Optomechanical Coupling between a Multilayer Graphene Mechanical Resonator and a Superconducting Microwave Cavity

Left to Right: (top row) V. Singh, S. J. Bosman, B. H. Schneider, (bottom row) Y. M. Blanter, A. Castellanos-Gomez, G. A. Steele.

Authors: 
V. Singh, S. J. Bosman, B. H. Schneider, Y. M. Blanter, A. Castellanos-Gomez, 
G. A. Steele

Affiliation:
Kavli Institute of NanoScience, Delft University of Technology, The Netherlands.

Introduction:

Mechanical resonators made from two dimensional exfoliated crystals offer very low mass, low stress, and high quality factor due to their crystalline structure [1]. These properties make them very attractive for application in mass sensing, force sensing, and exploring the quantum regime of motion by providing large quantum zero-point fluctuations over a small bandwidth. The most studied exfoliated crystal so far is graphene, where a considerable progress has been made in exploring its properties for mass sensing, study of nonlinear mechanics, and voltage tunable oscillators [2-9]. These properties also make graphene attractive for exploring the quantum regime of motion.

Past 2Physics articles by Andres Castellanos-Gomez and Gary A. Steele:

July 20, 2014: "Few-layer Black Phosphorus Phototransistors for Fast and Broadband Photodetection" by Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, Gary A. Steele, Herre S.J. van der Zant, Andres Castellanos-Gomez.

A possible route towards exploring the quantum regime of graphene motion is cavity optomechanics [10]. It has shown exquisite position sensitivity, enabled the preparation and detection of mechanical systems in the quantum ground state with conventional top-down superconducting mechanical resonators [11-18]. Therefore, a natural candidate for implementing cavity optomechanics with graphene resonator is to couple it to a high Q superconducting microwave cavity. However, coupling graphene resonators with superconducting cavities in such a way that both retain their excellent properties (such as their high quality factors) is technologically challenging. Using a deterministic dry transfer technique [19], we combine a multilayer graphene resonator to a high quality factor microwave cavity [20]. Although multilayer graphene has a higher mass than a mono-layer, it could be advantageous for coupling to a superconducting cavity because of its lower electrical resistance.

Results:

Device

To fabricate the superconducting cavities in coplanar waveguide geometry, we use an alloy of molybdenum and rhenium with superconducting transition temperature of 8.1 K. Using the dry transfer technique, we place a few layer thick graphene mechanical resonator near the coupler forming coupling capacitor for the cavity. Figure 1(a) shows a false color scanning electron microscope image of a device with a 10 nm thick multilayer graphene resonator coupled to a superconducting cavity. Figure 1(b) shows an equivalent schematic diagram with graphene resonator acting as a capacitor (C_g ) between the superconducting cavity (formed by L_sc and C_sc ) and the external microwave source. By cooling these cavities to very low temperatures (14 mK), we measured internal quality factor as high as 107,000.

FIG. 1: Coupling of a multilayer graphene mechanical resonator to a superconducting cavity. (a) A tilted angle scanning electron micrograph (false color) near the coupler showing 4 μm diameter multilayer (10 nm thick) graphene resonator (cyan) suspended 150 nm above the gate. (b) Schematic lumped element representation of the device with the equivalent lumped parameters as C_sc ≈ 415 fF and L_sc ≈ 1.75 nH.

Mechanical motion readout sensitivity

To the first order, the superconducting microwave cavity can be thought simply as motional transducer for the graphene resonator. To readout the motion of the graphene resonator, we inject a microwave near the cavity frequency given by

                                       
The motion of graphene resonator modulates the cavity frequency and hence its displacement gets imprinted on the phase of the reflected microwave signal from the cavity. By measuring the phase of the reflected signal (technically known as the homodyne detection), one can directly read the mechanical motion of the resonator [11]. The large quality factor of our cavity and its ability to sustain superconductivity with large number of the microwave photons enable us to measure the thermo-mechanical motion of the graphene resonator down to temperatures of 96 mK and a displacement sensitivity as low as 17 fm/√Hz.

Optomechanical coupling

In addition to detecting the motion of the graphene drum, we can also exert a force on the mechanical drum by using the radiation pressure of microwave photons trapped in the superconducting cavity. This force comes from the fact that light carries momentum: shining light from a flashlight at a piece of paper would in principle apply a force to it, pushing it away from the light source. The radiation pressure force that light exerts, however, is usually far too small to detect. Due to the tiny mass of the graphene sheet and the ability to detect small displacement, we could see the graphene sheet shaking in response to a "beat" set by the microwave light sent into the cavity.

By sending two microwave signals, a probe signal ω_p (near the cavity resonance frequency ω_c ) and another signal at ω_d (detuned by mechanical frequency ω_m , such that ω_d = ω_c +ω_m ), one can apply a a radiation pressure force on the mechanical resonator. This radiation pressure force beats at the mechanical resonance frequency, leading to coherent driven motion of the mechanical resonator, as shown schematically by process 1 in Figure 2(a). In presence of the significant optomechanical coupling, this coherent drive of the mechanical resonator down-converts the detuned drive photons exactly at the probe frequency (pink arrow) shown by process 2 in Figure 2(a). These two signals at probe frequency interfere with each other leading to a transmission window, appearing as a sharp peak in the cavity response, shown in Figure 2(b). This phenomena is known as "optomechanically induced transparency" (OMIT) and is a signature of the optomechanical coupling between the graphene mechanical resonator and the superconducting cavity [21-23]. As this effect rely on the coherent driven motion of the graphene mechanical resonator, the width of the transparency window is set by the mechanical resonator's linewidth as shown in the inset of Figure 2(b). Using the radiation pressure force driving, we measure the quality factor of the graphene resonator as high as 220,000.

FIG. 2: Optomechanically induced transparency (OMIT). (a) Schematic illustrate OMIT features in terms of the interference of the probe field (black arrow) with the microwave photons that are cyclically down- and then up- converted by the optomechanical interaction (pink arrow). (b) Measurement of the cavity reflection |S₁₁| in presence of sideband detuned drive tone. A detuned drive at ω_c+ω_m results in a window of optomechanically induced reflection (OMIR) in the cavity response. Inset: Zoom of the OMIR window. (c) Measurement of the cavity reflection |S₁₁| with a stronger detuned drive. At the center of the cavity response, the reflection coefficient exceeds 1, corresponding to mechanical microwave amplification of 17 dB by the graphene resonator.

By increasing the drive signal amplitude further, one can increase the strength of the optomechanical coupling. Using this, we make an amplifier in which microwave signals are amplified by the mechanical motion of the graphene resonator [16]. With a stronger detuned drive, we observed a microwave gain of 17 dB (equivalent to a photon gain of 50) as shown in Figure 2(c), before the nonlinear effects from the mechanical resonators come into play. Similarly, a different "beat" of the microwave photons (having ω_d = ω_c - ω_m ) allows one to store microwave photons into the mechanical motion of the resonator [24]. To this end we show a storage time up to 10 millisecond, which is equivalent to delay from a few hundreds of kilometer long coaxial cable.

The phenomena of OMIT also allow one to directly extract a quantity called "cooperativity" C without any fi t parameters. The quantity C is an important fi gure of merit in characterizing the optomechanical systems. For example, in sideband resolved limit (when mechanical frequency exceeds the cavity linewidth), the criteria for quantum-coherent regime can be simply written as C + 1 > n_th , where n_th is the average number of thermal phonon in the mechanical resonator. In our experiment, we have been able to achieve C = 8 close to the expected number of thermal phonon in the mechanical resonator at 14 mK, bringing this system close to the quantum coherent regime.

Summary and outlook:

In our work, we demonstrated the potential of exfoliated graphene crystal applied to form an optomechanical device, which so far have been realized using top-down technology. This opens up a new dimension to explore exfoliated two-dimensional crystals in optomechanical systems, and harnessing their unique properties such as extremely low mass and high quality factors. For future devices, two-dimensional superconducting exfoliated flakes could be of great interest for such applications. Superconducting cavity in our work is a very good detector for mechanical displacement with a bandwidth three orders of magnitude larger than the mechanical line-width. This would provide a new tool to study nonlinear restoring forces, nonlinear damping, and mode coupling in mechanical resonators from twodimensional crystals. The characterization of our device shows that in future by making little larger area mechanical resonators, devices operating in quantum regime can be easily realized, which can possibly be used as a memory element in a quantum computer. As many of the 2D crystals can be grown by chemical processes in large areas, they also hold the promise of scalability.

References:
[1] Andres Castellanos-Gomez, Vibhor Singh, Herre S.J. van der Zant, Gary A. Steele, "Mechanics of freely-suspended ultrathin layered materials". arXiv:1409.1173 [cond-mat] (2014).
[2] J. Scott Bunch, Arend M. van der Zande, Scott S. Verbridge, Ian W. Frank, David M. Tanenbaum, Jeevak M. Parpia, Harold G. Craighead, Paul L. McEuen, "Electromechanical resonators from graphene sheets". Science, 315, 490-493 (2007). Abstract.
[3] Changyao Chen, Sami Rosenblatt, Kirill I. Bolotin, William Kalb, Philip Kim, Ioannis Kymissis, Horst L. Stormer, Tony F. Heinz, James Hone, "Performance of monolayer graphene nanomechanical resonators with electrical readout". Nature Nanotechnology, 4, 861-867 (2009). Abstract.
[4] Vibhor Singh, Shamashis Sengupta, Hari S Solanki, Rohan Dhall, Adrien Allain, Sajal Dhara, Prita Pant, Mandar M Deshmukh, "Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene nanoelectromechanical systems resonators". Nanotechnology, 21, 165204 (2010). Abstract.
[5] Robert A. Barton, B. Ilic, Arend M. van der Zande, William S. Whitney, Paul L. McEuen, Jeevak M. Parpia, Harold G. Craighead, "High, size-dependent quality factor in an array of graphene mechanical resonators". Nano Letters, 11, 1232{1236 (2011). Abstract.
[6] A. Eichler, J. Moser, J. Chaste, M. Zdrojek, I. Wilson-Rae, A. Bachtold, "Nonlinear damping in mechanical resonators made from carbon nanotubes and graphene". Nature Nanotechnology, 6, 339-342 (2011). Abstract.
[7] Xuefeng Song, Mika Oksanen, Mika A. Sillanpää, H. G. Craighead, J. M. Parpia, Pertti J. Hakonen, "Stamp transferred suspended graphene mechanical resonators for radio frequency electrical readout". Nano Letters, 12, 198-202 (2012). Abstract.
[8] Robert A. Barton, Isaac R. Storch, Vivekananda P. Adiga, Reyu Sakakibara, Benjamin R. Cipriany, B. Ilic, Si Ping Wang, Peijie Ong, Paul L. McEuen, Jeevak M. Parpia, Harold G. Craighead, "Photothermal self-oscillation and laser cooling of graphene optomechanical systems". Nano Letters, 12, 4681-4686 (2012). Abstract.
[9] Changyao Chen, Sunwoo Lee, Vikram V. Deshpande, Gwan-Hyoung Lee, Michael Lekas, Kenneth Shepard, James Hone, "Graphene mechanical oscillators with tunable frequency". Nature Nanotechnology 8, 923{927 (2013). Abstract.
[10] Markus Aspelmeyer, Tobias J. Kippenberg, Florian Marquardt, "Cavity optomechanics". arXiv:1303.0733 [cond-mat.mes-hall] (2013).
[11] C. A. Regal, J. D. Teufel, K. W. Lehnert, "Measuring nanomechanical motion with a mi- crowave cavity interferometer". Nature Physics, 4, 555-560 (2008). Abstract.
[12] J. D. Teufel, T. Donner, M. A. Castellanos-Beltran, J. W. Harlow, K. W. Lehnert, "Nanomechanical motion measured with an imprecision below that at the standard quantum limit". Nature Nanotechnology 4, 820-823 (2009). Abstract.
[13] T. Rocheleau, T. Ndukum, C. Macklin, J. B. Hertzberg, A. A. Clerk, K. C. Schwab, "Preparation and detection of a mechanical resonator near the ground state of motion". Nature, 463, 72-75 (2010). Abstract.
[14] J. D. Teufel, Dale Li, M. S. Allman, K. Cicak, A. J. Sirois, J. D. Whittaker, R. W. Simmonds, "Circuit cavity electromechanics in the strong-coupling regime". Nature, 471, 204-208 (2011). Abstract.
[15] J. D. Teufel, T. Donner, Dale Li, J. W. Harlow, M. S. Allman, K. Cicak, A. J. Sirois, J. D. Whittaker, K. W. Lehnert, R. W. Simmonds, "Sideband cooling of micromechanical motion to the quantum ground state". Nature, 475, 359-363 (2011). Abstract.
[16] F. Massel, T.T. Heikkilä, J.-M. Pirkkalainen, S.U. Cho, H. Saloniemi, P.J. Hakonen, M.A. Sillanpää, "Microwave ampli fication with nanomechanical resonators". Nature, 480, 351-354 (2011). Abstract.
[17] Fredrik Hocke, Xiaoqing Zhou, Albert Schliesser, Tobias J Kippenberg, Hans Huebl, Rudolf Gross, "Electromechanically induced absorption in a circuit nano-electromechanical system". New Journal of Physics, 14, 123037 (2012). Abstract.
[18] T. A. Palomaki, J. D. Teufel, R. W. Simmonds, K. W. Lehnert, "Entangling mechanical motion with microwave fields". Science, 342, 710-713 (2013). Abstract.
[19] Andres Castellanos-Gomez, Michele Buscema, Rianda Molenaar, Vibhor Singh, Laurens Janssen, Herre S J van der Zant, Gary A Steele, "Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping". 2D Materials, 1, 011002 (2014). Abstract.
[20] V. Singh, S. J. Bosman, B. H. Schneider, Y. M. Blanter, A. Castellanos-Gomez, G. A. Steele, "Optomechanical coupling between a multilayer graphene mechanical resonator and a superconducting microwave cavity". Nature Nanotechnology 9, 820–824 (2014). Abstract.
[21] G. S. Agarwal, Sumei Huang, "Electromagnetically induced transparency in mechanical eff ects of light". Physical Review A, 81, 041803 (2010). Abstract.
[22] Stefan Weis, Rémi Rivière, Samuel Deléglise, Emanuel Gavartin, Olivier Arcizet, Albert Schliesser, Tobias J. Kippenberg, "Optomechanically induced transparency". Science, 330, 1520-1523 (2010). Abstract.
[23] A. H. Safavi-Naeini, T. P. Mayer Alegre, J. Chan, M. Eichenfield, M. Winger, Q. Lin, J. T. Hill, D. E. Chang, O. Painter, "Electromagnetically induced transparency and slow light with optomechanics". Nature, 472, 69-73 (2011). Abstract.
[24] X. Zhou, F. Hocke, A. Schliesser, A. Marx, H. Huebl, R. Gross, T. J. Kippenberg, "Slowing, advancing and switching of microwave signals using circuit nanoelectromechanics". Nature Physics, 9, 179-184 (2013). Abstract.

Atoms Under the Magnifying Glass: Direct Observation of the Nodal Structures of Electronic States

Aneta Stodolna (left) and Marc J.J. Vrakking

Authors: Aneta Stodolna¹, Marc J.J. Vrakking²  

Affiliation:
¹FOM Institute AMOLF, Amsterdam, Netherlands,
²Max-Born-Institut, Berlin, Germany.

To describe the microscopic properties of matter and its interaction with the external world, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms, electronic wave functions describe - among other things - charge distributions existing on length-scales that are many orders of magnitude removed from our daily experience. In physics laboratories, experimental observations of charge distributions are usually precluded by the fact that the process of taking a measurement changes a wave function and selects one of its many possible realizations. For this reason, physicists usually know the shape of charge distributions through calculations that are shown in textbooks. But in the last few years, this has started to change. Recent experiments have visualized the nodal structure of electronic states of hydrogen and helium on two-dimensional detectors.

The development of quantum mechanics in the early part of the last century had a profound influence on the way that scientists understand the world. Central to quantum mechanics is the concept of a wave function that satisfies the time-dependent Schrödinger equation. According to the Copenhagen interpretation, this wave function describes the probability of observing the outcome of measurements that are performed on a quantum mechanical system, such as measurements of the energy of the system or the position or momenta of its constituents. This allows reconciling the occurrence of non-classical phenomena on the micro-scale with manifestations and observations made on the macro-scale, which correspond to viewing one or more of countless realizations described by the wave function.

Despite the overwhelming impact on modern electronics and photonics, grasping quantum mechanics and the many possibilities that it describes continues to be intellectually challenging, and has motivated numerous experiments illustrating the intriguing predictions contained in the theory. For example, the 2012 Nobel Prize in Physics was awarded to Haroche and Wineland for their work on measurement and control of individual quantum systems in quantum non-demolition experiments, paving the way to more accurate optical clocks and, potentially, future quantum computers.

About thirty years ago, Russian theoreticians proposed an intriguing method for measuring properties of wave functions. They suggested studying atomic ionization in a static electric field that projects the electrons onto a two-dimensional detector and predicted interference patterns, with one of two possible origins. First of all, interference patterns result from path length differences between different trajectories that the electron can take between the atom and the detector. As clearly shown in the famous double-slit experiment on interference of single electrons (voted "the most beautiful physics experiment", in a poll conducted by Physicsworld about a decade ago) electrons exhibit both particle- and wave-like behavior.

The wave-like behavior derives from the de Broglie wavelength that quantum mechanics associates with any moving particle. The lower the kinetic energy of the electron, the larger the de Broglie wavelength is. Correspondingly, for low enough kinetic energies, the de Broglie wavelength becomes observable on macroscopic length scales. Secondly, in the case of hydrogen, the interference patterns can directly reflect the nodal structure of the electronic wave function. The fact that this is so, is due to the special status of hydrogen as nature´s only single-electron atom. Due to this circumstance, the hydrogen wave function can be written as the product of two functions that describe how the wave function changes as a function of two, so-called “parabolic coordinates”, which are linear combinations of the distance of the electron from the H+ nucleus “r”, and the displacement of the electron along the electric field axis “z”. Importantly, the shape of the two parabolic wave functions is independent of the strength of the static electric field, and therefore stays the same as the electron travels from the place where the ionization takes place to the two-dimensional detector.

Last year we published a paper, where we reported experiments for hydrogen atoms [1]. Ground state hydrogen atoms were optically excited to electronic states of interest, using two precisely tunable laser sources, and a delicate electrostatic lens was used to magnify the imprint of the electrons on the two-dimensional detector to millimeter-scale dimensions, so the nodal patterns of the wave functions could be observed with the naked eye. The main result is shown in Figure 1. This figure shows raw camera data for four measurements, where the hydrogen atoms were excited to states with 0, 1, 2 and 3 nodes in the wave function for the ξ = r+z parabolic coordinate. The nodes can be easily recognized. The experimental arrangement served as a microscope, allowing us to look deep inside the hydrogen atom, with a magnification of approximately a factor twenty-thousand.

Figure 1: (left) two-dimensional projection of electrons resulting from excitation of hydrogen atoms to four electronic states labeled with a set of quantum numbers (n1,n2,m) and having (from top to bottom) 0, 1, 2 and 3 nodes in the wave function for the ξ = r+z parabolic coordinate; (right) comparison of the experimentally measured radial distributions (solid lines) with results from quantum mechanical calculations (dashed lines), illustrating that the experiment has measured the nodal structure of the quantum mechanical wave function (copyright: American Physical Society).

More recently, we have performed similar experiments for the helium atom [2]. After the hydrogen atom, the helium atom is nature´s simplest atom, consisting of a doubly-charged nucleus surrounded by two electrons. The presence of two electrons in the atom introduces the concept of electron correlation. Remarkably, we saw that we could turn the electron correlation in helium on or off at will.

In the experiment, helium atoms were ionized by the absorption of an ultra-violet (UV) photon. Like in the hydrogen experiment, the photon energy of the UV light was tuned in such a manner that it was only just sufficient for ionization of the atom, thus producing very slow photoelectrons that were accelerated by an electric field towards a two-dimensional detector. At most of the UV photon energies, interference patterns were measured that could be explained by considering differences in the lengths of possible paths of the electron on the way to the detector (see Figure 2). Here, two paths differing by an integral number of de Broglie wavelengths interfere constructively, whereas two paths differing by a half-integer number of de Broglie wavelengths interfere destructively.

Figure 2: Sample images recorded for ionization of helium atoms. The four images contain interference patterns that result from path length differences along trajectories that the electron can take between the atom and the detector. The labels contained in the figures indicate the energy of the states used in the experiment defined with respect to the field-free ionization limit (copyright: American Physical Society).

However, at a number of UV photon energies the interference patterns looked markedly different, extending out to a much larger radius and containing a different number of nodes compared to measurements at slightly lower or higher photon energy (see Figure 3). A theoretical analysis revealed that at these energies the effect of electron correlation was momentarily suppressed. The suppression occurs when two electronic states, whose precise energies depend on the strength of the electric field, accidentally occur at almost identical energies. These two states then interact with each other, and for a particular value of the electric field, the energy exchange between the two parabolic coordinates is almost completely turned off. In other words, the atom becomes hydrogenic.

Figure 3: Sometimes helium behaves like a hydrogen atom, and interference patterns are measured that reveal the nodal structure of the electronic wave function that is excited (middle image). These cases stand out because the nodal pattern of these images is very different from those recorded at nearby excitation energies (left and right image), and the images extends farther radially. The labels contained in the figures indicate the energy of the states used in the experiment defined with respect to the field-free ionization limit (copyright: American Physical Society).

Correspondingly, the nodal pattern measured on the detector is once again the nodal pattern of the electronic state that is optically excited. The effect was found to be very subtle: tiny changes (<< 1%) in the strength of the electric field are sufficient to convert an atom that reveals the nodal pattern of its wave function in a hydrogen-like manner, into an atom where electron correlation removes the observability of this nodal pattern, and where the observed interference patterns are completely determined by path length differences between the atom and the detector.

In this manner, the hydrogen and helium atom constitute a wonderful nano-scale laboratory for studies of fundamental quantum mechanics, providing text-book images of nodal patterns in the case of hydrogen, and revealing the onset of electron correlation in the case of helium.

References:
[1] Aneta Stodolna, Ymkje Huismans, Arnaud Rouzée, Frank Lépine, Marc J. J. Vrakking, "Photoelectron holography in strong optical and dc electric fields". Journal of Physics: Conference Series 488, 012007 (2014). Full Article.
[2] A. S. Stodolna, F. Lépine, T. Bergeman, F. Robicheaux, A. Gijsbertsen, J. H. Jungmann, C. Bordas, M. J. J. Vrakking, "Visualizing the Coupling between Red and Blue Stark States Using Photoionization Microscopy". Physical Review Letters, 113, 103002 (2014). Abstract.   

Bio-inspired Plasmonic Structures Built on Virus Capsids and DNA Origami Tiles

Debin Wang (left) and James J. De Yoreo

Authors: Debin Wang^1,2, James J. De Yoreo^1,2

Affiliation:
¹Materials Sciences Division and the Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
²Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington, USA.

The use of biomolecular scaffolds to direct the organization of inorganic or organic nanomaterials addresses the grand challenge of assembling multiple functional units with precise control over their spatial arrangement at the molecular level [1-3]. Biomolecules, such as peptides, proteins, and nucleic acids have all been used as building blocks for bottom-up assembly of intricate structures thanks to their inherent chemical and biological addressability, structural precision, and efficiency of synthesis [4].

In photosynthetic bacteria, light-harvesting units are organized with molecular-level precision around the photochemical units [5-6]. In this way, they generate antenna complexes that ensure efficient photon absorption and energy transfer. In a similar way, biomimetic assembly of plasmonic nanostructures can provide molecular-level spatial precision, creating potential improvements in efficiency of light-harvesting platforms, light-emitting devices, and optical sensors [7-8].

Recently, we reported the bottom-up assembly of hierarchical plasmonic nanostructures using DNA origami tiles and MS2 virus capsids [9]. These bio-inspired structures serve as programmable scaffolds that provide molecular level control over the distribution of fluorescent dye molecules and nanometer-scale control over their distance from a gold nanoparticle antenna (Fig. 1). While previous studies on DNA origami assembly of plasmonic nanostructures focused on the distance-dependent response of single fluorescent dye molecules [10-11], these hybrid structures allowed us to investigate the plasmonic response of an entire ensemble of fluorescent molecules.

Figure 1. Bio-inspired assembly of plasmonic nanostructures using DNA origami and MS2 virus capsids. TEM imaging and profile analysis confirmed tight control over the distance between fluorophore labelled virus capsids and gold nanoparticle antennae. Correlated Raman-AFM imaging provided direct single-particle measurements of fluorescence intensities. Adapted with permission from ACS Nano 2014, 8, 7896-7904. Copyright 2014 American Chemical Society.

We studied the collective plasmon-coupled response of fluorophore-labeled capsids to the presence of an AuNP as a function of their separation distance. We demonstrated tight control over this distance by exploiting the programmable nature of DNA origami templates and the ability to site-specifically modify MS2 virus capsids (Fig.1). Using finite-difference time-domain (FDTD) numerical simulations in conjunction with atomic force microscopy (AFM) and correlated scanning confocal fluorescence microscopy, we then showed that the utilizing a 3D ensemble of dye molecules can effectively suppress the fluorescence quenching in the single molecule quenching regime, presumably due to the size effect of the capsid scaffold (Fig. 1). FDTD simulations also showed that increasing the size of the AuNPs to be commensurate with that of the capsids optimizes the fluorescence enhancement (Fig.2).

Figure 2. Finite-difference-time-domain (FDTD) numerical simulations predict the plasmon-coupled response of the bio-inspired nanostructures. Adapted with permission from ACS Nano 2014, 8, 7896-7904. Copyright 2014 American Chemical Society.

Looking forward, we plan to use this bio-inspired light harvesting platform to explore the effect of variations in nanoparticle size, choice of fluorophore, arrangement of fluorophores, and even the capsid shape on device performance. More generally, our assembly strategy establishes the possibility of using biological scaffolds to build hierarchical plasmonic nanostructures to address the need for energy harvesting in solar energy applications.

References:
[1] Trevor Douglas, Mark Young, "Viruses: Making Friends with Old Foes". Science, 312, 873-875 (2006). Abstract.
[2] James J. Storhoff, Chad A. Mirkin, "Programmed Materials Synthesis with DNA". Chemical Reviews, 99, 1849-1862 (1999). Abstract.
[3] Andre V. Pinheiro, Dongran Han, William M. Shih, Hao Yan, "Challenges and Opportunities for Structural DNA Nanotechnology". Nature Nanotechnology, 6, 763-772 (2011). Abstract.
[4] Shuguang Zhang, "Fabrication of Novel Biomaterials through Molecular Self-Assembly". Nature Biotechnology, 21, 1171-1178 (2003). Abstract.
[5] Svetlana Bahatyrova, Raoul N. Frese, C. Alistair Siebert, John D. Olsen, Kees O. van der Werf, Rienk van Grondelle, Robert A. Niederman, Per A. Bullough, Cees Otto, C. Neil Hunter, "The Native Architecture of a Photosynthetic Membrane". Nature, 430, 1058-1062 (2004). Abstract.
[6] Pascal Anger, Palash Bharadwaj, Lukas Novotny, "Enhancement and Quenching of Single-Molecule Fluorescence". Physical Review Letters, 96, 113002 (2006). Abstract.
[7] Stephan Link, Mostafa A. El-Sayed, "Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles". Journal of Physical Chemistry B, 103, 4212-4217 (1999). Abstract.
[8] Joanna Malicka, Ignacy Gryczynski, Zygmunt Gryczynski, Joseph R Lakowicz, "Effects of Fluorophore-to-Silver Distance on The Emission of Cyanine-Dye-Labeled Oligonucleotides". Analytical Biochemistry, 315, 57-66 (2003). Abstract.
[9] Debin Wang, Stacy L. Capehart, Suchetan Pal, Minghui Liu, Lei Zhang, P. James Schuck, Yan Liu, Hao Yan, Matthew B. Francis, James J. De Yoreo, "Hierarchical Assembly of Plasmonic Nanostructures Using Virus Capsid Scaffolds on DNA Origami Templates". ACS Nano, 8, 7896-7904 (2014). Abstract.
[10] G. P. Acuna, F. M. Möller, P. Holzmeister, S. Beater, B. Lalkens, P. Tinnefeld, "Fluorescence Enhancement at Docking Sites of DNA-Directed Self-Assembled Nanoantennas". Science, 338, 506-510 (2012). Abstract.
[11] Guillermo P. Acuna, Martina Bucher, Ingo H. Stein, Christian Steinhauer, Anton Kuzyk, Phil Holzmeister, Robert Schreiber, Alexander Moroz, Fernando D. Stefani, Tim Liedl, Friedrich C. Simmel, Philip Tinnefeld, "Distance Dependence of Single-Fluorophore Quenching by Gold Nanoparticles Studied on DNA Origami". ACS Nano, 6, 3189-3195 (2012). Abstract.

Imaging the Dynamics of Free-Electron Landau States

Transmission electron microscope that was used in the experiment. Foreground: Michael Stöger-Pollach (left) and Peter Schattschneider.

From Left to Right: Th. Schachinger, S. Löffler, A. Steiger-Thirsfeld, K. Y. Bliokh, Franco Nori

Authors: P. Schattschneider^1,2,3, Th. Schachinger¹, M. Stöger-Pollach², S. Löffler², A. Steiger-Thirsfeld², K. Y. Bliokh^4,5, Franco Nori^5,6

Affiliation:
¹Institute of Solid State Physics, Vienna University of Technology, Austria
²University Service Centre for Electron Microscopy, Vienna University of Technology, Austria
³LMSSMat (CNRS UMR 8579) Ecole Centrale Paris, France
⁴iTHES Research Group, RIKEN, Wako-shi, Saitama, Japan
⁵Center for Emergent Matter Science, RIKEN, Wako-shi, Saitama, Japan
⁶Department of Physics, University of Michigan, Ann Arbor, USA.

Inspired by theoretical calculations [1,2] from RIKEN (Japan), the group at Vienna University of Technology devised a way to generate free-electron Landau states [3], a form of quantized states that electrons adopt when moving through a magnetic field. Landau levels and states of electrons in a magnetic field are fundamental quantum entities underlying the quantum Hall and related effects in condensed matter physics [4]. Landau states can be envisaged as electron vortices occurring naturally in the presence of magnetic fields. The magnetic field plays the same role for electrons as the earth's rotation plays for the creation of cyclones, but here on a nanometer scale, where quantum effects become important [5].

Past 2Physics articles by Konstantin Y. Bliokh:
January 17, 2009: "Optical Magnus Effect: Topological Monopole Deflects Spinning Light".

Classical electrons in a uniform magnetic field propagate freely along the field and form confined circular orbits in the plane perpendicular to the field. The angular velocity of such orbiting is constant and is known as the cyclotron frequency. But quantum mechanics calls for a counter-intuitive behaviour [2]. The researchers were able to induce intrinsic rotation in single moving electrons. It was observed [3] that Landau modes with different azimuthal quantum numbers belong to three classes, which are characterized by rotations with zero, Larmor and cyclotron frequencies, respectively. This is in sharp contrast to the uniform cyclotron rotation of classical electrons, and in perfect agreement with recent theoretical predictions [2].

States with different quantum numbers are produced using nanometre-sized electron vortex beams, with a radius chosen to match the waist of the Landau states, in a quasi-uniform magnetic field. Scanning the beams along the propagation direction [3], the researchers reconstructed the rotational dynamics of the Landau wave functions with angular frequency of the order of 100 GHz.

Figure 1: A holographic fork mask generates a row of vortex beams with different azimuthal indices m. These beams are focused by a magnetic lens and are studied in the region of maximal quasi-uniform magnetic field (red arrow on the left). The focal plane is shifted a few Rayleigh ranges below the observation plane z=0 to reduce the Gouy-phase rotation. A knife-edge stop is placed at z_k, where it blocks half of each of the beams. Varying the position z_k of the knife edge allows the observation of the spatial rotational dynamics of the cut beams propagating to the observation plane [3].

The focusing lenses of a transmission electron microscope were used [3] to reconfigure the vortices so that they almost perfectly resembled Landau states. In an electron vortex beam, the wave current swirls around a common center similar to air in a tornado [6]. Measuring the rotation can be compared to determining how many times a thin wire is wound around a rod. When looking at the wire directly, it is extremely difficult to count the number of windings. But when stretching it along the direction of the rod, the wire takes the form of a well-spaced spiral, for which it is easy to count the revolutions. This is precisely what happens with Landau states: they were 'elongated' to vortex beams. That way, their rotation could be measured [3] with very high accuracy.

This is a very exciting finding that will contribute to a better understanding of the fundamental quantum features of electrons in magnetic fields [3]. In addition to showing that the rotational dynamics of the electrons are more complex and intriguing than was once believed, the new findings could have practical implications for technology. Taking Landau states into free space, away from the solids where they normally play a key role [4,7], can inspire new ideas in materials science.

This will certainly lead to novel insights and a better understanding of the delicate interaction between magnetic fields and matter, which might one day give rise to new and better technologies such as sensors, memory devices, or nanomanipulation.

References:
[1] Konstantin Yu. Bliokh, Yury P. Bliokh, Sergey Savel’ev, Franco Nori, "Semiclassical dynamics of electron wave packet states with phase vortices". Physical Review Letters, 99, 190404 (2007). Abstract.
[2] Konstantin Y. Bliokh, Peter Schattschneider, Jo Verbeeck, Franco Nori, "Electron vortex beams in a magnetic field: a new twist on Landau levels and Aharonov-Bohm states". Physical Review X, 2, 041011 (2012). Abstract.
[3] P. Schattschneider, Th. Schachinger, M. Stöger-Pollach, S. Löffler, A. Steiger-Thirsfeld, K. Y. Bliokh, Franco Nori, "Imaging the dynamics of free-electron Landau states". Nature Communications, 5, 4586 (2014). Full Article.
[4] Daijiro Yoshioka, "The Quantum Hall Effect" (Springer, 2002).
[5] J. Verbeeck, H. Tian, P. Schattschneider, "Production and Application of Electron Vortex Beams". Nature, 467, 301 (2010). Abstract.
[6] Giulio Guzzinati, Peter Schattschneider, Konstantin Y. Bliokh, Franco Nori, Jo Verbeeck, "Observation of the Larmor and Gouy rotations with electron vortex beams". Physical Review Letters, 110, 093601 (2013). Abstract.
[7] David L. Miller, Kevin D. Kubista, Gregory M. Rutter, Ming Ruan, Walt A. de Heer, Markus Kindermann, Phillip N. First, Joseph A. Stroscio, "Real-space mapping of magnetically quantized graphene states". Nature Physics, 6, 811–817 (2010). Abstract.

Over 100-bit Integrated All-Optical Memory on a Photonic Crystal Chip is Demonstrated

[From Left to Right] Eiichi Kuramochi, Kengo Nozaki, Akihiko Shinya, Masaya Notomi

Authors: Eiichi Kuramochi, Kengo Nozaki, Akihiko Shinya, Masaya Notomi

Affiliation: NTT Nanophotonics Center and NTT Basic Research Laboratories, NTT Corporation, Atsugi-shi, Kanagawa, Japan.

Random access memory (RAM) is used extensively in a wide variety of instruments. It is based on the bistable operation of electronic transistors and memorizes bit information. Recently, its optical counterpart, optical RAM (o-RAM), has become highly desirable in high-speed network processing, especially for routers, because it is more efficient to manage network information all-optically without power-consuming electric-to-optical (EO) or OE conversions. Various o-RAM devices have been proposed and fabricated, but so far they have been too big, too power-consuming, or too difficult to integrate.

In 2012, we demonstrated ultralow-power and ultra-small o-RAMs using a photonic crystal nanocavity [1]. Photonic crystals enable strong light confinement, so these o-RAMs were able to achieve a huge reduction of footprint and power consumption. However, the size of these o-RAMs was limited to just four bits. In the June 2014 issue of 'Nature Photonics' [2], we reported our successful integration of wavelength-addressable 105-bit o-RAMs. This is the first realization of an integrated o-RAM with more than four bits.

We implemented a 105-bit o-RAM in a small silicon photonic crystal chip less than 1.1 mm long (Fig. 1). This chip contained 128 nanocavities, each of which can serve as an o-RAM. This novel configuration enables wavelength-addressable o-RAM operation.

Figure 1: (a) Electron microscope images of large-scale nanocavity array integrated in a Si photonic crystal. (b) Transmission spectrum of nanocavity array coupled to an in-plane input/output waveguide. 105 cavities can be operated as bistable optical memories. (c) Schematic of o-RAM operation using the bistable output bias power of a nanocavity side-coupled to a bus waveguide. (d) o-RAM operation of 105 cavity modes in the nanocavity array.

First we explain how an optical memory works. It is well-known that a cavity with optical nonlinear medium exhibits optical bistability at a certain condition. In the present device, we make use of this phenomenon for bit memory operation. Figure 1(c) explains operation mechanism. When one injects light into a nonlinear cavity, the output light intensity shows hysteresis with two bistable states. We can switch between the two bistable states by applying an optical pulse or cutting the bias light. This operation can be used as a bit memory where the high output state (OFF) and low output state (ON) correspond to binary information of '0' and '1'. It should be noted that in a photonic crystal nanocavity, the optical nonlinearity effect is greatly enhanced by high quality factor (Q: ~10⁵) and ultra-small mode volume (< 1 (λ/n)³).

In the present study, we were able to achieve bistable o-RAM operation of 105 nanocavities integrated monolithically in a photonic crystal chip as shown in Fig. 1(d). The optical transmission spectrum is shown in Fig. 1(b). Here each dip corresponds to each cavity. Fabricated cavities have different resonant wavelengths with an averaged spacing of 0.23 nm, which was realized by precisely changing the lattice constant of photonic crystal by 0.125 nm. Such high level of nanofabrication accuracy was achieved by cutting-edge electron beam lithography.

In order to realize wavelength-addressable o-RAMs, each cavity should have a sharp resonance and a wide mode spacing to avoid undesired mode overlap. Conventional high quality factor (Q) cavities in a photonic crystal do not have sufficiently wide mode spacing. It is well known that a three-missing-hole defect cavity (so-called L3 cavity) in a photonic crystal has a wide mode spacing, but suffers from relatively low Q [3]. To overcome this problem, here we employed a modified L3 nanocavity as shown in Fig. 2, where we systematically tuned the 6 sets of the holes denoted A-F.

Figure 2: (a) New tuning design of an L3 nanocavity that improves Q over 10 times. (b) Spectrum of the fundamental cavity mode of an L3 nanocavity. The full-width half-maximum (FWHM) line-width (1.5 pm) corresponds to the experimental Q of 1.0 X 10⁶.

This novel design allows ~10 times enhancement of Q to the conventional L3 cavities with keeping the volume of the cavity mode. Not only the theoretical Q but also the experimental Q was enhanced to ~10⁶ which is 10 times higher than previously reported value [3]. The above-mentioned highly-dense wavelength division multiplexing scheme was enabled by this high performance of modified L3 cavities.

Although we employed Si for o-RAMs discussed above, we also employed InGaAsP for constructing multi-bit o-RAMs, which showed even small operation power (~100 nW) [2] owing to the efficient optical nonlinearity.

We are expecting that the present technology will be employed in an all-optical router as we mentioned. In this application, gigantic RAMs are not necessary, but kilobyte-sized RAMs would be sufficient. To realize this, we plan to combine the present wavelength addressing memory integration scheme with the parallel integration scheme. We have already demonstrated the parallel integration of o-RAM with an all-optical addresser in four-bit o-RAM [1]. The appropriate combination of two methods would be sufficient for our target. Of course, high-speed optical RAMs should also play various important roles in all-optical logical processing other than routers. In addition to that, the demonstrated wavelength addressing nanocavity integration technologies for o-RAM can also be applied for other types of devices such as multi-channel all-optical switches [4].

References: 
[1] Kengo Nozaki, Akihiko Shinya, Shinji Matsuo, Yasumasa Suzaki, Toru Segawa, Tomonari Sato, Yoshihiro Kawaguchi, Ryo Takahashi, and Masaya Notomi, “Ultralow-power all-optical RAM based on nanocavities”. Nature Photonics, 6, 248 (2012). Abstract.
[2] Eiichi Kuramochi, Kengo Nozaki, Akihiko Shinya, Koji Takeda, Tomonari Sato, Shinji Matsuo, Hideaki Taniyama, Hisashi Sumikura and Masaya Notomi, “Large-scale integration of wavelength-addressable all-optical memories on a photonic crystal chip”. Nature Photonics, 8, 474 (2014). Abstract.
[3] Yoshihiro Akahane, Takashi Asano, Bong-Shik Song, and Susumu Noda, “High-Q photonic nanocavity in a two-dimensional photonic crystal”. Nature, 425, 944 (2003). Abstract.
[4] Kengo Nozaki, Eiichi Kuramochi, Akihiko Shinya, and Masaya Notomi, "25-channel all-optical gate switches realized by integrating silicon photonic crystal nanocavities". Optics Express, 22, 14263 (2014). Abstract.

Few-layer Black Phosphorus Phototransistors for Fast and Broadband Photodetection

From Left to Right: (Top) Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, (Bottom) Gary A. Steele, Herre S.J. van der Zant and Andres Castellanos-Gomez

Authors:
Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, Gary A. Steele, Herre S.J. van der Zant, Andres Castellanos-Gomez.

Affiliation:
Kavli Institute of Nanoscience, Delft University of Technology, The Netherlands.

Introduction

The isolation of graphene has opened the door for the studying of the large family of layered two-dimensional (2D) materials, driven by the extraordinary properties that these materials show in their single and few-layer form [1-4]. Graphene, a one-atom thick layer of carbon atoms, has shown excellent electrical properties (e.g. mobility in the order of 170 000 cm²/Vs at room temperature) and large breaking strength [5,6]. However, its applicability in low-power field effect transistors (FETs) and optoelectronic devices (e.g. photodetectors) is hampered by its zero bandgap.

This absence of a bandgap has intensified the current research in other 2D materials with an intrinsic bandgap [7]. For instance, silicene, a single layer of silicon atoms, represents a semiconducting analogue to graphene but so far it has only been realized in an ultra-high-vacuum environment, a severe limitation for further studies and applications [8]. Other promising candidates for optoelectronic applications are the members of the transition metal dichalcogenides (TMDCs) material class [9-13]. The large and direct bandgap of their single-layer form provides strong light absorption – a necessary condition for large photoresponse – but operation is limited to part of the visible spectrum. A material with a direct and small bandgap is needed to extend the detection range accessible with 2D materials.

Black phosphorus

Few-layer black phosphorus is a new member of the 2D-materials family. Black phosphorus is a layered allotrope of the element phosphorus and, in bulk, it is a semiconductor with a direct bandgap of 0.35 eV [14]. In its few-layer form, the bandgap is predicted to strongly depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single-layer). Moreover, FETs based on few-layer black-phosphorus show promising electrical properties [15-18], making them an appealing candidate for tunable photodetection from the visible to the infrared part of the spectrum.

Main Results

In our recently published work [19], we characterized the response to light excitation of FETs based on few-layer black phosphorus (thickness ranging from 3nm to 8nm). Figure 1a shows a schematic of the device and of the measurement circuit. Without illumination, the black-phosphorus FETs show ambipolar behavior, as both holes and electrons can be induced in the conducting channel by the gate electric field. The measured mobilities are in the order of 100 cm²/Vs and current on/off ratio in the order of 10³, demonstrating good electrical behavior. Under illumination, we measure a sizable photoresponse to excitation wavelengths from the visible up to 940 nm (see Figure 1b). Figure 1c shows the photocurrent measured for a single pulse of light excitation from which we estimate a rise time of 1 ms, demonstrating broadband and fast photodetection. For comparison, photodetectors based on single-layer molybdenum disulphide (MoS₂) can reach higher responsivities (~ 880 X 10³ mA/W) but their response time is limited to 0.6 sec [20].

Figure 1: (a) Device schematics (b) Source-drain current vs. gate voltage in dark (black solid line), with λ = 940 nm illumination (purple solid line), λ = 640 nm illumination (red solid line) and λ = 532 nm illumination (green solid line). The total incident optical power is 750 μW for all wavelengths. (c) Source-drain current vs. time for a single period of light modulation with a mechanical chopper (different device from panel b).

Future trends and outlook

Taking advantage of the ambipolarity, one could think of electrostatically defining a PN junction in a few-layer black phosphorus flake, as already pioneered in single-layer tungsten diselenide (WSe₂) [9-11]. A PN junction could be used to boost the photoresponse and generate electrical power via the photovoltaic effect. Given the small and direct bandgap of few-layer black-phosphorus, it would be possible to harvest photons also in the near-infrared part of the spectrum.

References:
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, "Two-dimensional gas of massless Dirac fermions in graphene". Nature, 438, 197-200 (2005). Abstract.
[2] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, "Two-dimensional atomic crystals". Proceedings of the National Academy of Sciences of the United States of America, 102, 10451(2005). Full Article.
[3] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A. A. Firsov, "Electric Field Effect in Atomically Thin Carbon Films". Science, 306, 666 (2004). Abstract.
[4] Qing Hua Wang, Kourosh Kalantar-Zadeh, Andras Kis, Jonathan N. Coleman, Michael S. Strano, "Electronics and optoelectronics of two-dimensional transition metal dichalcogenides". Nature nanotechnology, 7, 699–712 (2012). Abstract.
[5] L. Wang, I. Meric, P.Y. Huang, Q. Gao, Y. Gao, H. Tran, T. Taniguchi, K. Watanabe, L. M. Campos, D.A. Muller, J. Guo, P. Kim, J. Hone, K. L. Shepard, C. R. Dean, "One-Dimensional Electrical Contact to a Two-Dimensional Material". Science, 342, 614-617 (2013). Abstract.
[6] Changgu Lee, Xiaoding Wei, Jeffrey W. Kysar, James Hone, "Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene". Science, 321, 385-388 (2008). Abstract.
[7] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, "Single-layer MoS₂ transistors". Nature Nanotechnology, 6, 147-150 (2011). Abstract.
[8] Patrick Vogt, Paola De Padova, Claudio Quaresima, Jose Avila, Emmanouil Frantzeskakis, Maria Carmen Asensio, Andrea Resta, Bénédicte Ealet, Guy Le Lay, "Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon". Physical Review Letters, 108, 155501 (2012). Abstract.
[9] Britton W. H. Baugher, Hugh O. H. Churchill, Yafang Yang, Pablo Jarillo-Herrero, "Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide". Nature Nanotechnology, 9, 262-267 (2014). Abstract.
[10] Andreas Pospischil, Marco M. Furchi, Thomas Mueller, "Solar-energy conversion and light emission in an atomic monolayer p–n diode". Nature Nanotechnology, 9, 257-261 (2014). Abstract.
[11] Jason S. Ross, Philip Klement, Aaron M. Jones, Nirmal J. Ghimire, Jiaqiang Yan, D. G. Mandrus, Takashi Taniguchi, Kenji Watanabe, Kenji Kitamura, Wang Yao, David H. Cobden, Xiaodong Xu, "Electrically tunable excitonic light-emitting diodes based on monolayer WSe₂ p–n junctions". Nature Nanotechnology, 9, 268-272 (2014). Abstract.
[12] Zongyou Yin, Hai Li, Hong Li, Lin Jiang, Yumeng Shi, Yinghui Sun, Gang Lu, Qing Zhang, Xiaodong Chen, Hua Zhang, "Single-Layer MoS₂ Phototransistors". ACS Nano, 6, 74-80 (2012). Abstract.
[13] Néstor Perea-López, Ana Laura Elías, Ayse Berkdemir, Andres Castro-Beltran, Humberto R. Gutiérrez, Simin Feng, Ruitao Lv, Takuya Hayashi, Florentino López-Urías, Sujoy Ghosh, Baleeswaraiah Muchharla, Saikat Talapatra, Humberto Terrones, Mauricio Terrones, "Photosensor Device Based on Few-Layered WS₂ Films". Advanced Functional Materials, 23, 5511-5517 (2013). Abstract.
[14] Yuichi Akahama, Shoichi Endo, Shin-ichiro Narita, "Electrical Properties of Black Phosphorus Single Crystals". Journal of the Physical Society of Japan, 52, 2148-2155 (1983). Abstract.
[15] Han Liu, Adam T. Neal, Zhen Zhu, David Tomanek, Peide D. Ye, "Phosphorene: A New 2D Material with High Carrier Mobility". arXiv:1401.4133 [cond-mat.mes-hall] (2014).
[16] Likai Li, Yijun Yu, Guo Jun Ye, Qingqin Ge, Xuedong Ou, Hua Wu, Donglai Feng, Xian Hui Chen, Yuanbo Zhang, "Black phosphorus field-effect transistors". Nature Nanotechnology, 9, 372–377 (2014). Abstract.
[17] Steven P. Koenig, Rostislav A. Doganov, Hennrik Schmidt, A. H. Castro Neto, Barbaros Özyilmaz, "Electric field effect in ultrathin black phosphorus". Applied Physics Letters, 104, 103106 (2014). Abstract.
[18] Han Liu, Adam T. Neal, Zhen Zhu, Zhe Luo, Xianfan Xu, David Tománek, Peide D. Ye, "Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility". ACS Nano, 8, 4033-4041 (2014). Abstract.
[19] Michele Buscema, Dirk J. Groenendijk, Sofya I. Blanter, Gary A. Steele, Herre S. J. van der Zant, Andres Castellanos-Gomez, "Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors". Nano letters, 14, 3347-3352 (2014). Abstract.
[20] Oriol Lopez-Sanchez, Dominik Lembke, Metin Kayci, Aleksandra Radenovic, Andras Kis, "Ultrasensitive photodetectors based on monolayer MoS₂" Nature Nanotechnology, 8, 497-501 (2013). Abstract.

Realizing Two-Dimensional Optics with Metal Antennas and Graphene Plasmons

(From Left to Right) Pablo Alonso-González, Alexey Nikitin and Rainer Hillenbrand.

Authors: 
Pablo Alonso-González¹, Alexey Nikitin^1,2, Federico Golmar^1,3, Alba Centeno⁴, Amaia Pesquera⁴, Saül Vélez¹, Jianing Chen¹, Gabriele Navickaite⁵, Frank Koppens⁵, Amaia Zurutuza⁴, Félix Casanova^1,2, Luis E. Hueso^1,2, Rainer Hillenbrand^1,2.

Affiliation:
¹CIC nanoGUNE, 20018 Donostia-San Sebastián, Spain.
²IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain.
³I.N.T.I-CONICET and ECyT-UNSAM, San Martín, Bs. As., Argentina.
⁴Graphenea SA, 20018 Donostia-San Sebastián, Spain.
⁵ICFO-Institut de Ciéncies Fotoniques, Mediterranean Technology Park, Barcelona, Spain.

Optical circuits and devices could make signal processing and computing much faster. However, although light is very fast, it needs too much space. In fact, propagating light needs at least the space of half its wavelength, which is much larger than state-of-the-art electronic building blocks in our computers. For that reason, a quest for squeezing light to propagate it through nanoscale materials arises.

Graphene, a single layer of carbon atoms with extraordinary properties, has been proposed as one solution. The wavelength of light captured by a graphene layer can be strongly shortened by a factor of 10 to 100 compared to light propagating in free space [1, 2]. As a consequence, this light propagating along the graphene layer - called graphene plasmon - requires much less space and promises ultra-compact photonic devices [3,4].

Past 2Physics articles by this Group:
July 22, 2012: "Capturing, Tuning and Controlling Light with a Single Sheet of Carbon Atoms"
by Jianing Chen, Michela Badioli, Pablo Alonso-González, Susokin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza, Nicolas Camara, Javier García de Abajo, Rainer Hillenbrand, Frank Koppens

Converting light efficiently into graphene plasmons, however, has been a major challenge. In our recent work [5], we demonstrate that the antenna concept of radio wave technology could be a promising solution. We show that a nanoscale metal rod on graphene (acting as an antenna for light) can capture infrared light and convert it into graphene plasmons, analogous to a radio antenna converting radio waves into electromagnetic waves in a metal cable. The excitation of graphene plasmons is purely optical, the device is compact and the phase and wavefronts of the graphene plasmons can be directly controlled by geometrically tailoring the antennas. The latter is essential for the development of applications that require focusing and guiding of graphene plasmons.

Fig. 1: Launching graphene plasmons with a gold antenna. The oscillations of the calculated electromagnetic field around the antenna reveal the graphene plasmons.

Based on calculations (Fig. 1), we fabricated gold nanoantennas on graphene provided by Graphenea. We then used the Neaspec near-field microscope to image how infrared graphene plasmons are launched and propagate along the graphene layer. In the experimental near-field images, we observed that indeed electromagnetic waves on the graphene propagate away from the antenna, with a wavelength that is about 30 times smaller than that of the incident light (Fig. 2).

Fig. 2: Top: Topography of a gold nanoantenna on graphene. Bottom: Near-field image showing the fields of the antenna and the graphene plasmons around the antenna. The image was taken at an illumination wavelength of 11.06 μm and shows the real part of the imaged field. The distance between fringes of the same color reveals the graphene plasmon wavelength.

In order to test whether the two-dimensional propagation of light waves along a one-atom-thick carbon layer follow the laws of conventional optics, we tried to focus and refract the waves. For the focusing experiment, we curved the antenna. The images then showed that the graphene plasmons focus away from the antenna, similar to the light beam that is concentrated with a lens or concave mirror.

We also observed that graphene plasmons refract (bend) when they pass through a prism-shaped graphene bilayer (Fig. 3), analogous to the bending of a light beam passing through a glass prism. The big difference is that the graphene prism is only two atoms thick. By measuring the graphene plasmon wavelengths in the bi- and monolayer, λ₁ and λ₂, as well as the propagation angles α₁ and α₂, we could demonstrate that the refraction of graphene plasmons qualitatively follows the fundamental law of refraction (Snell´s law): sin(α₁)/sin(α₂) = λ₁/λ₁.

Fig. 3: (a) Illustration of a graphene bilayer prism next to a gold antenna. (b) Near-field image (taken at an illumination wavelength of 10.20 μm) of graphene plasmons refracting at a graphene bilayer prism. The yellow lines and arrows illustrate the plasmon wavefronts and their refraction.

Intriguingly, the graphene plasmons are refracted because the conductivity in the two-atom-thick prism is larger than in the surrounding one-atom-thick layer. In the future, local conductivity changes in graphene could be generated by simple electronic means, such as gating, allowing for highly efficient electrical control of refraction, among others for steering applications.

Altogether, the experiments show that the fundamental and most important principles of conventional optics also apply for graphene plasmons, in other words, squeezed light propagating along a one-atom-thick layer of carbon atoms. Future developments based on these results could lead to extremely miniaturized optical circuits and devices that could be useful for sensing and computing, among other applications.

References:
[1] Jianing Chen, Michela Badioli, Pablo Alonso-González, Sukosin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza Elorza, Nicolas Camara, F. Javier García de Abajo, Rainer Hillenbrand, Frank H. L. Koppens, "Optical nano-imaging of gate-tunable graphene plasmons". Nature, 487, 77-81 (2012). Abstract. 2Physics Article.
[2] Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, D. N. Basov, "Gate-tuning of graphene plasmons revealed by infrared nano-imaging". Nature, 487, 82-85 (2012). Abstract. 2Physics Article.
[3] Ashkan Vakil, Nader Engheta, “Transformation optics using graphene”. Science, 332, 1291-1294 (2011). Abstract.
[4] A.N. Grigorenko, M. Polini, K.S. Novoselov, “Graphene plasmonics”, Nature Photonics, 6, 749-758 (2012). Abstract.
[5] P. Alonso-González, A.Y. Nikitin, F. Golmar, A. Centeno, A. Pesquera, S. Vélez, J. Chen, G. Navickaite, F. Koppens, A. Zurutuza, F. Casanova, L.E. Hueso and R. Hillenbrand. “Controlling grapheme plasmons with resonant metal antennas and spatial conductivity patterns”. Science,  344, 1369-1373 (2014). Abstract.

Institutes:

CIC nanoGUNE
The nanoGUNE Cooperative Research Center, located in Donostia-San Sebastian, Basque Country, is a research centre set up with the mission to conduct excellence research into nanoscience and nanotechnology with the aim of increasing the Basque Country’s business competitiveness and economic and social development.

GRAPHENEA S.A.
Graphenea is a pioneer graphene production start-up company founded in 2010 by private investors and CIC nanoGUNE. The company produces and commercializes graphene films by Chemical Vapor Deposition technology and graphene powders by Chemical Exfoliation techniques.

ICFO
ICFO is a young research institution located in Barcelona that aims to advance the very limits of knowledge in Photonics, namely the science and technology of harnessing Light. Its research programs target the global forefront of photonics, and aim to tackle important challenges faced by society at large. ICFO is focused on current and future problems in Health, Energy, Information, Safety, Security and caring for the Environment.

Micropillar Laser Mimics 'Excitability' of Neurons

Left to Right: S. Barbay and F. Selmi

Authors: S. Barbay, F. Selmi

Affiliation:
Laboratoire de Photonique et de Nanostructures, Marcoussis, France.

It is well known in neurophysiology textbooks (see e.g. [12]) that neurons are biological excitable systems possessing an absolute and a relative refractory period. These properties are fundamental for the propagation of nerve impulse and for information processing in the brain. Excitability is a generic property that is also found in biological [8], chemical [11], and optical systems [5]. An excitable system possess a rest state. If perturbed above a certain threshold - the excitable threshold - with a single perturbation, it emits a pulse with a characteristic shape (light pulse in optics, electrical pulse in neurons). If perturbed with two successive pulses above the excitable threshold, it can respond by emitting two identical pulses if the perturbation pulses are well separated temporally. It can emit a unique pulse if the second perturbation occurs too early after the first one : we are then in the absolute refractory period. However, in an intermediate regime, the relative refractory period, it is possible but harder to trigger a second response and this second response is inhibited and has smaller amplitude.

Figure 1: Schematic representation and SEM image of the micropillar laser with embedded saturable absorber. The pillar microcavity is coated with a thick, 2 microns, layer of SiN. The active zone is sandwiched between two aperiodic multilayer mirrors and is composed of a gain and an absorber section made by respectively 2 and 1 quantum wells. The laser emits around 980nm and is 4μm in diameter.

Our system is a special kind of micropillar laser embedding in its center a saturable absorber, and optimized for optical pumping [4]. Semiconductor lasers are interesting systems for the study of excitability [2, 3, 6, 7, 14-19] since they can have a small footprint and have short timescales. They can thus lead to short response times, necessary in view of their utilization in a neuromorphic information processing context. It was also recognized recently that microlasers with saturable absorber can work as leaky integrate-and-fire laser neurons [10] paving the way to fast cognitive computing.

We have shown in [13] that a micropillar laser with saturable absorber is a fast excitable unit with an absolute, and for the first time, a relative refractory period. The response time is of the order of 200ps, similar to the absolute refractory period, and the relative refractory period is about 350ps. This demonstrates that this system behaves analogously to a neuron but with much faster timescales (sub-nanosecond vs millisecond). The existence of a relative refractory period proves that the system keeps memory of its past state. Contrarily to a common belief, the excitable threshold is not a constant of the system and may be controlled either externally, via the bias pumping of the system, or dynamically via the past state memory. These properties can be utilized for neuro-mimetic optical processing of information, for instance by coupling several micropillar lasers together for building logic gates [1] or for neuromorphic processing [9,10].

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