Nonlinear laser with ultralow threshold

Physicists at Caltech coupled a 70-micron silica sphere to an optical fiber, which enabled light to race around near the surface of the sphere in a “whispering gallery” mode. Whispering modes have been produced before, for example, in micro-droplets, but practical applications seemed remote. The light buildup in these modes is characterized by a parameter Q, referred to as the quality factor; for the microsphere, Q exceeded 10⁸. The light can build up to such an extent that nonlinear interactions take place and engender coherent light emission. The result is a Raman laser, which is tunable and can be used as a pump for other lasers. Typically, Raman lasers need a highpower input to work at all. But the Caltech result achieved lasing with only tens of microwatts of input power—1000 times less than other Raman lasers and in a much smaller package—although the output was only picowatts. The nonlinear properties of light in the silica microspheres offer new avenues of exploring quantum optics. (S. M. Spillaneet al., Nature, 415 , 621, 2002 http://dx.doi.org/10.1038/415621a .)