All That Matters

Solar energy is obviously one of the key renewable energy resources available to us. At the same time researchers are hitting against a glass ceiling. A famous 1961 paper by William Shockley (who co-invented the transistor) and Hans Queisser comes to the conclusion that for a semiconductor such as silicon the maximum conversion efficiency of solar energy into electricity will never be more than about 30%.

Dye-sensitized solar cells. A design similar to these solar cells is now used to demonstrate the creation and extraction of multiple charge carriers per photons.

One reason for this limit is that each light particle only excites one electron. Even if the electron has enough energy to excite two electrons, all this energy is lost and only one electron is excited. And this is the case for pretty much all present commercial solar cell technology. Fortunately, however, there are possible exceptions. Bruce Parkinson and colleagues from the University of Wyoming in the USA have now built a photovoltaic cell that at certain wavelengths of light can generate more than one electron per photon of light. Their approach promises to beat the Shockley-Queisser limit and could lead to solar cells with considerably enhanced efficiency.

In silicon and other semiconductors, if a photon excites an electron all excess energy is predominantly lost as heat. Of course, there are attempts to harvest the heat generated in solar cells, and such approaches could beat the Shockley-Queisser limit. And so could nanostructured materials that use for example plasmonic effects. But a more direct solution would be if the excess energy could be used to excite more than one electron in the first place.

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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.

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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.”

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