A better way to build compact ultraviolet (UV) light sources with low power consumption has been discovered using an optimized optical resonator, which could lead to improved information storage, microscopy and chemical analysis. Researchers at the University of Michigan used a whispering gallery resonator to take an infrared signal from relatively cheap telecommunication-compatible lasers and, using a low-power, nonlinear process, boosted it to a higher-energy UV beam. In the setup for this experiment, a telecommunication-compatible infrared beam is coupled to the whispering-gallery resonator through a diamond prism, and the generated near-infrared, visible and ultraviolet light are collected by a multimode fiber. (Images: Mona Jarrahi) The optical resonator is a millimeter-scale disk with a precisely engineered shape and smooth surface polishing to encourage the input beam to gain power as it circulates inside, allowing researchers to make low-cost, wavelength-tunable UV sources using low-infrared power levels. The researchers used their resonator to generate the fourth harmonic of the infrared beam they started with. University of Michigan assistant professor Mona Jarrahi, doctoral students Jeremy Moore and Matthew Tomes, who performed the experiment, and assistant professor Tal Carmon. As with sound harmonics, engineers can generate harmonics of light: By pushing light beams through a nonlinear medium, they can coax out offshoot beams that are double or, in this case, quadruple the frequency and energy of the input beam, and one-quarter of the original wavelength. “As we go from green to blue [wavelengths], the efficiency of the laser goes down. Going to UV lasers is even harder,” said Mona Jarrahi, who led the research along with fellow assistant professor Tal Carmon. “This principle was first suggested by Einstein and is the reason why green laser pointers do not actually contain a green laser. It is actually a red laser, and its wavelength is divided by two to become green light.” UV light sources have applications in chemical detection, crisper medical imaging and finer lithography for more sophisticated integrated circuits and greater computer memory capacity, said the researchers. The paper was published in Optics Express. For more information, visit: www.umich.edu