Physics of Liquid Light

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Alexey Kavokin

Physics and Astronomy, University of Southampton, Highfield, Southampton, SO171BJ, UK

Mediterranean Institute of Fundamental Physics, 34, via Appia Nuova, Frattocchie, 00040, Rome, Italy

SOLAB, St-Petersburg State University, 1 Ulianovskaya, St-Petersburg, 198505, Russia

Phone: +44 7985647532

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ABSTRACT:

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300 years ago Isaak Newton invented the corpuscular theory of light, which was afterwards confirmed and developed by Max Plank and Albert Einstein. According to their theories, photons, the quanta of light, have no mass, propagate with a constant speed and do not interact with each other.

The modern physics of light-matter coupling went further. Now we know that inside crystals photons behave like molecules of water: they interact with each other, form vortices, droplets and waterfalls. Liquid light in crystals can become superfluid. I will discuss the recent experiments which revealed unusual properties of liquid light in crystals and comment on its applications in new lasers, transistors, computers and communication lines.

The central object of my study is a crystal quasi-particle named “exciton-polariton”. This is a quantum mechanical superposition of a quantum of light (photon) and an elementary excitation in a semiconductor crystal (exciton). Excitons are similar to giant atoms of hydrogen: they consist of a positively charge nucleus (hole) and a negatively charge electron that gravitates around the hole. Excitons have been experimentally discovered in crystals of cuprous oxide in 1953 by the Russian physicist Eugene Gross. Nowadays, they are being studied in the most part of semiconductor crystals, and their unique properties are exploited for the development of new lasers. Excitons are crucial for formation of liquid light: they repel each other due to the Coulomb force, that is why quasiparticles of liquid light (exciton-polaritons) tend to spread uniformly over the whole crystal. In contrast to the classical liquid that condenses into droplets, the liquid light condenses in the most delocalized of all states that is called Bose-Einstein condensate.

The physics of Bose-Einstein condensates has been strongly developed at the end of the XXth century and beginning of the new Millennium.  Such condensates may be formed by atoms of sodium or rubidium at ultralow temperatures. Due to their photonic component, the quasiparticles of liquid light have a very light mass (billions of times lighter than sodium atoms) that is why the critical temperature of their Bose-Einstein condensates is billions of times higher than one of the atoms. In certain structures liquid light can form bosonic condensates at room temperature.

Once the condensate of liquid light is form, it manifests itself by emitting the coherent and monochromatic light that we usually call “laser light”. Exciton-polaritons in a Bose-Einstein condensate occupy a single quantum state (the state characterized by one fixed energy and wave vector) in huge quantities.  Each of exciton-polaritons can escape from the crystal by converting to a conventional photon. As all polaritons in the condensate are identical, they generate a flow of identical photons: photons characterized by the same frequency and propagating in the same direction, that constitute the laser light.

Polariton lasers have been realized in GaAs, CdTe and GaN semiconductor microcavities during the recent decade. They constitute a valuable alternative to conventional semiconductor lasers and may be used not only for lighting but also for realization of optical integrated circuits, in information communication technologies, medicine and security.

The potentiality of liquid light for future optical computers is hard to overestimate. At certain conditions, its flows become superfluid and propagate without any losses. Future optical transistors and logic gates based on the superfluids of exciton-polaritons will consume much less energy and heat environment in much lesser extent than electronic chips used in the contemporary computers. This would constitute a huge economy in terms of energy consumption if one has in mind that already over 4% of all energy produced by humans is lost in the internet.

Finally, liquid light may be coupled to the electron gas, that may help realizing superconductivity at room temperature. The superconductivity is a unique property of cold metals to conduct electric current with zero losses. Since the discovery of superconductivity in 1911, the physicists are trying to find a system where this remarkable phenomenon would exist at room temperature. Such a discovery would have enormous economic benefits and it would constitute a true technological revolution. We hope that liquid light will help realizing this dream.