Array of Graphene dots absorbs light perfectly

http://physics.aps.org/articles/v5/12

http://prl.aps.org/abstract/PRL/v108/i4/e047401

The results may be used in e.g. solar cells ! How to broaden the absorption spectrum?

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.

BEC on an optical hexagonal lattice

The technology with lasers has incredibly enriched our understanding of a huge width of systems. The ability to prepare honeycomb lattice offers chances to study graphene-type physics and beyond. In this PRL paper, the authors addressed the issues of what would befall a Bose-Einstein condensate moving on a honeycomb lattice. They employed Gross-Pitaviskii equation, which is a mean-field theory for describing superfluids, to compute the band structure and found that, arbitrary interaction would drastically alter the structure around the Dirac points. Is it possible to observe similar stuff using graphene instead of an artificial lattice ? One needs to have superfluid to flow on graphene. The candidate is cooper pairs condensate, which may be created by placing a superconductor in contact with a layer of graphene.

The ability to prepare ultracold atoms in graphenelike hexagonal optical lattices is expanding the types of systems in which Dirac dynamics can be observed. In such cold-atom systems, one could, in principle, study the interplay between superfluidity and Dirac physics. In a paper appearing in Physical Review Letters, Zhu Chen at the Chinese Academy of Sciences and Biao Wu of Peking University use mean-field theory to calculate the Bloch bands of a Bose-Einstein condensate confined to a hexagonal optical lattice.

The Dirac point is a point in the Brillouin zone around which the energy-momentum relation is linear. Its existence in graphene is at the heart of this material’s unusual properties, in which electrons behave as massless particles. Chen and Wu’s study predicts, surprisingly, that in the analog cold-atom system, the topological structure of the Dirac point is drastically modified: intersecting tubelike bands appear around the original Dirac point, giving rise to a set of new Dirac points that form a closed curve. More importantly, this transformation should occur even with an arbitrarily small interaction between the atoms, upending the idea that such topological effects can only occur in the presence of a finite interaction between atoms.

The modified band structure prevents an adiabatic evolution of a state across the Dirac point, violating the usual quantum rule that a system remains in its instantaneous eigenstate if an external perturbation is sufficiently slow. This effect could be tested experimentally in a so-called triple-well structure, which is a combination of rectangular and triangular optical lattices. – Hari Dahal [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.107.065301]

Event Cloak

I just bumped into this funny stuff [http://physicsworld.com/cws/article/indepth/46376]:

But imagine if we could make a cloak that operates not only in space but in time as well. To understand how such a "space–time" cloak might work, consider a bank housing a money-filled safe. Initially, all incoming light continuously scatters off the safe and its surroundings, revealing the rather dull scene of an undisturbed safe visible to surveillance cameras. But imagine, near some specified time, splitting all the light approaching the safe into two parts: "before" and "after", with the "before" part sped up, and the "after" part slowed down. This would create a brief period of darkness in the stream of illuminating photons. If the photons were a stream of cars on a motorway, it is as if the leading cars were to speed up and those trailing behind were to decelerate, creating a gap in the traffic edged by bunches of cars (a dark period with bright edges – see t₃ in figure 1).

Now imagine that during the moment of darkness, a safe-cracker enters the scene and steals the money, being careful to close the safe door before he leaves. With the safe-cracker gone, the process of speeding up and slowing down the light is reversed, leading to an apparently untouched, uniform illumination being reconstituted. As far as the light reaching the surveillance cameras is concerned, everything looks the same as it did beforehand, with the safe door firmly shut. The dark interval when the safe was cracked has literally been edited out of visible history.

To complete our motorway analogy, it is as if the cars have acted to first open up and then close a gap in traffic, leaving no disturbance in the flow of vehicles. There is now no evidence of that temporary car-free interlude, during which the proverbial chicken may even have crossed the road without getting squashed. So by manipulating how light travels in time around a region of space, we can, at least in principle, make a space–time cloak that can conceal events – an "event cloak", if you will.

Light passes through without reflection

This is not an old concept: a light beam may not be reflected if the thickness of the glass it shines upon is carefully chosen so that the reflected wave from the second surface goes out of phase with the one from the first surface. Now that the proper thickness is proportional to the wavelength of the incident light, it is not possible to use a single piece of glass for reflectionless control of light with various colors. But, nature offers much more. One can make a more delicate refractive index profile of medium so that it invisible to a wide spectrum [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.193903].

Abrupt interfaces disrupt wave propagation. For example, light passing from air into a sheet of glass will partially reflect backwards. When the light exits back into air, there is a second reflection that can cancel the first for light of just the right frequency, given the refractive index and thickness of the sheet. The wave nature of electrons creates similar effects when they encounter a region with a changing electrostatic potential. But early in the history of quantum mechanics, theorists realized that certain smoothly varying potential profiles could eliminate the reflection of electrons over a wide range of frequencies.

As it turns out, the same concepts work for light: intense light pulses known as solitons create precisely this kind of profile in the refractive index of the surrounding medium, eventually becoming trapped. Creating permanent versions of such “reflectionless potentials” has, however, proved difficult. In Physical Review Letters, Alexander Szameit of the Technion in Haifa, Israel, and colleagues in Germany and Australia at last implement the lack of light reflection in the laboratory.

In their experiments, a beam of light travels along an array of closely spaced, parallel waveguides created in a glass sample through direct laser-writing. By changing the spacing between some of the waveguides, the researchers construct a stripe along the length of the array that has a different refractive index modulation relative to the rest of the array. For almost any change in spacing, light traveling diagonally across the stripe is partially reflected, as usual. But a stripe having the special variation suggested by theory generates almost no reflection. The technique adds to the bag of tricks that researchers have for manipulating light. – Don Monroe

No physical signal travels faster than c

Einstein's special relativity theory stipulates that no physical signal (i.e., anything that carries energy and obeys physical laws and is measurable) can not go faster than the vacuum light speed. There have been many 'dissidents' (mostly crackpots) don't like this and want to disprove this law, but all have been defied. Now an experiment that was recently done in HKUST demonstrated that, even a single photon cannot break it.

Einstein taught us that the speed of light was the traffic law of the universe—nothing could go faster. The development of media in which atomic gases can slow down or speed up the passage of light pulses initially caused a stir, at least until the difference between phase velocity and group velocity could be carefully explained. But what about the behavior of single photons, the fundamental quanta of light? Reporting in Physical Review Letters, Shanchao Zhang and colleagues at the Hong Kong University of Science and Technology have shown that photons obey the law too.

Zhang et al. study optical precursors, which are signals preceding the main wave packet in a light pulse with a sharply rising leading edge (as in a step function pulse). Past work has shown that even in “superluminal” media where the group velocity may be faster than light speed, the precursor is always in front of the pulse. The authors extend this work to the single-photon level with the help of cold atomic gases: a photon generated in one rubidium gas traverses a second collection of rubidium atoms. With careful use of electromagnetically induced transparency, the researchers can separate the precursor from the main pulse and confirm it travels at the speed of light. The results add to our understanding of how single-photon signals propagate but also confirm the upper bound on how fast information travels. – David Voss [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.243602]

1D is special

1D means enough space but little mobility. One always blocks another even if a bit of interaction gets in the way. And this makes the boundary between bosons and fermions fuzzy. Fermions naturally set hurdles to their compatriots for exclusion principle. While bosons, although without that famous principle, will also demobilize their partners in the presence of strong repulsions. If so, fermions and bosons will resemble each other. This is indeed what happens in this setup proposed in this work [Phys. Rev. Lett. 106, 153601 (2011) ]!

In this work we show that light-matter excitations (polaritons) generated inside a hollow-core onedimensional fiber filled with two types of atoms, can exhibit Luttinger liquid behavior. We first explain how to prepare and drive this quantum-optical system to a strongly interacting regime, described by a bosonic two-component Lieb-Liniger model. Utilizing the connection between strongly interacting bosonic and fermionic systems, we then show how spin-charge separation could be observed by probing the correlations in the polaritons. This is performed by first mapping the polaritons to propagating photon pulses and then measuring the effective photonic spin and charge densities and velocities by analyzing the correlations in the emitted photon spectrum. The necessary regime of interactions is achievable with
current quantum-optical technology.

Spins coupled to a mechanical resonator

Employing the spin-phonon coupling, they argued that, a tiny magnet welded with a torsional mechanical oscillator can be described by a spin-boson model [PRL, 106:147203(2011)]. What interests me is actually this, is it possible to filter spins using magnetoptical coupling schemes ? Especially, what is the implication for the DNA filtering effects that were reported earlier in this blog. See also the review [Phyics, 4:28(2011)].

Cloaks work with a mirror

Cloaks are such inspiring objects that intrigue both physical and atheistic innovative can only be quested by physicists to the end. Now a work from Singapore reported a cloak working with a mirrow: the thing of a 2mm to be hided is placed on a mirror and covered by a cloak made of calcite. One only perceives the mirror but not thing under carpet [PRL 106, 033901 (2011)].

Invisibility cloaks, a subject that usually occurs in science fiction and myths, have attracted wide interest recently because of their possible realization. The biggest challenge to true invisibility is known to be the cloaking of a macroscopic object in the broad range of wavelengths visible to the human eye. Here we experimentally solve this problem by incorporating the principle of transformation optics into a
conventional optical lens fabrication with low-cost materials and simple manufacturing techniques. A transparent cloak made of two pieces of calcite is created. This cloak is able to conceal a macroscopic object with a maximum height of 2 mm, larger than 3500 free-space-wavelength, inside a transparent liquid environment. Its working bandwidth encompassing red, green, and blue light is also demonstrated.

Cloak enough to cover visible objects

A team in UK recently fabricated a cloak that works under green light to make optical delusions [http://arxiv.org/ftp/arxiv/papers/1012/1012.2783.pdf]. This cloak is large and able to cover an object as large as 10mm^3. The physics is governed by Maxwell's equations. The substance is calcite, which has anisotropic optical properties.

Insights of the decade from Science

Now we are coming to the end of not only this year but also the first decade of this century. Science has its list of the insights of this decade in science. In materials physics, the meta-material and the related conformal optics which underlies the operation of these materials are enlisted. The ground breaking papers are as follows: [http://www.sciencemag.org/site/special/insights2010/]

• U. Leonhardt, "Optical Conformal Mapping," Science 312, 1777 (2006).
• J. Pendry et al., "Controlling Electromagnetic Fields," Science 312, 1780 (2006).
• D. Schurig et al., "Metamaterial Electromagnetic Cloak at Microwave Frequencies," Science 314, 977 (2006).
• T. Ergin et al., "Three-Dimensional Invisibility Cloak at Optical Wavelengths," Science 328, 337 (2010).

Photon generates upthrust

I think this is a very funny work [Nature Photon. doi:10.1038/nphoton.2010.266 (2010)], which demonstrates that photons carry energy and momentum, just as other forms of matter. This working mechanism is quite straightforward, much the way as air supports air crafts.

Grover Swartzlander at the Rochester Institute of Technology in New York and his colleagues shone a weakly focused laser beam through the roughly semi-cylindrical rods, which refracted the light rays. This refraction changed the direction of the rays' momentum, causing an equal and opposite momentum change on the rods themselves. Because of the rods' asymmetrical shape, the momentum shift was directed more towards one side, driving the rods upwards at around 2.5 micrometres per second.
[Nature, 468:734]

Invisible gateway

Manipulating light with meta-materials is under intensive study recently. In a work published in PRL, the researchers realized an invisible gateway, which is an open channel but appears not there for certain frequency light. The produces optical illusions. [Phys. Rev. Lett. 105, 233906 (Published December 2, 2010)]

In 2009, a team of researchers led by Che-Ting Chan at the Hong Kong University of Science and Technology theorized on using transformation optics and complementary media to produce optical illusion devices that change the optical response of an object into that of another object. Illusion optics, the science of making an object appear as something else, or reappear elsewhere in space, or even disappear altogether (cloaking) is full of exciting possibilities, pending experimental realization.

In a paper in Physical Review Letters, Chao Li and co-workers at the Chinese Academy of Sciences, Beijing, and colleagues at Soochow University, China, and Hong Kong University of Science and Technology, experimentally demonstrate the first illusion-optics device. They trick light to miss an open channel across a slab at a frequency range of interest, rendering the channel into an electromagnetically invisible gateway. Li et al.’s design involves carving out an open channel across a metamaterial slab that behaves as a perfect electric conductor, then replacing a trapezoidal region of the slab adjacent to the channel with another metamaterial having the exact opposite dielectric properties. This “double-negative” region complements the dielectric space inside the channel into an optically equivalent region that behaves as a perfect electric conductor, thereby giving the appearance of a blocked gateway to light that attempts to go through.

Li et al. use a transmission-line approach that allows them to design metamaterials with the desired optical properties and with minimal losses. Their illusion-optics prototype works at around ~50 MHz and has a ~15 MHz bandwidth. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.233906]

Toward engineering the color of metals by carving rings on the surface

The electrons in the metal are nimble and mobile and able to conduct electricity. The behaviors are controlled by two things: the band structure and the coulomb interactions. The elementary excitations of this sea of electrons may not be simply fermionic quasi-particles that partially resemble the original electrons. They can also be bosonic, for example, plasma. Such plasmons are hardly excitable by low energy probes, such as visible light. But they can indeed be created by X-ray. What controls the colors are basically visible light. To understand the colors of a particular metal, one needs know how the visible light interacts with which kind of elementary excitations of the similar energy scales. To describe this interaction, one may assume quantum mechanics, but the usual Maxwell equations will suffice, because the visible light has a wave length between 400nm to 760nm, which are indeed very long in comparison with the metallic band gaps of the order of nm (and hence, only the single partially filled band needs be considered). Basically, one has to treat an entangled system of light and electrons, the exact solution of which is a considerable problem. Usually, one treats the metal as a medium that is characterized by a complex dielectric function of frequency. This function determines which photon will be absorbed and which can be transmitted and which will be reflected. The reflected light decides the color. Most naturally occurring metals bear silver color. This is because, the spectrum encoded in the imaginary part of the dielectric function is a continuum in the visible light energy window, rather than a discrete set of resonances. Is it possible to tune the color of a metal without affecting its conductivity? The answer is yes. Due to the complex part of the dielectric function, visible light can hardly enter the bulk metal and can penetrate only a very thin layer near the surface, an effect called "skinning effect". Thus, the colors are actually controlled by the skin. By manipulating the surface electron spectrum, one should be able to tune the color. This has been achieved in a latest work by Jianfa Zhang at the University of Southampton and a few pals [arXiv:1011.1977v1 ]. See a review from Arxiv Blog [http://www.technologyreview.com/blog/arxiv/]:

Their idea is to carve a different type of repeating pattern on to the surface of a metal.

These patterns are smaller than the wavelength of visible light. Instead of causing the light to interfere, they work by changing the properties of the sea of electrons in the metal--in particular its resonant frequency. This alters the frequency of light it absorbs and reflects.

This is the same technique that researchers have been using for some time to build invisibility cloaks . The idea is that by carefully building repeating patterns of subwavelength structures, researchers can tailor the way a "metamaterial" can steer light.

But instead of creating 3D structures that steer light as it passes through the material, Zhang and co carve the relevant structures onto the surface to control the way light is absorbed and reflected.

The structures that do the trick are tiny rings carved into the surface. The team calculate that they can make gold or aluminium appear almost any colour simply by varying the size and depth of these rings. They've even demonstrated the technique on a thin layer of gold.

Seeing the image obscured by painted glasses

This is an interesting innovation. Unfortunately, I cant access the original article !

A new laser technique can capture an image of an object obscured behind painted glass.

Sylvain Gigan and his team at ESPCI ParisTech in France have devised a method that traces the scattered path that photons take as they pass through an opaque white material. On one side of a glass slide covered in thick white paint, the researchers projected the image of a flower. They illuminated this set-up with a laser and took a photograph from the slide's other side. After calculating the light's zigzagging journey through the painted glass, they were able to reconstruct the flower image.

With improvements, the technique might one day be used in medical imaging to see through opaque biological tissue such as skin.

Nature Commun. doi:10.1038/ncomms1078 (2010)

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

Dcoding the colors of butterfly wings

I would like to highlight this article, because I like it. It studied the mechanism behind colors of butterfly wings. It makes sense to many people who love nature. You can even do it yourself, even if you are not a scientist. [PHYSICAL REVIEW E 82, 021903 2010].

Simulating metric signature effects with metamaterials

A funny work here.

We demonstrate that the extraordinary waves in indefinite metamaterials experience an effective metric signature. During a metric signature change transition in such a metamaterial, a Minkowski space-time is created together with a large number of particles populating the space-time. Such metamaterial models provide a tabletop realization of metric signature change events suggested to occur in Bose-Einstein condensates and quantum gravity theories.

In one of my recent blogs, I said something on possible enhancement of photon-photon interactions. I suggested two basic elements: (1)BEC and (2) small polarization energy. An easy calculation has been just done to verify the idea. Here is a GoogleTech Talk also going under the name of photon-photon interactions, which lays their emphasis on frequency change, rather than momentum change as in my considerations, after effective interactions.

Spin orbit coupling of light

Light is described by a vector potential and thus has internal degrees of freedom, which represents its intrinsic spin carrying an internal momentum. Now, if this beam of light travels along a curved trajectory, it shall also posses an orbital momentum, which can be manifested in the way it transforms under coordinate rotation. These two momenta may interact with each other. Differing from the situation with electrons, the spin-orbit coupling of light is geometric by origin. Here is a description of the expriments observing this coupling.