Optical Nanoantennas Enable Multipurpose Particle Manipulation

By tuning the properties of laser light illuminating arrays of metal nanoantennas, researchers have shown that these nanoscale structures enable dexterous optical tweezing and size-sorting of particles.

In work conducted at the University of Illinois at Urbana-Champaign, assistant professor of mechanical science Kimani Toussaint Jr. and his research team have demonstrated for the first time the use of gold bowtie nanoantenna arrays (BNAs) for multipurpose optical trapping and manipulation of submicrometer- to micrometer-size objects. The study, which appeared in the journal Nano Letters, could prove useful for the growing interest in lab-on-a-chip devices.

This is a piece of concept art depicting the various potential BNA trapping states. (Image: Kimani C. Toussaint, Jr.)

The field enhancement and confinement properties of BNAs enable highly efficient optical tweezing and high-speed manipulation of submicrometer-size objects in aqueous environments using low-input power densities, according to the scientists. These characteristics could be useful for optofluidic applications (e.g., lab-on-a-chip devices), formation of optical matter, manipulation of biological matter with reduced specimen photo damage, and basic physics studies of colloidal dynamics.

“In contrast to other plasmonic tweezers, we find that BNAs permit particle trapping, manipulation and sorting utilizing only the optical parameter space, namely, low input power densities, wavelength and polarization,” said Brian Roxworthy, a graduate student in Toussaint’s research group and first author on the paper.

Using empirically obtained “optical trapping phase diagrams” to achieve the desired trapping response, the researchers demonstrated several types of particle manipulation, including single-beam optical tweezing of single particles over the entire nanoantenna area, single-beam optical tweezing of two-dimensional hexagonal-packed particles over the entire nanoantenna area, and optical sorting of particles by size; they also showed stacking of submicron- to micron-sized particles in three dimensions.

According to Toussaint, this is the first demonstration of a range of particle manipulation behavior for a given nanoantenna array.

“We actually excite our nanoantennas off resonance, which to our knowledge is a first, and at the right input optical power, we take advantage of thermal effects combined with optical forces to enable tweezing of tens of particles at a time,” Toussaint explained. “We show that very low power densities are required to achieve the aforementioned behavior. For example, we were able to carry out experiments using a standard laser pointer.”

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