In London's St. Paul's Cathedral, a whisper can be heard far across the circular whispering gallery as the sound curves around the walls. Now, an optical whispering gallery mode resonator developed by Penn State electrical engineers 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 resonators, which are basically optical resonators, have been intensely studied for at least 20 years. Typically, the end of an optical fiber has been touched 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. Recently the Penn State team developed a way to grow on-chip glass microspherical shells with high sensitivities that potentially can 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. The researchers grew arrays of spheres from 230 microns to 1.2 millimeters in diameter with wall thicknesses between 300 nanometers and 10 micrometers.
The bottom of the sphere is thinned until it is basically a hole. It enables you to put the light on the outside of the sphere but do all the chemistry on the inner face of the shell. According to the researchers, you can bring in any analyte that you want to identify, but it goes on the inner surface. That brings in a lot of possibilities. You can do chemical sensing, vapor sensing, biophysical sensing, pressure sensing, and outstanding temperature sensing.
After many failed attempts, the team discovered that the key to making a high-quality sensor lays in making sure that the equatorial plane of the sphere, its center, is above the surface of the chip.
The work at present is to gain an understanding of the quality of the spheres by measuring the resonance levels after the spheres are made.
The sensors will be useful for lab-on-a-chip biophysical disease sensing, or by adding a polymer coating on the inside of the bubble, they could be made into very sensitive humidity sensors.