Optomechanical measurements within fluids now a reality

Each December, Optics & Photonics News highlights the most exciting peer-reviewed optics research that has emerged during the preceding 12 months. The annual feature, called “Optics in …” includes the top 30 breakthroughs of interest to the optics community.

Research conducted by MechSE assistant professor Gaurav Bahl and his research group has made the cut not just once but twice. This year’s selection focuses on Bahl’s work on a microfluidic optomechanical oscillator system. This work is unique in that all previous optomechanics research has been on solid-state systems, working only with solids and dry, clean devices—i.e. free of liquids. “So far, no one has really built an optomechanical device that could operate with anything except solid materials. This is the first time that we have bridged the gap between what people have done with optomechanics, and the microfluidics and sensors work that mechanical engineers are familiar with,” said Bahl.

Optomechanics is interpreted as the use of light-matter interaction processes to enable the actuation and cooling of mechanical vibrations in a microdevice. Usually, in microscale and nanoscale engineering efforts, researchers try to keep devices free of particles, dust, liquids, or anything else that might interfere with operation. Similarly, in the case of optical resonators made of silica glass, moisture can get absorbed into the glass and reduce the quality-factor of the device, prohibiting the incorporation of water-based fluids.

Bahl’s team solved this problem by confining the liquid within a silica microcapillary device —a very thin 100-micron hollow glass filament—through which fluids flow. “Because fluids are now confined within the device, and the light only participates on the outside of the device, you can keep the outside pristine and it really doesn’t matter what’s inside. We can then send through any fluid-phase material we like, without affecting the device performance,” said Bahl. Optical radiation pressure and electrostriction are used to make the microfluidic device vibrate mechanically at extremely high frequencies ranging from 2 MHz to 11 GHz (a few million to a few billion times per second).

The implications of Bahl’s finding resonate not only in the mechanics and optics fields, but may improve biological analyses as well. “For example,” said Bahl, “in some cancers the mechanical properties of cells are known to change. If you have a test tube with a person’s unfiltered blood sample, and you want to find just one or two cancerous cells among many millions of normal cells, measuring the properties of all the cells one at a time is the only good way to do it.” Bahl believes this device could be used to build a very high-speed sensor for such single-cell mechanical analysis. Each flowing cell will perturb the mechanical oscillation frequency as a function of its own mass and stiffness, and these properties can be backed out.

Bahl also points out some other beneficial features of the device. “There is no other existing technology out there that can generate mechanical vibrations at GHz frequencies with fluids present. Furthermore, with this essentially being a transparent glass tube, you can also do an optical measurement on labeled cells, just like in a commercial flow cytometer, at the same time as the mechanical measurements,” he said.

Bahl will be offering a course on “Photonic MEMS” in Spring 2014 (ME498-PM4), designed for non-specialists, that explains the physics of optomechanical systems. In this project-based course students are encouraged to pursue original scientific questions in the broader optomechanics topic area, and to explore applications in sensors, actuators, and physics.

Optics & Photonics News is the monthly news magazine of the Optical Society of America (OSA). Last year’s “Optics in 2012” feature on Bahl’s research focused on his demonstration of a phenomenon called Brillouin cooling, using light-matter interaction to quench mechanical vibrations.