Researchers at Aalto University in Finland are the first to develop a plasmonic nanolaser that operates at visible light frequencies and uses what are called "dark lattice modes."

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The achievement open new prospects for on-chip coherent light sources such as lasers that are extremely small and ultrafast.

The laser works at length scales 1,000 times smaller than the thickness of a human hair. The lifetimes of light captured in such small dimensions are so short the light wave has time to wiggle up and down only a few tens or hundreds of times.

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, 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 all particles of the array radiate in unison.

Organic fluorescent molecules were used to provide the input energy (the gain) that is needed for lasing.

A major challenge in achieving lasing this way was that light might not exist long enough in such small dimensions to be helpful. Researchers found a smart way around this potential problem: they produced lasing in dark modes.

"A dark mode can be intuitively understood by considering regular antennas," said academy Professor Päivi Törmä.

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

Dark modes are attractive for applications where low power consumption is needed, said staff scientist Tommi Hakala.  He said 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, however, the team found an escape route for the light, said Ph.D. student Heikki Rekola. Towards the edges of the array, the nanoparticles start to behave more and more like regular antennas that radiate to the outer world.

The research team used the nanofabrication facilities and cleanrooms of the national OtaNano research infrastructure. The results were published in the journal Nature Communications.

Tagsplasmonic nanolaser, Aalto University, laser, silver nanoparticles, Dark modes