Generalized Snell's law

http://www.sciencemag.org/content/334/6054/333.full

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

Study blames plasma flow for quiet sun

Our sun is maintained by converting gravitational energy into electromagnetic energy (e.g., visible and invisible radiations). The temperature is so high that the solar matter exists only in gas of light ions and electrons. These are charged particles. Their motions may take the form of the so-called 'plasma', which has these charges moving in a coherent and collective way, appearing as waves. The dynamics are quite nonlinear and intricate. Associated with such plasma, the electromagnetic fields will also change, and have been well known following a very regular pattern. One way to notice this pattern is by counting the intensity of the so-called sunspots, which are caused by pop-ups of strong magnetic energy flow from interior to the surface, like a cork decorated with flares carrying a huge amount of energy that may partly reach the earth. The latest solar cycle peaked in 2001 and was supposed to end in 2008. However, there is something unusual with this cycle: the sun appears too sluggish, with an extra number of spotless days. Now researchers came up with an explanation, which seems under debate [http://www.wired.com/wiredscience/2011/03/spotless-sun-model/]:

Now, Dibyendu Nandy of the Indian Institute of Science Education and Research and colleagues offer an explanation: A “conveyor belt” of plasma inside the sun ran quickly at first and then slowed down.

Nandy and colleagues at Montana State University and the Harvard-Smithsonian Center for Astrophysics ran a computer simulation of magnetic flow inside the sun for 210 sunspot cycles. They randomly varied the speed of plasma flow around a loop called the meridional circulation, which carries magnetic fields from the sun’s interior to its surface and from the equator to the poles.

Observations suggest that the fastest flow runs around 22 meters per second (49 miles per hour). Nandy’s model looked at speeds between 15 and 30 meters per second (33 to 67 miles per hour).

The model found that a fast flow followed by a slow flow reproduced both the weak magnetic field and the dearth of sunspots observed in the last solar minimum.

....

Unfortunately, observations of the sun’s surface seem to directly contradict the new model.

“We’re in this quandary, this clash between theory and observations,” said NASA astronomer David Hathaway, who analyzed 13 years of data from the Solar and Heliospheric Observatory (SOHO) that tracked the movement of charged material near the surface of the sun.

Hathaway agrees that a fast flow can cause weak magnetic fields and fewer sunspots. But his observations, published March 12, 2010 in Science, suggest that the meridional flow was slow in the first half of the last solar cycle, from about 1996 to 2000. Only after the solar maximum did the flow speed up.

“That’s where there’s a problem,” Hathaway said. “We see one thing, they want the opposite to explain the observations.”

Nandy and colleagues point out that the SOHO observations only see plasma moving at the surface of the sun, not in the deep interior where sunspots are born. The surface flows might not reflect what’s going on underneath, he says.

“In an analogy that you might be able to relate to, one could ask, do ripples on the surface of the sea indicate how ocean currents determine the migration of aquatic animals deeper inside?” Nandy said.

Hathaway argues that changes in the surface should be transmitted to the interior at the speed of sound, and should reach the creation zone in half an hour or less. The disagreement between theory and data means there must be a problem with the models, he says.

Producing fake solar flares in lab

Here is a synopsis of this experiment:

Solar flares are caused by eruptions of the magnetic field near the sun’s surface that send out a blast of charged particles and intense electromagnetic radiation. The flares occasionally interfere with radio communication and electrical lines, but also play a role in the creation of the beautiful aurorae (the northern and southern lights).

Writing in Physical Review Letters, Shreekrishna Tripathi and Walter Gekelman, at the University of California, Los Angeles, US, describe their efforts to understand certain types of magnetic flux eruptions in the solar atmosphere by creating and imaging similar bursts in a laboratory-scale plasma chamber.

Tripathi and Gekelman focus on reproducing what are called “arched magnetic flux ropes,” literal arcs of magnetic flux on the sun’s surface that keep plasma confined for up to days at a time, before erupting. Within the confines of a 4-m-long cylindrical chamber that contains an ambient plasma, they create an arched magnetic field (using two electromagnets) and generate a second plasma that is confined by this magnetic field. The arched magnetic field and the plasma it confines remain stable until two lasers ablate carbon targets near each of the arc’s feet, sending two jetlike blasts of positively charged carbon—roughly 800 amperes—into the flux rope. The current produces its own magnetic field, creating a destabilizing kink in the flux rope that causes it to erupt with a wave of energy.

Tripathi and Gekelman’s images of the outward wave of plasma following the eruption provide a rare, albeit scaled down, glimpse of how such solar events evolve in time. – Jessica Thomas

Plasmon enhanced microalgal growth

Plasmon is the quantum of collective motions of mobile electrons. They interact strongly with light at certain wavelengths. The properties of the plasmon of nano particles can be varied by changing the size the particle. Now it was used to enhance the backscattering of light so as to promote the growth of microalgals.

Photoactivity of green microalgae is nonmonotonic across the electromagnetic spectrum. Experiments on Chlamydomonas reinhardtii green alga and Cyanothece 51142 green-blue alga show that wavelength specific backscattering in the blue region of the spectrum from Ag nanoparticles, caused by localized surface plasmon resonance, can promote algal growth by more than 30%. The wavelength and light flux of the backscattered field can be controlled by varying the geometric features and/or concentration of the nanoparticles. © 2010 American Institute of Physics.
doi:10.1063/1.3467263

An opaque fishing net ?

Light, because of its wave-corpulse duality, shows many surprising behaviors. A recent PRL paper [1] adds one more. Imagine you fabricate a gold film on a glass substrate and punch a regular array of sub-wavelength holes in this film. Now you shine light upon it. It is so thin--about 20 nm--that it becomes semi-transparent. Now you look at the transmission, which unexpectedly turns out being smaller than without holes. That is, the brutalized film, contrary to expectations, makes an obacure view. Nevertheless, if the film is much thicker, say, 100nm thick, the scenario will be the opposite: the transmission is greatly increased [2].

The authors think that, Fano analysis may lend an explanation. According to them, there are two interfering wavelets contributing to the transmitted waves, which are the resonantly scattered and the nonresonantly scattered, respectively. The former involves resonant excitations of surface plasmon, whereas the latter enters directly through holes. It turns out that, Fano resonance hinges on a single parameter, which is the quotient of a ratio to the line width (which measures how coherent a light is). The ratio concerns the resonant wave amplitude and the directly transmited amplitude. They argue that, this parameter is large for thick films but rather small for extremely thin films, which may then give rise to the observations.

It is worth seeing that, the surface plasma may play a central role. The interaction between light and plasma is obviously an interesting subject. This interaction can carry light to pass through very small holes. The light is at first coupled to the plasma and then the plasma carries it to the destination [2].

[1]Phys. Rev. Lett. 103, 203901 (2009)

[2]Nature (London) 391, 667 (1998)

Wield light

Here is a fascinating work on counter-intuitive facets of light [1].

If a metal film, thick enough to be totally opaque, is perforated by tiny subwavelength holes in an orderly fashion, the transmission will be enhanced extraordinarily [2]. Here, we investigate the transmission through an ultrathin semitransparent Au film with a square array of subwavelength holes and observe the opposite behavior: less light is transmitted through the pierced metal compared to the closed film.

These authors blame surface plasma for the light block, although not every one thinks so. Here is a comment.

The way light moves, with its fixed speed and its ability to act like either a wave or a particle, often leads to some of the most curious paradoxes of physics. A new one has just been found: Make holes in a film of gold so thin that it's already semitransparent, and less light gets through.

Because of its wave nature, light generally can't squeeze through a hole whose width is smaller than the wavelength of the light. In 1998, however, researchers discovered that light could zip through certain patterns of such holes punched into thin metal plates. Physicists figured out that the light created waves in the metal's electrons--called plasmons--that move across the material's surface in much the same way that ripples move through water. The plasmons, which have wavelengths much shorter than light, couple with each other across the tiny holes and pull the light along for the ride. One possible application is to use plasmons to build better light-based integrated circuits that would be as fast as fiber optics but less bulky.

Toward this end, researchers from the University of Stuttgart in Germany laid very thin films of gold onto pieces of glass and then used ion beams to etch the film with holes arranged in a regular, square array. These holes were smaller than the wavelength of light and, despite being so tiny, are just the kind of openings that have been shown to let light through the thicker, opaque film used in the 1998 experiment. But in the new experiment, the gold film was so thin--only 20 nanometers--that light could already shine through it. And surprisingly, less light went through the holey gold than through the original semitransparent film.

Why? The researchers blame the semitransparent nature of the gold film, which allows 40% of the light to flow directly through it, preventing it from stopping at the surface to help form plasmons. Plasmons are formed by the kick of energy they get from the incoming light, combined with how the electron waves of the plasmons skitter around the hole geometry, so the light needs to be tuned to the specific hole geometry to maximize plasmons. In this case, that leftover 60% of the light simply doesn't combine with the geometry to create plasmons that can cross through the gold holes, the team reports this week in Physical Review Letters.

Physicist Martin P. van Exter of Leiden University in the Netherlands says that interference between hole geometry and light transmission is expected, so the results shouldn't come as too much of a surprise. However, he also points out that gold always absorbs light in peculiar ways--indeed, this is what leads to its golden color instead of most metals' more typical silver--and it's possible that this contributed to the results.

Team member Bruno Gompf says that the next step is to see whether other hole patterns--hexagonal, rectangular, aperiodic--show the same effect. Perhaps a particular pattern could serve as a filter to block certain wavelengths of light in those future plasmonic integrated chips, he says.

The origin of this seems still elusive.

[2]Nature (London) 391, 667 (1998)
[1]Phys. Rev. Lett. 103, 203901 (2009)