3-D bow-tie nanolaser defies diffraction limit

Nanolaser devices operating at room temperature defy the diffraction limit, thanks to a lasing cavity composed of 3-D bow-tie-shaped metal nanoparticle dimers designed by researchers at Northwestern University.

The diffraction limit has been a challenging obstacle in nanoscale laser design. Because the metal dimers – the structure of which forms the lasing cavity – support localized surface plasmons, the resulting nanolaser has no size restrictions when confining light.

“Coherent light sources at the nanometer scale are important not only for exploring phenomena in small dimensions, but also for realizing optical devices with sizes that can beat the diffraction limit of light,” said Teri W. Odom, a materials science and engineering professor at the university’s McCormick School of Engineering and Applied Sciences. The sources include silicon-based photonic devices, all-optical circuits and nanoscale biosensors.

“There has been a lot of excitement in our proof-of-concept studies of room-temperature lasing from a cavity the size of a virus particle,” Odom said. “The dimensionality of 3-D-shaped particles is critical for observing lasing. Our fabrication approach to produce 3-D bow-tie lasers is scalable and simple.”

Bow-tie geometry offers plasmon lasers two significant benefits compared with previous designs: First, because of an antenna effect, the bow-tie structure provides a well-defined electromagnetic hot spot in a nanosize volume; second, because of its discrete geometry, the individual structure has only minimal metal losses.

“The gap between the nanoparticles is an important design criterion for boosting the confined electrical fields and, thus, reducing the lasing threshold onset,” she said.

The Northwestern nanolaser, as with most such devices, requires external optical pumping, but “for commercialization purposes, we need to design a laser that can be electrically pumped,” Odom said. “We are investigating the possibility of combining these bow-tie nanostructures into conventional PN junction devices.”

Next is the fabrication of single bow-tie nanolasers instead of arrays for quantum optics studies, she said, and the researchers are testing different resonator designs and gain materials.

The results were published in Nano Letters (doi: 10.1021/nl303086r).