Pablo Jarillo-Herrero

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

Are you Pablo Jarillo-Herrero?

Claim your profile

Publications (108)954.25 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hexagonal boron nitride (h-BN) is a natural hyperbolic material 1 , in which the dielectric constants are the same in the basal plane (ε t ≡ ε x = ε y) but have opposite signs (ε t ε z < 0) in the normal plane (ε z) 1–4. Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons 1,2,5 —collective modes that originate from the coupling between photons and electric dipoles 6 in phonons. However, control of these hyperbolic phonon polari-tons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN 1,2,7. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure 8 composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene 9–13 with hyperbolic phonon polaritons in h-BN 1,2 , so that the eigenmodes of the gra-phene/h-BN heterostructure are hyperbolic plasmon–phonon polaritons. The hyperbolic plasmon–phonon polaritons in gra-phene/h-BN suffer little from ohmic losses, making their propagation length 1.5–2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon–phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial 14 as the resulting properties of these devices are not present in its constituent elements alone. Van der Waals (vdW) heterostructures assembled from mono-layers (one or a few) of graphene, hexagonal boron nitride (h-BN), MoS 2 and other atomic crystals in various combinations are emerging as a new paradigm with which to attain desired electronic 8,15
    Nature Nanotechnology 06/2015; DOI:10.1038/NNANO.2015.131 · 33.27 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Interference of standing waves in electromagnetic resonators forms the basis of many technologies, from telecommunications and spectroscopy to detection of gravitational waves. However, unlike the confinement of light waves in vacuum, the interference of electronic waves in solids is complicated by boundary properties of the crystal, notably leading to electron guiding by atomic-scale potentials at the edges. Understanding the microscopic role of boundaries on coherent wave interference is an unresolved question due to the challenge of detecting charge flow with submicron resolution. Here we employ Fraunhofer interferometry to achieve real-space imaging of cavity modes in a graphene Fabry-Perot resonator, embedded between two superconductors to form a Josephson junction. By directly visualizing current flow using Fourier methods, our measurements reveal surprising redistribution of current on and off resonance. These findings provide direct evidence of separate interference conditions for edge and bulk currents and reveal the ballistic nature of guided edge states. Beyond equilibrium, our measurements show strong modulation of the multiple Andreev reflection amplitude on an off resonance, a direct measure of the gate-tunable change of cavity transparency. These results demonstrate that, contrary to the common belief, electron interactions with realistic disordered edges facilitate electron wave interference and ballistic transport.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hybrid graphene-topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI Bi$_2$Se$_3$ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the Bi$_2$Se$_3$ density of states. Similar results were obtained for BLG on top of Bi$_2$Se$_3$, with 10-fold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both Bi$_2$Se$_3$ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI $\Gamma$ and graphene $K, K'$ points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge-neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A far-reaching goal of graphene research is exploiting the unique properties of carriers to realize extreme nonclassical electronic transport. Of particular interest is harnessing wavelike carriers to guide and direct them on submicron scales, similar to light in optical fibers. Such modes, while long anticipated, have never been demonstrated experimentally. In order to explore this behavior, we employ superconducting interferometry in a graphene Josephson junction to reconstruct the real-space supercurrent density using Fourier methods. Our measurements reveal charge flow guided along crystal boundaries close to charge neutrality. We interpret the observed edge currents in terms of guided-wave states, confined to the edge by band bending and transmitted as plane waves. As a direct analog of refraction-based confinement of light in optical fibers, such nonclassical states afford new means for information transduction and processing at the nanoscale.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. In such materials light propagation is unusual, leading to novel and often non-intuitive optical phenomena. Here we report infrared nano-imaging experiments demonstrating that crystals of hexagonal boron nitride (hBN), a natural mid-infrared hyperbolic material, can act as a "hyper-focusing lens" and as a multi-mode waveguide. The lensing is manifested by subdiffractional focusing of phonon-polaritons launched by metallic disks underneath the hBN crystal. The waveguiding is revealed through the modal analysis of the periodic patterns observed around such launchers and near the sample edges. Our work opens new opportunities for anisotropic layered insulators in infrared nanophotonics complementing and potentially surpassing concurrent artificial hyperbolic materials with lower losses and higher optical localization.
    Nature Communications 04/2015; 6(6963). DOI:10.1038/ncomms7963 · 10.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications.
    Nature Nanotechnology 04/2015; 10(5). DOI:10.1038/nnano.2015.54 · 33.27 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report high quality graphene and WSe2 devices encapsulated between two hexagonal boron nitride (hBN) flakes using a pick-up method with etched hBN flakes. Picking up pre-patterned hBN flakes to be used as a gate dielectric or mask for other 2D materials opens new possibilities for the design and fabrication of 2D heterostructures. In this letter, we demonstrate this technique in two ways: first, a dual-gated graphene device that is encapsulated between an hBN substrate and pre-patterned hBN strips. The conductance of the graphene device shows pronounced Fabry-Pérot oscillations as a function of carrier density, which implies strong quantum confinement and ballistic transport in the locally gated region. Second, we describe a WSe2 device encapsulated in hBN, with the top hBN patterned as a mask for the channel of a Hall bar. Ionic liquid selectively tunes the carrier density of the contact region of the device, while the hBN mask allows independent tunability of the contact region for low contact resistance. Hall mobility larger than 600 cm(2)/(V·s) for few-layer p-type WSe2 at 220 K is measured, the highest mobility of a thin WSe2 device reported to date. The observations of ballistic transport in graphene and high mobility in WSe2 confirm pick-up of pre-patterned hBN as a versatile technique to fabricate ultra-clean devices with high quality contact.
    Nano Letters 02/2015; 15(3). DOI:10.1021/nl504750f · 12.94 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have implemented and investigated the tunable hyperbolic response in heterostructures comprised of a monolayer graphene deposited on hexagonal boron nitride (G-hBN) slabs. Electrostatic gating of the graphene layer enables electronic tunability of phonon polaritonic properties of hBN: a layered material with well-documented hyperbolic response in the mid-infrared (mid-IR) frequencies. The tunability originates from the hybridization of surface plasmon polaritons in graphene to hyperbolic phonon polaritons in hBN: an effect that we examined via nano-IR imaging and spectroscopy. The hybrid polaritons possess combined virtues from plasmons in graphene and phonon polaritons in hBN. Therefore, G-hBN structures fulfill the definition of the electromagnetic metamaterial since the attained property of these devices is not revealed by its constituent elements. Our results uncover a practical approach for realization of agile nano-photonic metamaterials by exploiting the interaction of distinct types of polaritonic modes hosted by different constituent layers of van der Waals heterostructures.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.
    Journal of Physics Condensed Matter 11/2014; 27(16). DOI:10.1088/0953-8984/27/16/164207 · 2.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Controlling the energy flow processes and the associated energy relaxation rates of a light emitter is of high fundamental interest, and has many applications in the fields of quantum optics, photovoltaics, photodetection, biosensing and light emission. While advanced dielectric and metallic systems have been developed to tailor the interaction between an emitter and its environment, active control of the energy flow has remained challenging. Here, we demonstrate in-situ electrical control of the relaxation pathways of excited erbium ions, which emit light at the technologically relevant telecommunication wavelength of 1.5 $\mu$m. By placing the erbium at a few nanometres distance from graphene, we modify the relaxation rate by more than a factor of three, and control whether the emitter decays into either electron-hole pairs, emitted photons or graphene near-infrared plasmons, confined to $<$15 nm to the sheet. These capabilities to dictate optical energy transfer processes through electrical control of the local density of optical states constitute a new paradigm for active (quantum) photonics.
    Nature Physics 10/2014; 11:281-287. DOI:10.1038/nphys3204 · 20.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on electronic transport measurements of dual-gated nano-devices of the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all devices the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the inter-surface capacitance $C_{TI}$ and the surface state energy-density relationship $\mu$(n), which is found to be consistent with independent ARPES measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.
    Physical Review Letters 10/2014; 113(20). DOI:10.1103/PhysRevLett.113.206801 · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as $\gamma_0 = 3.1$ eV, $\gamma_1 = 0.39$ eV, and $\gamma_4 = 0.22$ eV.
    06/2014; 2(9). DOI:10.1063/1.4890543
  • Hugh O H Churchill, Pablo Jarillo-Herrero
    Nature Nanotechnology 05/2014; 9(5):330-1. DOI:10.1038/nnano.2014.85 · 33.27 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The crystal structure of a material plays an important role in determining its electronic properties. Changing from one crystal structure to another involves a phase transition that is usually controlled by a state variable such as temperature or pressure. In the case of trilayer graphene, there are two common stacking configurations (Bernal and rhombohedral) that exhibit very different electronic properties. In graphene flakes with both stacking configurations, the region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon-carbon bond distance. Here we show the ability to move this strain soliton with a perpendicular electric field and hence control the stacking configuration of trilayer graphene with only an external voltage. Moreover, we find that the free-energy difference between the two stacking configurations scales quadratically with electric field, and thus rhombohedral stacking is favoured as the electric field increases. This ability to control the stacking order in graphene opens the way to new devices that combine structural and electrical properties.
    Nature Material 04/2014; 13(8). DOI:10.1038/nmat3965 · 36.43 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on temperature dependent photocurrent measurements of high-quality dual-gated monolayer graphene (MLG) p-n junction devices. A photothermoelectric (PTE) effect governs the photocurrent response in our devices, allowing us to track the hot electron temperature and probe hot electron cooling channels over a wide temperature range (4 K to 300 K). At high temperatures ($T > T^*$), we found that both the peak photocurrent and the hot spot size decreased with temperature, while at low temperatures ($T < T^*$), we found the opposite, namely that the peak photocurrent and the hot spot size increased with temperature. This non-monotonic temperature dependence can be understood as resulting from the competition between two hot electron cooling pathways: (a) (intrinsic) momentum-conserving normal collisions (NC) that dominates at low temperatures and (b) (extrinsic) disorder-assisted supercollisions (SC) that dominates at high temperatures. Gate control in our high quality samples allows us to resolve the two processes in the same device for the first time. The peak temperature $T^*$ depends on carrier density and disorder concentration, thus allowing for an unprecedented way of controlling graphene's photoresponse.
    Physical Review Letters 03/2014; 112(24). DOI:10.1103/PhysRevLett.112.247401 · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The p-n junction is the functional element of many electronic and optoelectronic devices, including diodes, bipolar transistors, photodetectors, light-emitting diodes and solar cells. In conventional p-n junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Ambipolar semiconductors, such as carbon nanotubes, nanowires and organic molecules, allow for p-n junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here, we report ambipolar monolayer WSe2 devices in which two local gates are used to define a p-n junction within the WSe2 sheet. With these electrically tunable p-n junctions, we demonstrate both p-n and n-p diodes with ideality factors better than 2. Under optical excitation, the diodes demonstrate a photodetection responsivity of 210 mA W(-1) and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising values for a nearly transparent monolayer material in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe2. These devices provide a building block for ultrathin, flexible and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.
    Nature Nanotechnology 03/2014; DOI:10.1038/nnano.2014.25 · 33.27 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: van der Waals heterostructures assembled from atomically thin crystalline layers of diverse two-dimensional solids are emerging as a new paradigm in the physics of materials. We used infrared nanoimaging to study the properties of surface phonon polaritons in a representative van der Waals crystal, hexagonal boron nitride. We launched, detected, and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens. The measured dispersion of polaritonic waves was shown to be governed by the crystal thickness according to a scaling law that persists down to a few atomic layers. Our results are likely to hold true in other polar van der Waals crystals and may lead to new functionalities.
    Science 03/2014; 343(6175):1125-9. DOI:10.1126/science.1246833 · 31.48 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We explore the photoresponse of an ambipolar graphene infrared thermocouple at photon energies close to or below monolayer graphene's optical phonon energy and electrostatically accessible Fermi energy levels. The ambipolar graphene infrared thermocouple consists of monolayer graphene supported by an infrared absorbing material, controlled by two independent electrostatic gates embedded below the absorber. Using a scanning infrared laser microscope, we characterize these devices as a function of carrier type and carrier density difference controlled at the junction between the two electrostatic gates. Based on these measurements, conducted at both mid- and near-infrared wavelengths, the primary detection mechanism can be modeled as a thermoelectric response. By studying the effect of different infrared absorbers, we determine that the optical absorption and thermal conduction of the substrate play the dominant role in the measured photoresponse of our devices. These experiments indicate a path toward hybrid graphene thermal detectors for sensing applications such as thermography and chemical spectroscopy.
    Nano Letters 01/2014; 14(2). DOI:10.1021/nl4042627 · 12.94 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: licenses/by-nc-sa/4.0/ The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires and graphene. Recently, a new method has emerged with the recognition that symmetry-protected topological (SPT) phases, which occur in systems with an energy gap to quasiparticle excitations (such as insulators or superconductors), can host robust surface states that remain gapless as long as the relevant global symmetry remains unbroken. The nature of the charge carriers in SPT surface states is intimately tied to the symmetry of the bulk, resulting in one- and two-dimensional electronic systems with novel properties. For example, time reversal symmetry endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, fixing the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations have yet to be demonstrated in two-dimensional topological insulators, where the primary example of a SPT phase is the quantum spin Hall state. Here we demonstrate experimentally that charge-neutral monolayer graphene has a quantum spin Hall state when it is subjected to a very large magnetic field angled with respect to the graphene plane. In contrast to time-reversal-symmetric systems, this state is protected by a symmetry of planar spin rotations that emerges as electron spins in a half-filled Landau level are polarized by the large magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with a tunable bandgap and an associated spin texture.
    Nature 12/2013; 505(7484). DOI:10.1038/nature12800 · 42.35 Impact Factor

Publication Stats

4k Citations
954.25 Total Impact Points

Institutions

  • 2009–2015
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2007
    • Columbia University
      • Department of Physics
      New York City, New York, United States
  • 2004–2006
    • Delft University of Technology
      • Applied Geophysics and Petrophysics
      Delft, South Holland, Netherlands