Low-power optical tweezers can trap, manipulate and analyze nanoparticles — including delicate biological samples — new work demonstrates. University of Illinois at Urbana-Champaign engineers showed that near-field optical forces can be enhanced even further by exploiting the high-peak powers associated with a femtosecond optical source, and without making alterations in the fabrication process. “We used an average power of 50 microwatts to trap, manipulate and probe nanoparticles,” said Kimani Toussaint Jr., assistant professor of mechanical science and engineering. “This is 100 times less power than what you would get from a standard laser pointer.” The experimental setup schematic showing laser source, microscope, and imaging detector and spectrometer. Inset illustrates the two different 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. It is already known that plasmonic nanoantennas enhance local fields by several orders of magnitude, and the UIUC group previously showed that these structures can be used with a regular continuous-wave laser source to make very good optical tweezers. “This is exciting because, for the first time, we’re showing that, the near-field optical forces can be enhanced even further, without doing anything extra in terms of fabrication, but rather simply by exploiting the high-peak powers associated with using a femtosecond optical source,” Toussaint said. Their system 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. “This system offers increased local diagnostic capabilities by permitting the probing of the nonlinear optical response of trapped specimens, enabling studies of in vitro fluorescent-tagged cells, or viruses using a single line for trapping and probing rather than two or more laser lines,” he said. 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. In their paper, which appeared Sept. 17 in Scientific Reports, the investigators discuss how the trapping strength of gold bow-tie nanoantenna arrays (BNAs) is greatly 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,” said Brian Roxworthy, a graduate student in Toussaint’s PROBE (Photonics Research of Bio/nano Environments) lab group. Demonstrating controlled particle fusing, he added, could pave the way to the creation of novel nanostructures; it could also lead to improved local magnetic field response, which will be essential for magnetic plasmonics. The paper also demonstrated trap stiffness improvement of up to 5 times as compared to traditional optical tweezers using a femtosecond source, and up to 2 times as compared to continuous-wave nanotweezers. They also showed successful trapping and tweezing of spherical dielectric, metal, fluorescent and nonfluorescent particles varying between 80 nm and 1.2 µm in diameter; silver nanoparticle fusing to BNAs; approximately 3.5 times enhancement of the second-harmonic signal for the combined nanoparticle-BNA system as compared to bare BNAs; and improvement of two-photon fluorescent signal received from trapped microparticles as compared to the response without BNAs. For more information, visit: www.engineering.illinois.edu