Highly unidirectional “whispering gallery” microlasers

Highly collimated unidirectional microlasers have been demonstrated, and their creators say they will have a wide range of new photonics applications, including sensing and communications.

Researchers at Harvard University, in collaboration with colleagues from Hamamatsu Photonics in Hamamatsu City, Japan, and from the Institute of Theoretical Physics of the University of Magdeburg, Germany, employed “whispering gallery modes” to develop the microlasers.

The new microlaser uses an elliptical resonator with a wavelength-size notch (seen at top right) on the boundary to create unidirectional rays (bottom left). Courtesy of Harvard School of Engineering and Applied Sciences/Capasso Group.

“One of the crucial unsolved problems of these microlasers for practical applications has been that their emission is nondirectional and their optical power output is negligible,” said team leader Federico Capasso of Harvard’s School of Engineering and Applied Sciences.

Faint isotropic emission has been just about the only way for light to escape the whispering gallery modes, he added, and efforts to deform the disks involved have been disappointing. So the team shaped the microlaser as an ellipse with a wavelength-size notch carved into its edge and found that the cycling whispering gallery modes scatter efficiently off the notch. They then emerge as nearly parallel beams from the microlaser.

The prototypes are 10-μm quantum cascade lasers emitting 5 mW; the researchers say that the notched elliptical microlaser should perform even better at shorter wavelengths, near 1 μm, which could be especially useful for optical communications.

The work was published online in the Proceedings of the National Academy of Sciences in December 2010.