Trapped photons lead to new class of lasers

The world's smallest laser cavities, each less than a single cubic wavelength, have led to the first continuous-wave UV and visible random lasers and could open the door to a host of other new devices, including brighter computer displays, improved fluorescent light bulbs, and batteries that store energy in photons.

Physicist Stephen Rand and materials scientist Richard Laine at the University of Michigan (Ann Arbor, MI) first developed a process for creating doped nanophosphor powders of almost any composition. When stimulated by a low-voltage (1 keV–10 keV) electron gun, alumina nanopowders doped with the rare-earth metal ions cerium (Ce³⁺) and praesodymium (Pr³⁺) at concentrations of ±100 ppm, emitted photons in the UV (357–392 nm), and red bands (625–632 nm), respectively.

One of the most remarkable results of the experiment is that it achieves continuous laser action by Anderson localization of electromagnetic radiation in the optical range for what is believed to be the first time. Because the nanophosphor particle sizes and inter-particle separations are less than a wavelength, the photons experience nearly total reflection without loss over distances of less than a single wavelength, oscillating in time but not in space. These points of light do not exhibit coherent properties commonly associated with laser light, although peers agree that the 'random laser' does indeed lase.

"Random lasers are counter-intuitive because people think that scattering is bad for lasing because it destroys coherence. But the scattering makes random lasers—it increases amplification and gain until you reach a threshold and start to generate light," says Vladimir Shalaev of Purdue University (West Lafayette, IN), who also studies laser effects in scattering systems.

Although the powders, compressed into a 3-mm layer inside a vacuum chamber, do evidence clear lasing threshold, gain, and modest frequency narrowing on a 5d-4f electron transition in Ce³⁺ and a 4f-4f electron transition in Pr³⁺, the light does not display speckle, or emerge in any particular direction, or show evidence of standard mode selectivity. The output is incoherent and omni-directional. These properties are due to the stationary nature of light generated as an evanescent field within the powders. Light only leaks out of the medium near the surface of the powder where the Anderson localization condition is compromised by the presence of the boundary.

A large part of the experiment's success comes from Laine's flame spray pyrolysis fabrication process. Metallo-organic precursors dissolved in ethanol produce powders by combustion in oxygen at about 2000°C. Sprayed into the combustion chamber at rates greater than 50g/h, charged particles with approximately 75 ions per 20-nm Ce particle and 800 ions per 40-nm Pr particle are collected electrostatically downstream from the flame.

Researchers have created stationary light in dense ionized alumina powders inside an ultrahigh vacuum chamber. (University of Michigan)

"For our approach to work, the particles need to be quite a bit smaller than lambda for strong localization and packed more densely than one per cubic wavelength. That's what's different from previous work [on random lasers]," says Rand. In dense media with lossless reflection arising from multiple scattering, the threshold for laser action is greatly reduced.

"I'm very impressed with this work because it has huge potential in terms of applications," says Purdue's Shalaev. "Compared to quantum well and semiconductor lasers that are very expensive and hard to use, random lasers are cheap. You can have lasing paint, for example, and that could lead to incredible applications that could be sprayed on glass, walls, or any other object to produce light or displays." Laine has already developed an ink-jet deposition process for the doped powder.

Rand has yet to make radiometric measurements on the random laser, "so we can't answer efficiency questions or total output, but we expect the power [from the electron beam] is being very efficiently re-radiated."