World-record Performance using a Silicon-based Avalanche Photodetector

Mario Paniccia [Photo courtesy: Intel]

In an article published today in online version of Nature Photonics, a team led by Intel researchers reported a path-breaking advancement in the field of Silicon Photonics by achieving world-record performance with a silicon-based Avalanche Photodetector (APD), a light sensor that gains superior sensitivity by detecting light and amplifying weak signals as light is directed onto silicon. This could lower costs and improve performance as compared to commercially available optical devices.

Silicon Photonics is an emerging technology using standard silicon to send and receive optical information among computers and other electronic devices. The technology aims to address future bandwidth needs of data-intensive computing applications such as remote medicine and lifelike 3-D virtual worlds.

The photodetector developed by the team is Ge/Si-based and has built-in amplification, which makes it much more useful in instances where very little light falls on the detector. It is called an avalanche photodetector because an avalanche process occurs inside the device. First, a negative and a positive charge (electrons and holes in semiconductor terminology) are created when the light strikes the detector. The electron is accelerated by an electric field until it attains a high enough energy to slam into a silicon atom and create another pair of positive and negative charges. Each time this happens the number of total electrons doubles, until this “avalanche” of charges are collected by the detection electronics.

This amplification effect (called gain) is the key to the device, and it serves as the motivation for why anyone would try to do this in silicon and not just continue to use traditional InP (Indium phosphide)-based APDs. The materials properties of silicon inherently led to lower noise and better performance in this avalanche process.

Image: A ladybug crawls across an experimental Avalanche Photodetector chip containing silicon optical devices that are only a fraction of a millimeter [Photo courtesy: Intel]

The APD device developed by the Intel team used silicon and CMOS processing to achieve a "gain-bandwidth product" of 340 GHz -- the best result ever measured for this key APD performance metric. This opens the door to lower the cost of optical links running at data rates of 40Gbps or higher and proves, for the first time, that a silicon photonics device can exceed the performance of a device made with traditional, more expensive optical materials such as indium phosphide (InP).

"This research result is another example of how silicon can be used to create very high-performing optical devices," said Dr. Mario Paniccia, Intel Fellow and director of the company's Photonics Technology Lab. "In addition to optical communication, these silicon-based APDs could also be applied to other areas such as sensing, imaging, quantum cryptography or biological applications."

Reference
"Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain–bandwidth product"
Yimin Kang, Han-Din Liu, Mike Morse, Mario J. Paniccia, Moshe Zadka, Stas Litski, Gadi Sarid, Alexandre Pauchard, Ying-Hao Kuo, Hui-Wen Chen, Wissem Sfar Zaoui, John E. Bowers, Andreas Beling, Dion C. McIntosh, Xiaoguang Zheng & Joe C. Campbell,
Nature Photonics (7 December 2008 doi:10.1038/nphoton.2008.247). Abstract.

Squeezing of Quantum Noise successfully used to develop First Tunable, ‘Noiseless’ Amplifier

Konrad Lehnert [Photo Courtesy: JILA, Boulder]

By significantly reducing the uncertainty in delicate measurements of microwave signals, a team of researchers from the National Institute of Standards and Technology (NIST) and Joint Institute of Laboratory Astrophysics (JILA) could successfully develop the first tunable “noiseless” amplifier which could boost the speed and precision of quantum computing and communications systems.

Conventional amplifiers add unwanted “noise,” or random fluctuations, when they measure and boost electromagnetic signals. Amplifiers that theoretically add no noise have been demonstrated before, but the JILA/NIST technology offers better performance and is the first to be tunable, operating between 4 and 8 GHz, according to JILA group leader Konrad Lehnert. It is also the first amplifier of any type ever to boost signals sufficiently to overcome noise generated by the next amplifier in a series along a signal path, Lehnert says, a valuable feature for building practical systems.

Noisy amplifiers force researchers to make repeated measurements of, for example, the delicate quantum states of microwave fields—that is, the shape of the waves as measured in amplitude (or power) and phase (or point in time when each wave begins). The rules of quantum mechanics say that the noise in amplitude and phase can’t both be zero, but the JILA/NIST amplifier exploits a loophole stipulating that if you measure and amplify only one of these parameters—amplitude, in this case—then the amplifier is theoretically capable of adding no noise. In reality, the JILA/NIST amplifier adds about half the noise that would be expected from measuring both amplitude and phase.

The JILA/NIST amplifier could enable faster, more precise measurements in certain types of quantum computers—which, if they can be built, could solve some problems considered intractable today—or quantum communications systems providing “unbreakable” encryption. It also offers the related and useful capability to “squeeze” microwave fields, trading reduced noise in the signal phase for increased noise in the signal amplitude. By combining two squeezed entities, scientists can “entangle” them, linking their properties in predictable ways that are useful in quantum computing and communications. Entanglement of microwave signals, as opposed to optical signals, offer some practical advantages in computing and communication such as relatively simple equipment requirements, Lehnert says.

[Image Credit: M. Castellanos-Beltran/JILA] In the JILA/NIST “noiseless” amplifier, a long line of superconducting magnetic sensors (beginning on the right in this photograph) made of sandwiches of two layers of superconducting niobium with aluminum oxide in between, creates a 'metamaterial' that selectively amplifies microwaves based on their amplitude rather than phase.

The new amplifier is a 5-millimeter-long niobium cavity lined with 480 magnetic sensors called SQUIDs (superconducting quantum interference devices). The line of SQUIDs acts like a “metamaterial,” a structure not found in nature that has strange effects on electromagnetic energy. Microwaves ricochet back and forth inside the cavity like a skateboarder on a ramp. Scientists tune the wave velocity by manipulating the magnetic fields in the SQUIDs and the intensity of the microwaves. An injection of an intense pump tone at a particular frequency, like a skateboarder jumping at particular times to boost speed and height on a ramp, causes the microwave power to oscillate at twice the pump frequency. Only the portion of the signal which is synchronous with the pump is amplified.

Reference
"Amplification and squeezing of quantum noise with a tunable Josephson metamaterial",
M.A. Castellanos-Beltran, K.D. Irwin, G.C. Hilton, L.R. Vale and K.W. Lehnert,
Nature Physics, published online: 5 Oct. 5 2008; doi:10.1038/nphys1090. Abstract

[We thank Media Relation, NIST for materials used in this posting]

Phase Transitions of Dirac Electrons in Bismuth in a High Magnetic Field

N. Phuan Ong

[This is an invited article based on recent works of the authors.
-- 2Physics.com]

Authors: N. P. Ong, Lu Li, Joseph G. Checkelsky and R. J. Cava

Affiliation: Dept of Physics, Princeton University
[Link to Ong Lab -->]

In most metals and semiconductors, the motion of an electron is well described by the Schrödinger equation. The kinetic energy increases as the square of the momentum just as for electrons in vacuum (Fig. 1a). However, in certain materials, the electronic energy increases linearly with the momentum (Fig. 1b). The linear dispersion is reminiscent of that of neutrinos and photons. Examples of such materials are graphene [1] (a single layer of carbon peeled from graphite), bismuth and antimony and their alloy BiSb, and a class of organic metals called “ET” salts. The quasiparticles (broken Cooper pairs) in the unconventional superconductors based on copper oxide and on strontium ruthenate also display a linear dispersion. In these “Dirac materials”, the electrons are accurately described by the Dirac Hamiltonian, except that the effective velocity of light is reduced by a factor of about 300. The effective fine structure constant is roughly 3 instead of 1/137.

Lu Li [Left], Joe Checkelsky [Right]

In comparison with relativistic electrons in vacuum, Dirac electrons living in solids have several distinguishing features. First, a magnetic field of a few Tesla is sufficient to quantize the orbits of Dirac electrons to form a series of Landau levels. To obtain similar effects in relativistic electrons in vacuum, one would need magnetic fields in excess of a million Tesla. As the applied field is increased, the Landau levels are successively emptied. This leads to the well-known quantum oscillations in curves of the resistivity and magnetization versus field (Fig. 1c) (the periods of these oscillations are routinely used to measure the caliper of Fermi Surfaces).

R. J. Cava

If the field is strong enough, all the Dirac electrons are forced into the lowest Landau Level. In this situation, the effects of mutual Coulomb repulsion between the electrons are greatly enhanced. This is particularly true for Dirac electrons which (unlike the Schrödinger case) do a poor job of screening each other’s Coulomb potential. Secondly, in solids, Dirac electrons occupy distinct but equivalent Fermi Surface pockets or “valleys”. The additional degree of freedom, akin to “flavor” in particle physics, is known as valley degeneracy. These ingredients suggest that, in a steadily increasing magnetic field, Dirac electrons may undergo a sudden phase transition to a collective state in order to relief the effects of mutual repulsion.

The Fermi surface of bismuth is comprised in part of 3 equivalent electron ellipsoids which are accurately described as Dirac electrons (the Fermi Surface encloses all occupied states in momentum space). Hence bismuth presents a gas of Dirac electrons which come in 3 flavors, corresponding to the 3 ellipsoids. The electrons coexist with an equal concentration of holes, which are positively charged carriers obeying the ordinary Schrödinger equation. If the electrons can be studied free from interference from the holes, we could investigate the question posed. Unfortunately, Nature has diabolically arranged matters so that the interference from the holes is a maximum when the magnetic field is pointed along the most interesting direction -- the trigonal axes (along this axis, strict equivalency between the 3 electron flavors is maintained). In this field direction, the period of the quantum oscillations is nearly the same for both holes and electrons. Because the hole oscillations have a larger amplitude, they completely obscure the electron oscillations. For 4 decades, this has prevented researchers from “seeing” what the Dirac electrons are doing in a magnetic field.

In Ref. 2, we extended torque magnetometry, a technique pioneered by David Shoenberg, to fields of 32 Tesla. In bismuth, the dynamics of the electrons are conveniently described in terms of effective-mass parameters which assume different values along the 3 symmetry axes. If the magnetic field is tilted at an angle to a symmetry axis, the mass anisotropy leads to a magnetic moment that is at an angle to the field. This immediately leads to a torque on the sample. As shown by Shoenberg, the torque signal provides a remarkably sensitive way to detect individual Landau Levels as they cross the chemical potential. By careful choice of the torque axes, we were able to tease out the torque signals of the Dirac electrons and distinguish them from the holes (see the low-field regions in Fig. 2). This solved the problem mentioned.

Fig. 1a: Quadratic energy dispersion of electrons obeying Schrödinger equation. Fig. 1b: Linear dispersion of Dirac electrons. Fig. 1c: Curves of the torque signal versus magnetic field in bismuth at 0.3 K at selected tilt angles of the field to the trigonal axis. Rapid oscillations at low fields are Landau Level crossings. The red and black arrows indicate sharp electronic transitions to a collective state [2].

With the ability to see what the Dirac electrons are doing, we proceeded to investigate their behavior in an intense magnetic field aligned nearly parallel with the trigonal axis. We found that at low temperature (below 2 K), the electrons exhibit a sharp transition to a collective state (red arrows in Fig. 1c). In contrast with the torque signal at lower fields which are replete with Landau oscillations (Fig. 2), the torque flat lines at a value close to zero above the transition field. In this region, which extends to very high fields over a narrow range of tilt angles, Landau levels are completely absent (upper right quadrant of Fig. 2). We have dubbed this the “dead zone”. Interestingly, when we exit the dead zone by further increasing the field at a finite tilt angle (see black arrows in Fig. 1c), the remaining Landau levels reappear.

Fig. 2 Curves of the derivative of the torque signal versus magnetic field at selected tilt angles q [2]. Peaks (labeled by 1+ or 0-) occur when Landau Levels cross the chemical potential. In the dead zone (upper right quadrant bounded by the gray curve), the curves assume flat-line behavior.

What is the state in the dead zone? Our best guess is that the Dirac electrons have transitioned to a collective state in which their total Coulomb energy is dramatically lowered. We may draw an analogy with ferromagnetism. In an ordinary metal, each electron can align its spin up or down. If the Coulomb repulsion is large compared with the kinetic energy, the electrons transition to the ferromagnetic state in which all spins are aligned up (say). The Pauli Principle then guarantees that the electrons are always kept maximally apart from each other. This results in a large lowering of the mutual repulsion energy (at the price of a small increase in kinetic energy). The 3 equivalent valleys for the Dirac electrons in bismuth constitute a degree of freedom that mimics the spin degree in a ferromagnet. We propose that, in the dead zone, all Dirac electrons occupy one of the valleys (or a linear combination of the 3) so that they can stay maximally apart from each other. This collective state, dubbed “valley ferromagnetism”, was previously proposed for bilayer GaAs in the quantum Hall effect state [3].

The present results share a common thread with recent developments in several areas of condensed matter physics. The Dirac electrons in (2 dimensional) graphene occupy 2 equivalent valleys. There is recent evidence [4] that, in an intense field, they also undergo a phase transition to a high-field state in which the degeneracy between the 2 valleys is lifted. Interesting Dirac states are predicted to exist on a 2D interface separating regions in which the Dirac mass has opposite signs. The massless Dirac states at the interface are chiral, i.e. they propagate in only one direction depending on the spin. The recent prediction [5] of quantized spin Hall currents carried by these chiral states has received strong support from experiments [6] on HgTe quantum wells. The 3-dimensional alloy Bi-Sb has recently drawn strong interest because of the prediction [7] that it harbors surface Dirac states that are chiral as well as “topological” in nature. Using angle-resolved photomission spectroscopy (ARPES), Hasan and collaborators recently observed directly the surface states [8]. Very recent spin-resolved ARPES experiments have confirmed the unique spin polarization of the surface states in both pure Sb and Bi-Sb [9]. Lastly, the possibility of Majorana fermions living at the edge of a p-wave superconductor has been discussed but the experimental situation is still uncertain.

References
[1] "Electric Field Effect in Atomically Thin Carbon Films", K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 306, 666 (2004). Abstract.
[2] “Phase transitions of Dirac electrons in bismuth”, Lu Li, J. G. Checkelsky, Y.S. Hor, C. Uher, A. F. Hebard, R. J. Cava and N. P. Ong, Science 321, 547 (2008). Abstract.
[3] “Spontaneous interlayer coherence in double-layer quantum Hall systems: Charged vortices and Kosterlitz-Thouless phase transitions,” K. Moon, H. Mori, Kun Yang, S. M. Girvin, A. H. MacDonald, L. Zheng, D. Yoshioka, Shou-Cheng Zhang, Phys. Rev. B 51, 5138 (1995). Abstract.
[4] "Zero-Energy State in Graphene in a High Magnetic Field".
Joseph G. Checkelsky, Lu Li, and N. P. Ong, Phys. Rev. Lett. 100, 206801 (2008). Abstract.
[5] “Quantum Spin Hall Effect and Topological Phase Transition in HgTe Quantum Wells”, B.A. Bernevig, T.L. Hughes and S.–C. Zhang, Science, 314, 1757-1761 (2006). Abstract.
[6] “Quantum Spin Hall Insulator State in HgTe Quantum Wells”, M. König, S. Wiedmann, C. Brüne, A. Roth, H. Buhmann, L. W. Molenkamp, X. L. Qi, and S. C. Zhang, Science 318, 766 (2007). Abstract.
[7] “Topological insulators with inversion symmetry”,
L. Fu and C.L. Kane, Phys. Rev. B, 76, 045302 (2007). Abstract.
[8] “A topological Dirac insulator in a quantum spin Hall Phase”,
D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava and M. Z. Hasan,
Nature 452, 970 (2008). Abstract.
[9] M. Z. Hasan, private communication.

New Technique Probes Ultracold Atomic Gases

Deborah Jin [photo courtesy: JILA, Boulder, CO]

In a paper published in August 7th issue of the journal Nature, a team of physicists from JILA (link to Deborah Jin Group->), a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, reported a new powerful technique that reveals hidden properties of ultracold atomic gases. The idea behind the technique originates from 'photoemission spectroscopy' which has been used for nearly a century in the study of materials and, specifically, for probing the energy of electrons in a material. The team of scientists led by Deborah Jin adapted this technique to study potassium atoms in an ultracold gas.

Photoemission spectroscopy is particularly powerful in revealing details of the pairing of electrons in high-temperature superconductors, which are solids that have zero resistance to electrical current at relatively high temperatures (but still below room temperature). The scientists at JILA study a very similar phenomenon: superfluidity (fluids that can flow with zero friction). Specifically, they study how atoms in a Fermi gas behave as they "cross over" from acting like a Bose Einstein Condensate (in which fermions pair up to form tightly bound molecules) to behaving like pairs of separated electrons in a superconductor.

In the crossover region, atoms in an ultracold gas exert very strong forces on each other, which masks their individual properties. To see the hidden behavior, JILA scientists apply a radio frequency field to a cloud of trapped, paired potassium atoms, ejecting a few atoms from the strongly interacting cloud. Then the laser trap is turned off so the gas can expand. Scientists make images and count the numbers of escaping atoms at different velocities. With this information, scientists can calculate the atoms' original energy states and momentum values back when they were inside the gas. Scientists then map the energy levels for all the original states of the atoms and can identify a particular pattern that shows the appearance of a large "energy gap," which represents the amount of energy needed to break apart a pair of atoms.

The new photoemission technique represents a huge jump in the information available to physicists who study ultracold gases. Traditionally, scientists could probe either the energy or momentum of these gases, not both. The new technique simultaneously probes the energy and momentum, allowing the scientists to study the microscopics involved in the pairing of two atoms.

"This technique is a clean probe of the microscopics in this system, and it allows us to see interesting things like a very large energy gap that seems to appear before the superfluid state," says group leader Deborah Jin. Ultimately, the JILA work studying superfluidity in atomic gases may one day help in understanding the energy gap that appears in high-temperature superconductors, which may have applications such as more efficient transmission of electricity across power grids. In addition, the new technique can be extended beyond the study of pairing to include, for example, the study of atoms trapped in crisscrossed "lattices" of laser light, a building block for some atomic clock and quantum computer designs.

Reference
"Using photoemission spectroscopy to probe a strongly interacting Fermi gas"
J. T. Stewart, J. P. Gaebler, D. S. Jin,
Nature, 454, p744-747 (7 August, 2008) Abstract.

[We thank Media Relations, National Institute of Standards and Technology (NIST) for materials used in this posting]

Anderson Localization of Matter-Waves

Philippe Bouyer

[This is an invited article based on recent work of the authors and their collaborators. -- 2Physics.com]

Authors: Philippe Bouyer and Vincent Josse

Affiliation: Groupe d'Optique Atomique, Laboratoire Charles Fabry, Institut d'Optique Graduate School, Palaiseau, France

How to understand the conducting properties of metals and semiconductors? From the quantum theory of conduction, in which electrons are described as matter waves, we can draw a naïve picture based on the idea that electrons with certain momenta can travel freely through the crystal, while others cannot as they diffract from the periodic structure played by the lattice.

Fifty years ago, Philip Anderson, 1977 Physics Nobel Prize winner, worked out that tiny modifications of the lattice, such as the introduction of impurities or defects, can dramatically modify this behavior : the electron that would move freely inside the solid does not simply diffuse on the defects as expected for classical particles but they can be completely stopped.

On a macroscopic scale, that would be like saying that a few blades of grass scattered haphazardly over a golf course could completely stop a full-speed golf ball in its tracks : this would be a surprising situation, since we all know that small perturbations can only slow the movement of material objects, but can never stop them. In the light of fundamental discoveries made in the 1930s about semi-conductors that led to the invention of the transistor and then to integrated circuits, this phenomenon called 'Anderson Localization' created and is still creating strong interests among physicists.

While theoretical physicists strived to understand its underlying nature and its significance, experimental physicists tried to observe the phenomenon. However, experiments in real materials had to struggle mainly against these two disturbances : residual thermal excitations inside the solid and the unavoidable strong interactions among electrons. For these reasons, even if convincing experiments existed, direct observation of these phenomena for particle matter remained an unattainable goal.

In a recent communication published in Nature, our team of researchers at the Institut d'Optique reported on the direct observation of Anderson localization of matter-waves in a controlled disorder [1]. In our experiment, ultra-cold atoms play the role of electrons. They are chilled to a temperature close to absolute zero (-459.67 degrees Fahrenheit) to generate a Bose-Einstein condensate (BEC), in which all the atoms can be described as a single wave function.

Fig.1: Artist's view of the experimental apparatus. © A. Bernard / P. Bouyer / Institut d'Optique

We allowed these BECs to expand from a small starting spot along a single direction imposed by a laser-induced atomic waveguide. To “simulate” the disordered environment, we created a perfectly controlled disorder by shining laser light through finely ground glass onto the expanding atoms — creating then a random distribution of light and dark regions. Without disorder, the atoms propagate freely, but when disorder is present, all atomic movement stop within a fraction of a second. We then observed the atomic density profile. Its exponential form, characteristic of Anderson Localization (see figure 2 below), is the awaited direct proof that random diffusion of matter can hinder the diffusion process.

Fig. 2: The exponential atomic density profile, in green, reflects the localization of atomic waves. This immobilization is caused by minor optic disorder (represented in blue) which has stopped the movement of free atoms along the red light guide axis. © Vincent Josse / Philippe Bouyer / CNRS

Thanks to the joint effort with the team of theorists in the institute, we were able to prove the high level of accurate control that we have on all parameters in this simple model. Our results indeed show that we do have this level of control, and do not so far reveal any surprising properties. This is compliant with the fact that in 1D, Anderson localisation is rather well understood theoretically.

This is no more the case in 2D and 3D, where the role of interactions, for example, is not fully understood, and hard to calculate. We believe that our work represents a crutial step that can lead to an additional kind of quantum simulator, where, with atoms, we can build experiments with high level of control to "mimic" these complex situations. We want now to simulate these systems. Extending the technique to two and three dimensions, and better controlling interactions, it might be possible to better understand the behavior of real materials. We could experience situations that theory can not currently precisely predict in these complex systems. May be then, in the long run, these simulators can be used to improve semi-conductors devices, such as amorphous silicon-based electronic devices, for example.

Theorists and experimentalists at the Institute of optics involved in the observation of Anderson Localisation: (from L to R) Vincent Josse, Juliette Billy, Jean-Francois Schaff, Philippe Bouyer, Patrick Cheinet, Pierre Lugan, Alain Bernard, Alain Aspect, Ben Hambrecht, Laurent Sanchez-Palencia.

More information can be found in this special webpage on Anderson localization: Link>>.



References:
[1] "Direct observation of Anderson localization of matter-waves in a controlled disorder”
Juliette Billy, Vincent Josse, Zhanchun Zuo, Alain Bernard, Ben Hambrecht, Pierre Lugan, David Clément, Laurent Sanchez-Palencia, Philippe Bouyer & Alain Aspect,
Nature 453, p891-894 (June 12, 2008). Abstract Link.
[2] "Condensed-matter physics: Paralysed by disorder"
Daniel A. Steck, Nature, 453, p866 (Jun 12, 2008). Abstract Link.
[3] "Anderson localization of a non-interacting Bose–Einstein condensate"
Giacomo Roati, Chiara D'Errico, Leonardo Fallani, Marco Fattori, Chiara Fort, Matteo Zaccanti, Giovanni Modugno, Michele Modugno & Massimo Inguscio,
Nature, 453, p895-898 (Jun 12, 2008). Abstract Link.
[4] "Transport and Anderson localization in disordered two-dimensional photonic lattices"
Tal Schwartz, Guy Bartal, Shmuel Fishman & Mordechai Segev,
Nature, 446, p52-55 (Mar 1, 2007). Abstract Link.

Nonlinear Cerenkov Radiation and its Modulation

Shining Zhu (right) and Yong Zhang (left)

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Authors: Yong Zhang and Shining Zhu

Affiliation: Physics Department, Nanjing University, China

Link to Dielectric Superlattice Laboratory

In 1934, P. V. Cerenkov observed the emission of blue light from a bottle of water subjected to radioactive bombardment. This phenomenon is so-called Cerenkov radiation (CR), associated with charged particles moving at speed faster than the speed of light in the medium. The coherent radiation is observable at a conical wavefront defined by Cerenkov angle θ = arccos(v′/v), where v is the speed of the moving charged particle, and v′ is the phase velocity of the radiation wave. CR has been proved to be of great importance in subsequent experimental work in nuclear physics, and extensively used in experiments for counting and identifying relativistic particles. A recent work reported the observations of coherent, impulsive radio Cerenkov radiation from electromagnetic showers in solid ice.

Light can also create such an emission, which always involves a nonlinear optical process. So we call that nonlinear Cerenkov radiation (NCR). Unlike CR from particles, NCR results from nonlinear polarization driven by light field. The earliest work is Cerenkov second harmonic generation (SHG) in ZnS waveguide. The waveguide configuration has been widely studied since 1970. Cerenkov SHG has been considered as an important method to effectively achieve visible light because it has no strict requirements of waveguide parameters and the maximum nonlinear coefficient of the material can be used. And recently we reported Cerenkov sum-frequency generation in waveguide configuration [1]. Another NCR configuration is optical beating by extreme short pulse, which is a hot field because it can realize THz emission.

Image: Cerenkov SHG in 2D nonlinear photonic crystal waveguide

When a new physical phenomenon is discovered, people will think about two questions, how to use it and how to modify it for better use. CR is important in particle physics. NCR also has a lot of potential applications, for example, multiple photonic detection, THz waves and spectral analysis etc. However, both CR and NCR have a limiting condition, the speed of the source (charged particle for CR and nonlinear polarization for NCR) has to exceed the velocity of emission in the medium. This makes it not convenient to apply this phenomenon in some cases. Can we change that? The answer is yes, at least for NCR.

Up to now, there are three methods to finish this task, phonon-assistance, photonic crystal and nonlinear photonic crystal. All the three methods can realize Cerenkov radiation below the light threshold. And all these methods are based on the fact that the phase velocity of radiation source, linear or nonlinear polarization, could be changed by the interaction between the light and the medium. In our work, nonlinear photonic crystal waveguide is used. This is a waveguide with the periodical modulation of χ⁽²⁾, which may assist in accelerating or retarding the phase velocity v_p of nonlinear polarization, even changing its direction, thereby modulating the behavior of NCR, such as threshold value, radiation angle and direction. The increase of velocity depends on the period of nonlinear photonic crystal. Smaller the period, larger the increase. Therefore, in principle, this method can realize NCR without velocity threshold and with any second-order optical parameter process. This can realize more applications of NCR, such as photonic entanglement, quantum communication and computation networks.

References
[1] “Nonlinear Cerenkov radiation in nonlinear photonic waveguide”
Y. Zhang, Z. D. Gao, Z. Qi and S. N. Zhu, Phys. Rev. Lett. 100, 163904 (2008). Abstract Link.
[2] “Visible glow of pure liquids under g-irradiation”
P. A. Čerenkov, Dokl. Akad. Nauk SSSR 2, 451 (1934).
[3] “Optical second harmonic generation in form of coherent Cerenkov radiation form a thin-film waveguide”, P. K. Tien, R. Ulrich, and R. J. Martin, Appl. Phys. Lett. 17, 447 (1970). Abstract Link.
[4] “Cherenkov radiation at speeds below the light threshold: phonon-assisted phase matching”
T. E. Stevens, J. K. Wahlstrand, J. Kuhl, R. Merlin, Science 291, 627 (2001). Abstract Link.
[5] “Cerenkov radiation in photonic crystals”
Chiyan Luo, Mihai Ibanescu, Steven G. Johnson, J. D. Joannopoulos,
Science 299, 368 (2003). Abstract Link.
[6] “Observations of the Askaryan Effect in Ice”
P. W. Gorham, S. W. Barwick, J. J. Beatty, D. Z. Besson, W. R. Binns, C. Chen, P. Chen, J. M. Clem, A. Connolly, P. F. Dowkontt, M. A. DuVernois, R. C. Field, D. Goldstein, A. Goodhue, C. Hast, C. L. Hebert, S. Hoover, M. H. Israel, J. Kowalski, J. G. Learned, K. M. Liewer, J. T. Link, E. Lusczek, S. Matsuno, B. Mercurio, C. Miki, P. Miočinović, J. Nam, C. J. Naudet, J. Ng, R. Nichol, K. Palladino, K. Reil, A. Romero-Wolf, M. Rosen, L. Ruckman, D. Saltzberg, D. Seckel, G. S. Varner, D. Walz, and F. Wu,
Phys. Rev. Lett. 99, 171101 (2007). Abstract Link.
[7] “Quasi-phase-matched Čerenkov second-harmonic generation in a hexagonally poled LiTaO3 waveguide”, Y. Zhang, Z. Qi, W. Wang, and S. N. Zhu,
Appl. Phys. Lett. 89, 171113 (2006). Abstract Link.

Observation of the Spin Hall Effect of Light

Onur Hosten (left) and Paul Kwiat (right) [Photo credit: L. Brian Stauffer]

Physicists Onur Hosten and Paul G. Kwiat, at the University of Illinois at Urbana-Champaign showed that light exhibits a Spin Hall effect, analogous to the Spin Hall effect in electronic systems, showing the universality of the effect for particles of different nature. The researchers used a novel technique from quantum weak measurements to enhance the tiny Spin Hall displacements prior to observation.

Whenever the propagation direction of a beam of light changes due to a variation in the refractive index of the medium (in the experiment, refraction at an air-glass interface serves this purpose), the beam center experiences a spin-dependent (or circular polarization-dependent) displacement perpendicular both to the initial propagation direction and the change in the propagation direction, i.e. a lateral shift. Two different spin components (parallel and anti-parallel to the propagation direction) acquire opposite displacements. This is the spin Hall effect as it applies to light. Therefore, when a beam of linearly polarized light (which is an equal combination of spin parallel and anti-parallel to the propagation direction) changes direction, the beam slightly splits into two beams, each containing different spin states.

Figure 1: Spin Hall effect of Light. (Click on figure to watch it) In the animation, a beam of linearly polarized light incident on an air-glass interface slightly splits into its two spin components -- spin parallel or anti-parallel to the propagation direction (or right and left circular polarizations) -- upon refraction at the interface. [Animation credit: Onur Hosten]

The effect takes place due to conservation of angular momentum (spin plus orbital). Due to the rotational symmetry around the axis perpendicular to the interface (z-axis), the total angular momentum of light around this axis has to be conserved. Assume that, initially the spin angular momentum of light is either parallel or anti-parallel to the propagation direction, and has a certain component along the z-axis. When light refracts at the interface, the spin still remains either parallel or anti-parallel to the new propagation direction. But this time the spin makes a different angle with the z-axis, therefore the spin angular momentum component along the z-axis changes. The spin Hall effect compensates for this change in the angular momentum component, and light acquires an orbital angular momentum by shifting itself laterally from the z-axis.

In the experiment a linearly polarized laser beam was incident on a glass prism at an angle. Upon refraction, the two different spin components acquired opposite displacements out of the plane of incidence. Because the separation between these two beams was only on the order of nanometers, and the beam widths themselves on the order of millimeters, the two beams overlap to a great extent. The researchers measured the separation between the two beams using a novel metrological method (quantum weak measurements in pre- and post-selected systems) to measure the miniscule effect.

Essentially, the spin Hall effect performs a weak measurement of the spin state of the photons. If the measurement were to be strong, the beams corresponding to different spin states would completely separate from each other, and one would be able to tell the spin state by looking at the beam position. But, in the University of Illinois experiment, the spin state measurement was a weak measurement, because the beams were still overlapping to a great extent and one could tell only very little about the spin state by looking at the position of the beam.

When the researchers made a particular pre- and post-selection on the polarization state of the photons before and after the weak measurement (i.e., the spin Hall effect), due to an interference effect between the two beams, there resulted an enhancement of the original displacement by a factor of ten thousand. This pre- and post-selection step experimentally amounts to sending the photons through two calcite polarizers, one before and one after the spin Hall effect, oriented at angles almost perpendicular to each other. Therefore, for instance, an angstrom displacement was enhanced to a micron displacement. Then the enhanced displacement was read by a position-sensitive photodiode (a photodiode split into two halves – the difference signal is proportional to the beam displacement). The researchers were thus able to characterize the Spin Hall effect of light with angstrom resolution.

The measurement technique holds further promise for achieving better resolutions. In particular, the researchers believe that by incorporating standard signal modulation and lock-in detection techniques, a resolution of picometers can be achieved. Moreover, the technique is not limited to position measurements; similar tricks in the appropriate experimental conditions will enhance any kind of signal, e.g., position or momentum of any particle, intensity (e.g. photon number) or amplitude (e.g. electric field) of a field.

The researchers think that it would be interesting to demonstrate the case when the index of refraction varies continuously (as opposed to observing the effect at a discrete air-glass interface), which is the analogous case for the spin Hall effect in semiconductors. The researchers are also theoretically looking for systems where they can separate the spin states into two completely separate beams, and use this for both quantum and classical optical information processing applications.

Reference:
"Observation of the Spin Hall Effect of Light via Weak Measurements"
O. Hosten and P. Kwiat,
Science 319, 787 (2008); published online 10 January 2008 (10.1126/Science.1152697).
Link to Abstract
Link to Full text in the website of Kwiat Quantum Information Group

'Invisibility Cloaks' Could Break Sound Barriers

Steven Cummer

Contrary to earlier predictions, Duke University engineers report in Physical Review Letters that a three-dimensional sound cloak is possible, at least in theory.

Such an acoustic veil would do for sound what the "invisibility cloak" previously demonstrated by the research team does for microwaves -- allowing sound waves to travel seamlessly around it and emerge on the other side without distortion. Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering said that such a cloak might hide submarines in the ocean from detection by sonar, he said, or improve the acoustics of a concert hall by effectively flattening a structural beam.

As in the case of the microwave cloak, the properties required for a sound cloak are not found among materials in nature and would require the development of artificial, composite metamaterials. (For more about metamaterials, click this link) The engineering of acoustic metamaterials lags behind those that interact with electromagnetic waves (i.e. microwaves or light), but "the same ideas should apply," Cummer said.

In 2006, researchers at Duke and the Imperial College London used a new design theory to create a blueprint for an electromagnetic invisibility cloak. Only a few months later,the team demonstrated the first such cloak, designed to operate at microwave frequencies.

Cummer and David Schurig, a former research associate at Duke who is now at North Carolina State University, later reported in "The New Journal of Physics" a theory showing that an acoustic cloak could be built. But that theory relied on a "special equivalence" between electromagnetic and sound waves that is only true in two dimensions. A report by another team had also suggested that a 3-D acoustic cloak couldn't exist. It appeared they had reached a dead end.

This time, he started instead from a shell like the microwave cloak his team had already devised and attempted to derive the mathematical specifications required to prevent such a shell from reflecting sound waves, a key characteristic for achieving invisibility. On paper, at least, it worked.

Image Description: Analytical computation of the interaction of an acoustic wave with the ideal cloaking shell. The peaks and valleys in the color scale represent the instantaneous pressure field of a uniform plane wave traveling from left to right as it impinges on a rigid sphere surrounded by a cloaking shell whose thickness is equal to the radius of the interior sphere. The shell smoothly bends the acoustic wavefronts around the interior rigid sphere so that they smoothly pass around the object. No acoustic wave is scattered back toward the source, nor does the object cast any acoustic shadow. From the pressure field outside the shell, there's no way to know whether there is an object there or not.

Although the theory used to design such acoustic devices so far isn't as general as the one used to devise the microwave cloak, the finding nonetheless paves the way for other acoustic devices, for instance,those meant to bend or concentrate sound. The existence of an acoustic cloaking solution also indicates that cloaks might possibly be built for other wave systems, including seismic waves that travel through the earth and the waves at the surface of the ocean.

Reference
"Scattering Theory Derivation of a 3D Acoutic Cloaking Shell"
S. Cummer, B.-I. Popa, D. Schurig, D.R. Smith, J. Pendry, M. Rahm and A.Starr,
Physical Review Letters, 100, 024301 (2008), Abstract Link.

Physics Nobel Prize 2007 for 'Giant Magnetoresistance'

Albert Fert (left) and Peter Grünberg (right) [Photo Courtesy: Unité mixte de physique CNRS/Thales, Orsay and Institut für Festkörperforschung, Forschungszentrum Jülich ]

Laptops, iPods and so many other small-sized devices that have defined a new generation of our civilization owe a large part of their existence to the discovery of a fundamental effect in Physics about 19 years back and this year's Nobel Prize celebrates this path-breaking advancement that influenced so much the growth of the computer industry and days of our lives.

The Royal Swedish Academy of Sciences has awarded the Nobel Prize in physics for 2007 jointly to Albert Fert (France) and Peter Grünberg (Germany) for the discovery of 'Giant Magnetoresistance’ or GMR.

Albert Fert is currently professor at Université Paris-Sud, Orsay, since 1976 and scientific director of the Unité mixte de physique CNRS/Thales, Orsay, since 1995. He earned his PhD in 1970 at the Université Paris-Sud. He was born on 7 March 1938 at Carcassonne. Peter Grünberg is a Professor at Institut für Festkörperforschung, Forschungszentrum Jülich, Germany, since 1972. He was born on May 18, 1939. Grünberg received his Ph.D in 1969 at Darmstadt University of Technology in Germany.

About 'Giant Magnetoresistance’ (GMR): In 1988 the Frenchman Albert Fert and the German Peter Grünberg each independently discovered this totally new physical effect. They observed that very weak magnetic changes give rise to major differences in electrical resistance in a GMR system. A system of this kind is the perfect tool for reading data from hard disks when information registered magnetically has to be converted to electric current.

A hard disk stores information, such as music, in the form of microscopically small areas magnetized in different directions. The information is retrieved by a read-out head that scans the disk and registers the magnetic changes. The smaller and more compact the hard disk, the smaller and weaker the individual magnetic areas. More sensitive read-out heads are therefore required if information has to be packed more densely on a hard disk. A read-out head based on the GMR effect can convert very small magnetic changes into differences in electrical resistance and therefore into changes in the current emitted by the read-out head. The current is the signal from the read-out head and its different strengths represent ones and zeros.

Soon after the discovery of Fert and Grünberg, researchers and engineers began work to enable use of the effect in read-out heads. In 1997 the first read-out head based on the GMR effect was launched and this soon became the standard technology. Thanks to this technology that it has been possible to miniaturize hard disks so radically in recent years. Sensitive read-out heads are needed to be able to read data from the compact hard disks used in laptops and some music players, for instance. Even the most recent read-out techniques of today are further developments of GMR.

"The GMR effect was discovered thanks to new techniques developed during the 1970s to produce very thin layers of different materials. If GMR is to work, structures consisting of layers that are only a few atoms thick have to be produced. For this reason GMR can also be considered one of the first real applications of the promising field of nanotechnology", The Royal Swedish Academy of Sciences said.

Homepage of Albert Fert: http://www2.cnrs.fr/en/338.htm
Homepage of Peter Grünberg: http://www.fz-juelich.de/portal/gruenberg/

Those Historic Papers:
"Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices",
M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, J. Chazelas,
Phys. Rev. Lett. 61, 2472, (1988), Abstract.
&
"Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange",
G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn,
Phys. Rev. B 39 (7), 4828-4830 (1989). Abstract.

A Single-Photon Transistor using Nanoscale Surface Plasmons

Author: Darrick Edward Chang

Affiliation: Physics Department, Harvard University

[This is an invited article based on a recent work done by the author and his collaborators and published in 'Nature']

Finding ways to make pulses of light interact with each other has been an active area of research for several decades. In fact, the study of “nonlinear optics” has led to countless breakthroughs and technological advances in fields as diverse as imaging, spectroscopy, laser physics, communications, and signal processing [1]. Interactions between pulses of light are achieved by their common interaction with some material medium. However, because such processes are generally very weak, optical nonlinearities typically become significant only when very large light intensities are used.

The ability to achieve nonlinear interactions at low optical powers would enable a new generation of devices that consume much less power than their predecessors and enable new applications as well. The ultimate limit would be to achieve nonlinear interactions between individual photons, the constituent particles that comprise light. Recently, there has been great interest in this area in part because of potential applications in quantum computing and quantum information science [2,3].

The interaction strength between matter and light can be increased by confining the light in space to very small dimensions, which causes the associated optical fields to become very intense. In normal dielectric media, light cannot be confined to regions smaller than an optical wavelength. However, the situation changes dramatically when light is coupled to the free electrons in a conductor. The unique properties of these coupled excitations of light and charge (known as surface plasmons) [4] allow them to be confined to arbitrarily small dimensions.

Image: An illustration of how a single atom near a nanowire can prevent light from propagating past it

Recently, we proposed [5] and experimentally investigated [6] the strong interaction between single atoms (or other optical emitters) and individual surface plasmons tightly confined to a conducting nanowire. The strong coupling causes the nanowire to act as a “super-lens” that directs the majority of emission into the surface plasmon modes. More recently, we have theoretically shown that such a system also leads to remarkable nonlinear optical effects [7]. In particular, the confinement of the surface plasmons is so strong that when a single surface plasmon (i.e., a single photon) is incident on a single emitter, the two must interact, and this interaction prevents the photon from being transmitted past the emitter. However, because the emitter cannot interact with more than a single photon at a time, its response to a second incident photon becomes fundamentally different and transmission is now much more likely. In this sense, the single emitter behaves as an efficient, single-photon switch.

One can gain even further control over the nonlinear optical interactions in this system by using techniques from quantum optics to coherently manipulate the emitter. In fact, we have shown that the system can behave as a single-photon transistor, where the presence or absence of a single photon in a “gate” field can prevent or allow the propagation of a whole stream of “signal” photons. In analogy to the role that electronic transistors play in electronic computing devices, a single-photon transistor would open the door to optical computing devices and many other possibilities.

Our experimental efforts to explore the nonlinear properties of this system are just beginning, and considerable work remains to be done before large-scale, integrated quantum plasmonic devices can be practically realized. More broadly, however, work such as this suggests the great promise of merging the tools of quantum optics with plasmonics and the many other novel optical materials that have recently arisen. Ultimately this merger may help us to achieve unprecedented control over the interactions of light quanta.

This work was done in collaboration with Mikhail Lukin and Eugene Demler, both in the Physics Dept. at Harvard University, and Anders Sorensen in the Physics Dept. at the Niels Bohr Institute, Copenhagen, Denmark.

References:
[1] R.W. Boyd, Nonlinear Optics (Academic, New York, 1992).
[2] L.-M. Duan and H.J. Kimble, Phys. Rev. Lett. 92, 127902 (2004) Abstract.
[3] M.D. Lukin and A. Imamoglu, Phys. Rev. Lett. 84, 1419 (2000) Abstract.
[4] H.A. Atwater, Sci. Am. 296, 53 (2007).
[5] D.E. Chang, A.S. Sorensen, P.R. Hemmer, M.D. Lukin, Phys. Rev. Lett. 97, 053002 (2006) Abstract.
[6] A.V. Akimov et al., accepted by Nature (2007).
[7] D.E. Chang, A.S. Sorensen, E.A. Demler, and M.D. Lukin, Nature Physics advance online publication, doi:10.1038/nphys708 (2007) Abstract

Toward Coherent Control in the Nanoscale

Author: Tamar Seideman

Professor of Chemistry and Physics,
Northwestern University, Evanston, IL.
Link to Seideman Group>>

[This is an invited article from Prof. Seideman on the recent work published in J. Physics B which illustrated how the effect of the interaction of light with plasmons on the surfaces of metal nanoparticles could be utilized to create sources of coherent and polarized light -- A work that opens up lot of other possibilities.
-- 2Physics.com Team]

Coherent control has been applied in recent years to a wide variety of problems, ranging from atomic physics and gas-phase molecular dynamics through solid-state physics and semiconductor device technology to solution chemistry and biology [1]. The broad applicability of this method owes to its origin in a simple and very general concept, namely wave interference. As traditionally practiced, however, coherent control applies to large ensembles – laser beams are macroscopic objects whose focal spot size is constrained by the diffraction of light to the 10^-7-10^-4 m range.

Molecular-scale electronics, the application of single molecules or ordered networks of molecules to make electronic or electromechanical devices, has grown during the past two decades from an inspiring dream[2] to a technology-facing area of science. The use of electronic and nuclear molecular degrees of freedom to create transport, logic, or memory elements is conceptually intriguing, since it makes use of the quantum states of single or small collections of molecules, where coherence may be expected to have a special role. The application of coherent control concepts and tools to control electronic transport and molecular dynamics in the nano-constructs, however, requires the development of nanoscale light sources with controlled polarization and phase properties.

It is here that a third established and yet rapidly growing field, namely nanoplasmonics [3], presents an opportunity. The physics underlying plasmonic phenomena is familiar from related fields such as surface enhanced Raman spectroscopy and tip-assisted spectroscopies. Similar to metal surfaces and sharp tips, metal nanoparticles enhance light that is shine upon them at frequencies (the plasmon resonance frequencies) that depend sensitively on the particle shape and size and on the dielectric constants of both the system and the surrounding medium. The origin of the enhancement is collective coherent excitation of conductive electrons in the particles, which leads to buildup of polarization charges on the particle surface.

The possibility of using metal nanoparticles to focus and enhance incident light, and the sensitivity of the electromagnetic fields generated to the structural parameters and chemical composition of the material system, have been illustrated numerically and experimentally and are well understood. It is this sensitivity that is responsible for the application of nanoplasmonics to make sensors, medical diagnostics and markers. The question how to design nanoparticles and arrays thereof to have specific, predetermined optical properties and hence desired functionalities is thus both fundamentally interesting and potentially tied to applications.

Recent work by the Seideman group at Northwestern University has extended concepts and tools developed for coherent control of molecular dynamics to guide light in the nanoscale via metal nanoparticle arrays and to develop nanoplasmonics with predetermined functionalities [4]. Several simple elements in what the group envisions developing into coherently controlled nanoplasmonics are schematically illustrated in Fig. 1.

The T-junction of Fig. 1a was applied to guide electromagnetic energy traveling down the leg into one or the other of the two symmetry-equivalent arms of the junction. The challenge of inducing light to bend about corners was achieved by choice of the incident field polarization. The symmetry breaking was achieved by choice of the incident field phase. Figure 1b depicts a hybrid construct, which combines elements that provide local enhancement (such as nanospheres) with elements that provide long distance propagation (such as nanowires) in order to minimize losses. The structural parameters of the construct are optimized using a genetic algorithm. Fig. 1c depicts a plasmonic nanocrystal, developed to separate an incident plane wave into two frequency components and funnel each component in a different direction normal to the direction of incidence, in parallel to the surface plane. Elsewhere the group applies genetic algorithms to iteratively build-in other optical properties into metallic and hybrid metal-semiconductor nanoparticle arrays, designing elements such as lenses and antennas with predetermined properties.

The challenge of numerically developing coherent nanoscale light sources and applying them to control in the nanoscale is one of the topics of ongoing work of the Seideman group. By proper construction of single metal nanoparticles and arrays thereof, the group is able to produce spatially localized, time-resolved electromagnetic fields with predetermined phase and polarization properties. It is hoped that such sources will enable the extension of the machinery of coherent control to the nanoworld, with potential applications in the control of molecular-scale electronics, electromechanics and possibly spintronics.

References:
[1] S. A. Rice and M. Zhao, Optical Control of Molecular Dynamics, (John Wiley & Sons, 2000);
M. Shapiro and P. Brumer, Principles of the Quantum Control of Molecular Processes (Hoboken, N.J.:
Wiley-Interscience, 2003); H. A. Rabitz, M. M. Hsieh and C. M. Rosenthal, Science 303, 1998 (2004).
[2] A. Ariram and M.A. Ratner, Chem.Phys.Lett. 29, 277 (1974).
[3] S. A. Maier, M. L. Brongersma, P.G. Kik, S. Meltzer, A. A. G. Requicha, and H. A. Atwater, Adv. Mat. 13, 1501 (2001); S. Link and M. A. El-Sayed, Annu. Rev. Phys. Chem. 54, 331 (2003); E. Hutter and J. H. Fendler, Adv. Mat. 16, 1685 (2004).
[4] M. Sukharev and T. Seideman, J.Phys. B 40 S283 (2007); J.Chem.Phys. 126, 204702 (2007); NanoLett. 6, 715 (2006);

Merging Spintronics and Plasmonics: Evidence of Spinplasmonics

Photo: Prof. Abdulhakem Elezzabi, Professor & Canada Research Chair, Department of Electrical and Computer Engineering, University of Alberta, Canada

Researchers at the University of Alberta (Edmonton, Canada) and the Naval Research Laboratory (Washington, D.C., U.S.A.) have demonstrated a novel approach for the active control of terahertz plasmonic propagation. Using an ensemble of sub-wavelength size ferromagnetic/nonmagnetic spintronic structures, their experiments provide the first evidence of low frequency plasmonic conduction controlled via the electron-spin. Such phenomenon can be conceptualized as the photonic analog to the electrically-driven spin accumulation that serves as a basis for spintronic devices. The team is led by Prof. Elezzabi of University of Alberta.

In their experiments, the researchers employ a rudimentary plasmonic system consisting of ferromagnetic particles coated with nonmagnetic nano-layers. The excitation of the particles with a single-cycle, 1 picosecond wide electric field pulse induces nonresonant particle plasmons on the surface of the bimetallic particles. The dipolar electric fields associated with the particle plasmons on individual particles couple from particle to particle via nearest neighbor interaction and radiate into the far-field at the edge of the sample as coherent terahertz radiation.

When a magnetic field is applied to the sample, electron spin induced resistivity changes within the skin depth of the particles are mapped onto a modulation of the radiated electromagnetic fields. The researchers demonstrate that terahertz radiation propagated through the spintronic particles exhibits increased magnetic field dependent amplitude attenuation and phase modulation, nearly an order of magnitude larger than that of bare ferromagnetic particles.

The electron spin induced attenuation increases as the surface coverage of the nonmagnetic layer increases, showing that the striking enhancement of the magnetically dependent attenuation is attributed to the nonmagnetic layer.

The physical mechanism underlying the enhanced attenuation arises from non-equilibrium accumulation of electron spin electromagnetically driven from the ferromagnet into the nonmagnet (spin polarized surface currents). A quantitative measurement of the dependence of the attenuation on the nonmagnet layer thickness is in very good agreement with the spin diffusion length predicted by the spin accumulation model, as well as with other experimental measurements of this length scale.

Conceptual illustration of a nonresonant particle plasmon excited on a spintronic structure consisting of a sub-wavelength size ferromagnetic (Co) particle that has been coated with nonmagnetic (Au) layers. Shown below are the density of spin-up and spin-down electron states, N(E), in the ferromagnetic and nonmagnetic media. In an applied magnetic field, spin polarized electrons in the ferromagnet are electromagnetically driven into the nonmagnet layer, which results in excess interface resistance. The electron-spin induced resistivity change is mapped onto a modulation of the fields re-radiated from the non-resonant particle plasmon.

The demonstration of a spin-dependent photonic phenomenon opens up a novel avenue in both the fields of spintronics and photonics. The ability to magnetically manipulate near-field mediated light transport on metallic particles via electron spin promises another degree of freedom in the design of photonic devices. The researchers envision the development of solid-state, magnetically sensitive terahertz photonic switches, modulators, and band-pass filters based on electron spin.

Reference:
"Electron-Spin-Dependent Terahertz Light Transport in Spintronic-Plasmonic Media,”
by K. J. Chau, Mark Johnson, and A. Y. Elezzabi,
Physical Review Letters 98, 133901 (Link to Abstract)

"Existence of Axion" -- R.N. Mohapatra

Rabindra N. Mohapatra (photo courtsey: U. of Maryland)

[This is an invited article. In a recent publication in Physical Review Letters, R.N. Mohapatra (U. of Maryland) and Salah Nasri (U. of Florida) have put forward a theory that can reconcile conflicting results from two experiments that tried to test the existence of "axion" -- an ultralight particle that could make up dark matter. We thank Prof. Mohapatra for contributing this article on our request. -- 2Physics.com Team]

Axions were proposed by Roberto Peccei and Helen Quinn as a way to solve one of the fundamental mysteries of nuclear forces i.e. an inordinately large amount of CP violation in the otherwise successful theory of these forces, Quantum Chromodynamics (QCD) proposed by D. Gross, F. Wilczek and H. D. Politzer. Since their debut into the world of theoretical physics, axions have also been found useful in another context: being ultralight particles (believed to be a billion times or more lighter than the electron) they are capable of populating the Universe so abundantly that they could be candidates for the dark matter of the Universe and thereby resolve another fundamental mystery of cosmology.

Salah Nasri (photo courtsey: U. of Florida)

Because of these twin attributes (solving the problem of QCD and being a candidate for dark matter), considerable amount of research is being devoted to establishing the existence of axions. One of their key properties is that they couple to two photons (one being the magnetic and the other the electric component of light). Therefore interaction of laser beams with strong magnetic fields is considered to be an efficient way to search for them[1].

Two recent attempts that use this technique to search for axions are the CERN CAST (CERN Axion Solar Telescope) experiment[2] and PVLAS experiment at INFN-LNL[3]. The CAST experiment searched for axions produced by light-by-light collision at the center of the Sun and gave a negative result setting strong limits on the axion-photon coupling and its mass. The PVLAS experiment on the other hand looked for axions produced by laser-magnetic field interaction in the laboratory and seems to have a positive evidence for an axion like particle. Their observations can be understood only if the axion-photon coupling are considerably larger than the upper limits set by the CAST result. This has posed a major challenge for theory and in the very least implies that the axion solution to the problems of QCD may be much more complex than previously envisioned or the PVLAS experiment could be the result of completely new kind of phenomenon, not related to the axion.

In a recent Phys. Rev. Lett. Paper [4], we have proposed a new way to reconcile the CAST and the PVLAS results. We use the axion possibility in our approach except the theory has several new features compared to the conventional axion models. We use the phenomenon of phase transition so well known in the study of condensed matter physics (e.g. loss of magnetism of ferromagnets at high temperature). Our basic observation is that the axion photon coupling is not a primordial coupling but is induced by the formation of a vacuum condensate. Therefore the strength of the coupling depends on the environment temperature.

Note that the solar axions are produced at a very high temperature of about 10 million degrees in the core of the Sun whereas the PVLAS axions are produced at room temperature. Therefore if the vacuum condensate responsible for axion-photon coupling undergoes phase transition to zero value in the solar core due to its high temperature, there would be no axion production in the solar core explaining the CAST result. On the other hand, the PVLAS experiment is taking place at the room temperature and therefore the vacuum condensate has nonzero value and the axion-photon coupling is present giving rise to the PVLAS signal for the axion.

This idea is consistent with all known experimental observations in particle physics and astrophysics. We predict that the axion must be accompanied by a twin particle with mass about 100 times that of the electron which undergoes the condensation and is responsible for our effect. It can be produced in the decay of heavy quark bound states, which can provide a way to test our model.

[1] P.Sikivie, Phys. Rev. Lett. 51, 1415 (1983)
[2] S.Andriamonje [CAST Collaboration], arXiv:hep-ex/0702006.
[3] E.Zavattini et al. [PVLAS Collaboration], Nucl. Phys. Proc. Suppl. 164, 264 (2007).
[4] R. N. Mohapatra and S. Nasri, "Reconciling the CAST and PVLAS Results", Phys. Rev. Letters 98, 050402 (2007) Link to Abstract

Ken Libbrecht & Physics of Snowflakes

Last week, the U.S. Postal Service issued a set of four commemorative stamps featuring images of snowflakes based on photographs taken by Ken Libbrecht, a professor of physics at the California Institute of Technology.

For several years Libbrecht has been investigating the basic physics of how patterns are created during crystal growth and other simple physical processes. He has delved particularly deeply into a case study of the formation of snowflakes. His research is aimed at better understanding how structures arise in material systems, but it is also visually compelling and, from the start, has been a hit with the public. His snowflake website, snowcrystals.com, is getting about two million hits a year. He is also the author of a new book, Ken Libbrecht's Field Guide to Snowflakes, a 112-page guide for anyone who wants to know more about the many different types of snow crystals and how to find them.

Libbrecht began his research by growing synthetic snowflakes in his lab, where they can be created and studied under well-controlled conditions. Precision micro-photography was necessary for this work, and over several years Libbrecht developed some specialized techniques for capturing images of snow crystals. Starting in 2001, he expanded his range to photographing natural snowflakes as well.

This interesting research project was aimed essentially as a case study of the growth of ice crystals from the vapor phase, the purpose of which is to better understanding pattern formation in nonlinear nonequilibrim systems. The diverse morphologies seen in snow crystals are largely due to the bizarre temperature dependence of ice crystal growth rates, a phenomenon that was discovered 75 years ago and remains unexplained to this day. Libbrecht has been making precise measurements of the growth rates of the different facets of ice crystals under controlled conditions to gain insights into the temperature dependent molecular structure of the ice surface and how it affects crystal growth.

Libbrecht can grow many different snowflake forms at will in his lab, but says there are still many subtle mysteries in crystal growth that are of interest to physicists who are trying to understand and control the formation of various materials. A real-world application of research on crystals is the growth of semiconductors for our electronic gadgets. These semiconductors are made possible in part by painstakingly controlling how certain substances condense into solid structures.

Libbrecht's research activities are not limited to snowflakes. He is also involved in the Laser Interferometer Gravitational-Wave Observatory (LIGO), an NSF-funded project that seeks to confirm the existence of gravitational waves from exotic cosmic sources such as colliding black holes. In LIGO, Libbrecht has lots of professional company; in fact, the field was essentially founded by Albert Einstein, who first predicted the existence of gravitational waves as a consequence of general relativity. Kip Thorne and Ron Drever at Caltech, along with Rai Weiss at MIT, were instrumental in initiating the LIGO project in the 1980s.

Spin Hall Effect

David Awschalom (courtsey: Awschalom Group, Physics Dept, Univ. of California, Santa Barbara)

The Hall effect, as discovered by Edwin Hall in 1879, refers to the potential difference induced on opposite sides of a thin sheet of conducting or semiconducting material called 'Hall element' when the element is subject to crossed electric and magnetic fields. As a result of this potential diffference, a charge current transverse to both the applied fields appears.

In a research paper published in September 1st issue of Physical Review Letters (full reference below), David Awschalom and colleagues at the Center for Spintronics and Computation at the University of California, Santa Barbara reported the current-induced spin-polarization of electrons and the spin Hall effect in thin surface layers of ZnSe at room temperature.

The spin Hall effect was first observed in GaAs at 20K by Awschalom and Yuichiro Kato in 2004. In an analogy to the conventional Hall effect, the spin Hall effect refers to a pure spin current transverse to an applied electric field in the absence of applied magnetic fields, and was theoretically predicted more than 30 years ago. A pure spin current is a flow of spin angular momentum without any charge current, which can be realized, for example, by spin-up and spin-down electrons moving in opposite directions. At the edges of a sample, the spin current results in accumulation of spins, similar to charge accumulation in the conventional Hall effect.

The unique advantage of using the spin Hall effect is that it does not require a magnetic field or magnetic materials to generate and separate spins in the solid state. The effect could be of use in the growing field of spintronics, in which the intrinsic spin of the electron (in addition to its electrical charge) is exploited in the development of logic devices. The spin Hall effect could provide a source of spin-polarized electrons for injection into semiconductor devices. Such electrons could carry information based on the state (up or down) of their spin polarization.

The spin polarization at 20K was about ten times stronger than at room temperature. Currently, the research team is working to boost the level of spin polarization to levels where nearly all electrons are polarized.

Reference: "Generating Spin Currents in Semiconductors with the Spin Hall Effect", V. Sih, W. H. Lau, R. C. Myers, V. R. Horowitz, A. C. Gossard, and D. D. Awschalom, Phys. Rev. Lett. 97, 096605 (2006).
Study of background materials:
Wikipedia page on 'Hall effect'
Wikipedia page on 'Quantum Hall effect'
"Observation of the spin Hall effect in semiconductors", Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, Science Express 1105514 (2004)