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2Physics Quote:
"From the textbooks we know that the propagation of light through an inhomogeneous medium is strongly influenced by the refractive index distribution. We experimentally investigated this phenomenon with the purpose of realizing if such propagation along a free-space path may induce a useful randomness. An intuition of such effect manifested during the campaigns for the experiments we carried out at the Canarias on the quantum Communications along extremely long links. The atmospheric turbulence in the path is very strong, preventing for example the direct application of interferometry... We tried to turn it here instead into a useful resource for randomness."
-- Davide G. Marangon, Giuseppe Vallone, Paolo Villoresi
(Read Full Article: "A True Randomness Generator Exploiting a Very Long and Turbulent Path" )

Saturday, October 18, 2014

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.

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

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


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.



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) between the superconducting cavity (formed by Lsc and Csc ) 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 Csc ≈ 415 fF and Lsc ≈ 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 ωp (near the cavity resonance frequency ω) and another signal at ωd (detuned by mechanical frequency ω, such that ωd = ω+ω), 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 ωc+ωm 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 ωd = ωc - ω) 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 > nth , where nth 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.

[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.

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Sunday, October 12, 2014

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  

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.

[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.  

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Tuesday, October 07, 2014

Physics Nobel Prize 2014: Blue LED

(From Left to Right) Isamu Akasaki, Hiroshi Amano and Shuji Nakamura

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2014 to Isamu Akasaki (Meijo University, Nagoya, Japan and Nagoya University, Japan), Hiroshi Amano (Nagoya University, Japan) and Shuji Nakamura (University of California, Santa Barbara, CA, USA) “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”.

This year’s Nobel Laureates are rewarded for having invented a new energy-efficient and environment-friendly light source – the blue light-emitting diode (LED). In the spirit of Alfred Nobel the Prize rewards an invention of greatest benefit to mankind; using blue LEDs, white light can be created in a new way. With the advent of LED lamps we now have more long-lasting and more efficient alternatives to older light sources.

When Isamu Akasaki, Hiroshi Amano and Shuji Nakamura produced bright blue light beams from their semi-conductors in the early 1990s, they triggered a fundamental transformation of lighting technology. Red and green diodes had been around for a long time but without blue light, white lamps could not be created. Despite considerable efforts, both in the scientific community and in industry, the blue LED had remained a challenge for three decades.

They succeeded where everyone else had failed. Akasaki worked together with Amano at the University of Nagoya, while Nakamura was employed at Nichia Chemicals, a small company in Tokushima. Their inventions were revolutionary. Incandescent light bulbs lit the 20th century; the 21st century will be lit by LED lamps.

White LED lamps emit a bright white light, are long-lasting and energy-efficient. They are constantly improved, getting more efficient with higher luminous flux (measured in lumen) per unit electrical input power (measured in watt). The most recent record is just over 300 lm/W, which can be compared to 16 for regular light bulbs and close to 70 for fluorescent lamps. As about one fourth of world electricity consumption is used for lighting purposes, the LEDs contribute to saving the Earth’s resources. Materials consumption is also diminished as LEDs last up to 100,000 hours, compared to 1,000 for incandescent bulbs and 10,000 hours for fluorescent lights.

Blue light has a shorter wavelength than other colors such as red and green, and therefore can be used to read and write smaller and smaller bits of information. Creating blue LEDs and lasers was a technologically difficult feat. While compact disc players were on the scene since 1982, Blu-Ray players and the Playstation 3, introduced in late 2006, were among the first consumer electronics devices to use these shorter-wavelength diode lasers. (Fun fact: Even though they're called Blu-Ray, the lasers in the players and Playstation are actually violet, an even shorter-wavelength color.)

The LED lamp holds great promise for increasing the quality of life for over 1.5 billion people around the world who lack access to electricity grids: due to low power requirements it can be powered by cheap local solar power.

The invention of the blue LED is just twenty years old, but it has already contributed to create white light in an entirely new manner to the benefit of us all.

Homepage of Isamu Akasaki >>
Homepage of Hiroshi Amano >>
Homepage of Shuji Nakamura >>

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Sunday, September 28, 2014

When Magnetism Meets Optics

S. Mangin (Left) and E. E. Fullerton


C.H. Lambert, M. Salah, N. Bergeard, G. Malinowski, M. Hehn, S. Mangin,
Equipe Nanomagnetisme et Electronique de Spin de l’Institut Jean Lamour UMR CNRS 7198, Université de Lorraine, France

Y. Fainman, E. E. Fullerton,
Center For Magnetic Recording Research, University of California San Diego (UCSD), USA 

M. Cinchetti, M. Aeschlimann, 
Department of Physics and Research Center OPTIMAS, University of Kaiserlautern- Allemagne, Germany 

B. Varaprasad, Y. Takahashi, K. Hono, 
National Institute for Materials Science, Japan

With the fast development of mass storage units all around the world (clouds, data centers…) the pressure to increase the density, speed and energy efficiency of conventional hard disk drives is becoming stronger and stronger. The discovery of “All-optical control of ferromagnetic thin films and nanostructures” might open up new technological horizons in magnetic recording. This work is the results of a collaboration between scientists and engineers from University of California San Diego, Universite de Lorraine, Kaiserlauter Universitat and National Institute for Materials Science in Tsukuba, Japan published in Science on September 14th 2014 [1].

 The authors found that they could control the final state of the magnetization of a broad range of magnetic materials using laser pulses of circularly polarized light instead of an applied magnetic fields. In particular these researchers find out that the magnetization of some magnetic material similar to those used in the recording industry can be manipulated directly with a laser beam. The ability to optically control magnetic materials the density and access time of data on hard drives could be increased dramatically.

Image: Writing with a laser on a magnetic thin film.

The first observation of “all optical switching” of magnetic materials was performed in 2007 by the group from T. Rasing in Nijmegen on a very particular ferrimagnetic alloy GdFeCo [2]. Since this discovery there has been extensive studies of optical switching of this material class including detailed studies of the magnetic response to optical excitations of both the rare-earth (Gd) and transition metal (Fe and Co) elements. Based on these studies a detailed understanding has emerged of the ultra-fast physics of rare-earth-transition-metal alloys [3,4]. However, the extent of the practical impact of this research is limited by the materials that are not compatible with many modern technologies. By extending these exciting studies to new classes of materials such as ferromagnets, the “all-optical” magnetization switching has made a significant step to demonstrate its potential for technological impact.

These results further show that theoretical understanding of all-optical switching needs to be re-examined. Most recent theories predicted that the all-optical reversal should only occur in ferrimagnetic materials, where the overall magnetization is the result of the competition between two magnetic sub-lattices that are antiferromagnetically coupled. Our results show that all-optical switching is not exclusive to ferrimagnetic materials and therefore antiferromagnetic exchange coupling between two magnetic sublattices is not required. The results do suggest that heating near the Curie point is important for the all-optical switching in ferromagnetic materials. Near the Curie point then a small symmetry-breaking from circularly polarized light (e.g. the inverse Faraday effect or transfer of angular momentum from the light to the magnetic system) can deterministically determine the magnetization direction. However details of this process still need to be determined.

Video: Writing with a laser on a magnetic thin film : Micrometer size "Etch A Sketch".

[1] C-H. Lambert, S. Mangin, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, M. Hehn, M. Cinchetti, G. Malinowski, K. Hono, Y. Fainman, M. Aeschlimann, E. E. Fullerton, "All-optical control of ferromagnetic thin films and nanostructures".  Science, 345, 1337-1340 (2014). Abstract.
[2] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, Th. Rasing, "All-optical magnetic recording with circularly polarized light". Physical Review Letters, 99, 047601 (2007). Abstract.
[3] Andrei Kirilyuk, Alexey V Kimel, Theo Rasing, "Laser-induced magnetization dynamics and reversal in ferrimagnetic alloys". Reports on Progress in Physics, 76, 026501 
(2013). Abstract.
[4] S. Mangin, M. Gottwald, C-H. Lambert, D. Steil, V. Uhlíř, L. Pang, M. Hehn, S. Alebrand, M. Cinchetti, G. Malinowski, Y. Fainman, M. Aeschlimann, E.E. Fullerton, "Engineered materials for all-optical helicity-dependent magnetic switching".  Nature Materials, 13, 286–292 (2014). Abstract.


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Sunday, September 21, 2014

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

Debin Wang (left) and James J. De Yoreo

Authors: Debin Wang1,2, James J. De Yoreo1,2

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.

[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.

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Sunday, September 14, 2014

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. Schattschneider1,2,3, Th. Schachinger1, M. Stöger-Pollach2, S. Löffler2, A. Steiger-Thirsfeld2, K. Y. Bliokh4,5, Franco Nori5,6

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 zk, where it blocks half of each of the beams. Varying the position zk 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.

[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.

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