Physicists Store Images in Vapor

By Lisa Zyga, Nanotechnology / Physics

The original image (left), the image slowed for 6 microseconds (middle), and the image stored for 2 microseconds (right). The technique to improve image resolution was not used for these images. Credit: M. Shuker, et al. ©2008 APS.

Books are written on solid pieces of paper for an obvious reason: the atoms in a solid don’t move around much, keeping the words and pictures in place for centuries. Trying to store letters and images in a gas medium, on the other hand, seems a little far-fetched. Atoms in a gas are constantly moving around, which would move the images around with them.

But physicists from the Technion-Israel Institute of Technology in Haifa, Israel, and the Weizmann Institute of Science in Rehovot, Israel, have recently demonstrated how to store images in a warm atomic vapor. With their method, which is based on electromagnetically induced transparency, the researchers could store complex images for up to 30 microseconds in rubidium vapor. To improve the resolution of the retrieved images, the physicists also developed a method to minimize the effect of the diffusion of the gas atoms on the images’ visibility.

Images stored for 30 microseconds. The left table shows actual and predicted images without the technique to improve image resolution. The right table shows actual and predicted images with the technique to improve image resolution, where a phase shift was applied to cancel light emission between the lines. Credit: M. Shuker, et al. ©2008 APS.

“The basic concepts of the storage of light have been known for several years now, as well as possible applications,” Moshe Shuker of the Technion-Israel Institute of Technology told PhysOrg.com. “What triggered our work was a paper by Howell’s group in Rochester in which they showed they can slow images and delay them for several nanoseconds. We wondered would it be possible to store images – and for how long? Since we used slowing delays and storage durations in the range of microseconds, we immediately noticed the effect of the diffusion of the atoms.”

In their technique, the researchers first stored an image (for example, the number “2”) in a light pulse. When that light pulse hits a gas of atoms, it is strongly absorbed, and excites the atoms. But when a second light beam is aimed at the gas, it drives the atoms to a unique quantum state, and causes the first pulse to pass through the vapor. This phenomenon is called electromagnetically induced transparency.

As previous experiments have shown, when the second light beam is shut off while the first pulse is inside the vapor, the first pulse can be completely stopped (and be temporarily stored inside the vapor). Then, by starting up the second beam again, the first pulse can be recovered.

Here, the physicists used this method to capture, store, and restore complex 3D light fields. The scientists slowed images on a light pulse to a group velocity of 8,000 meters per second, a velocity that allowed the images to be stored in atomic vapor for several microseconds.

They directed two light beams to a 5-cm-long vapor cell containing 52°C rubidium gas and a neon gas for buffering. Once half of the first light pulse (containing the image) had exited the vapor cell, the researchers turned off the second beam, so that the remaining half of the image was stored in the vapor. As the researchers explained, during storage, the image was encoded in the quantum state of the ensemble of atoms. After 30 microseconds, the researchers turned the second pulse back on, and the image was then recovered as it left the vapor cell.

“During the storage time, there are no light fields in the experimental system,” Shuker explained. “All the information carried by the light (in our case the 3D intensity and phase pattern) is converted to the quantum state of the atoms in the vapor (specifically, the coherence between the sub-levels of the ground state). If it was easy to detect the coherence level of the quantum state of the atoms, we would notice that an effective ‘image’ exists – but this is not easily performed (maybe the easiest way is to convert it back to light – as we do in the restoring stage of the experiment).”

Due to the diffusion of the gas atoms, the recovered images looked somewhat blurry and had a decreased signal-to-noise ratio. To improve the image resolution, the researchers developed a technique to minimize image degradation caused by the movement of the atoms. The technique is similar to the phase-shift lithography technique used to reduce optical spreading, where the phases of neighboring image features are flipped so that light between them will interfere destructively. The researchers shifted the phases of image features by 180 degrees, so that atoms of opposite phases that diffused to the areas between lines in the image had amplitudes that cancelled, and no light was emitted that blurred the image lines.

Storing images in vapor – or, as the researchers describe, “converting optical information to atomic coherence” – could be useful for various image processing and correlation applications, as well as quantum information processing and even quantum communication. The scientists also predict that it should be possible to store more elaborate images, including temporal images, or movies.

“The storage-of-light technique (generally, not only images) might have important applications in future quantum information devices,” Shuker said. “The most ‘straight-forward’ application is a ‘memory device’ for the basic information unit of quantum information – the qubit.

“Furthermore, the ability to convert quantum information from one representation (a light pulse) to another (atomic coherence) might prove very useful, since each of them has its unique benefits. Photons are excellent carriers of information, and atomic coherence is a good place to store the information – and maybe even process it, since atoms interact with the environment much better than photons.”

Ultrafast look into atoms and molecules

Generation of the attosecond pulses. Physicists of the Max-Planck-Institute for Quantum Optics (Prof. Ferenc Krausz) produced light pulses lasting just approx. 80 attoseconds with ultrashort laser flashes. The laser pulses are focused on a neon gas target streaming out of a thin tube. The intense (ionizing) laser field induces electron oscillations in the neon atoms, which emit attosecond pulses of extreme ultraviolet light. Image: Thorsten Naeser

New record in ultrafast metrology: Physicists at Max-Planck Institute of Quantum Optics and the Ludwig-Maximilians-University Munich are the first to produce light pulses lasting only 80 attoseconds.

To observe the motion of electrons in atoms one has to be fast. The speed needed has once again been achieved by a team of physicists of the "Munich-Centre for Advanced Photonics" (MAP). In cooperation with their colleagues at the Advanced Light Source in Berkeley (USA) researchers of the team of Professor Ferenc Krausz at Max-Planck Institute of Quantum Optics (MPQ) in Garching and Ludwig-Maximilians-University of Munich (LMU) and Prof. Ulf Kleineberg at LMU have produced the first light pulses lasting just approx. 80 attoseconds with ultrashort laser flashes. An attosecond is a billionth of a billionth of a second. This is the first time that scientists have advanced metrology into the temporal range below 100 attoseconds. This affords access to real-time observation of the fastest electron motions inside atoms, molecules and solids. Insight into electron processes can lead to the development of new light sources, exploration of the microscopic origin of serious illnesses, or gradual advancement of electronic data processing towards the ultimate limits of electronics (Science, 20 June 2008).

In the microcosm, electrons move with awesome rapidity. Within just a few attoseconds the particles in atoms jump between adjacent atoms in a molecule or solid from one location to another. These jumps are the cause of light being emitted in the region of the visible, ultraviolet or X-ray spectrum. They are also responsible for deformation and resulting malfunctioning of biomolecules, or transmission of biological information in the nerves. Observation of such jumps calls for technologies allowing such short times to be measured. This is done with light pulses.

To generate attosecond pulses the Garching physicists use the strong electric field of flashes in the near, infrared laser light. In the hyper short laser flashes this field performs hardly more than a single strong oscillation with a period of approx. 2.5 femtoseconds (a femtosecond is 1000 attoseconds). That is: the light wave now comprises just two high wave peaks and a deep wave valley between them. The force exerted by the electric light field on the electrons is strongest at the summits and the lowest point of the valley; strong enough to liberate electrons which are ejected from rare-gas atoms in the experiment at Garching. This leaves ion rumps. With the oscillation of the light field the force changes direction and very soon hurls the electrons back to the ion rumps. The re-colliding free electrons induce extremely fast electron oscillations which last just attoseconds and thereby emit light flashes of the same duration. These flashes are then in the region of extreme ultraviolet light (XUV, approx. 10 to 20 nanometers wavelength).
Controlled production of this single strong light oscillation within a hyper short flash has now allowed the Garching research team for the first time to release electrons exactly three times during a single laser pulse. On returning to the ion they then emit exactly three attosecond pulses. Each femtosecond laser flash thus generates three attosecond pulses. One of these pulses has a particularly high intensity, providing more than 100 million photons in a period of just 80 attoseconds. This pulse is filtered out with special x-ray mirrors from Prof. Ulf Kleineberg, resulting in a single isolated x-ray pulse of 80 attosecond duration. The new generation of attosecond pulses, with unprecedented brevity and intensity, owes its existence to the physicists’ ability to use their laser flashes to limit ionisation (release of electrons) to a single oscillation period of the laser light.

With their experiments the Garching physicists are advancing measurement technology into hitherto unknown time dimensions. "Pulses shorter than 100 attoseconds will provide access to hitherto unresolved electron dynamics, particularly electron-electron interactions in real time", states Dr. Eleftherios Goulielmakis, who is head of the team, conducting the experiments in Prof. Krausz’ research group.

"Electrons are omnipresent in vital microscopic processes just as in technology. Their ultrafast motion governs the course of all biological and chemical processes, as well as the speed of the microprocessors constituting the core of computers", explains Ferenc Krausz. Many of these processes, such as the energy transfer between electrons or the reaction of particles to external influences, can take place within just a few attoseconds. "With our light pulses we are making these phenomena more clearly visible", adds the Austro-Hungarian physicist. Just as with high speed-photography, the images from the microcosm are made all the sharper, the shorter the light pulses with which they are flashed. "By means of attosecond technology we shall one day be able to observe in real time how the microscopic motion of electrons in molecules initiates diseases such as for example, cancer. We shall likewise be able to switch electric current in atomic circuits with infrared light many trillionth times per second", says Ferenc Krausz.

Source: Max Planck Institute of Quantum Optics

Carbon Nanotubes Compromise the Functions of Certain Protozoa, Study Shows

By Laura Mgrdichian

A Tetrahymena thermophila culture exposed to a moderate concentration of single-walled carbon nanotubes. The image shows the nanotubes (red) both outside and within the protozoa.

A new study by researchers from the University of Waterloo in Ontario, Canada, hints that carbon nanotubes may be toxic to microorganisms. When cultures of a certain key protozoan, a single-cell organism, were exposed to the nanotubes their ability to ingest and digest bacteria was hindered.

The protozoan studied, Tetrahymena thermophila, lives in water, propelling itself using many arm-like cilia. The group it belongs to, the “grazing protists,” are ecologically important because they are active in water ecology at many levels.

Tetrahymena thermophila helps regulate microbial populations by ingesting and digesting bacteria. It is also an important organism in wastewater treatment and is an indicator of the quality of treated wastewater. For these reasons, it is often studied by ecotoxicologists.

When the University of Waterloo group exposed Tetrahymena thermophila to single-walled carbon nanotubes (SWNTs), the researchers found that the protozoa unnaturally clumped together initially and then ingested SWNTs and bacteria alike. One troubling effect of this, if such exposure ever occurred in the wild, is that the nanotubes could move up the food chain. In addition, because the protozoa's ability to ingest and digest their prey bacteria species is compromised, certain bacteria populations could balloon. This could have untold ecological effects.

“There is a pressing need for research into the health and environmental impact of nanoparticles,” said Xiaowu (Shirley) Tang, the study's corresponding researcher, to PhysOrg.com. Tang is an assistant professor in the University of Waterloo Department of Chemistry. “But although the importance of grazing protists to the environment and public health is well known, few reports can be found on exposure of such organisms to carbon nanotubes.”

Part of the reason for the lack of studied on carbon nanotubes effects on microorganisms is that scientists generally believe that the nanotubes are insoluble in water. However, at least one recent study challenges this belief.

With that study in mind, Tang and her colleagues exposed Tetrahymena thermophila cultures to different concentrations of nanotubes in solution and monitored them for three days using video microscopy. Besides clearly showing that the protozoa ingested the nanotubes, the video revealed that the control cultures remained healthy while the nanotube-exposed cultures exhibited various negative responses depending on the concentration. These ranged from diminished mobility to death, with the most prevalent effect being cell clumping.

“We hope that our work will stimulate a line of research towards better understanding of the effects of nanomaterials on diverse organisms, especially on single-cell organisms that are ecological important,” says Tang.

The researchers do note one potential positive effect of Tetrahymena thermophila nanotube uptake that could make controlled exposure useful in wastewater treatment: The protozoa released extra “exudates,” fluids rich in proteins and cellular debris, which help solidify impurities in the wastewater. This, in turn, could make the protozoa more efficient water-cleaners.

Chemists Create Cancer-Detecting Nanoparticles

Magnetic resonance imaging (MRI) can be a doctor’s best friend for detecting a tumor in the body without resorting to surgery. MRI scans use pulses of magnetic waves and gauge the return signals to identify different types of tissue in the body, distinguishing bone from muscle, fluids from solids, and so on.

Scientists have found that magnetic nanoparticles can be especially helpful in locating cancerous cell clusters during MRI scans. Like tiny guide missiles, the nanoparticles seek out tumor cells and attach themselves to them. Once the nanoparticles bind themselves to these cancer cells, the particles operate like radio transmitters, greatly aiding the MRI’s detection capability.

Now, a team of researchers led by Shouheng Sun, Ph.D., of Brown University, and Xiaoyuan Chen, Ph.D., a member of the Stanford University Center for Cancer Nanotechnology Excellence Focused on Therapy Response, have created the smallest magnetic nanoparticles to date that can be employed on such seek-and-find missions. With a thinner coating, the particles also emit a stronger signal for the MRI to detect. Their work appears in the Journal of the American Chemical Society.

The team created iron oxide nanoparticles coated with a peptide-based targeting agent. The researchers injected the particles into mice and tested their ability to locate a brain tumor cell called U87MG. The investigators concentrated specifically on the nanoparticle’s size and the thickness of the peptide coating, which ensures that the nanoparticle attaches to the tumor cell.

Size is important because the trick is to create a nanoparticle that is small enough to navigate through the bloodstream and reach the diseased area. Bigger particles tend to stack up, creating the circulatory system’s version of a traffic jam. The investigators developed a nanoparticle that is about 8.4 nanometers in overall diameter—some six times smaller than the size of particles currently used in medicine.

The coating, while integral to the nanoparticles’ attachment to the tumor cell, also is crucial to establishing the “signal-to-noise” ratio that a MRI uses. The thinner the coating, the stronger the emitted signal and vice versa. The research team outfitted its nanoparticles with a 2-nanometer thick peptide coating—10 times thinner than the coating available in popular MRI contrast agents such as Feridex.

Another important feature of the team’s work is discovering that the RGD peptide coating binds almost seamlessly to the U87MG tumor cell. The team plans to test the particle’s ability to bind with other tumor cells in additional animal experiments.

'Nanoglassblowing' Seen as Boon to Study of Individual Molecules

Left: Schematic of a T-junction nanofluidic device with a "nanoglassblown" funnel-shaped entrance to a nanochannel. The funnel tapers down to 150 micrometers (about the diameter of a human hair) at the nanochannel entrance. Right: Photomicrograph of the T-junction with the first section of the nanochannel visible at the bottom. The colors are a white light interference pattern caused by the changing depth of the curved glass funnel. Credit: Elizabeth Strychalski, Cornell University

While the results may not rival the artistry of glassblowers in Europe and Latin America, researchers at the National Institute of Standards and Technology and Cornell University have found beauty in a new fabrication technique called “nanoglassblowing” that creates nanoscale (billionth of a meter) fluidic devices used to isolate and study single molecules in solution—including individual DNA strands. The novel method is described in a paper posted online this week in the journal Nanotechnology.

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Traditionally, glass micro- and nanofluidic devices are fabricated by etching tiny channels into a glass wafer with the same lithographic procedures used to manufacture circuit patterns on semiconductor computer chips. The planar (flat-edged) rectangular canals are topped with a glass cover that is annealed (heated until it bonds permanently) into place.

About a year ago, the authors of the Nanotechnology paper observed that in some cases, the heat of the annealing furnace caused air trapped in the channel to expand the glass cover into a curved shape, much like glassblowers use heated air to add roundness to their work. The researchers looked for ways to exploit this phenomenon and learned that they could easily control the amount of “blowing out” that occurred over several orders of magnitude.

As a result, the researchers were able to create devices with “funnels” many micrometers wide and about a micrometer deep that tapered down to nanochannels with depths as shallow as 7 nanometers—approximately 1,000 times smaller in diameter than a red blood cell. The nanoglassblown chambers soon showed distinct advantages over their planar predecessors.

“In the past, for example, it was difficult to get single strands of DNA into a nanofluidic device for study because DNA in solution balls up and tends to bounce off the sharp edges of planar channels with depths smaller than the ball,” says Cornell’s Elizabeth Strychalski. “The gradually dwindling size of the funnel-shaped entrance to our channel stretches the DNA out as it flows in with less resistance, making it easier to assess the properties of the DNA,” adds NIST’s Samuel Stavis.

Future nanoglassblown devices, the researchers say, could be fabricated to help sort DNA strands of different sizes or as part of a device to identify the base-pair components of single strands. Other potential applications of the technique include the manufacture of optofluidic elements—lenses or waveguides that could change how light is moved around a microchip—and rounded chambers in which single cells could be confined and held for culturing.

This work was supported in part by Cornell’s Nanobiotechnology Center, part of the National Science Foundation’s Science and Technology Center Program. It was performed while Samuel Stavis held a National Research Council Research Associateship Award at NIST.

Citation: E.A. Strychalski, S.M. Stavis and H.G. Craighead. Non-planar nanofluidic devices for single molecule analysis fabricated using nanoglassblowing. Nanotechnology, Posted online the week of June 8, 2008.

Carbon Nanotubes as a Single-Photon Source

By Lisa Zyga

This atomic force microscope image shows a single-walled CNT with a height of 0.8 nm and length of 800 nm. Researchers found that, under low temperatures, CNTs emit one photon at a time, marking the first demonstration of non-classical optical emission from a CNT. Credit: Högele, Alexander, et al.

Carbon nanotubes, as true multi-purpose materials, have potential applications in everything from electrical circuits and drug delivery to golf clubs and space elevators. Recently, physicists have investigated single-walled carbon nanotubes (CNTs) for one more use: as a single-photon source, where they could help make quantum communication networks extremely secure and efficient.

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Over the past few years, researchers have identified several different systems that can act as single-photon sources. All these systems have a common feature: they confine the motion of charged particles to a very small space in all three dimensions. Physicists describe such systems with restricted particle motion as “quasi-zero-dimensional.”

The corresponding phase-space of localized particles is made into discrete areas due to the laws of quantum mechanics. The Pauli Exclusion Principle prevents two particles from occupying identical quantum states, so that only one particle at a time can undergo a transition between two distinct states. The principle inhibits simultaneous two-photon generation, and ensures that the system generates only a single photon.

Physical systems such as atoms, ions, molecules, and quantum dots are quasi-zero-dimensional and operate as single-photon sources. In the new study, the researchers show that the same principle applies to CNTs – despite the fact that nanotubes are spatially extended in one dimension.

“The fact that carbon nanotubes are nearly-perfect single photon emitters is surprising,” lead author Alexander Högele told PhysOrg.com. “Single photon emission is characteristic of systems with quantum confinement in all spatial dimensions. Carbon nanotubes, however, are axially extended and represent a one-dimensional model system.”

The single-walled CNTs that the team studied had diameters of just 0.8 nanometers, along with an average length of about 500 nanometers. The researchers used a laser to excite individual CNTs at temperatures as low as 4.2 K, and caused them to emit light with a wavelength of around 880 nanometers.

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At first, the researchers observed that individual CNTs emitted photons at a delay of a few nanoseconds, consistent with the repetition rate in the laser pulse train. The researchers then investigated statistic correlations between such consecutively emitted photons. Significantly, they did not detect any photon correlation at zero time delay (t=0), indicating strong photon antibunching.

“A photon correlation experiment measures the probability of arrival of two photons with a given time delay,” Högele explained. “When two photons travel together, an ideal photon correlation setup would detect with a probability of one the arrival of one photon upon the arrival of another. When one of the two traveling photons is delayed, then the photons will arrive consecutively at the detector and the probability of simultaneous photon detection is zero.

“The meaning of the vanishing peak at zero time delay is that photons ‘avoid’ each other, and that it is impossible to find a pulse that contains two or more photons. It has been shown previously that such photon statistics cannot be explained using classical Maxwell's equations; a quantum description of light is necessary to understand the photon antibunching phenomenon.”

The researchers found that two different mechanisms ensure the antibunching: Augur processes and low temperatures that suppress the motion of charged particles along the nanotube axis. Together, these two mechanisms lead to a two-photon emission probability as low as 3% at low temperatures.

“We were quite puzzled by our first measurement that showed strong antibunching in the photo-emission of single carbon nanotubes,” said Högele. “In consecutive experiments, we identified two key mechanisms that ensure single-photon generation: first, non-linear Auger processes [which cause one out of two pairs of electrons and electron holes to annihilate non-radiatively] play an important role. Second, electron-hole pairs in carbon nanotubes appear to be strongly localized at low-temperatures by trapping centers. The result is a carbon nanotube quantum dot with an anharmonic spectrum.”

The researchers hope that these observations could lead to the development of new single-photon sources for applications such as long-distance quantum communication and quantum cryptography.

“The potential advantage of using CNTs as single-photon sources is the fact that their emission wavelength can be tuned into the optical communication wavelength window,” said Högele. “The emission wavelength of single-walled carbon nanotubes depends on the tube diameter and is tunable by growth in the range between around 1 and 2 microns.”