All That Matters

Light is special. In our everyday experience it behaves like a wave, which gets reflected, refracted and shows interference with other light of the same wavelength. At the same time, light also consists of particles, so-called photons. This duality is quite fundamental: the Hanbury Brown and Twiss experiment for example only works because of the particle-like properties of light.

The experimental setup. A laser beam injects photons into a cavity filled with light, and a camera observes the photons coming out of the cavity. Reprinted by permission from Macmillan Publishers Ltd. Nature 468, 545-548 (2010).

This amazing and perhaps confusing duality, where light in one experiment appears to be a wave and in others it behaves like particles, is now laid bare in a paper published in Nature. There, Jan Klaers, Martin Weitz and colleagues from the University of Bonn in Germany take one of the classical properties of light waves and turn it upside down — by demonstrating a related effect that only works when considering the particle qualities of light!

The classical effect they use is that light waves can all oscillate synchronously. This is exactly what happens in a laser, and is typical behaviour for a class of particles to which photons belong to, the bosons. Bosons love to be all in the same state.

A similar synchronous behaviour can also occur for other bosons, including certain atoms, which then all assume the same quantum state. This state is called a Bose-Einstein condensate, after Satyendra Nath Bose (after whom bosons are named) and Albert Einstein, who described it first in 1924. It is a Bose-Einstein condensate of light that Weitz and colleagues have now demonstrated. […]

Earlier this week I wrote about some of the exciting polaritons in semiconductors. And just a few days later, there is another intriguing paper on this topic out. Something that I speculate(!) might lead to new types of quantum computers.

But to recapitulate, polaritons are object that form when light interacts with electronic excitations. What I also mentioned was that if cooled down to about 30 degrees above absolute zero the polaritons for a condensate. In this condensate all polaritons are identical — they oscillate synchronously. Mathematically, they all have the same phase.

Typical semiconductor structure in which polaritons can be created. Two mirrors (DBR) on top and bottom of the device confine light between them. In the center is a layer (red) where the high light intensity between the mirrors creates polaritons. In the present study two such layers are placed right next to each other. Image reprinted by permission from Macmillan Publishers Ltd: Nature Materials 9, 655 (2010).

In my last blog post, I described how acoustic waves can separate this condensate into thin wires. What Benoit Deveaud-Plédran and colleagues from the École Polytechnique Fédérale de Lausanne in Switzerland have now realised is along the same idea: they fabricated two polariton condensates right next to each other.

What happens if the two thin layers with polariton condensates come close to each other? Something very similar to what superconductors do. The particles in superconductors also are in the same quantum state, with the same phase. And what we also know is that if two superconductors are brought in close contact to each other and if an electrical voltage is applied across the barrier that separates the two, an electric current flows.  But unlike the static current of normal conductors, for superconductors it shows a characteristic modulation: the magnitude of the current that flows oscillates over time. This is the Josephson effect.

[…]

Ultracold atoms might no longer be the only hot game in the town of cold condensates. A few weeks ago I highlighted the analogies between the science of ultracold atoms and other areas of physics, down to lasers even. Now meet the new kids on the block: the polaritons. Even though they sound more like the name of a 1960s rock’n’roll band, polaritons are the basis of some of the hottest research not only in condensed-matter physics but also in photonics.

Polaritons form when light couples to electronic excitations in a material. A widely studied type of polaritons, which I mentioned previously in the context of enhanced solar cells, is surface plasmon polaritons. Surface-plasmon polaritons are successfully used in photonics because they enable a versatile, highly local control of light by nanoscale structures. Applications range from sensing and the guiding of light to solar cells and other optical devices.

Schematic of the acoustic waves applied to a polariton condensate. The polaritons are shown in red. DBR are the mirror layers between which the polaritons are confined. (c) 2010 American Physical Society

A perhaps lesser known variety of polaritons are exciton-polaritons, which are quickly turning into a hot research area themselves because they enable the study of fundamental quantum physics phenomena directly in a semiconductor.

Exciton-polaritons form in semiconductors such as GaAs, which have a band structure where the lower energy band, the valence band, is occupied by electrons, and the higher energy band, the conduction band, is empty. If an electron is excited to the conduction band, an empty unoccupied space, a hole, remains in the valence band. The hole left behind in the sea of valence band electrons has a positive charge. The electrostatic interaction between the positive hole in its low-energy state and the negative electron in the high-energy state leads to the formation of a combined entity known as an excition. And these excitons can interact with light to form exciton-polaritons. Usually, they are simply referred to as polaritons.

The high materials quality that can be achieved in semiconductors such as GaAs means that polaritons exist long enough to do experiments with them. Even though the interaction between polaritons is much different than that between ultracold atoms, there are also similarities. Bose-Einstein condensation for example, known from ultracold atom systems, has been observed in polariton systems.

Maurice Skolnick from the University of Sheffield in the UK and his colleagues have now shown in a paper published in Physical Review Letters that polariton condensates can be dynamically controlled by sound waves applied to the semiconductor. “The dynamic modulation allows for the first time a tunable periodic potential to be applied to the polariton condensate,” says Skolnick. This, so Benoit Deveaud-Plédran, a physicist from École Polytechnique Fédérale de Lausanne in Switzerland who also works on polaritons, offers the opportunity to study the reactions of polaritons to these changes in environment: “I really like the idea that is proposed here, playing with condensates with a surface acoustic wave is indeed great.”

[…]