When a laser beam is intense enough, it can interact with the air around it in ways that lead to surprising effects. According to computer simulations to be published in the 12 November Physical Review Letters, the beam can act like a gas of quantum particles (fermions) or like a liquid droplet–and switch between the two as intensity is increased. Observing this transition in the lab would help researchers confirm that they understand the behavior of high intensity lasers in air, which they hope to use for improved transmission of signals across long distances.
An optical fiber confines light to its interior in part because its internal index of refraction–the factor by which it slows light compared with the vacuum–is larger than that for the outer coating material. A very intense beam of light propagating through a material can do something similar. It can align and distort nearby molecules, making the index of refraction larger at the intense center of the beam than at its edges, which prevents the light from spreading as it travels.
The possibility of sending this type of “self-focused” light pulse long distances could be important for remote sensing applications, such as LIDAR, which uses laser light the way radar uses radio waves. But nailing down the details of the interactions between intense light and atmospheric gases has been an experimental challenge. In 2009 a team at the University of Bourgogne in Dijon, France, managed to measure the refractive indices of nitrogen, oxygen, and air, for a high intensity, infrared laser. They found that as the laser intensity rises, the refractive index first increases, but then rapidly decreases beyond an intensity of a few tens of terawatts per square centimeter .
“We were inspired by these results,” says Daniele Tommasini, of the University of Vigo in Spain. In their earlier research, Tommasini and his colleagues showed theoretically that this kind of intensity dependence would lead to more than simply self-focusing, which keeps light confined for only a limited distance. A light pulse could maintain a fixed shape in all directions, as a so-called soliton . The new measurements gave them a chance to use real experimental parameters in their model.
The team came up with a theoretical model based on the new data and also ran computer simulations. They were surprised to find that, depending on the intensity of the light, the solitons had strikingly different shapes: a high intensity pulse generated a soliton with intensity uniformly distributed across its cross-section, while a lower intensity pulse produced a soliton with high intensity at the center of the pulse and a gradual drop-off toward the edges.
The team describes these states as having an effective outward “pressure”–a tendency to spread because of standard optical effects–that is exactly canceled by their interaction with the gas. For the high intensity pulses, the pressure and light intensity are mathematically analogous to the pressure and density of a liquid droplet. The light is squeezed into a uniform distribution with a sharp boundary, similar to the way the surface tension of a liquid droplet maintains a well-defined boundary. In contrast, for lower intensity pulses, the mathematical analogy is with a gas of uncharged fermions. These particles, unlike photons, repel each other because of the Pauli exclusion principle.
The team has explored “liquid light” in earlier work  but the idea of fermionic light is new. Intrigued by the possibility of driving a phase transition between these two stable forms of light, they simulated focusing a grid of the “Fermi-like” light filaments into a single beam. They found that the filaments collapsed into a single “liquid-like” soliton, much like droplets coalescing.
Yuri Kivshar at the Australian National University in Canberra finds the prediction of the two types of light behavior “really surprising,” though he notes that observing the transition from fermionic to liquid light may prove to be challenging in the lab. Still, Bruno Lavorel, a member of the University of Bourgogne experimental team, “strongly agrees” with Tommasini and his colleagues that the predicted transition can serve as an experimental test of researchers’ understanding of the behavior of high-intensity light in air.
- V. Loriot, E. Hertz, O. Faucher, and B. Lavorel, “Measurement of High Order Kerr Refractive Index of Major Air Components,” Opt. Express 17, 13429 (2009).
- D. Novoa, H. Michinel, and D. Tommasini, “Pressure, Surface Tension, and Dripping of Self-trapped Laser Beams,” Phys. Rev. Lett. 103, 023903 (2009); H. Michinel, J. Campo-Táboas, R. García-Fernández, J. R. Salgueiro, and M. L. Quiroga-Teixeiro, “Liquid Light Condensates,” Phys. Rev. E 65, 066604 (2002).