An optical whispering gallery mode resonator was developed that can spin light around the circumference of a tiny sphere millions of times, creating an ultrasensitive, microchip-based sensor for multiple applications.
Whispering gallery mode sensors have been created by taking an optical fiber and touching the end with a blow torch. When the melted fiber re-condenses, it forms a sphere at the tip. This can be coupled to a light source to make a sensor. That type of sensor consists of solid spheres, and is not compatible with microfabrication methods.
The new way to grow on-chip glass microspherical shells with ultra-high sensitivity can potentially be used for motion, temperature, pressure, or biochemical sensing. The hollow borosilicate glass spheres are blown from sealed and pressurized cylindrical cavities etched into a silicon substrate. Using a glassblowing technique, the thin glass wafer — under high heat and external vacuum pressure — forms an almost perfect bubble. Arrays of spheres from 230 microns to 1.2 millimeters in diameter were grown with wall thicknesses between 300 nanometers and 10 micrometers.
The bottom of the sphere is thinned until it is basically a hole. The light is put on the outside of the sphere, but all the chemistry takes place on the inner face of the shell. A key to making a high-quality sensor is making sure the equatorial plane of the sphere — its center — is above the surface of the chip.
This result could have particular significance for lab-on-a-chip biophysical sensing for disease sensing. By adding a polymer coating on the inside of the bubble, an ultra-sensitive humidity sensor could be made.