Optical resonator builds better UV beam

Ashley N. Paddock, ashley.paddock@photonics.com

An optimized optical resonator can be a better way to build compact ultraviolet light sources with low power consumption, which could lead to improved data storage and chemical analysis.

Researchers at the University of Michigan optimized a whispering gallery resonator to take an infrared signal from relatively cheap telecommunications-compatible lasers, using a low-power nonlinear process to boost it to a higher-energy UV beam. 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 with which they started.

“We experimentally demonstrate, for the first time, continuous-wave cascaded harmonic generation up to the fourth-harmonic in a lithium niobate whispering gallery resonator – which allows us to convert a telecommunications-compatible infrared light beam to an ultraviolet light beam at pump power levels as low as 200 mW,” said Mona Jarrahi, an assistant professor in the department of electrical engineering and computer science at the university.

A telecommunications-compatible IR beam is coupled to a whispering gallery resonator through a diamond prism, and the generated near-infrared, visible and ultraviolet light are collected by a multimode fiber. GRIN = gradient index. Courtesy of Mona Jarrahi, University of Michigan.

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 work appeared online in Optics Express (http://dx.doi.org/10.1364/OE.19.024139).

UV light sources have applications in crisper medical imaging, chemical detection and finer lithography for more sophisticated integrated circuits and greater computer memory capacity.

“Our results can also transform many nonlinear optical studies which are currently only possible using femtosecond pulses to offer the required high pump power levels,” Jarrahi said. “For example, and despite many challenges on the way, extreme light-matter interactions may be extended to be continuous in time. Also, imaging crystals and molecular states with subatomic-scale resolution will benefit from a continuous-in-time source.”

The researchers hope to extend their work to generate light beams at other wavelengths that are difficult to achieve through standard solid-state laser sources, such as terahertz wavelengths and extreme UV.