Tiny spherical drops in liquid self-organize into lasing cavities that demonstrate low threshold, emit in all directions, and can be designed with wavelengths from ultraviolet to infrared.
A cut-away view of a microlaser droplet. The reddish center represents lasing dye molecules, while the radial yellowish helices indicate cholesteric liquid crystal molecules.
Tiny spherical drops in liquid self-organize into lasing cavities that demonstrate low threshold, emit in all directions, and can be designed with wavelengths from ultraviolet to infrared. These lasers are made of droplets of dye-doped cholesteric liquid crystals (CLCs) suspended in a carrier fluid (Opt. Express 18, 26995), and they are part of a long-term project on soft-matter photonic devices carried out in Igor Musevic's lab at the J. Stefan Institute and University of Ljubljana, Slovenia.
Matjaz Humar and Musevic demonstrated that the CLCs in glycerol form into microdroplets (15-50 µm in diameter) that lase when optically pumped. Within the droplets, the CLCs self-organize into concentric layers with periodic changes in the index of refraction—i.e., into concentric Bragg gratings that can act as an optical cavity. When optically pumped, the dye molecules in the droplet emit light, which the cavity reflects. At a low threshold, lasing occurs in all directions. The researchers found that the lasing wavelength depends on the pitch (the length of the helix) in the cholesteric material. “You can select the range of wavelengths you want,” Musevic explains, by choosing the material or mix of cholesteric liquid crystals. The lasers are also temperature tunable over tens of nanometers.
Low-threshold microlasers would be attractive light sources for investigating both the physics of lasing and as tiny coherent light sources for commercial applications. The rotational symmetry offers the advantage of light confinement in all directions. Similar lasers have been made using 2-D structures, but the difficulty of making solid 3-D microcavities has thus far prevented development of solid microsphere lasers. The liquid-crystal-in-fluid lasers, by contrast, are very simple to make. “Millions of microlasers can be formed simply by mixing a liquid crystal, a laser dye and a carrier fluid, thus providing microlasers for soft-matter photonic devices,” says Musevic.
Potential applications include holography, telecommunications, optical computing, imaging, sensing or even standard illumination that doesn’t depend on coherence. The researchers suggest that the laser could be made more stable if it were coated with a protective shell, or if the liquid crystal were polymerized. It may also be possible to manipulate the core of the cavity or to couple arrays of the microlasers.
Future applications aside, these microlasers are fascinating. Musevic says, “From my perspective as a physicist, these liquid microlasers are just beautiful objects built on very simple principles.”
Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics.