Narrow waveguide strengthens optical trap

15 Jan 2009

Channelling light through a sub-wavelength waveguide is allowing researchers to manipulate the smallest particles to date.

A slot waveguide that focuses light to nanometre scales is overcoming prior limitations caused by light diffraction to trap and transport particles as small as 75 nm in diameter. The trap may allow researchers to boost the accuracy of biological sensors and create a new range of lab on a chip diagnostic tools (Nature doi:10.1038/nature07593).

"We've demonstrated a way to condense photons down to a very small area and stream them along a special type of waveguide, a device that acts like a nanoscale optical fibre," said David Erickson, a researcher at Cornell University. "When DNA or nanoparticles float near these streaming photons, they are sucked in and pushed along with the flow."

The waveguide consists of two parallel silicon bars 60 nm apart. A number of these waveguides ranging from 60–120 nm wide are located parallel to one another and spaced closely together. Light from a 1550 nm laser is channelled through each of the slots via a coupling fibre. The optical power at the entrance of each slot is less than 300 mW.

Because the channels are smaller than the wavelength of the light, an evanescent field extends beyond the boundaries of each waveguide. The evanescent field can exert a downward force on tiny particles, including DNA molecules, pulling them down into the slot. Light pressure then moves the trapped objects along the slot.

In the set-up, the researchers used water solutions containing either DNA or tiny nanoparticles and washed the fluids over the waveguide microchannels at a speed of 80 µm per second. The system traps less than one in four of the target particles, but with smaller channel sizes, slower flows and higher energy lasers, the success rate increases.

"We're hoping to better understand some of the underlying physics to see what else might be possible with this approach," commented Erickson. "Ultimately, we imagine taking all of the ultrafast and highly efficient optical devices that have been developed for communications and other applications over the last 20 years and applying them to manipulate matter in different types of nanosystems."