Optical “Bernoulli” Forces

Light scattering from a rotating dielectric cylinder

Light scattering from a rotating dielectric cylinder

Ramis Movassagh and Steven G. Johnson
By Bernoulli’s law, an increase in the relative speed of a fluid around a body is accompanies by a decrease in the pressure.
Therefore, a rotating body in a fluid stream experiences a force perpendicular to the motion of the fluid because of the unequal relative speed of the fluid across its surface. It is well known that light has a constant speed irrespective of the relative motion.
Does a rotating body immersed in a stream of photons experience a Bernoulli-like force?
We show that, indeed, a rotating dielectric cylinder experiences such a lateral force from an electromagnetic wave.
In fact, the sign of the lateral force is the same as that of the fluid-mechanical analogue as long as the electric susceptibility is positive (ε>ε0), but for negative-susceptibility materials (e.g. metals) we show that the lateral force is in the opposite direction.
Because these results are derived from a classical electromagnetic scattering problem, Mie-resonance enhancements that occur in other scattering phenomena also enhance the lateral force.
Read more at http://arxiv.org/pdf/1305.0317v2.pdf

Reab also: Optical Bernoulli Forces Could Steer Objects Bathed in Light, Say Theorists

Metamaterials and the mathematical Science of invisibility

Principle of cloaking : (left) A point source radiating in an homogeneous isotropic medium; (middle) A point source radiating in an homogeneous isotropic medium in the  presence of an infinite conducting F-shaped scatterer; (right) A point source radiating in  an homogeneous isotropic medium in the presence of an infinite conducting F-shaped  scatterer surrounded by an invisibility cloak (an anisotropic heterogeneous ring)

Principle of cloaking : (left) A point source radiating in an homogeneous isotropic
medium; (middle) A point source radiating in an homogeneous isotropic medium in the
presence of an infinite conducting F-shaped scatterer; (right) A point source radiating in
an homogeneous isotropic medium in the presence of an infinite conducting F-shaped
scatterer surrounded by an invisibility cloak (an anisotropic heterogeneous ring)

Andre Diatta, Sebastien Guenneau, Andre Nicolet, Frederic Zolla
A review of some recent developments in the field of photonics: cloaking, whereby an object becomes invisible to an observer, and mirages, whereby an object looks like another one (say, of a different shape). Such optical illusions are made possible thanks to the advent of metamaterials, which are new kinds of composites designed using the concept of transformational optics. Theoretical concepts introduced here are illustrated by finite element computations…..
Read more: http://arxiv.org/ftp/arxiv/papers/1212/1212.5408.pdf

Invisibility cloaking in ‘perfect’ demonstration

The trick included developing a diamond-shaped cloaking region – invisible only from one direction

By Jason Palmer

Scientists have succeeded in “cloaking” an object perfectly for the first time, rendering a centimetre-scale cylinder invisible to microwaves.

Many “invisibility cloak” efforts have been demonstrated, but all have reflected some of the incident light, making the illusion incomplete.

A Nature Materials study has now shown how to pull off the trick flawlessly.

However, the illusion only works from one direction and would be difficult to achieve with visible light.

The idea of invisibility cloaking got its start in 2006 when John Pendry of Imperial College London and David Schurig and David Smith of Duke University laid out the theory of “transformation optics” in a paper in Science, demonstrating it for the first time using microwaves (much longer wavelengths than we can see) in another Science paper later that year.

The papers sparked a flurry of activity to move the work on to different wavelengths – namely those in which we see.

As the “Where’s my cloak of invisibility?” article points out The field has moved on considerably since then.

But no effort to date has been able to achieve the “perfect” cloaking that the theory originally described.

The structures that can pull off this extraordinary trick of the light are difficult to manufacture, and each attempt has made an approximation to the theoretical idea that results in reflections.

So someone would not see a cloaked object but rather the scene behind it – however, the reflections from the cloak would make that scene appear somewhat darkened.

Now, Prof Smith and his Duke colleague Nathan Landy have taken another tack, reworking how the edges of a microwave cloak line up, ensuring that the light passes around the cloak completely with no reflections.

The trick was to use a diamond-shaped cloak, with properties carefully matched at the diamond’s corners, to shuttle light perfectly around a cylinder 7.5cm in diameter and 1cm tall.

“This to our knowledge is the first cloak that really addresses getting the transformation exactly right to get you that perfect invisibility,” Prof Smith told BBC News.

However, the cloaking game is always one of trade-offs; though the illusion is perfect, it only works in one direction.

“It’s like the card people in Alice in Wonderland,” Prof Smith explained. “If they turn on their sides you can’t see them but they’re obviously visible if you look from the other direction.”

The design principles that make the cloak work in microwaves would be difficult to implement at optical wavelengths. But microwaves are important in many applications, principally telecommunications and radar, and improved versions of cloaking could vastly improve microwave performance.

“The cloak we demonstrated in 2006 as a kind of microwave device would be very poor, but this one gets us to something that could be potentially useful,” Prof Smith said.

“I think it’s something that a lot of people can build on. Everything in this field is going to come down to what you can make, what you can design. And I think this steps up the design.”

Read more: www.bbc.co.uk

A Double Green Flash

Click to Enlarge

At sunset, the sky is often painted with an array of oranges, reds and yellows, and even some shades of pink. There are, however, occasions when a green flash appears above the solar disc for a second or so. One such occurrence was captured beautifully in this picture taken from Cerro Paranal, a 2600-metre-high mountain in the Chilean Atacama Desert, by ESO Photo Ambassador Gianluca Lombardi. Cerro Paranal is home to ESO’s Very Large Telescope.

The green flash is a rather rare phenomenon; seeing such a transient event requires an unobstructed view of the setting (or rising) Sun and a very stable atmosphere. At Paranal the atmospheric conditions are just right for this, making the green flash a relatively common sight (see for example eso0812). But a double green flash such as this one is noteworthy even for Paranal.

The green flash occurs because the Earth’s atmosphere works like a giant prism that bends and disperses the sunlight. This effect is particularly significant at sunrise and sunset when the solar rays go through more of the lower, denser layers of the atmosphere. Shorter wavelength blue and green light from the Sun is bent more than longer wavelength orange and red, so it appears slightly higher in the sky than orange or red rays from the point of view of an observer.

When the Sun is close to the horizon and conditions are just right, a mirage effect related to the temperature gradient in the atmosphere can magnify the dispersion — the separation of colours — and produce the elusive green flash. A blue flash is almost never seen as the blue light is scattered by molecules and particles in the dense blanket of air towards the horizon.

The mirage can also distort the shape of the Sun and that of the flash. We see two bands of green light in this image because the weather conditions created two alternating cold and warm layers of air in the atmosphere.

This stunning photo was taken by ESO Photo Ambassador Gianluca Lombardi on 28 March 2011. The phenomenon was captured on camera as the Sun was setting on a sea of clouds below Cerro Paranal.

Credit: G. Lombardi/ESO

Laser Meets Lightning

On Thursday 18 August, the sky above the Allgäu Public Observatory in southwestern Bavaria was an amazing sight, with the night lit up by two very different phenomena: one an example of advanced technology, and the other of nature’s dramatic power.

As ESO tested the new Wendelstein laser guide star unit by shooting a powerful laser beam into the atmosphere, one of the region’s intense summer thunderstorms was approaching — a very visual demonstration of why ESO’s telescopes are in Chile, and not in Germany. Heavy grey clouds threw down bolts of lightning as Martin Kornmesser, visual artist for the ESO outreach department, took timelapse photographs of the test for ESOcast 34. With purely coincidental timing this photograph was snapped just as lightning flashed, resulting in a breathtaking image that looks like a scene from a science fiction movie. Although the storm was still far from the observatory, the lightning appears to clash with the laser beam in the sky.

Laser guide stars are artificial stars created 90 kilometres up in the Earth’s atmosphere using a laser beam. Measurements of this artificial star can be used to correct for the blurring effect of the atmosphere in astronomical observations — a technique known as adaptive optics. The Wendelstein laser guide star unit is a new design, combining the laser with the small telescope used to launch it in a single modular unit, which can then be placed onto larger telescopes.

The laser in this photograph is a powerful one, with a 20-watt beam, but the power in a bolt of lightning peaks at a trillion (one million million) watts, albeit for just a fraction of a second! Shortly after this picture was taken the storm reached the observatory, forcing operations to close for the night. While we may have the ability to harness advanced technology for devices such as laser guide stars, we are still subject to the forces of nature, not least among them the weather!
http://www.eso.org/public/images/potw1136a/

A Single Atom as a Mirror of an Optical Cavity

a) Single ion+mirror set-up. The probe feld is coupled to the atom-mirror cavity through the dielectric mirror that is mounted on piezo stages.The intensity of the probe is measured in transmission by PMT1 and in reflection by PMT2. PMT3 is used for measuring the ion fluorescence.The main properties of the single atom operated as a mirror are shown in b): positioning, c): central frequency and d):transmission, as measured without the dielectric mirror.


By tightly focussing a laser eld onto a single cold ion trapped in front of a far-distant dielectric mirror, we could observe a quantum electrodynamic e ect whereby the ion behaves as the optical mirror of a Fabry-Perot cavity. We show that the amplitude of the laser eld is signi cantly altered due to a modi cation of the electromagnetic mode structure around the atom in a novel regime in which the laser intensity is already changed by the atom alone. We propose a direct application of this system as a quantum memory for single photons….
Read more: news.sciencemag.orgarxiv.org

This cloud has a rainbow lining

Image: Esther Hevens

The multicoloured halo surrounding the peak of this dark cloud looks heavenly, but it’s actually an iridescent pileus cloud, also called a cap cloud. These smooth, round clouds form on top of a puffy cumulus cloud when it rises into higher, colder air.
The pileus cloud is made up of uniformly sized water droplets that diffract sunlight, creating a rainbow of colours. Usually, the effect is outshone by the brightness of the sun, but in this photo taken in Ethiopia, the darker cloud is helpfully blocking the glare to reveal the spectrum of light behind.
http://www.newscientist.com