New Bose-Einstein Condensate Couples Light with Metal Electrons

Researchers from Aalto University have created a new Bose-Einstein condensate, where the condensed particles were mixtures of light and metal electrons (so-called surface plasmon polaritons), in motion in gold nanorods arranged into a periodic array. Unlike most previous Bose-Einstein condensates created experimentally, the new condensate does not need to be cooled down to temperatures near absolute zero. Because the particles are mostly light, condensation can be induced at room temperature.

The wavelength of emitted light grows, that is, the energy decreases, along the gold nanorod array. A Bose-Einstein condensate forms when an energy minimum of the lattice is reached. Courtesy of Aalto University/Tommi Hakala and Antti Paraoanu.

Interaction of the nanoscale-confined surface plasmons with a room-temperature bath of dye molecules enabled thermalization and condensation in picoseconds. The researchers needed to demonstrate proof of the new condensate — a challenge because it came into existence so quickly.

“According to our calculations, the condensate forms in only a picosecond. How could we ever verify the existence of something that only lasts one trillionth of a second?” said researcher Antti Moilanen.

Researchers showed the ultrafast thermalization and condensation dynamics in an experiment that exploited thermalization under propagation and the open-cavity character of the system.

Essentially, researchers “kickstarted” the condensation process so that the particles forming the condensate would start to move.

“As the condensate takes form, it will emit light throughout the gold nanorod array. By observing the light, we can monitor how the condensation proceeds in time,” said researcher Tommi Hakala.

The light that the condensate emits is similar to laser light. A crossover from the condensate to traditional lasing was achieved by tailoring the band structure.

“We can alter the distance between each nanorod to control whether Bose-Einstein condensation or the formation of ordinary laser light occurs. The two are closely related phenomena, and being able to distinguish between them is crucial for fundamental research. They also promise different kinds of technological applications,” said professor Päivi Törmä.

Both lasing and Bose-Einstein condensation provide bright beams, but the coherences of the light they offer have different properties. These, in turn, affect the ways the light can be tuned to meet the requirements of a specific application. The new condensate can produce light pulses that are extremely short and could offer faster speeds for information processing and imaging applications.

“The gold nanoparticle array is easy to create with modern nanofabrication methods. Near the nanorods, light can be focused into tiny volumes, even below the wavelength of light in vacuum. These features offer interesting prospects for fundamental studies and applications of the new condensate,” said Törmä.

Bose-Einstein condensation is based on a prediction made by Albert Einstein and Satyendra Nath Bose nearly 100 years ago that quantum mechanics could force a large number of particles to behave in concert, as if they were a single particle. Although Bose-Einstein condensation has been observed in several systems, the limits of the phenomenon could be pushed to faster timescales, higher temperatures, and smaller sizes, say the researchers. The researchers believe that the easier it becomes to create these condensates, the more routes will become open for new technological applications. Light sources, for example, could be extremely small in size and allow fast information processing. The condensate discovered by the Aalto team shows promise because of its ultrafast, room-temperature, and on-chip nature.

The research was published in Nature Physics (doi:10.1038/s41567-018-0109-9).

Researchers at Aalto University in Finland are the first to create a Bose-Einstein condensate of light coupled with metal electrons, so-called surface plasmon polaritons. Courtesy of Aalto University/Kalle Kataila, Antti Moilanen, Tommi Hakala, and Päivi Törmä.