Optical-fiber-based grating plus microcavity could become ultrasensitive sensor

Optical cavities can enhance the interaction between light and matter, ideally serving as sensitive sensors, for example. Xia Yu and his team at the A*STAR Singapore Institute of Manufacturing Technology have now developed an optical-fiber-based structure that harnesses the potential of light trapped in a microcavity.¹

Yu and colleagues melted silica glass to form a sphere with a diameter of 182 μm. They then patterned the end of an optical fiber with a gold grating and fixed it close to the microsphere. The grating coupled light propagating along the fiber into the sphere. Light with the right wavelength traveled around in a whispering-gallery resonant mode within the sphere.

The A*STAR team investigated the properties of the structure by measuring the resulting spectral response. The typical wavelength-dependent response of a microsphere is a sharp, symmetric peak centered on the resonant wavelength of the cavity. Instead, the researchers observed an asymmetric spectral peak, which they recognized as a clear signature of the so-called Fano effect, indicating strong interaction or interference between the whispering-gallery mode and the light in the fiber directly reflected back from the grating.

"This interfering effect makes Fano resonances especially sensitive to changes in either of the participating systems: a slight perturbation results in dramatic alteration in the optical characteristics," says Yu. "An obvious application of Fano resonance is for use in ultrasensitive detection."

In previous investigations of the optical Fano effect, researchers inserted (and extracted) light into the cavity through the side of an optical fiber -- an approach that proved unstable and inefficient. The method used by Yu and colleagues of directly inserting light into the cavity through the end of the fiber is far more robust, making the technology a plausible platform for cheap and compact optical-resonator-based photonic devices.

Another possible application for the technology is as an optical switch. "A good switching device must be fast," says Yu. "Therefore, the next step in our research will be to attempt to control the speed of the whispering-gallery mode Fano resonance."

Source: http://www.sciencedaily.com/releases/2014/05/140509131603.htm

REFERENCE:

1. Zhou, Y. et al., Applied Physics Letters 103, 151108 (2013). dx.doi.org/10.1063/1.4823531