Using “dark lattice modes,” researchers at Aalto University have created a plasmonic nanolaser that operates at visible light frequencies. Researchers at Aalto University have made an array of nanoparticles combined with dye molecules to act as a tiny laser. The lasing occurs in a dark mode and the laser light leaks out from the edges of the array. Courtesy of Antti Paraoanu. The laser works at length scales 1000 times smaller than the thickness of a human hair. The results open new prospects for on-chip coherent light sources, such as lasers, that are extremely small and ultrafast. A major challenge in achieving lasing this way was that light may not exist long enough in such small dimensions to be helpful. Professor Päivi Törmä said they found a way around this potential problem by producing lasing in dark modes. "A dark mode can be intuitively understood by considering regular antennas: A single antenna, when driven by a current, radiates strongly, whereas two antennas — if driven by opposite currents and positioned very close to each other — radiate very little," explained Törmä. "A dark mode in a nanoparticle array induces similar opposite-phase currents in each nanoparticle, but now with visible light frequencies.” The laser operation in this work is based on silver nanoparticles arranged in a periodic array. In contrast to conventional lasers, where the feedback of the lasing signal is provided by ordinary mirrors, this nanolaser utilizes radiative coupling between silver nanoparticles. These 100-nanometer-sized particles act as tiny antennas. To produce high intensity laser light, the interparticle distance was matched with the lasing wavelength so that all particles of the array radiate in unison. "Dark modes are attractive for applications where low power consumption is needed,” said staff scientist Tommi Hakala. “But without any tricks, dark mode lasing would be quite useless because the light is essentially trapped at the nanoparticle array and cannot leave.” By utilizing the small size of the array, the researchers found an escape route for the light. The results have been published in the journal Nature Communications (doi:10.1038/ncomms13687).