College of Science

Mohanty team's optical wrench can move microparticles

UT Arlington physicists have developed a new fiber-optic spanner, or wrench, that uses two laser beams to stably rotate and move microscopic objects, such as living cells, an innovation that will help scientists to work more efficiently at the microscopic level.

The new technology surpasses current methods of fiber-optic rotation because it allows the object to be rotated at any axis, giving a fuller view. Through this method, cancer cells could be imaged during rotation or oocyte cells could be moved during in vitro fertilization, researchers said. The spanner also can use a "rotating bead handle" to twist and untwist DNA molecules to allow its sequencing more rapidly than current methods.

The innovation is detailed in a new report, "Fiber-optic Spanner," published today in the journal Optics Letters and available online (http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-37-24-5030). Samarendra Mohanty, assistant physics professor, led the research team and co-authored the paper with doctoral student Bryan Black.

"This technique overcomes many of the challenges to working with optically trapped microscopic objects and has numerous possibilities for nanotechnology and biotechnology," Mohanty said. "It is widely applicable because it is not limited by the sample's shape and does not require any mechanical motion of the fiber. Also, because the tools are fiber-optic, they can be used at a larger depth inside closed environment such as the body."

A research paper, based on advanced application of this technology, by Black, Mohanty and Dijun Luo, a recent UT Arlington graduate from the College of Engineering's Department of Computer Science, was recently accepted for publication in Applied Physics Letters, which is published by the American Institute of Physics.

The fiber-optic spanner uses two laser beams emanating from optic fibers. The fibers are placed on opposite sides of the object with a transverse offset, or parallel, but not co-linear.

Through a process of counter propagation, the beams use gradient and scattering forces to trap and rotate an object. The axis on which the object is rotated can be adjusted by changing the direction of offset between the two fibers. This gives a fuller, deeper view than what is available with most existing microscope objective-based laser tweezers systems, Mohanty said. By adjusting the power of one beam, the object can also be moved from one place to another. An ultrafast laser beam in one fiber optic arm can also be used to analyze fluorescence of trapped objects.

"The attention that Dr. Mohanty and his team are receiving for their

A fiber-optically trapped and rotated human smooth muscle cell in the center of two transversely offset fibers. (Photo courtesy S. Mohanty)
work in the lab is well-deserved," said Carolyn Cason, UT Arlington's vice president for research. "His enhancement of current technology could yield results for a number of fields and it is a demonstration of the strides that come from determined exploration."

Mohanty joined the UT Arlington College of Science in 2009 and leads the biophysics and physiology group. He is a co-principal investigator on a $300,000 grant from the Molecular and Cellular Biosciences division of the National Science Foundation to explore rotational dynamics of motor proteins.

His research team focuses on methods to improve the manipulation of microscopic objects by using existing laboratory tools, such as optical tweezers, and developing their own. They have published numerous scientific articles, including a December 2011 Nature Photonics paper that described the use of spinning microparticles to direct the growth of nerve fiber. (Available online: http://www.nature.com/nphoton/journal/v6/n1/full/nphoton.2011.287.html#/).

The recently developed fiber-optic spanner also can be used to rotate a micro-motor, or birefringent particle, and create microfluidic flow. That motion could be used to move objects into a flow for analyzing or to test variations in viscosity of fluid, Mohanty said. The method also is translatable to "lab-on-a-chip" devices, tiny devices that can perform multiple laboratory functions of chemical/physical synthesis or analysis in parallel. (More information is available here: http://pubs.rsc.org/en/content/articlelanding/2012/LC/c2lc40538e.)

Published December 5, 2012