"Our optical trapping innovation reduces bench-top optics to a small device on a chip," says Michelle Wang, a single-molecule biophysicist at Cornell University. Traditional optical trapping set-ups can take years for even a skilled postdoc to put together. "This is more like plug-in-and-play," she adds. "For an experiment that typically takes a year to do, we hope to reduce that time to a month."

The Cornell approach also reduces the laser power used, which tends to escalate when attempting to speed things up by running several experiments in parallel. "Usually if you want a hundred traps you need a hundred beams," explains Wang. "Here the laser beam is recycled – to have more traps you just lengthen the waveguide without needing more laser power."

Wang developed the device with Michal Lipson at the Cornell University Kavli Institute, along with researchers at Cornell’s physics department and Howard Hughes Medical Institute. They use an on-chip interferometer to split and recombine an evanescent wave into two counter-propagating waves that form a standing wave. The antinodes of the standing wave provide an array of traps for tens to hundreds of particles.

Position precision

The technique also allows the position of the particles to be controlled with nanometre precision. An on-chip microheater located close to the waveguide induces phase changes in the evanescent wave – a result of the thermo-optic effect – which in turn changes the position of the antinode traps.

Wang describes how in previous nanophotonic waveguides the trapped particles are continuously propelled along the waveguide with no way of precisely controlling their position. But she adds, "These previous designs served as inspirations for our current work."

Tap-resistant stability

The nanophotonic standing-wave approach developed by the Cornell team also inherently offers excellent stability, with video tracking revealing no drift in the position of the trapped particle relative to the waveguide for more than 10 minutes. Previous attempts to set up multiple optical traps based on time-shared lasers have suffered from perturbations because the laser beam is not steady. With the Cornell team’s approach the path differences between the counter-propagating beams are reduced down to around 100 µm with all the components mounted on the chip. "We even tapped it and the set-up remained stable," says Wang.

Next steps

Wang believes that the technique offers a promising approach for single-molecule studies relating to DNA and DNA-based processes, RNA and RNA-based processes, and protein-folding and unfolding, while there could also be applications in cell biology.

"Our challenges now are to optimize every aspect of the devices to make single-molecule measurements routine and streamlined," says Wang. "This requires substantial work to improve measurement and manipulation capabilities, dovetail the device to commercial microscopes and develop software to fully automate the data acquisition and control."

Full details are reported in Nature Nanotechnology.