Researchers at Northwestern University have developed a laser the size of a virus particle that can operate at room temperature. The "nanolaser," which uses gold nanoparticles instead of mirrors, is claimed to be the first demonstration to make use of a so-called bowtie arrangement of metal nanoparticles, though nano-scale lasers have been previously demonstrated.
In many ways the bowtie nanolaser works like an ordinary laser, laser being an acronym, standing for Light Amplification by Stimulated Emission of Radiation. Like other sources of light, a laser is in the business of exciting electrons into higher energy states, so they can emit photons as they calm back down. Unlike other light sources, lasers achieve this in an extremely regimented way, producing a very focused, single-color beam of light.
This nanolaser, like ordinary-sized lasers, achieves this through the "optical pumping" (effectively flashing light at) a gain material in which electrons are to be excited. The light stimulates electrons which in turn emit photons of the same wavelength (and therefore color). The nanolaser's gain material is a single molecule of organic dye.
In an ordinary laser, mirrors are used to reflect the stimulated light back and forth through the gain material to further excite electrons (though a mirror at one end lets out some of the light, resulting in the controlled laser beam.) The space between the mirrors is known as the optical cavity. In the bowtie nanolaser, however, the optical cavity is a gap in the middle of the bowtie.
What gap, exactly? Well, when it comes to molecules, "bowtie" is another term for a dimer molecule or molecule group, which is one composed of two identical molecular parts: here, individual gold nanoparticles. (A nanoparticle, you'll recall, is simply a name for a particle under 100 nanometers across.) The nanolaser's cavity, then, is the gap between the two nanoparticles.
There's no reflection as such, though. In fact, the gap between the nanoparticles is less than the wavelength of the resultant laser beam. "The surface plasmons [units of electron oscillation] of the metal nanoparticles effectively squeeze the light into a very small volume," Northwestern Professor of Materials Science and Engineering Teri Odom told Gizmag. "When the gain material is optically pumped, this energy is transferred into the cavity, and lasing can occur."
Key to this research is the bowtie, without which, the researchers claim, the gain cannot overcome the losses that occur in the system. The team reports coherent light emission in the face of "slight geometric inhomogeneities," meaning the technology should prove robust when it comes to real-world applications.
The researchers claim that the development is a step towards ultrafast optical applications which could improve data storage capacities, and help bring about compact ultra-fast photonic sensors and better biological sensors.
The team's paper, Plasmonic Bowtie Nanolaser Arrays, was published recently in the journal Nano Letters.
Source: Northwestern University