Transition from Superfluid to Mott Insulator

Karina Jiménez-García [photo courtesy: Joint Quantum Institute, Maryland]

Researchers studying a gas of trapped ultracold atoms have identified a set of conditions, never before observed but in excellent agreement with new theoretical predictions, that determine the onset of a critical “phase transition” in atomic arrays used to model the behavior of condensed-matter systems.

The findings provide a novel insight into the way collections of atoms suddenly cease to be a superfluid, which flows without resistance, and switch to a very different state called a “Mott insulator.” That transition and similar phenomena are of central interest to the science of solid-state materials, including superconductors.

“This work shows that the transition can be precisely controlled and confirms that it can be described by only two independent variables,” says lead researcher Karina Jiménez-García, a member of Ian Spielman’s group at the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute (JQI). The group reports its findings in a forthcoming issue of Physical Review Letters [1].

In order to understand the behavior of materials on the atomic and molecular scale, researchers often cannot experiment directly with samples. In many cases, they need model systems – analogous, at microscopic dimensions, to the physical models built by engineers to test the dynamics of a planned structure – that allow them to change one or two experimental parameters at a time while holding the rest constant. That can be prohibitively difficult, if not impossible, in bulk samples of real material.

But in recent years, quantum science has made it possible to create accurate and highly illuminating models of condensed-matter systems by using ensembles of individual atoms which are confined by electrical and magnetic forces into patterns that mimic the fundamental physics of the repeating structural pattern, or “lattice,” of a solid material.

Improving these quantum-mechanical models is an important research area at JQI, and Spielman’s group has been investigating a model for the superfluid-to-Mott insulator (SF-MI) phase transition – the point at which the atoms cease to share the same quantum properties, as if each atom were spread over the entire lattice, and change into a set of individual atoms trapped at specific locations, that do not communicate with one another.

Figure 1

The group’s experimental setup at NIST’s Gaithersburg, MD facility uses a cloud of about 200,000 atoms of rubidium that have been cooled to near absolute zero and confined in a combination of magnetic and optical potentials. In those conditions, a majority of the atoms forms a Bose-Einstein condensate (BEC), an exotic condition in which all the atoms coalesce into exactly the same quantum state.

Then the team loads the BEC – which is about 10 micrometers in diameter, or about one-tenth the width of a human hair – into an “optical lattice” that forms at the intersection of three laser beams placed at right angles to one another [See Figure 1], two horizontal and one vertical. Interference patterns in the beams’ waves cause regularly spaced areas of higher and lower energy; atoms naturally tend to settle into the lowest-energy locations like eggs in an egg carton.

The depth of the lattice wells (the cavities in the egg carton) is adjusted by varying the intensity of the laser beams. [See Figure 2] In a relatively shallow lattice, atoms can easily “tunnel” from one site to another in the condensate superfluid state, whereas deep lattice wells tend to hold each atom in place, producing the non-condensate insulator state. “We can tune the depth of all the wells in the carton by adjusting the intensity of the laser beams which create it,” Jiménez-García explains. “We can go from a flat carton to a carton with very deep wells.”

Figure 2 (click to view hi resolution image)

That general lab arrangement – ultracold trapped atoms suspended in an optical lattice – is the current standard worldwide for experiments on condensed-matter models. But it has a serious problem: The mathematical theory behind the model is predicated on a completely homogenous system, whereas arrays such as the JQI group uses are only homogenous on small spatial scales. Globally, they are inhomogenous because the magnetic trapping potential is not uniform across the width of the trap. As a result, the equations used to calculate expected outcomes do not accurately predict the SF-MI transition, compromising their utility.

Last year, however, an international collaboration of theorists determined [2] that in such configurations, where there were spatially separated SF and MI phases, the quantum state of the system could be fully specified by the relationship between only two variables: the characteristic density of the system (a composite of trapping potential, total number of trapped atoms, tunneling energy, lattice spacing and dimensionality); and the strength of the interactions between neighboring atoms.

Jiménez-García and colleagues in the JQI group set out to see if they could make an experimental system that performed according to the theorists’ specifications.

They set the depth of the vertical lattice beam such that it partitioned the roughly spherical BEC into about 60 two-dimensional, pancake-shaped segments, and then used a method similar to medical MRI scanning to select and analyze just a couple of individual 2D segments at the same time. The inhomogeneity of the originally 3D atomic sample results in the selection of 2D systems with different total number of atoms, ranging from 0 (at the edges of the system) to 4000 atoms (in the center of the system), allowing the researchers to examine a broad range of total atom numbers and lattice depths.

Because the trapping potential was not homogenous across the BEC, the group’s lattices were not completely orthogonal. “What we get instead,” Jiménez-García says, “is an array of egg cartons which have a parabolic curvature. Imagine each egg carton with the overall shape of a bowl, and the whole system as a stack of egg carton bowls.”

To determine the state of the atoms in the 2D slice, the scientists abruptly turn off the trap and let the atoms begin to fly apart. After a few thousandths of a second, they take a picture of the expanding population. If the atoms were deep into the SF state, the images will show a tightly focused bunch. If they were in the MI state, the bunch will have dispersed farther and appear more diffuse. “We detect a sharp peak in the momentum distribution which we associate with the condensate fraction,” Jiménez-García says. “Wider dispersion – that is, less condensate fraction -- would mean more MI.”

After measuring about 1300 different samples, the group was able to determine that the two-variable theory completely described the state of each slice.

References
[1] K. Jimenez-Garcia, R.L. Compton, Y.-J. Lin, W.D. Phillips, J.V. Porto and I.B. Spielman, "Phases of a 2D Bose Gas in an Optical Lattice", accepted for publication in Physical Review Letters. arXiv:1003.1541.
[2] Marcos Rigol, George G. Batrouni, Valery G. Rousseau, Richard T. Scalettar, "State diagrams for harmonically trapped bosons in optical lattices", Phys. Rev. A 79, 053605 (2009). Abstract.

Unpeeling Atoms and Molecules from the Inside Out

Nora Berrah [photo courtesy: Western Michigan University]

The first published scientific results from the world's most powerful hard X-ray laser -- Linac Coherent Light Source (LCLS) -- located at the Department of Energy's SLAC National Accelerator Laboratory, show its unique ability to control the behaviors of individual electrons within simple atoms and molecules by stripping them away, one by one—in some cases creating hollow atoms.

These early results describe in great detail how the Linac Coherent Light Source's intense pulses of X-ray light change the very atoms and molecules they are designed to image. Controlling those changes will be critical to achieving the atomic-scale images of biological molecules and movies of chemical processes that the LCLS is designed to produce.

In a report published June 22 in Physical Review Letters [1], a team led by physicist Nora Berrah of Western Michigan University—the third group to conduct experiments at the LCLS—describes the first experiments on molecules. Her group also created hollow atoms, in this case within molecules of nitrogen gas, and found surprising differences in the way short and long laser pulses of exactly the same energies stripped and damaged the nitrogen molecules.

"We just introduced molecules into the chamber and looked at what was coming out there, and we found surprising new science," said Matthias Hoener, a postdoctoral researcher in Berrah's group at WMU and visiting scientist at Lawrence Berkeley National Laboratory who was first author of the paper. "Now we know that by reducing the pulse length, the interaction with the molecule becomes less violent. "

Linda Young [photo courtesy: Argonne National Laboratory]

In another report published in the July 1 issue of Nature [2], a team led by Argonne National Laboratory physicist Linda Young describes how they were able to tune LCLS pulses to selectively strip electrons, one by one, from atoms of neon gas. By varying the photon energies of the pulses, they could do it from the outside in or—a more difficult task—from the inside out, creating so-called "hollow atoms."

"Until very recently, few believed that a free-electron X-ray laser was even possible in principle, let alone capable of being used with this precision," said William Brinkman, director of DOE's Office of Science. "That's what makes these results so exciting."

Young, who led the first experiments in October with collaborators from SLAC and five other institutions, said, "No one has ever had access to X-rays of this intensity, so the way in which ultra-intense X-rays interact with matter was completely unknown. It was important to establish these basic interaction mechanisms."

SLAC's Joachim Stöhr, director of the LCLS, said, "When we thought of the first experiments with LCLS ten years ago, we envisioned that the LCLS beam may actually be powerful enough to create hollow atoms, but at that time it was only a dream. The dream has now become reality."

The world's first hard X-ray free-electron laser started operation with a bang. First experiments at SLAC National Accelerator Laboratory's Linac Coherent Light Source stripped electrons one by one from neon atoms (illustrated above) and nitrogen molecules, in some cases removing only the innermost electrons to create "hollow atoms." Understanding how the machine's ultra-bright X-ray pulses interact with matter will be critical for making clear, atomic-scale images of biological molecules and movies of chemical processes. (Artwork by Gregory Stewart, SLAC)

While the first experiments were designed to see what the LCLS can do and how its ultra-fast, ultra-bright pulses interact with atoms and molecules, they also pave the way for more complex experiments to come. Its unique capabilities make the LCLS a powerful tool for research in a wide range of fields, including physics, chemistry, biology, materials and energy sciences.

The LCLS forms images by scattering X-ray light off an atom, molecule or larger sample of material. Yet when the LCLS X-rays are tightly focused by mirrors, each powerful laser pulse destroys any sample it hits. Since certain types of damage, like the melting of a solid, are not instantaneous and only develop with time, the trick is to minimize the damage during the pulse itself and record the X-ray snapshot with a camera before the sample disintegrates.

Both teams found that the shorter the laser pulse, the fewer electrons are stripped away from the atom or molecule and the less damage is done. And both delved into the detailed mechanisms behind that damage.

Atoms are a little like miniature solar systems, with their electrons orbiting at various distances from the nucleus in a sort of quantum fuzz. To make things simpler, scientists describe the electrons as orbiting in "shells" at specific distances from the nucleus. The innermost shell contains up to two electrons, the next one up to eight, the third one up to 18, and so on.

Since they're closest to the positively charged nucleus, the two innermost electrons are generally the hardest to wrest away. But they also most readily absorb photons of X-ray light, and so are the most vulnerable to getting stripped away by intense X-rays.

Although previous experiments with intense optical lasers had stripped neon atoms of most of their electrons, Young's was the first to discover how ultra-intense X-ray lasers do this. At low photon energies, the outer electrons are removed, leaving the inner electrons untouched. However, at higher photon energies, the inner electrons are the first to be ejected; then the outer electrons cascade into the empty inner core, only to be kicked out by later parts of the same X-ray pulse. Even within the span of a single pulse there may be times when both inner electrons are missing, creating a hollow atom that is transparent to X-rays, Young said.

"This transparency associated with hollow atoms could be a useful property for future imaging experiments, because it decreases the fraction of photons doing damage and allows a higher percentage of photons to scatter off the atom and create the image," Young said. She said application of this phenomenon will also allow researchers to control how deeply an intense X-ray pulse penetrates into a sample.

Berrah's team bombarded puffs of nitrogen gas with laser pulses that ranged in duration from about four femtoseconds, or quadrillionths of a second, to 280 femtoseconds. No matter how short or long it was, though, each pulse contained the same amount of energy in the form of X-ray light; so you might expect that they would have roughly the same effects on the nitrogen molecules.

But to the team's surprise, that was not the case, Hoener said. The long pulses stripped every single electron from the nitrogen molecules, starting with the ones closest to the nucleus; the short ones stripped off only some of them.

Their report attributes this to the "frustrated absorption effect": Since the molecule's electrons are preferentially stripped from the innermost shells, there is simply not enough time during a short pulse for the molecule's outermost electrons to refill the innermost shells and get kicked out in turn.

With all this activity going on inside the atom, scientists have a new way to explore atomic structure and dynamics. Further experiments have investigated nanoclusters of atoms, protein nanocrystals and even individual viruses, with results expected to be published in coming months.

Reference
[1] M. Hoener, L. Fang, O. Kornilov, O. Gessner, S. T. Pratt, M. Gühr, E. P. Kanter, C. Blaga, C. Bostedt, J. D. Bozek, P. H. Bucksbaum, C. Buth, M. Chen, R. Coffee, J. Cryan, L. DiMauro, M. Glownia, E. Hosler, E. Kukk, S. R. Leone, B. McFarland, M. Messerschmidt, B. Murphy, V. Petrovic, D. Rolles and N. Berrah, "Ultraintense X-Ray Induced Ionization, Dissociation, and Frustrated Absorption in Molecular Nitrogen", Phys. Rev. Lett. 104, 253002 (2010). Abstract.
[2] L. Young, E. P. Kanter, B. Krässig, Y. Li, A. M. March, S. T. Pratt, R. Santra, S. H. Southworth, N. Rohringer, L. F. DiMauro, G. Doumy, C. A. Roedig, N. Berrah, L. Fang, M. Hoener, P. H. Bucksbaum, J. P. Cryan, S. Ghimire, J. M. Glownia, D. A. Reis, J. D. Bozek, C. Bostedt & M. Messerschmidt, "Femtosecond electronic response of atoms to ultra-intense X-rays", Nature 466, 56-61 (1 July 2010). Abstract.

Visualizing the Electron Wind Force in Nanostructures

Ellen D. Williams

[This is an invited article based on a recently published work by the authors -- 2Physics.com]

Authors: Chenggang Tao, William G. Cullen, and Ellen D. Williams

Affiliation: Materials Research Science and Engineering Center & Department of Physics, University of Maryland, USA

Link to the Williams Lab >>


As electronic devices get smaller and smaller, they are more susceptible to effects of the charge carriers flowing through them. The charge carriers (electrons in metals) can push atoms around by collisions. For some specific types of atomic structures (for example, atomic “steps”, where the surface height changes by one layer of atoms), the scattering force is much stronger than had been thought [1]. These structures are ubiquitous for the surfaces of solid materials, and this becomes very important for nanoscale electronics where surfaces make up a much bigger fraction of the material.

W. G. Cullen (left) and Chenggang Tao (right)

A very careful measurement is needed to directly observe the forces that electrons exert on the atoms of the material which they are passing through. Yet over a long time, the effects of this force accumulate and can lead to failure of wires which connect components in integrated circuits - a process known as electromigration [2, 3]. In our experiment, we carefully created different types of nanoscale structures on top of a very thin wire of silver. One type of structure consists of single-atom high “islands” that contain between 100 and 100,000 atoms. Another type consists of single-atom high “steps” decorated by C₆₀ buckyballs. We then used a scanning tunneling microscope to watch the structures move or change shape when we ran current through the wire. Amazingly, when we changed the direction of the current, we could move the structures back and forth.

The force exerted by the electrons on island edge atoms is up to 20 times larger than previous theoretical calculations had predicted. However, when we decorate the island edge with a chain of C₆₀ molecules (which tend to mildly withdraw electrons locally from the silver atoms, and also change their local configuration) we find that the force is reduced by over a factor of 10. This indicates that the force is very dependent on the local environment of the atoms which comprise the step and island boundaries.

Fig. 1 Schematic of the experimental setup; inset shows STM image of silver wire surface.

The fundamentally interesting idea here is that all the different ways that electrons can move through the wire can be described by how easily an electron can be “transmitted”. Most atomic structures in a solid allow easy transmission, but the defect sites impede the transmission. This results in a local “resistivity dipole” which means that the defect sites have a local resistance. Our measurements detected the motion of atomic-scale surface structures which results from forces exerted by the passing electrons – as the atoms resist the electron flow, they in turn feel a larger “push” from the electrons.

Fig. 2A-B: Island pushed by moving electrons. The current direction is downward, and the island displacement is upward.

Here we have demonstrated that nanoscale surface structures can be moved (and even turned around) using the scattering force from electrons. Further, the scattering force can be significantly reduced by attaching C₆₀ molecules to the structures. On the other hand, a particularly exciting implication is the use of this effect to move atoms around intentionally in nanoelectronic devices, or to harness it to do work [4]. This effect might be used to self-assemble or to create structures that could be cycled through different structures under an alternating current.

Our work was supported by the NSF Materials Research Science and Engineering Center at the University of Maryland, including the use of shared experimental facilities. Additional support was provided by the University of Maryland NanoCenter and the Center for Nanophysics and Advanced Materials.

Reference:
[1] Chenggang Tao, W. G. Cullen, E. D. Williams, “Visualizing the electron scattering force in nanostructures”, Science 328, 736–740 (2010). Abstract.
[2] P. S. Ho and T. Kwok, “Electromigration in metals”, Reports on Progress in Physics, 52, 301 (1989). Abstract.
[3] H. Yasunaga and A. Natori, “Electromigration on semiconductor surfaces”, Surface Science Reports 15, 205 (1992). Abstract.
[4] D. Dundas, E. McEniry and T. N. Todorov, “Current-driven atomic waterwheels”, Nature Nanotechnology, 4, 99 (2009). Abstract.

Playing Tug-of-War at Atomic-Scale

Douglas Smith [Photo courtesy: Nanomechanical Properties Group, NIST]

How hard do you have to pull on a single atom of—let’s say—gold to detach it from the end of a chain of like atoms?
[For answer, see the end of this article*]

It’s a measure of the astonishing progress in nanotechnology that questions that once would have interested only physicists or chemists are now being asked by engineers. To help with the answers, a research team at the National Institute of Standards and Technology (NIST) has built an ultra-stable instrument for tugging on chains of atoms, an instrument that can maneuver and hold the position of an atomic probe to within 5 picometers, or 0.000 000 000 5 centimeters.

The basic experiment uses a NIST-designed instrument inspired by the scanning tunneling microscope (STM). The NIST instrument uses as a probe a fine, pure gold wire drawn out to a sharp tip. The probe is touched to a flat gold surface, causing the tip and surface atoms to bond, and gradually pulled away until a single-atom chain (see figure) is formed and then breaks.

The trick is to do this with such exquisite positional control that you can tell when the last two atoms are about to separate, and hold everything steady; you can at that point measure the stiffness and electrical conductance of the single-atom chain, before breaking it to measure its strength.

The NIST team used a combination of clever design and obsessive attention to sources of error to achieve results that otherwise would require heroic efforts at vibration isolation, according to engineer Jon Pratt. A fiber-optic system mounted just next to the probe uses the same gold surface touched by the probe as one mirror in a classic optical interferometer capable of detecting changes in movement far smaller than the wavelength of light. The signal from the interferometer is used to control the gap between surface and probe. Simultaneously, a tiny electric current flowing between the surface and probe is measured to determine when the junction has narrowed to the last two atoms in contact. Because there are so few atoms involved, electronics can register, with single-atom sensitivity, the distinct jumps in conductivity as the junction between probe and surface narrows.

[Image credit: F. Tavazza, NIST] A quantum-mechanics-based simulation demonstrates how a new NIST instrument can delicately pull a chain of atoms apart. The chart records quantum jumps in conductivity as a gold contact is stretched 0.6 nanometer. The junction transitions from a 2-dimensional structure to a one-dimensional single-atom chain, with a corresponding drop in conductivity. Following the last point, at a wire length of 3.97 nm, the chain broke.

The new instrument can be paired with a parallel research effort at NIST to create an accurate atomic-scale force sensor—for example, a microscopic diving-board-like cantilever whose stiffness has been calibrated on NIST’s Electrostatic Force Balance.

Physicist Douglas Smith says the combination should make possible the direct measurement of force between two gold atoms in a way traceable to national measurement standards. And because any two gold atoms are essentially identical, that would give other researchers a direct method of calibrating their equipment.

“We’re after something that people that do this kind of measurement could use as a benchmark to calibrate their instruments without having to go to all the trouble we do, " Smith says. "What if the experiment you’re performing calibrates itself because the measurement you’re making has intrinsic values? You can make an electrical measurement that’s fairly easy and by observing conductance you can tell when you’ve gotten to this single-atom chain. Then you can make your mechanical measurements knowing what those forces should be and recalibrate your instrument accordingly.”

In addition to its application to nanoscale mechanics, say the NIST team, their system’s long-term stability at the picometer scale has promise for studying the movement of electrons in one-dimensional systems and single-molecule spectroscopy.

[* The answer, calculated from atomic models, should be something under 2 nanonewtons, or less than 0.000 000 007 ounces of force.]

Reference
D.T. Smith, J.R. Pratt, F. Tavazza, L.E. Levine and A.M. Chaka. An ultra-stable platform for the study of single-atom chains. J. Appl. Phys., Accepted for publication. [Link will be added, once published -- 2Physics]

[We thank NIST for materials used in this report]

Filming Photons of Nanoscale Structures with Electrons

Ahmed Zewail [Image courtesy: Caltech]

In two recent papers published in the December 17 issue of Nature [1] and the October 30 issue of Science [2], a team of researchers from the California Institute of Technology (Caltech) reported the invention of techniques that allow the real-time, real-space visualization of fleeting changes in the structure of nanoscale matter. These novel techniques have been used to image the evanescent electrical fields produced by the interaction of electrons and photons, and to track changes in atomic-scale structures.

Four-dimensional (4D) microscopy-the methodology upon which the new techniques were based-was developed at Caltech's Physical Biology Center for Ultrafast Science and Technology. The center is directed by Ahmed Zewail, the Linus Pauling Professor of Chemistry and professor of physics at Caltech, and winner of the 1999 Nobel Prize in Chemistry.

Zewail was awarded the Nobel Prize for pioneering the science of femtochemistry, the use of ultrashort laser flashes to observe fundamental chemical reactions occurring at the timescale of the femtosecond (one-millionth of a billionth of a second). The work "captured atoms and molecules in motion," Zewail says, but while snapshots of such molecules provide the "time dimension" of chemical reactions, they don't give the dimensions of space of those reactions-that is, their structure or architecture.

Zewail and his colleagues were able to visualize the missing architecture through 4D microscopy, which employs single electrons to introduce the dimension of time into traditional high-resolution electron microscopy, thus providing a way to see the changing structure of complex systems at the atomic scale.

Image 1: The diffraction obtained for silicon with 4D electron microscopy. From the patterns the structure can be determined on the nanoscale. [Credit: AAAS / Science / Zewail / Caltech]

In the research detailed in the Science paper [2], Zewail and postdoctoral scholar Aycan Yurtsever were able to focus an electron beam onto a specific nanoscale-sized site in a specimen, making it possible to observe structures within that localized area at the atomic level.

In electron diffraction, an object is illuminated with a beam of electrons. The electrons bounce off the atoms in the object, then scatter and strike a detector. The patterns produced on the detector provide information about the arrangement of the atoms in the material. However, if the atoms are in motion, the patterns will be blurred, obscuring details about small-scale variations in the material.

The new technique devised by Zewail and Yurtsever addresses the blurring problem by using electron pulses instead of a steady electron beam. The sample under study - in the case of the Science paper [2], a wafer of crystalline silicon-is first heated by being struck with a short pulse of laser light. The sample is then hit with a femtosecond pulse of electrons, which bounce off the atoms, producing a diffraction pattern on a detector.

Since the electron pulses are so incredibly brief, the heated atoms don't have time to move much; this shorter "exposure time" produces a sharp image. By adjusting the delay between when the sample is heated and when the image is taken, the scientists can build up a library of still images that can be strung together into a movie.

"Essentially all of the specimens we deal with are heterogeneous," Zewail explains, with varying compositions over very small areas. "This technique provides the means for examining local sites in materials and biological structures, with a spatial resolution of a nanometer or less, and time resolution of femtoseconds."

The new diffraction method allows the structures of materials to be mapped out at an atomic scale. With the second technique-introduced in the Nature paper [1], which was coauthored by postdoctoral scholars Brett Barwick and David Flannigan - the light produced by such nanostructures can be imaged and mapped.

The concept behind this technique involves the interaction between electrons and photons. Photons generate an evanescent field in nanostructures, and electrons can gain energy from such fields, which makes them visible in the 4D microscope.

Image 2: Photons imaged in nanoscale structures (carbon nanotubes) using pulsed electrons at very high speed. Shown are the evanescent fields for two time frames and for two polarizations. [Credit: Zewail/Caltech]

In what is known as the photon-induced near-field electron microscopy (PINEM) effect, certain materials-after being hit with laser pulses-continue to "glow" for a short but measurable amount of time (on the order of tens to hundreds of femtoseconds).

In their experiment, the researchers illuminated carbon nanotubes and silver nanowires with short pulses of laser light as electrons were being shot past. The evanescent field persisted for femtoseconds, and the electrons picked up energy during this time in discrete amounts (or quanta) corresponding to the wavelength of the laser light. The energy of an electron at 200 kilo-electron volts (keV) increased by 2.4 electron volts (eV), or by 4.8 eV, or by 7.2 eV, etc.; alternatively, an electron might not change in energy at all. The number of electrons showing a change is more striking if the timing is just right, i.e., if the electrons are passing the material when the field is at its strongest.

The power of this technique is that it provides a way to visualize the evanescent field when the electrons that have gained energy are selectively identified, and to image the nanostructures themselves when electrons that have not gained energy are selected.

"As noted by the reviewers of this paper, this technique of visualization opens new vistas of imaging with the potential to impact fields such as plasmonics, photonics, and related disciplines," Zewail says. "What is interesting from a fundamental physics point of view is that we are able to image photons using electrons. Traditionally, because of the mismatch between the energy and momentum of electrons and photons, we did not expect the strength of the PINEM effect, or the ability to visualize it in space and time."

References
[1] "Photon-induced near-field electron microscopy",
Brett Barwick, David J. Flannigan & Ahmed H. Zewail,
Nature, Vol. 462, pp 902-906 (17 December 2009). Abstract.
[2] "4D Nanoscale Diffraction Observed by Convergent-Beam Ultrafast Electron Microscopy",
Aycan Yurtsever and Ahmed H. Zewail,
Science, Vol. 326. no. 5953, pp 708 - 712 (October 30, 2009). Abstract.

[We thank Media relations, Caltech for materials used in this posting]

Creation of ‘Synthetic Magnetic Fields’ for Neutral Atoms

Ian Spielman [photo courtesy: Joint Quantum Institute, University of Maryland]

The current (December 3) issue of the journal 'Nature' carries an article describing the creation of the so-called “synthetic” magnetic fields for ultracold gas atoms, in effect “tricking” neutral atoms into acting as if they are electrically charged particles subjected to a real magnetic field.

This important new capability in ultracold atomic gases is achieved by a team of researchers at the Joint Quantum Institute (JQI), a collaboration of the University of Maryland and the National Institute of Standards and Technology (NIST). The demonstration of this capability not only paves the way for exploring the complex natural phenomena involving charged particles in magnetic fields, but may also contribute to an exotic new form of quantum computing.

As researchers have become increasingly proficient at creating and manipulating gaseous collections of atoms near absolute zero, these ultracold gases have become ideal laboratories for studying the complex behavior of material systems. Unlike usual crystalline materials, they are free of obfuscating properties, such as impurity atoms, that exist in normal solids and liquids.

However, studying the effects of magnetic fields is problematic because the gases are made of neutral atoms and so do not respond to magnetic fields in the same way as charged particles do. So how would you simulate, for example, such important exotic phenomena as the quantum Hall effect, in which electrons can “divide” into quasiparticles carrying only a fraction of the electron’s electric charge?

The answer Ian Spielman and his colleagues came up with is a clever physical trick to make the neutral atoms behave in a way that is mathematically identical to how charged particles move in a magnetic field. A pair of laser beams illuminates an ultracold gas of rubidium atoms already in a collective state known as a Bose-Einstein condensate. The laser light ties the atoms' internal energy to their external (kinetic) energy, modifying the relationship between their energy and momentum. Simultaneously, the researchers expose the atoms to a real magnetic field that varies along a single direction, so that the alteration also varies along that direction.

A pair of laser beams (red arrows) impinges upon an ultracold gas cloud of rubidum atoms (green oval) to create synthetic magnetic fields (labeled Beff). (Inset) The beams, combined with an external magnetic field (not shown) cause the atoms to "feel" a rotational force; the swirling atoms create vortices in the gas [Image courtesy: JQI]

In a strange inversion, the laser-illuminated neutral atoms react to the varying magnetic field in a way that is mathematically equivalent to the way a charged particle responds to a uniform magnetic field. The neutral atoms experience a force in a direction perpendicular to both their direction of motion and the direction of the magnetic field gradient in the trap. By fooling the atoms in this fashion, the researchers created vortices in which the atoms swirl in whirlpool-like motions in the gas clouds. The vortices are the “smoking gun,” Spielman says, for the presence of synthetic magnetic fields.

A harbinger of the synthetic magnetic fields is the formation of vortices (spots). These spots, the number of which increases with increasing synthetic field, mark the points about which atoms swirled with a whirlpool-like motion. The measurement units in each panel indicate the size of the external magnetic field gradient applied to the gas of atoms, with larger external fields producing more vortices. [Image courtesy: JQI]

Previously, other researchers had physically spun gases of ultracold atoms to simulate the effects of magnetic fields, but rotating gases are unstable and tend to lose atoms at the highest rotation rates.

In their next step, the JQI researchers plan to partition a nearly spherical system of 20,000 rubidium atoms into a stack of about 100 two-dimensional “pancakes” and increase their currently observed 12 vortices to about 200 per-pancake. At a one-vortex-per-atom ratio, they could observe the quantum Hall effect and control it in unprecedented ways. In turn, they hope to coax atoms to behave like a class of quasiparticles known as “non-abelian anyons,” a required component of “topological quantum computing,” in which anyons dancing in the gas would perform logical operations based on the laws of quantum mechanics.

Reference
"Synthetic magnetic fields for ultracold neutral atoms"
Y.-J. Lin, R. L. Compton, K. Jiménez-García, J. V. Porto & I. B. Spielman.
Nature, 462, 628-632 (3 December, 2009). Abstract.

[We thank National Institute of Standards and Technology for materials used in this report]

Bose-Einstein Condensation of Strontium

Figure 1: The SrBEC team. From left to right: Bo Huang, Meng Khoon Tey, Rudolf Grimm, Florian Schreck (Author), and Simon Stellmer



[This is an invited article based on the recently published work of the author and his collaborators -- 2Physics.com]




Author: Florian Schreck

Affiliation: Institut für Quantenoptik und Quanteninformation (IQOQI), Austria
Link to the 'Ultracold Atoms and Quantum Gases' Group >>

Atoms are particles as well as waves. The wave nature of atoms becomes evident when cooling a gas of bosonic atoms to extremely low temperatures. The de Broglie wavelength describing the atomic wave packets grows and as soon as it exceeds the interatomic spacing, the gas undergoes a phase-transition and enters a collective state of matter, known as Bose-Einstein condensate (BEC). This behavior was predicted in 1924 by Bose and Einstein and realized for the first time in 1995 in gases of alkali-metal atoms [1, 2, 3]. Research on these degenerate quantum gases has since grown strongly and has now connected to many other fields as, for example, condensed-matter physics, molecular physics and precision measurements.

Since then, ten elements have been Bose-condensed: all stable alkali-metal atoms and hydrogen, metastable helium, chromium, ytterbium [4] and very recently calcium [5]. Oftentimes new isotopes cooled to quantum degeneracy have, with their unique properties, opened the doors to the investigation of novel phenomena. Atoms with two electrons in their outer shell, as ytterbium and the alkaline-earth elements, have properties unlike any other of the condensed species: a non-magnetic ground-state (for bosonic isotopes), metastable states and a combination of broad and narrow linewidth optical transitions. This has made these elements, especially ytterbium and strontium, prime choices for neutral atom optical clocks. In addition, numerous proposals employ these properties to realize quantum simulation and computation schemes, mHz-linewidth lasers or to probe the time dependence of natural constants.

These applications either rely on or would benefit from the availability of quantum degenerate samples. Already ten years ago, strontium atoms have been cooled to near quantum degeneracy [6], but a BEC could not be reached. The problem resided in the last cooling stage used in all experiments that have produced degenerate quantum gases: evaporative cooling. This cooling process works by removing hot atoms from the sample and using elastic collisions to rethermalize the remaining gas at a lower temperature. For strontium evaporative cooling worked very badly: the most abundant isotope, ⁸⁸Sr with 83% abundance, essentially does not collide. ⁸⁶Sr with 10% abundance has the opposite problem: the atoms collide so strongly that often molecules are formed, releasing the molecular binding energy, which leads to heating and the loss of atoms. Sr has yet another bosonic isotope: ⁸⁴Sr with only 0.56% natural abundance. Apparently for this reason nobody had undertaken experiments using this isotope.

However, we had experience working with low abundance isotopes and took a closer look at ⁸⁴Sr. We asked Roman Ciuryło to calculate the scattering properties of ⁸⁴Sr by scaling the known properties of the abundant isotopes with the mass. Based on measurements by Thomas Killians group he deduced an elastic scattering cross section in the Goldilocks zone: neither too small nor too big. This was shortly afterwards confirmed by two other groups [7, 8]. Therefore we decided to make ⁸⁴Sr our main approach to Sr BEC.

To overcome the low natural abundance, a combination of Sr properties turned out to be very beneficial. To produce a sample of cold Sr atoms suitable for evaporative cooling, an atomic beam is slowed and then held and cooled in a magneto-optical trap (MOT) using laserlight near-resonant with a broad-linewidth transition. A small fraction of the atoms in the excited state of that transition will not decay back to the ground-state, but to a metastable state with a lifetime of several minutes. Atoms in this state are magnetic and can be trapped in the quadrupole magnetic field used for the MOT. Within 10 seconds we can accumulate about 100 million atoms in this state, overcoming the low natural abundance and giving us enough material for evaporative cooling.

Temperature and density achievable in a MOT depend on the linewidth of the transition used. Strontium has also a narrow-linewidth transition that is suitable for a MOT. It is too narrow to allow slowing of an atomic beam or capture of atoms from that beam, but it can be used to further cool the atoms accumulated in the metastable state (after optically pumping them back to the ground-state). The figure of merit for a cooling process with the goal to reach BEC is phase-space density, the product of density and the de Broglie wavelength cubed. The phase-transition to BEC will occur if the phase-space density exceeds 2.6. For alkali-metal atoms, only one MOT transition exists, which allows to obtain phase-space densities on the order of 10^-6. The combination of broad-linewidth and just perfectly sized narrow-linewidth transition in strontium allows to achieve remarkably high phase-space densities of 10^-2 already after the MOT stage. This means that only very little evaporative cooling is required to obtain quantum degeneracy.

For evaporative cooling, the atoms have to be held in a conservative potential. Atoms in the ground-state have no magnetic moment, so a magnetic trap can not be used. We confine them using a so-called optical dipole trap, which consists of two crossed infrared laser beams. About one million atoms are loaded into the crossing region after switching off the MOT. Evaporative cooling proceeds now by letting hot atoms escape from the trap and waiting for the remaining atoms to thermalize. To force evaporation to continue at ever lower temperatures, the potential depth is lowered over the course of a few seconds. We knew that the elastic scattering properties of ⁸⁴Sr would be ideal for evaporative cooling, but it was impossible to predict the inelastic scattering properties that can lead to detrimental atom loss and heating. To our great pleasure we discovered that inelastic processes were very weak. Evaporative cooling worked the first time we tested it and only minutes later we created the first Bose-Einstein condensate of strontium.

Figure 2: Density distribution of strontium atoms released from a trap. To the left a thermal sample is shown. Further cooling results in the appearance of a dense and cold Bose-Einstein condensate in the middle of the cloud. Finally the thermal component is too small to be detectable.

Figure 3: Expansion of a ⁸⁴Sr BEC from an elongated trap. The repulsion between the atoms leads to a faster expansion along the initially strongly confined directions. The sequence of images shows the temporal evolution in 5ms steps [Ref: Physical Review Letters, 103, 200401 (2009)]

Figure 2 shows images of the density distributions of clouds of ⁸⁴Sr atoms across the phase transition from a thermal cloud to a pure BEC. As soon as the phase-transition is crossed, a dense central peak appears, the BEC. Figure 3 highlights another property of the BEC: after release from the trap it expands fastest along the direction in which it was initially strongest confined leading to a disk-shaped density distribution, shown here from the side. Thermal atoms would expand isotropically and show a spherical density distribution.

About two weeks after we had achieved BEC of Sr [9], the group of Thomas Killian arrived at the same goal [10]. It is clear from both experiments that BEC of Sr is very robust. Simple scaling up of the volume of the optical dipole trap should result in BECs in excess of one million atoms. The two other species with two electrons in the outer shell that have been Bose-condensed, Yb and Ca, have so far only produced relatively small BECs of up to 6 X 10⁴ atoms. This puts Sr in a prime position for experiments with BECs of two-electron systems.

Using sympathetic cooling it should be possible to cool also the other Sr isotopes to quantum degeneracy. ⁸⁸Sr is nearly non-interacting, which would be useful for precision sensors, for example force sensors. The fermionic isotope ⁸⁷Sr has a nuclear magnetic moment, which can be used to store quantum information. It is at the heart of proposals for quantum computation and is the key to the study of a new class of many-body systems. Sr₂ molecules can be created and used to measure the stability of fundamental constants. Cooling of the alkali-metal rubidium is compatible with the scheme employed for Sr. SrRb ground-state molecules would possess both, an electric and a magnetic dipole moment. This can be used to design many-body systems with spin-dependent long range interactions.

It will be exciting to explore all the new possibilities opened up by the Bose-Einstein condensation of strontium.

References
[1] M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, “Observation of bose-einstein condensation in dilute atomic vapor,” Science, vol. 269, pp. 198–201 (1995). Abstract.
[2] K. B. Davis, M. O. Mewes, M. R. Andrews, N. J. van Druten, D. S. Durfee, D. M. Kurn, and
W. Ketterle, “Bose-einstein condensation in a gas of sodium atoms,”
Phys. Rev. Lett., vol. 75, pp. 3969–3973 (1995). Abstract.
[3] C. C. Bradley, C. A. Sackett, J. J. Tollett, and R. G. Hulet, “Evidence of bose-einstein condensation in an atomic gas with attractive interactions,” Phys.Rev. Lett., vol. 75, pp. 1687–1690 (1995). Abstract.
[4] Y. Takasu, K. Maki, K. Komori, T. Takano, K. Honda, M. Kumakura, T. Yabuzaki, and Y. Takahashi, “Spin-singlet bose-einstein condensation of two-electron atoms,”
Phys. Rev. Lett., vol. 91, p.040404 (2003). Abstract.
[5] S. Kraft, F. Vogt, O. Appel, F. Riehle, and U. Sterr, “Bose-einstein condensation of alkaline earth atoms: ⁴⁰Ca,” Phys. Rev. Lett., vol. 103, p. 130401, 2009. Abstract.
[6] H. Katori, T. Ido, Y. Isoya, and M. Kuwata-Gonokami, “Laser cooling of strontium atoms toward quantum degeneracy,” in Atomic Physics 17 (E. Arimondo, P. DeNatale, and M. Inguscio, eds.), pp. 382–396, American Institute of Physics, Woodbury (2001).
[7] A. Stein, H. Knöckel, and E. Tiemann, “Fourier-transform spectroscopy of Sr₂ and revised groundstate potential,” Phys. Rev. A, vol. 78, p.042508 (2008). Abstract.
[8] Y. N. Martinez de Escobar, P. G. Mickelson, P. Pellegrini, S. B. Nagel, A. Traverso, M. Yan, R. Côté, and T. C. Killian, “Two-photon photoassociative spectroscopy of ultracold ⁸⁸Sr,”
Phys.Rev. A, vol. 78, p.062708 (2008). Abstract.
[9] S. Stellmer, M. K. Tey, B. Huang, R. Grimm, and F. Schreck, “Bose-Einstein condensation of strontium,” Physical Review Letters, vol. 103, no. 20, p.200401 (2009). Abstract.
[10] Y. N. M. de Escobar, P. G. Mickelson, M. Yan, B. J. DeSalvo, S. B. Nagel, and T. C. Killian, “Bose-einstein condensation of ⁸⁴Sr,” Physical Review Letters, vol. 103, no. 20, p.200402 (2009). Abstract.

Science Begins At The World's Most Powerful X-Ray Laser

Joachim Stöhr, Director of LCLS (Photo courtesy: Stanford Linear Accelerator Laboratory)

The first experiments are now underway using the world's most powerful X-ray laser, the Linac Coherent Light Source (LCLS), located at the Department of Energy's SLAC National Accelerator Laboratory. LCLS produces ultrafast pulses of X-rays millions of times brighter than even the most powerful synchrotron sources — pulses powerful enough to make images of single molecules. Illuminating objects and processes at unprecedented speed and scale, the LCLS has embarked on groundbreaking research in physics, structural biology, energy science, chemistry and a host of other fields.

In early October, researchers from around the globe began traveling to SLAC to get an initial glimpse into how the X-ray laser interacts with atoms and molecules. The LCLS is unique, shining light that can resolve detail the size of atoms at ten billion times the brightness of any other manmade X-ray source.

"No one has ever had access to this kind of light before," said LCLS Director Jo Stöhr. "The realization of the LCLS isn't only a huge achievement for SLAC, but an achievement for the global science community. It will allow us to study the atomic world in ways never before possible."

The LCLS is a testament to SLAC's continued leadership in accelerator technology. Over 40 years ago, the laboratory's two-mile linear accelerator, or linac, was built to study the fundamental building blocks of the universe. Now, decades later, this same machine has been revitalized for frontier research underway at the LCLS.

After SLAC's linac accelerates very short pulses of electrons to 99.9999999 percent the speed of light, the LCLS takes them through a 100-meter stretch of alternating magnets that force the electrons to slalom back and forth. This motion causes the electrons to emit X-rays, which become synchronized as they interact with the electron pulses over this long slalom course, thereby creating the world's brightest X-ray laser pulse. Each of these laser pulses packs as many as 10 trillion X-ray photons into a bunch that's a mere 100 femtoseconds long—the time it takes light to travel the width of a human hair.

Atomic, Molecular and Optical Science (AMO) Instrument (Image courtesy: Stanford Linear Accelerator Laboratory)

Currently, user-assisted commissioning is underway, with researchers conducting experiments using the Atomic, Molecular and Optical science instrument, the first of six planned instruments for the LCLS. In these first AMO experiments, researchers are using X-rays from the LCLS to gain an in-depth understanding of how the ultra-bright beam interacts with matter.

Early experiments are already revealing new insights into the fundamentals of atomic physics and have successfully proven the machine's unique capabilities to control and manipulate the underlying properties of atoms and molecules. Earlier this month, researchers used the LCLS's strobe-like pulses to completely strip neon atoms of all their electrons. Researchers also watched for two-photon ionization—an event where two photons pool their energy to eject a single electron from an atom. Normally difficult to observe at X-ray facilities, researchers at the LCLS were able to study these events using the extreme brightness of the laser beam.

Future AMO experiments will create stop-action movies of molecules in motion. The LCLS's quick, short, repetitive X-ray bursts enable researchers to take individual photos as molecules move and interact. By stringing together many such images to make a movie, researchers will for the first time have the ability to watch the molecules of life in action, view chemical bonds forming and breaking in real time, and see how materials work on the quantum level.

By 2013, all six LCLS scientific instruments will be online and operational, giving researchers unprecedented tools for a broad range of research in material science, medicine, chemistry, energy science, physics, biology and environmental science.

"It's hard to overstate how successful these first experiments have been," said AMO Instrument Scientist John Bozek. "We look forward to even better things to come."

[We thank Stanford Linear Accelerator Laboratory for materials used in this posting.]

Finer Atomic Matchmaking by Radio Frequency Tuning

The image shows, in the sequence of green arrows, how a pair of ultracold gas atoms collides, briefly forms a molecule, and flies apart, in the presence of an external magnetic field (not shown) that influences this process. By adding RF radiation (lightning bolts) of the right frequency, the atoms can experience being in many different molecular states (red arrows), providing even more extensive and detailed control of the collision. The size of the yellow bursts indicate the amount of absorption/emission of RF radiation. [Image Credit: Eite Tiesinga, Joint Quantum Institute, University of Maryland/ National Institute of Standards and Technology]

In a paper accepted for publication in Physical Review A, a team of scientists at the Joint Quantum Institute (JQI) of the National Institute of Standards and Technology (NIST) and the University of Maryland have reported that properly tuned radio-frequency waves can influence how much the atoms attract or repel one another, opening up new ways to control their interactions.

While investigating mysterious data in ultracold gases of rubidium atoms, they found that the radio-frequency (RF) radiation could serve as a second "knob," in addition to the more traditionally used magnetic fields, for controlling how atoms in an ultracold gas interact. Just as it is easier to improve reception on a home radio by both electronically tuning the frequency on the receiver and mechanically moving the antenna, having two independent knobs for influencing the interactions in atomic gases could produce richer and more exotic arrangements of ultracold atoms than ever before.

Previous experiments with ultracold gases, including the creation of Bose-Einstein condensates, have controlled atoms by using a single knob—traditionally, magnetic fields. These fields can tune atoms to interact strongly or weakly with their neighbors, pair up into molecules, or even switch the interactions from attractive to repulsive. Adding a second control makes it possible to independently tune the interactions between atoms in different states or even between different types of atoms.

Such greater control could lead to even more exotic states of matter. A second knob, for example, may make it easier to create a weird three-atom arrangement known as an Efimov state, whereby two neutral atoms that ordinarily do not interact strongly with one another join together with a third atom under the right conditions [Read past 2Physics report on Efimov state]

For many years, researchers had hoped to use RF radiation as a second knob for atoms, but were limited by the high power required. The new work shows that, near magnetic field values that have a big effect on the interactions, significantly less RF power is required, and useful control is possible.

In the new work, the JQI/NIST team examined intriguing experimental data of trapped rubidium atoms taken by the group of David Hall at Amherst College in Massachusetts. This data showed that the RF radiation was an important factor in tuning the atomic collisions. To explain the complicated way in which the collisions varied with RF frequency and magnetic field, NIST theorist Thomas Hanna developed a simple model of the experimental arrangement.

"The model reconstructed the energy landscape of the rubidium atoms and explained how RF radiation was changing the atoms' interactions with one another. In addition to providing a roadmap for rubidium, this simplified theoretical approach could reveal how to use RF to control ultracold gases consisting of other atomic elements", Hanna says.

Reference
"Radio-frequency dressing of multiple Feshbach resonances"
A.M. Kaufman, R.P. Anderson, T.M. Hanna, E. Tiesinga, P.S. Julienne, and D.S. Hall,
To appear in Physical Review A.
Abstract: We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in 87Rb using radiofrequency radiation (rf). We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.

[We thank NIST for materials used in this posting]

Laser Pulses Control Single Electrons in Complex Molecules

Garching and Munich researchers (L to R) Regina de Vivie-Riedle, Philipp von den Hoff, Matthias Kling, and Irina Znakovskaya. [Photograph compliments of Thorsten Naeser of Max Planck Institute of Quantum Optics]



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



Authors: Matthias Kling and Regina de Vivie-Riedle

Affiliations: Max Planck Institute of Quantum Optics, Garching and
Ludwig-Maximilians University, Munich, Germany

Link to 'Attosecond Imaging' Group >>

Predatory fish are well aware of the problem: In a swarm of small fish it is hard to isolate the prey. A similar situation can be found in the microcosm of atoms and molecules, whose behavior is influenced by “swarms” of electrons. In order to achieve control over single electrons in a bunch, ultrashort light pulses of a few femtoseconds duration are needed.

Electrons are extremely fast. In atoms and molecules they move on attosecond timescales [1]. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the hydrogen molecular ion and its isotopes, their control with such light pulses is relatively easy and was demonstrated by Kling et al. [2, 3]. These first studies on controlling electrons in molecules with waveform controlled light pulses sparked, however, the question, whether the steering of electrons in more complex systems is feasible.

In our recent work [4] we have now managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes more difficult as their states lie energetically very close to each other. Control of the electric field waveform E(t) = E₀(t) cos(ωt + φ), with envelope E₀(t), and frequency ω, is achieved by stabilization and control of the carrier envelope phase (CEP) φ and constitutes a new paradigm of coherent control that can be applied to steer electrons in atomic and molecular systems [1].

We used visible (740 nm) laser pulses with only 4 femtoseconds duration. The control of electron dynamics in the system was experimentally determined via an asymmetric distribution of C⁺ and of O⁺ fragments after the breaking of the molecular bond. The measurement of C⁺ and O⁺ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the CEP of the laser pulse.

Theoretical methods based on ab initio quantum mechanics were developed for multi electron systems and applied to analyze the ionization and subsequent electron localization process. The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process takes only ca. 1.7 femtoseconds. The collision produces an electronic wave packet which induces a directional motion of electrons along the molecular axis. The excitation and subsequent interaction with the remaining part of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry.

From theoretical analysis it became clear that in the performed experiments, it was also possible to image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed to explore this process in the outermost HOMO (highest occupied molecular orbitals) and HOMO-1 orbitals. The ionization of the molecules is found to take place with a distinct angular dependence with respect to the laser polarization direction. This dependence is seen in fragments from the dissociation following ionization (see Fig. 1 for recorded C+ fragments). Our observation is in good agreement with theoretical calculations (shown as black line in Fig. 1) [4]. The ionization process itself also gives a contribution to the observed asymmetry, which strongly depends on the duration of the laser pulses [4].

Fig. 1: The detachment of electrons from carbon monoxide molecules by femtosecond laser pulses leads to a characteristic angular distribution of the molecular ions and their fragments. The angular distribution resembles the structure of orbitals from which electrons have been ionized. The figure shows the angular distribution of C⁺ ions measured in the laser-induced dissociative ionization of CO via velocity-map imaging (VMI) for CEP = Π. The laser polarization is vertical and the number of observed ions is displayed in color.

Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors – technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics [5] aiming at computing speeds on attosecond timescales.

References
[1] M. F. Kling, and M. J. J. Vrakking, "Attosecond Electron Dynamics", Annu. Rev. Phys. Chem. 59, 463 (2008). Abstract.
[2] M. F. Kling, Ch. Siedschlag, A. J. Verhoef, J. I. Khan, M. Schultze, Th. Uphues, Y. Ni, M. Uiberacker, M. Drescher, F. Krausz, M. J. J. Vrakking, "Control of Electron Localization in Molecular Dissociation", Science 312, 246 (2006). Abstract.
[3] M. F. Kling, Ch. Siedschlag, I. Znakovskaya, A. J. Verhoef, S. Zherebtsov, F. Krausz, M. Lezius, M. J. J. Vrakking, "Strong-field control of electron localization during molecular dissociation", Mol. Phys. 106, 455 (2008). Abstract.
[4] I. Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M. J. J. Vrakking, R. de Vivie-Riedle, M. F. Kling, "Attosecond Control of Electron Dynamics in Carbon Monoxide", Phys. Rev. Lett. 103, 103002 (2009). Abstract.
[5] E. Goulielmakis, V. S. Yakovlev, A. L. Cavalieri, M. Uiberacker, V. Pervak, A. Apolonski, R. Kienberger, U. Kleineberg, F. Krausz, "Attosecond Control and Measurement: Lightwave Electronics", Science 317, 769 (2007). Abstract.

Phonon Laser Demonstrated Using Trapped Ions

For decades there has been interest in phonon lasers, a device that operates in analogy to optical lasers, except replacing light with vibrational energy. Phonon lasers would amplify motion to the point of coherence through the process of stimulated emission, whereby the presence of energy quanta increases the chances that additional quanta are emitted. Now, researchers at the Max Planck Institute of Quantum Optics in Garching, Germany have realized such a device using a cold, harmonically bound, magnesium ion.

Reporting in the journal Nature Physics [1], the ion was first laser cooled to a temperature of around 1 milliKelvin in an electromagnetic trap, and then a second laser was applied, whose wavelength was precisely adjusted so as to create mechanical (not optical) amplification through the process of stimulated emission of phonons. The phonons are associated with the center-of-mass motion of the ion in the electromagnetic trap. As such, phonon stimulated emission causes the ion to oscillate, once a threshold condition is achieved (figure).

Sequence of time-averaged images showing coherent motion of the phonon laser. At the far left, the magnesium ion is cooled to approximately 1 milli-Kelvin using a red-detuned laser. In each subsequent image, an additional, blue-detuned pump laser is stepped in intensity. Beginning around the fifth trace, laser threshold is achieved at which point mechanical amplification compensates mechanical damping. Beyond this threshold pumping level, the motion is sustained by stimulated emission of phonons and stabilized by saturation of the amplification. The ions oscillate vertically in the figure, and the brighter end points in each image result from the ion slowing and eventually stopping to reverse motion.

“The possibility of phonon laser action has been considered dating back to the earliest days of the optical laser, nearly 50 years ago,” notes Kerry Vahala, who is currently a guest scientist at the Max Planck Institute of Quantum Optics in Garching, on leave from Caltech. He adds that the possible ways to realize these devices are as numerous as those used for conventional optical lasers, and ultimately, many other types of phonon lasers including those in the solid-state are possible.

“Laser-cooled ions are, however, a very good starting point to explore the physics of these intriguing devices,” notes Thomas Udem. Udem observes that they can be precisely controlled and manipulated, because of a multi-decade effort directed towards exploration of quantum phenomena using chains of cold ions. That history is also interesting in another respect.

“The physical mechanism that is responsible for amplification and stimulated emission of phonons has, over the last decades, been misunderstood as a different phenomenon referred to as heating,” says Maximilian Herrmann. He explains that the regime of heating was always associated with laser pumping using a wavelength that is blue-detuned, relative to an optical transition of the trapped ion. In stark contrast, the Garching team was able to show that the regime is, in fact, one of phonon laser action.

In the experiment, the ion was simultaneously cooled using a red-detuned laser and also pumped to create stimulated phonon amplification using the blue-detuned beam. The cooling laser also allowed the researchers to introduce a controlled amount of damping into the system. To understand the origin of amplification in the ion system, consider the so-called small-signal regime with the blue detuned pumping beam. This interaction involves a second-order quantum process in which a pump photon is absorbed, followed by emission of center-of-mass phonon.

Herein, the role of atomic transition damping is crucial. As is also true in a conventional optical laser oscillator, the transition damping is fast enough so as to quench the underlying Rabi dynamic. As a result, the rate of emission of phonons can be shown to contain a stimulated component that is proportional to the number of phonons, and also a component that is purely spontaneous (and responsible for starting the motion in the first place). The stimulated term, as in a conventional laser, produces the amplification that ultimately creates the coherent motion.

The transition from purely thermal motion to coherent motion is marked by a threshold pumping power, where stimulated amplification balances ion motional damping. As pumping is further increased through this threshold, coherence emerges and is visible in images of the ion motion (figure). Ultimately, the large signal motion of the ion saturates the stimulated amplification. The saturation process in the ion is, itself, interesting as it involves a competing phonon absorption process. In effect, as the coherent motion grows in amplitude, this phonon absorption competes more effectively with the stimulated emission and saturates the net amplification. Generally speaking, saturation of amplification is an essential feature of all regenerative oscillators, and makes possible stable coherent motion.

The mechanism of amplification through scattering of a pump photon and generation of a stimulated phonon is in some ways analogous to the process of Raman optical amplification in which a pump photon scatters to produce a stimulated (and amplified) optical Stokes wave [2]. In that process, a phonon is also produced, but is so strongly damped that it effectively serves to only assist the overall amplification of light as an intermediate process. In the ion phonon laser system, on the other hand, this situation changes dramatically.

The researchers note that the pump photon creates both a polarization excitation and a phonon; but in a peculiar twist, the phonon switches out of its conventional “supporting” role, and, because of the very low damping rate of phonons in the ion system, takes-on the lead role analogous to the optical Stokes mode in the conventional optical Raman process. This feature of the stimulated phonon process in the ionic system can be made mathematically precise; and also helps to explain, as is true in optical Raman lasers, operation without an obvious inversion.

Current research is directed towards controlling the phonon laser using tools and techniques that can be adapted from the laser world. One example is a process called injection locking whereby a weak, external control field is used to phase-synchronize the phonon laser with an external reference. The team has recently used this method to image the coherent motion of the ion as it oscillates. Also interesting is the possibility of studying excitations in chains of ions and even two-dimensional arrays of ions.

Concerning applications, Professor Theodor Hänsch notes, “the fact that the forces involved here are so weak suggests that this phenomenon might prove useful as a weak force probe.” He adds, “it is always important when a phenomenon or idea can be reduced to practice.” Along these lines, the Garching work has already inspired realization in a very different system of phonon laser action [3].

Reference
[1] K. Vahala, M. Herrmann, S. Knünz, V. Batteiger, G. Saathoff, T. W. Hänsch & Th. Udem, “A phonon laser,” Nature Physics, 5, 682 – 686 (2009). Abstract.
[2] Shen, Y. R. & Bloembergen, N. “Theory of stimulated Brillouin and Raman
scattering,” Phys. Rev. 137, 1787–1805 (1965). Abstract.
[3] Ivan S. Grudinin, O. Painter, Kerry J. Vahala, “Phonon laser action in a tunable, two-level photonic molecule,” arXiv:0907.5212v1 (2009).

IBM Researchers First to Image the 'Anatomy' of a Molecule at Atomic Resolution

IBM Research - Zurich scientists Nikolaj Moll, Reto Schlittler, Gerhard Meyer, Fabian Mohn and Leo Gross (L to R) standing behind a scanning tunneling/atomic force microscope similar to the one they used to image the "anatomy" of the Pentacene molecule at an atomic resolution. [Photo by Michael Lowry. Image courtesy of IBM Research - Zurich]


In a paper published in the August 28 issue of Science magazine [1], IBM Research – Zurich scientists Leo Gross, Fabian Mohn, Nikolaj Moll and Gerhard Meyer, in collaboration with Peter Liljeroth of Utrecht University -- Netherlands, have reported capturing the first image of the “anatomy” -- or chemical structure -- inside a molecule with unprecedented resolution, using a complex technique known as noncontact atomic force microscopy (AFM). The results push the exploration of using molecules and atoms at the smallest scale and could greatly impact the field of nanotechnology.

"Though not an exact comparison, if you think about how a doctor uses an x-ray to image bones and organs inside the human body, we are using the atomic force microscope to image the atomic structures that are the backbones of individual molecules," said Gerhard Meyer. "Scanning probe techniques offer amazing potential for prototyping complex functional structures and for tailoring and studying their electronic and chemical properties on the atomic scale.”

The team’s current publication follows on the heels of another experiment published just two months ago [2] where IBM scientists measured the charge states of atoms using an AFM. These breakthroughs will open new possibilities for investigating how charge transmits through molecules or molecular networks. Understanding the charge distribution at the atomic scale is essential for building smaller, faster and more energy-efficient computing components than today’s processors and memory devices.

The team used an AFM operated in an ultrahigh vacuum and at very low temperatures ( –268⁰C or –451⁰F) to image the chemical structure of individual pentacene molecules. With their AFM, the IBM scientists, for the first time ever, were able to look through the electron cloud and see the atomic backbone of an individual molecule. While not a direct technological comparison, this is reminiscent of x-rays that pass through soft tissue to enable clear images of bones.

Imaging the "anatomy" of a pentacene molecule - 3D rendered view: By using an atomically sharp metal tip terminated with a carbon monoxide molecule, IBM scientists were able to measure in the short-range regime of forces which allowed them to obtain an image of the inner structure of the molecule. The colored surface represents experimental data. [Image courtesy of IBM Research – Zurich]

The AFM uses a sharp metal tip to measure the tiny forces between the tip and the sample, such as a molecule, to create an image. In the present experiments, the molecule investigated was pentacene. Pentacene is an oblong organic molecule consisting of 22 carbon atoms and 14 hydrogen atoms measuring 1.4 nanometers in length. The spacing between neighboring carbon atoms is only 0.14 nanometers—roughly 1 million times smaller then the diameter of a grain of sand. In the experimental image, the hexagonal shapes of the five carbon rings as well as the carbon atoms in the molecule are clearly resolved. Even the positions of the hydrogen atoms of the molecule can be deduced from the image.

“The key to achieving atomic resolution was an atomically sharp and defined tip apex as well as the very high stability of the system,” said Leo Gross. To image the chemical structure of a molecule with an AFM, it is necessary to operate in very close proximity to the molecule. The range, where chemical interactions give significant contributions to the forces, is less than a nanometer. To achieve this, the IBM scientists were required to increase the sensitivity of the tip and overcome a major limitation: Similar to the way two magnets would attract or repel each other when getting close, the molecules would easily be displaced by or attach to the tip when the tip was approached too closely—rendering further measurements impossible.

Gross added, “We prepared our tip by deliberately picking up single atoms and molecules and showed that it is the foremost tip atom or molecule that governs the contrast and resolution of our AFM measurements.” A tip terminated with a carbon monoxide (CO) molecule yielded the optimum contrast at a tip height of approximately 0.5 nanometers above the molecule being imaged and—acting like a powerful magnifying glass—resolved the individual atoms within the pentacene molecule, revealing its exact atomic-scale chemical structure.

Furthermore, the scientists were able to derive a complete three-dimensional force map of the molecule investigated. “To obtain a complete force map the microscope needed to be highly stable, both mechanically and thermally, to ensure that both the tip of the AFM and the molecule remained unaltered during the more than 20 hours of data acquisition,” says Fabian Mohn, who is working on his Ph.D. thesis at IBM Research – Zurich.

A topography of forces: The forces exerted on the tip above the pentacene molecule create a landscape that resembles a mountain ridge in this 3-D force map. The overall elevation represents the attractive forces between the tip and the molecule. The finer features on the ridge stem from repulsive forces between the tip and the pentacene molecule at closer tip–molecule distance. [Courtesy: IBM Research –Zurich]

To corroborate the experimental findings and gain further insight into the exact nature of the imaging mechanism, IBM scientist Nikolaj Moll performed first-principles density functional theory calculations of the system investigated. He explains, “The calculations helped us understand what caused the atomic contrast. In fact, we found that its source was Pauli repulsion between the CO and the pentacene molecule.” This repulsive force stems from the quantum mechanical effect of Pauli exclusion principle which states that two identical electrons can not approach each other too closely.

Reference
[1] “The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy”,
Leo Gross, Fabian Mohn, Nikolaj Moll, Peter Liljeroth, and Gerhard Meyer,
Science, Volume 325, Issue 5944, pp. 1110 – 1114 (28 August 2009). Abstract.
[2] "Measuring the Charge State of an Adatom with Noncontact Atomic Force Microscopy",
Leo Gross, Fabian Mohn, Peter Liljeroth, Jascha Repp, Franz J. Giessibl, Gerhard Meyer,
Science, Volume 324, Issue 5933, pp. 1428 – 1431 (June 12, 2009). Abstract.