Whispering glass

A new type of optical whispering gallery mode resonator has been developed by electrical engineers at Pennsylvania State University. The device spins light around the circumference of a tiny sphere in an analogous way to the acoustic whispering galleries of architecture, such as that seen (and heard) at St Paul's Cathedral London. The new device offers the possibility of building an ultrasensitive microchip-based sensor for various applications.

"Whispering gallery mode resonators, which are basically optical resonators, have been intensely studied for at least 20 years," explains Srinivas Tadigadapa. "What people have done is to take an optical fiber and touch 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." Sensors built in that way consist of solid spheres and are not compatible with microfabrication methods.

Now, Tadigadapa and his team have developed an alternative fabrication method of growing innovative way to grow on-chip glass microspherical shells. These are incredibly sensitive structures that could be adapted for motion, temperature, pressure or biochemical sensing.

The team made their hollow borosilicate glass spheres from sealed and pressurized cylindrical cavities etched into a silicon substrate and blown using a glassblowing under at high temperature and external vacuum pressure. They could thus grow arrays of almost perfect spheres with diameters from 230 micrometers to 1.2 millimeters and a wall thickness of between 300 nanometers and 10 micrometers.

"The bottom of the sphere is thinned until it is basically a hole," Tadigadapa explains. "You can put the light on the outside of the sphere but do all the chemistry on the inner surface of the shell. 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 really outstanding temperature sensing." [Tadigadapa et al., Sci Rep (2017), 7, 14965; DOI:10.1038/s41598-017-14049-w].

The team suggests that their work will have particular significance for lab-on-a-chip biophysical sensing for medical diagnostics. Equally by adding specific polymer coatings to the interior of the bubbles it would be possible to make a very sensitive humidity sensor. "There are some really exciting possibilities. I think it will spawn a large follow-up work," Tadigadapa adds.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.