In optical tweezing, a process which involves using beams of light to trap microscopic objects, the smaller the volume of light that confines a microparticle, the more tightly the particle can be trapped. Maximizing the stiffness of the optical trap can lead to more precise measurements at the nanoscale and more photon-efficient tweezing of objects. Building on conventional optical tweezing techniques, a University of Exeter-led team, including researchers at the University of Glasgow and the Vienna University of Technology, devised a way to optimize particle trapping through customization. The researchers tailored the shape of the light fields in the optical trap to suit the specific particle, with the aim of optimizing trapping stiffness in all three dimensions. The team focused on optimizing the trapping of larger particles, which include many types of biological cells. When a particle is large, most of the light from optical tweezing is concentrated near the particle’s center. However, light interacts more strongly with the surface of a particle than with its center — a phenomenon that affects the ability of larger particles to make the most of the light coming from the tweezers. Light intensity in conventional optical tweezers (left) and a custom-tailored optical trap (right). Projections show cross-sections through the middle of the particle, which is 6 μm in diameter. Courtesy of the University of Exeter. “We hypothesized that, if instead of being concentrated in the middle of the particle, the light enveloped it, that would confine the particle more strongly, giving it a sort of a tight hug,” professor David Phillips said. Determining the shape of light that would yield the strongest confinement was challenging both computationally and experimentally. “There is no one-size-fits all solution here,” researcher Une Butaite said. “For best performance, every different particle requires a custom suit-of-light, if you will.” Typically, the spatial shape of the laser beam in an optical tweezer is created using a Gaussian beam profile. While Gaussian beams are versatile and straightforward to create, they usually do not have the optimal shape of light field needed to tightly trap a microparticle — especially a larger particle whose size is greater than the trapping wavelength. To determine to what extent 3D trap stiffness could be enhanced by tailoring the spatial profile of the laser beam, instead of by increasing the laser power, the researchers explored the problem through various mathematical and numerical methods and rigorous experimental techniques. They designed bespoke trapping beams using an integrated, multiparameter optimization strategy that allowed all three dimensions to be considered simultaneously, in terms of both stiffness enhancement and trap stability. A particle is not completely immobilized by optical tweezers. “It is experiencing thermal motion of the molecules surrounding it,” Butaite said. “A bit like a boat in a lake rocked about by the wind and the waves but prevented from drifting away by the anchor, a particle in optical tweezers is constantly jiggling about, but its motion is confined to a certain volume.” The researchers estimate that custom-tailored trap shapes could confine microsphere motion to a volume up to 200 times smaller than a Gaussian trap of equivalent power. The researchers found the implementation of such highly optimized trapping fields to be extremely sensitive to precise experimental conditions. To validate their approach experimentally, they developed a real-time optimization routine that iteratively adapts the trapping field to the shape of the particle in situ. Using a strategy inspired by wavefront-shaping, the researchers passively suppressed the Brownian fluctuations of microspheres in every direction concurrently, demonstrating order-of-magnitude reductions in their confinement volumes. The team showed that significant gains in 3D optical trapping efficiency are possible by judiciously structuring light fields, and developed theoretical and experimental routes to achieving these gains. The work of the Exeter-led team could set scientists on a path toward reaching the limits of optical control when using optical tweezers to manipulate nano- and micromaterials. The research was published in Science Advances (www.doi.org/10.1126/sciadv.adi7792).
