Ultraviolet lasers get smarter

U. MICHIGAN (US) — Scientists have discovered a way to build compact and efficient ultraviolet light sources that could enhance information storage, microscopy, and chemical analysis.

Engineers modified an optical resonator to take an infrared signal from relatively cheap telecommunication-compatible lasers and, using a low-power, nonlinear process, boost it to a higher-energy ultraviolet beam. The 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 the resonator.

“We optimized the structure to achieve high gain over a broad range of optical wavelengths,” says Mona Jarrahi, assistant professor of electrical engineering and computer science at the University of Michigan. “This allows us to make low-cost, wavelength-tunable ultraviolet sources using low-infrared power levels.”

The resonator was used to generate the fourth harmonic of the infrared beam they started with. Like the harmonic distortions you get from new sound frequencies when you crank up a loudspeaker, engineers can generate harmonics of light by using the right materials. 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.

The research is reported in the journal Optics Express.

Lasers get progressively more difficult to generate and more inefficient, as engineers aim for shorter wavelength, Jarrahi says.

“As we go from green to blue, the efficiency of the laser goes down. Going to UV lasers is even harder. 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.”

The research was funded by the National Science Foundation and the Air Force Office of Scientific Research.

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