength of the electromagnetic waves used. For the visible region of the electromagnetic spectrum, this periodicity must be $\sim$ 200-350 nm making the fabrication of PCs expensive, cumbersome and complicated. An optical diode based on the transmission of optical pulses through structures with an abrupt variation in the longitudinal nonlinear absorption coefficient, as opposed to periodic variation of refractive index or dielectric constant is demonstrated. In particular, we present the studies performed on an all carbon optical diode with C60 and graphene coated on quartz cover slips. We find that the reverse saturable absorption of C60 and the saturable absorption of graphene can be combined to obtain modest reciprocity factors for a solid-state all-carbon optical diode.   

Photoconductivity of biased graphene

The origin of photosensitivity of graphene devices has been attributed to either thermoelectric, photovoltaic, or bolometric effects. Here we report on the intrinsic photoresponse of electrically biased, but otherwise homogeneous single-layer graphene. In this simple, yet unstudied experimental condition, the photocurrent shows polarity reversal, as it alternates between two of these effects while sweeping the electronic potential. Near the Dirac point, the photovoltaic effect dominates, and the photocurrent adds to the transport current. Away from the Dirac point, the bolometric effect dominates, and reduces the transport current. Magnitude and polarity of the photocurrent allow us to infer the hot carrier and phonon temperatures under light illumination. The electron temperature is found to be an order of magnitude higher than the phonon temperature, shedding light on energy loss pathways other than via intrinsic graphene phonons. (M. Freitag et al., Nature Photonics, accepted for publication (2012).)   

Far-IR Spectroscopy and FDTD Simulations of Graphene Plasmonic Structures

Plasmonics, the field of manipulating charge density waves, is uniquely suited to graphene due to graphene's high mobility and tunable plasma frequency in the THz range. Graphene microstructures, such as strips, discs, and rings confine plasmon modes, leading to plasma resonances with THz frequencies. These micro- and nanostructures form the building blocks of graphene plasmonic devices for tunable terahertz generation, detection, filtering, and switching. We present experimental results on the spectroscopy of plasmon resonances in the far-IR wavelength range in various graphene microstructures. Analytical methods of modeling even the simplest graphene plasmonic structures are not quantitatively accurate, and as such, we developed a 3D finite-difference time-domain (FDTD) tool for simulating the plasmon modes. By fitting simulations to the measured data, we have quantitatively extracted the parameters characterizing graphene's intraband conductivity and carrier scattering time with good accuracy. We have also investigated the interaction between plasmon modes of nearby structures and found them to be strong when the distance between structures is less than the dimension of the structures. FDTD simulations enable a quantitative characterization of such interactions.   

Ultra-Amplification of Surface Plasmon Coupled Emission in Graphene-Silver Hybrid Films

Surface Plasmon Coupled Emission (SPCE) stems from an interaction between fluorophores and thin metallic films and leads to strongly directional $p$-polarized emission with signal intensities that are 10-1000 times greater than isotropic fluorescence emission. Conventional SPCE methods use silver thin films with a SiO$_{2}$ spacer layer to prevent oxidation of silver, and the latter has no role in the signal generation. Here we employ single- and bi- layer graphene (SLG-BLG) as the spacer layer and demonstrate a 10 fold enhancement in comparison to the isotropic fluorescence intensity for rhodamine B fluorophore doped in PVA matrix. A fiber optic spectrometer was used to record the emission which was strongly directional (at 50$^{\circ}$ relative to the incident excitation) and 97{\%} $p$-polarized. Base on our preliminary simulations, we attribute the synergistic interaction between the $\pi $-plasmons of graphene and the surface plasmons of silver as the most important factor in the amplification of the SPCE.   

Single layer graphene plasmonic detector for broadband THz spectroscopy

Among many possible applications of graphene, THz detection is one of the most promising. The Drude-type absorption of THz radiation by free carriers is much stronger than the frequency-independent ~2.3\% absorption for interband transitions. By patterning the graphene sheet strips the Drude-type response is transformed into a Lorentzian peak corresponding to a THz plasmon resonance on the width $w$ of each strip. The plasmon resonance frequency $\omega_0 \propto n^{1/4} w^{1/2}$, where $n$ is carrier concentration which is tunable by gate(s) as was reported in Ref. 1 for graphene grown by chemical vapor deposition. We have reproduced results of Ref. 1 on our single layer graphene on Si-face SiC with electrolyte top gate. The next step to a detector is extraction of DC photocurrent without destroying plasmons. We will present our solution to this problem and compare the performance of our room-temperature detector to existing THz detector technologies. Other aspects of our graphene photodetectors such as device fabrication, response time, and response mechanism will be presented in other talks at this meeting. [1] L. Ju et al, Nature Nanotechnology, 6 (2011) 630-634.   

Graphene electrically reconfigurable patterns for THz imaging applications

THz waves are attractive for several imaging applications, since they can propagate through non metallic media such as paper, cloth, plastics, and ceramics, and do not scatter over nano-scale defects or ionize the material under imaging -as might shorter wavelengths do- while offering an image resolution similar to that of the human eye. In this work we propose and experimentally demonstrate electrically reconfigurable patterns for single-pixel terahertz imaging based on arrays of graphene THz electro-absorption modulators. In an optical setup, in conjunction with mirrors, the modulator array can transform the output radiation from a CW THz source into a pixelated and collimated beam of illumination. Single-atom-thick graphene is employed as the active element of these modulators, achieving a modulation of the THz wave reflectance \textgreater 50{\%} with a potential modulation depth approaching 100{\%} (i.e. each region of the pixelated collimated beam can be potentially completely turned-off). Although the proof-of-concept device here discussed only consists of 4x4 elements, we foresee that this technology can enable low-cost video rate THz imaging systems.   

Graphene Based Tunable SPR Sensors

Today's highly mobile world requires widely deployable disease detection and monitoring systems. We need compact, sensitive, and cost-effective biosensors, which can also tolerate a wide range of operating conditions to be field-deployable. Especially for point-of-care diagnostics, where the testing environment can be highly variable, it would be advantageous to have sensors with tunable operating ranges. To address this need, we propose tunable, localized surface plasmon resonance (SPR) based biosensors using graphene layers and metal nanoparticle arrays. Tuning capability is achieved by bias voltage applied to the thin layers of the substrate, where on metal nanoparticle arrays are fabricated. The key component of the design is graphene. The applied voltage changes not only optical properties of graphene but also the induced dipole moment of each nanoparticle and hence the resonance wavelength of the sensor. For the modeling of proposed tunable biosensors, we use both a frequency domain approximate solver (layer medium coupled dipole approximation) and a full wave time-domain electromagnetic solver (Wavenology). Numerical results obtained with these two independent solvers reveal the tuning capability of the proposed structures.   

Ferroelectric-Gated Terahertz Plasmonics on Graphene

Inspired by recent advancement of low-power ferroelectic-gated memories and transistors, we propose a design of ferroelectic-gated nanoplasmonic devices based on graphene sheets clamped in ferroelectric crystals. We show that the two-dimensional plasmons in graphene strongly couple with the phonon-polaritons in ferroelectrics at terahertz frequencies, leading to characteristic modal wavelength of the order of 100--200 nm at only 3--4 THz. By patterning the ferroelectrics into different domains, one can produce compact on-chip plasmonic waveguides, which exhibit negligible crosstalk even at 50 nm separation distance. Harnessing the memory effect of ferroelectrics, low-power electro-optical switching can be achieved on these plasmonic waveguides.   

Graphene nano-photonics and carrier dynamics

Graphene, a two-dimensional sheet of carbon atoms, has recently emerged as a novel material with unique electrical and optical properties, with great potential for novel opto-electronic applications, such as ultrafast photo-detection, optical switches, strong light-matter interactons etc. In the first part of this talk, I will review recent experimental work on exploiting graphene as a host for guiding, switching and manipulating light and electrons at the nanoscale [1]. This is achieved by exploiting surface plasmons: surface waves coupled to the charge carrier excitations of the conducting sheet. Due to the unique characteristics of graphene, light can be squeezed into extremely small volumes and thus facilitate strongly enhanced light-matter interactions. Additionally, I will discuss novel types of hybrid graphene photodetectors [2] and recent findings on carrier dynamics and hot carrier multiplication in graphene. By studying the ultrafast energy relaxation of photo-excited carriers after excitation with light of varying photon energy, we find that electron-electron scattering dominates the energy relaxation cascade rather than electron-phonon interaction [3]. This solves a long- standing debate on the relative contribution of electron-electron scattering versus optical phonon emission.\\[4pt] [1] J. Chen, M. Badioli, P. Alonso-Gonz\'{a}lez, S Thongrattanasiri, F Huth, J Osmond, M. Spasenovic, A. Centeno, A. Pesquera, P. Godignon, A. Zurutuza, N. Camara, J. Garcia de Abajo, R. Hillenbrand, F. Koppens, ``Optical nano- imaging of gate-tuneable graphene plasmons'', Nature (2012).\\[0pt] [2] G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, P. Garcia de Arquer, F. Gatti, F. Koppens, ``Hybrid graphene-quantum dot phototransistors with ultrahigh gain'', Nature Nanotechnology (2012).\\[0pt] [3] Photo-excitation Cascade and Multiple Carrier Generation in Graphene. K.J. Tielrooij, J.C.W. Song, S.A. Jensen,~A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L.S. Levitov, and F.H.L. Koppens. ArXiv 1210.1205 (2012)   

High-performance Photoconductive devices based on Graphene-Nanowire Hybrid Structures

The photoconductivity effect in various semiconducting materials has been extensively utilized for optoelectronic applications. However, conventional photoconductive channels exhibited rather slow responses to external light pulses because the photogenerated electrons and holes survive for a rather long time even after the lights are turned off. On the other hand, single-layer graphene (SLG) was reported to exhibit quite a fast photoconductivity, while its rather small photocurrent levels may limit the practical applications. Herein, we developed graphene-CdS nanowire (NW) hybrid structures for high-speed photoconductivity and large photoresponse. The hybrid structure consists of CdS NWs which were selectively grown in specific regions on a SLG sheet. The photosensor based on graphene-CdS NW hybrid structures exhibited rather large photocurrents as well as much faster operation speed than those based only on CdS NW networks. This simple but efficient strategy takes advantages of both graphene and NWs, and it should enable the fabrication of high performance optoelectronic devices for practical applications.   

Liquid-Gated Epitaxial Graphene: How Leakage Currents Affect Ionic Strength Sensing

Graphene is a promising material for the fabrication of miniaturized biological and chemical sensors. Epitaxial graphene is an exciting candidate due to its compatibility with standard processing techniques and its intrinsic robustness. We have fabricated liquid-gated FET-like devices based upon sub-millimeter wide epitaxial graphene strips defined using optical lithography methods. The devices exhibit a bipolar conductance versus gate voltage behavior with the minimum conductance point being dependent upon the ionic strength of a KCl solution. Measurements of the graphene conductance and gate-leakage currents during the stepping of the gate voltage demonstrate the presence of time dependent nA-scale leakage currents which limit signal stability at short times. Notably, these currents depend upon the gate voltage and the composition of the gate electrode. These and other electrode dependent effects have ramifications for graphene sensor design and implementation such as the need to limit gate voltage operating windows as and carefully design electrodes. With high transconductance and controlled doping, such devices should be able to function at low gate voltages if a full understanding of charge and charge transport at the graphene interface is obtained.   

Graphene as a Platform for Hybrid Optomechanical Devices

Graphene is known for providing a flat 2D material with outstanding optical, electrical and mechanical properties. We propose to take advantage of all three features by developing an optomechanical platform based on cantilevers made of freestanding multilayer graphene connected to an electrode. In this talk I will present several examples of a simple optomechanical systems involving a multilayer graphene suspended cantilevers that can act as a mirror closing an optical cavity. By varying the gate voltage applied on the mirror, its angle can be adjusted on a wide range (exceeding the wavelength of the incoming light) and its motion can be actuated and followed in real time from DC up to the tens of MHz range. Detection of elastic and inelastic scattered light can be performed. It allows simultaneous detection of motion, local stress and temperature of the membrane. A fully spectral detection of NEMS resonance is presented (1) and allows a novel optomechanical scheme based on coupling between motion and light through the dynamic mechanical stress. Further applications are presented as well such as a gate tunable enhancement of the Raman signal of molecular species adsorbed on the graphene platform. (1) Reserbat-Plantey, A., et al, Nature Nanotechnology, vol. 7, 151-155. (2012).