LET THERE BE LIGHT: Scientists Observe Superfluorescence in Semiconductors

Researchers have found evidence of cooperation in the unlikeliest of places — among atoms within solids.

Dr. Alexey Belyanin, professor of physics at Texas A&M University, and Belyanin research group members Yongrui Wang and Dr. Aleksander Wojcik are part of a team of scientists that first predicted and recently observed this effect, known as superfluorescence, in solid-state materials — in this case, electron-hole pairs created in a semiconductor using lasers. Their results are featured in a recent issue of the journal Nature Physics.

“We predicted the possibility of this self-organization and collective recombination in semiconductors a long time ago, but nobody was able to observe it,” Belyanin says. “We also pointed out at that time that you might get a better chance of observing superfluorescence if you have a layered semiconductor (so-called quantum wells) and impose a strong magnetic field to prevent electrons from bumping into each other too often and to increase their coupling to light. We are happy that our collaborators Dr. Junichiro Kono and his group from Rice University were able to test these ideas and observe the effect.”

Belyanin says superfluorescence is a rare phenomenon — one almost counterintuitive in semiconductors — that can only happen when “many bodies” decide to work together. He credits his Rice colleagues for conducting a meticulous and difficult experiment that required a strong magnetic field and low temperatures obtained at Florida State University’s National High Magnetic Field Laboratory in collaboration with physicists at the University of Florida to prove it.

“It took several years to plan everything and get the data,” Belyanin adds. “The Rice group did it beautifully, and I think that the results are absolutely convincing.”

Belyanin says the true beauty of the experiment lies in the after-effects of that powerful femtosecond laser pulse, which near-instantaneously creates a huge number of free electrons that bump into each other and into the atoms of a crystal. But instead of gradually losing their energies and eventually disappearing in the course of a few nanoseconds, thereby returning the crystal back to equilibrium, he says they rally together to emit a giant and extremely short burst of light that consumes all particles within a few picoseconds.

“We think that during this waiting time a kind of ‘self-organization’ occurs: electrons ‘talk’ to each other by exchanging photons and establish cooperation,” Belyanin explains. “Billions of tiny antennas align together to form a giant single antenna, which then emits a powerful coherent pulse of radiation. Each antenna is formed by a pair of particles with opposite charge, like a tiny nano-dipole. You would expect that these antennas should never align because they constantly bump into each other, but we found that they do align and cooperate.”

Although similar cooperation of tiny atomic dipoles previously has been observed in gases and rarefied impurities in crystals, Belyanin says people did not believe superfluorescence was possible in such a “messy” semiconductor plasma in which charged particles collide with each other thousands of times before they emit even one photon.

“I think this is a beautiful example of self-organization in many-body physics — when a collective, organized behavior emerges from seemingly chaotic motion of myriads of particles,” he says. “In this case, the cooperation emerges through interaction with electromagnetic radiation, which brings an extra bonus: It is accompanied by a spectacular outburst of light!”

Beyond its literal brilliance, Belyanin says the observed effect should be fairly universal, given that it relies on fundamental principles of quantum mechanics and electromagnetism.

“The radiation was in the near-infrared range, which is dictated by the material we used, but with different semiconductor materials you can have pulses of visible light, or shift them toward longer, mid-infrared wavelengths,” he adds.

Belyanin notes that, by playing with different materials and their layout, it could be possible to observe superfluorescence at higher temperatures and with a weaker or no magnetic field at all — meaning that superfluorescence may become a practical source of ultrashort pulses of light. In addition, he says the prefix “super” has particular relevance, considering the combined burst of radiation may be up to many million times brighter than the emission from the same number of independent particles.

“Picture a self-alignment of myriads of tiny antennas into one giant radiating antenna, and you would have a pretty accurate analogy,” he says. “Another picture is a collective ‘suicide’ of billions of particles, when charges of opposite sign recombine in a fiery outburst of radiation. They would live much longer if they did not cooperate. For them, individualism would be much better than collectivism, but we physicists do enjoy a beautiful firework.”

Belyanin, an internationally recognized leader in the rapidly developing interdisciplinary research field of optics of nanostructured materials, joined the Texas A&M faculty in 2003. World-renowned for his breakthrough ideas and developments in research fields that span semiconductor physics, quantum optics, photonic devices and high-energy astrophysics, he currently leads Texas A&M efforts in two National Science Foundation-funded, multi-university consortiums. He is widely respected by his peers for his breadth of knowledge, his uncanny ability to bridge the gap between theory and experimentalists, and his ability to attract research funding.

For more information on Belyanin and his research, visit http://faculty.physics.tamu.edu/belyanin/.

-aTm-

Contact: Shana K. Hutchins, (979) 862-1237 or shutchins@tamu.edu or Dr. Alexey Belyanin, (979) 845-7785 or belyanin@physics.tamu.edu