Scientists slow down the speed of light travelling in free space

Scientists have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass.

However, it has generally been thought impossible for particles of light, known as photons, to be slowed as they travel through free space, unimpeded by interactions with any materials.

In a new paper published in Science Express today (Friday 23 January), researchers from the University of Glasgow and Heriot-Watt University describe how they have managed to slow photons in free space for the first time. They have demonstrated that applying a mask to an optical beam to give photons a spatial structure can reduce their speed.

The team compare a beam of light, containing many photons, to a team of cyclists who share the work by taking it in turns to cycle at the front. Although the group travels along the road as a unit, the speed of individual cyclists can vary as they swap position.

The group formation can make it difficult to define a single velocity for all cyclists, and the same applies to light. A single pulse of light contains many photons, and scientists know that light pulses are characterised by a number of different velocities.

The team's experiment was configured like a time trial race, with two photons released simultaneously across identical distances towards a defined finish line. The researchers found that one photon reached the finish line as predicted, but the structured photon which had been reshaped by the mask arrived later, meaning it was travelling more slowly in free space. Over a distance of one metre, the team measured a slowing of up to 20 wavelengths, many times greater than the measurement precision.

The work demonstrates that, after passing the light beam through a mask, photons move more slowly through space. Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material -- it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.

The work was carried out by a team from the University of Glasgow's Optics Group, led by Professor Miles Padgett, working with theoretical physicists led by Stephen Barnett, and in partnership with Daniele Faccio from Heriot-Watt University.

Daniel Giovannini, one of the lead authors of the paper, said: "The delay we've introduced to the structured beam is small, measured at several micrometres over a propagation distance of one metre, but it is significant. We've measured similar effects in two different types of beams known as Bessel beams and Gaussian beams."

Co-lead author Jacquiline Romero said: "We've achieved this slowing effect with some subtle but widely-known optical principles. This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum.

"Although we measure the effect for a single photon, it applies to bright light beams too. The effect is biggest when the lenses used to create the beam are large and when the distance over which the light is focused is small, meaning the effect only applies at short range."

Professor Padgett added: "It might seem surprising that light can be made to travel more slowly like this, but the effect has a solid theoretical foundation and we're confident that our observations are correct.

"The results give us a new way to think about the properties of light and we're keen to continue exploring the potential of this discovery in future applications. We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example."

The team's paper, titled 'Spatially Structured Photons that Travel in Free Space Slower than the Speed of Light', is published in Science Express, which provides electronic publication of selected papers in advance of print in the journal Science.

Selengkapnya »»  

Breakthrough lights up metamaterials

This is a schematic of active metamaterial which shows enhanced light emission and extraction.

A City College of New York led-team has successfully demonstrated how to both enhance light emission and capture light from metamaterials embedded with light emitting nanocrystals. The breakthrough, headed by physicist Dr. Vinod Menon, could lead to a range of applications including ultrafast LEDs, nanoscale lasers and efficient single photon sources.

In the demonstration, the team used metamaterials having hyperbolic dispersion to enhance the light emission properties of the nanocrystals and simultaneously engineered an efficient light extraction scheme.

"The idea of metamaterials in the context of optics is that you can manipulate light and decide how you want it to behave in this medium," said Professor Menon a photonics expert whose specialty includes control of light-matter interaction at the nanoscale.

While enhancement in light emission from such systems was shown by different groups, including Dr. Menon's, in the past, they were not useful since light did not come out easily from them, making their practical application an issue. The present work alleviates this issue and takes the first step towards developing practical light emitters based on metamaterials.

"We've shown both an increase in light emission and were able to extract light." added Professor Menon, whose team included City College PhD students Tal Galfsky and H.N.S. Krishnamoothy

Also part of the research team were scientists from the University of Alberta (Canada), Purdue University in Indiana. Part of the research was also carried out at the Center for Functional Nanomaterials at Brookhaven National Laboratory.

The paper appears in the latest issue of Optica.

Selengkapnya »»  

New technology enables ultra-fast steering and shaping of light beams

New optical beam-forming device making 'twisted light.'

A team of engineers has developed a new acousto-optic device that can shape and steer beams of light at speeds never before achieved. The new technology will enable better optical devices to be made, such as holographs that can move rapidly in real time.

The research led by Bruce Drinkwater, Professor of Ultrasonics at the University of Bristol and Dr Mike MacDonald at the University of Dundee is published in the journal, Optics Express.

The array consists of 64 tiny piezo-electric elements which act as high frequency loudspeakers. The complex sound field generated deflects and sculpts any light passing through the new device. As the sound field changes, so does the shape of the light beam.

Professor Drinkwater from the Department of Mechanical Engineering said: "This reconfigurability can happen extremely fast, limited only by the speed of the sound waves. The key advantage of this method is that it potentially offers very high refresh rates -- millions of refreshes per second is now possible. This means that in the future laser beam-based devices will be able to be reconfigured much faster than is currently possible. Previously, the fastest achieved is a few thousand refreshes per second."

The advancement will enable reconfigurable lenses that can automatically compensate for aberrations allowing for improved microscopy and a new generation of optical tweezers that will make them more rapidly reconfigurable and so allow better shaped traps to be produced.

Dr Mike MacDonald, Head of the Biophotonics research group at the University of Dundee, explained: "What we have shown can be thought of as a form of optical holography where the hologram can be made in real time using sound. Previous attempts to do this have not had the level of sophistication that we have achieved in the control of our acoustic fields, which has given us much greater flexibility in the control we have over light with these devices.

"The device can potentially be addressed much more quickly than existing holographic devices, such as spatial light modulators, and will also allow for much higher laser powers to be used. This opens up applications such as beam shaping in laser processing of materials, or even fast and high power control of light beams for free space optical communications using orbital angular momentum to increase signal bandwidth, as shown recently by a demonstration in Vienna."

Professor Drinkwater added: "The number of applications of this new technology is vast. Optical devices are everywhere and are used for displays, communications as well as scientific instruments."

The capabilities of laser beam shaping and steering are crucial for many optical applications, such as optical manipulation and aberration correction in microscopy. Depending on specific requirements of each application, these capabilities are currently achieved using different methods which are based on establishing a certain level of control over the phase of the laser beam. Deformable mirrors are used for aberration corrections in astronomy and spatial light modulators (SLMs) are the common choice in a wide range of applications such as holography, optical tweezers and microscopy.

Selengkapnya »»  

Atoms queue up for quantum computer networks

The experiment is carried out in a glass cell with very low pressure. In here is an ultra-thin glass fiber and a gas of cesium atoms. Using lasers and a magnetic field, the atoms are cooled down to almost absolute zero (minus 273 degrees Celsius) and the atoms gather as a cloud around the glass fiber. Then two laser beams with very different frequencies are transmitted into the fiber, thereby capturing atoms above the fiber surface. By measuring the difference in the speed of light for two other light beams on each side of the atoms' absorption line, you can measure the number of atoms.

In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute have developed a method with a trap that captures the atoms along an ultra thin glass fiber, where the atoms can be controlled. The results are published in the scientific journal, Physical Review Letters.

The research is carried out in the quantum optics laboratory in the basement of the Niels Bohr Institute in Copenhagen. The underground laboratory is set back from the road so there are no vibrations from traffic. Here, the researchers have designed experiments in which they can perform ultrasensitive trials with quantum optics.

"We have an ultra-thin glass fiber with a diameter of half a micrometer (a hundred times smaller than a strand of hair). Along this glass fiber we capture cesium atoms. They are cooled down to 100 micro Kelvin using a laser -- this is almost absolute zero, which is equivalent to minus 273 degrees Celsius. This system acts like a trap that holds the atoms on the side of the glass fiber," explains Jürgen Appel, Associate Professor in the research group Quantop at the Niels Bohr Institute, University of Copenhagen.

Atoms and light linked together

When light is transmitted through the glass fiber thread, the light will also move along the surface because the fiber is thinner than wavelength of the light. This creates strong interaction between the light and the atoms sitting securely above the surface of the fiber.

"We have developed a method where we can measure the number of atoms. We send two laser beams with different frequencies through the glass fiber. If there were no atoms on the fiber, the speed of light would be the same for both light beams. However, the atoms affect the two frequencies differently and by measuring the difference in the speed of light for the two light beams on each side of the atoms' absorption lines, you can measure the number of atoms along the fiber. We have shown that we can hold 2,500 atoms with an uncertainty of just eight atoms," says Jürgen Appel.

These are fantastic results. Without this method, you would have to use resonant light (light that the atoms absorb) and then you would scatter photons, which would kick the atoms out of the trap, says Jürgen Appel and explains that with this new method they can measure and control the atoms so that only 14 percent are kicked out of the trap and are lost.

"Our resolution is only limited by the natural quantum noise (the laser light's own minimal fluctuations) so our method could be used for so-called entangled states of atoms along the fiber. Such an entangled system with strongly interacting atoms and light is of great interest for future quantum computer networks," notes Jürgen Appel.

Selengkapnya »»  

American cities are many times brighter at night than German counterparts

Berlin at night, seen from the ISS.

German cities emit several times less light per capita than comparably sized American cities, according to a recent publication in the journal Remote Sensing. The size of the gap grew with city size, as light per capita increased with city size in the USA but decreased with city size in Germany. The study also examined regional differences, and surprisingly found that light emission per capita was higher in cities in the former East of Germany than from those in the former West.

The lead author, Dr. Christopher Kyba, studies visible light at night as a member of the Remote Sensing section of the German Research Center for Geosciences (GFZ). "The size of the difference in light emission is surprisingly large. This work will allow us to identify comparable cities in order to uncover the reasons behind the differences." These could include differences in the type of lamps, but also architectural factors like the width of the streets and the amount of trees. The LED lamps currently being installed in many cities are expected to greatly change the nighttime environment, for example by reducing the amount of light that shines upwards.

A main point of the study is to emphasize the great improvement in the quality of nighttime imagery of Earth since 2012. The European Space Agency's NightPod instrument has allowed astronauts to take high resolution images of individual cities. In addition, the entire world is now imaged nightly at 750 meter resolution by the Visible Infrared Imaging Radiometer Suite Day-Night Band onboard the Suomi National Polar-Orbiting Program weather satellite. This new imagery has made it possible to identify and measure the output of individual bright sources of light pollution for the first time. The study found that in Megacities in developing countries, the brightest light sources were typically airports or harbors. In contrast, the brightest areas in the capital cities of Europe are often associated with leisure, for example stadiums and city centers.

While artificial light at night is a problem for astronomers and nocturnal animals, it has the potential to be an important tool in understanding human activity. In order to make the most use out of it, the researchers say they will need to study urban light emissions in detail, including their spectrum, the directions in which light is emitted, and changes in light use and lit area over time.

The study demonstrated one practical use of the new data: since maps of nighttime light emission highlight the areas where light pollution is especially prevalent, they provide information about which areas can best be targeted for energy savings. Coauthor Dr. Franz Hölker from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) explains, "artificial light is responsible for a sizable portion of all nighttime electricity consumption. Identifying areas where light could be more efficiently used will make it possible to save energy, reduce costs, and reduce the impact of artificial light on the nighttime environment."

The study was performed at the Leibniz Institute of Freshwater Ecology and Inland Fisheries, the Free University of Berlin, and the Universidad Complutense de Madrid.

Selengkapnya »»