Femtosecond laser-powered nanotweezers benefit cell studies

Ashley N. Rice, ashley.rice@photonics.com

Low-power plasmonic tweezers can trap, manipulate and analyze nanoparticles – including fragile biological samples – using ultralow input power densities.

University of Illinois at Urbana-Champaign engineers have demonstrated for the first time that near-field optical forces can be further enhanced by exploiting the high peak powers associated with a femtosecond optical source and without altering the fabrication process.

“We used an average power of 50 microwatts to trap, manipulate and probe nanoparticles,” assistant professor of mechanical science and engineering Kimani C. Toussaint Jr. said in a university statement. “This is 100 times less power than what you would get from a standard laser pointer.”

Scientists already know that plasmonic nanoantennas enhance local fields by several orders of magnitude, and the group had previously demonstrated that these structures can be used with a regular continuous-wave laser source to make very good optical tweezers.

Femtosecond sources offer several advantages over continuous-wave ones with regard to plasmonic nanotweezers.

“We have experimentally demonstrated that the femtosecond source leads to higher trap stiffness compared to a continuous-wave source in our nanotweezer system for a given input average power,” graduate student Brian Roxworthy told BioPhotonics. “Furthermore, the femtosecond source allows for a much lower input average power (<10 percent that of a continuous-wave source) to be used for trapping and manipulating particles. This lowers the focal power density considerably compared to continuous-wave nano-tweezers, which is attractive for tweezing biological species.”

The experimental setup schematic showing laser source, microscope, and imaging detector and spectrometer. Inset illustrates the two sample configurations that were explored; the red arrows correspond to the input polarization directions, and the black arrows depict the propagation vector. Courtesy of the University of Illinois.

High peak powers associated with femtosecond pulses also are attractive because they provide access to the nonlinear optical response of trapped specimens and plasmonic nanostructures, he said.

“This may be particularly useful for simultaneous trapping and probing of fluorescent-tagged biological samples using a single femtosecond-laser source.”

In their paper, which appeared in Scientific Reports (doi: 10.1038/srep00660), the investigators explain how the trapping strength of gold bow-tie nanoantenna arrays (BNAs) is exponentially improved using a femtosecond pulsed laser beam. They also describe using a femtosecond source to perform optical trapping with plasmonic nanotweezers.

“We present strong evidence that a [femtosecond] source could actually augment the near-field optical forces produced by the BNAs, and most likely, other nanoantenna systems as well. To our knowledge, this has never been demonstrated,” Roxworthy said. “It is possible that, by exploring additional parameters not covered in this study (e.g., nanostructure shape, pulse duration), even larger near-field forces can be generated.”

This image shows experimentally collected spectra from the trapped fluorescent microbead-BNA system (inverted orientation) with horizontal and vertical polarization (parallel and perpendicular to the bowtie axis, respectively). The reference is taken from a microbead trapped away from the arrays in the inverted orientation. The inset image depicts the dramatic fluorescence enhancement when the particle is moved onto the array (indicated by the yellow outline). The scale bar is 5 µm. Courtesy of K. Toussaint, University of Illinois.

The key to the femtosecond nanotweezer’s trapping performance and nonlinear optical response lies in the orientation of nanostructures with respect to the incident optical field, Roxworthy said. When using a femtosecond source in an inverted orientation – a 2-D plasmonic trap – “the trapped particle can act as a miniature lens, which further focuses the incident field onto the nanostructures. The result is that trap stiffness is reduced slightly, but the nonlinear optical response of the overall system is significantly enhanced.”

Demonstrating controlled particle fusing, he added, could pave the way to creating novel nanostructures and could lead to improved local magnetic field response, which will be essential for magnetic plasmonics. The system also is suitable for biological (lab on a chip) applications such as cell manipulation because it runs at average power levels roughly three orders of magnitude lower than the estimated optical damage threshold for biological structures, Toussaint said.

The team is trying its experimental study on biological specimens.

“While we have experimentally demonstrated an increase in trap performance using femtosecond rather than continuous-wave illumination, the actual mechanism for the increased forces is not well understood theoretically,” Roxworthy said. “Thus, it would be very interesting to further investigate the trapping dynamics of femtosecond plasmonic nanotweezers.” The team also will pursue the fusing mechanism that occurs when trapped species bond to the nanoantenna surface, to fabricate 3-D nanostructures.