When lasers start getting down to the nanoscale, they run up against the diffraction limit where the size of the laser cannot be smaller than the wavelength of light it emits. But researchers have shown that nanoscale plasmonic lasers can reach an optical mode well below this limit by confining light of very short wavelengths through the use of surface plasmons—oscillations of electrons that occur at the junction of a metal and an insulator. This has revitalized the hope that chips populated with these plasmonic nanolasers could make possible computer processors run by light rather than electrons.
Now researchers at Northwestern University have developed a new design for plasmonic nanolasers that are the size of a virus particle and capable of operating at room temperature. They described the discovery in the journal Nano Letters, (“Plasmonic Bowtie Nanolaser Arrays”).
"The reason we can fabricate nano-lasers with sizes smaller than that allowed by diffraction is because we made the lasing cavity out of metal nanoparticle dimers -- structures with a 3-D 'bowtie' shape," says Teri Odom, the leader of the research and professor of Chemistry at Northwestern, in a press release.
The bowtie geometry allowed the nanoparticles to achieve an antenna effect and suffer only minimal metal “losses”. Typically, plasmon nanolaser cavities have suffered from both metal and radiation losses that required them to be operated at cryogenic temperatures.
Odom also explains that the antenna effect allows for lasing to occur from an "electromagnetic hot spot"—a capability not demonstrated previously. "Surprisingly, we also found that when arranged in an array, the 3-D bowtie resonators could emit light at specific angles according to the lattice parameters," Odom adds in the release.
Of course, nanolasers that are capable of operating at room temperature are not unique. Researchers at the University of California, San Diego reported earlier this year on a room temperature nanolaser design that requires less power to generate a coherent beam than other designs. The key difference between the two plasmonic nanolasers seems to be the bowtie geometry the Northwestern team developed.
At least one of the aims of both lines of research seems to be to integrate these nanolasers with CMOS electronics. Whether they can reach this lofty goal remains to be seen, but these nanolasers are a key step in their realization.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.