Dark lattice modes used to create laser light

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A plasmonic nanolaser that operates at visible light frequencies and uses dark lattice modes has been made by researchers at Aalto University, Finland. The results are said to open new prospects for on chip coherent light sources.

The laser 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, the nanolaser uses radiative coupling between silver nanoparticles. The 100nm particles act as 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.

According to the researchers, the challenge was that light may not exist long enough in such small dimensions to be helpful. The researchers found a way around this potential problem: they produced lasing in dark modes.

“A dark mode can be 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 close to each other – radiate little,” explains Professor Päivi Törmä.

“A dark mode in a nanoparticle array induces similar opposite-phase currents in each nanoparticle, but now with visible light frequencies,” she continues.

“Dark modes are attractive for applications where low power consumption is needed. But without any tricks, dark mode lasing would be quite useless because the light is trapped at the nanoparticle array and cannot leave,” adds scientist Tommi Hakala.

“But we found an escape route for the light. Towards the edges of the array, the nanoparticles start to behave more like regular antennas,” concludes PhD student Heikki Rekola.

The edge nanoparticles therefore channel out the light and this nanolight is said to behave like a laser.