A typical laser is a cavity where light bounces between mirrors and gets amplified. But you can ditch the mirrors and make light circulate inside a tiny particle smaller than the light wave itself. That's how the new lasers work: nanoparticles of indium phosphide sitting on perfectly smooth gold.
The particles, ranging from 115 to 480 nanometers in length, hundreds of times thinner than a human hair, were placed on a gold substrate polished to atomic smoothness. When such a particle is hit with short pulses of light, it starts lasing — emitting coherent light that doesn't go in random directions but travels along the gold surface. The smallest laser in the experiment emitted at a wavelength of 730 nanometers, which is deep red, almost at the edge of the infrared.
The key trick is operating near the surface plasmon resonance — the collective oscillation of electrons at the metal-dielectric boundary, which can squeeze light into extremely small volumes. It was previously thought that losses near the resonance were too high, but the authors showed that if you use a material with very high gain, that very indium phosphide, lasing is possible. Moreover, the farther the wavelength from the resonance, the narrower the emission line and the lower the threshold pump energy. In larger particles, they managed to excite second- and third-order plasmon modes — the light inside the particle formed not one but two or three nodes of a standing wave.
This approach pushes the limits of laser miniaturization. Such lasers could be used for ultra-fast data transfer within computer chips, for ultra-sensitive sensors capable of detecting single molecules, or as optical barcodes for tracking cells in biological experiments. And all this fits in a device no thicker than a spider web.
Indium phosphide, the material these nanoparticles are made of, is the same stuff used in high-speed transistors and lasers for fiber-optic communications. Now it also works at a scale comparable to viruses.