Finnish researchers develop nanolaser

Laser produces high intensity light and works at length scales 1,000 times smaller than the thickness of human hair

Researchers at Finland’s Aalto University have developed a nanolaser that operates at visible light frequencies and uses so-called dark lattice modes.

 The laser operation 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 utilises radiative coupling between silver nanoparticles. These 100-nanometer-sized particles act as small antennas. To produce high intensity laser light, the distance between particles was matched with the lasing wavelength so that all particles of the array radiate in unison. Organic fluorescent molecules were used to provide the input energy that is needed for lasing.

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 that the light wave has time to oscillate up and down only a few tens or hundreds of times.

A challenge in achieving lasing this way was that light may not exist long enough in such small dimensions to be helpful. The researchers found a way around this potential problem by producing lasing in dark modes.

Päivi Törmä, a professor of physics at the university, said: “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 very close to each other, radiate very little. A dark mode in a nanoparticle array induces similar opposite-phase currents in each nanoparticle, but with visible light frequencies.”

Dark modes are desirable for low power consumption applications. However, one issue with dark mode lasing is that the light becomes trapped in the nanoparticle array and cannot escape.

Heikki Rekola, a PhD student at the university, said: “By utilising the small size of the array, we found an escape route for the light. Towards the edges of the array, the nanoparticles start to behave more and more like regular antennas that radiate to the outer world.”

The results open prospects for coherent light sources, such as lasers, that are extremely small and ultrafast.

The results have been published in the journal Nature Communications.