Nature Commun. 4, 1869 (2013)

Lenses are usually fashioned from glass or other solid transparent materials, but, in principle, they could be made from pretty much anything — as long as the material transmits light and has a refractive index different from that of the medium in which the light is propagating. To make a gas lens, for instance, air is heated to create a gradient of refractive index that focuses the light. Using the same principle, Max Michaelis and colleagues have now demonstrated a flame lens for laser light.

Michaelis et al. combined a tubular gas-lens of limited focusing power with a spiral flame, which generated a steep gradient of temperature and refractive index. The result was an improved lens that was successfully tested with different types of laser: from imaging a pattern of low-power LEDs, to drilling a hole with a high-power Nd:YAG laser. However, the concept has yet to be optimized to eliminate the relatively high aberrations caused by uncontrolled hot air plumes.

Solid lenses are damaged by repeated exposure to high-power lasers — petawatt laser applications being the most challenging for conventional optics. The appeal of flame lenses is that they are virtually indestructible.