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NANOFABRICATION


Tiny laser emerges from Finnish national cleanroom facility


R


esearchers at Aalto University, Finland claim to be the first to develop a plasmonic nanolaser that operates at visible light frequencies and uses so-called


dark lattice modes. The research team used the nanofabrication facilities and cleanrooms of the national OtaNano research infrastructure, which include 4700m2


a 2600m2


of laboratory space including class 10–1000 cleanroom.


Researchers and companies can use the cleanroom as well as special facilities for testing and developing new devices at cryogenic temperatures. New users are given training in the equipment and process, safe ways of working and working with chemicals. Consultancy is available concerning process flows and equipment preference, small-scale processing, measurement techniques and imaging. Some customers rent the cleanroom space and use their own dedicated equipment. The Aalto laser works at length scales 1000 times smaller than the thickness of a human hair. The lifetimes of light captured in such small dimensions are extremely short. But the potential is for on-chip coherent light sources, such as lasers, that are extremely small and ultrafast. 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 utilises radiative coupling between silver nanoparticles. These 100nm 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. Organic fluorescent molecules were used to provide the input energy (the gain) that is needed for lasing.


20 /// Cleanrooms & Contamination Control 2017


❱ ❱ 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. Image: Antti Paraoanu.


LIGHT FROM THE DARK A major 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 smart way around this potential problem: they produced 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,” explains Academy Professor Päivi Törmä. “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. But without any tricks, dark mode lasing would be quite useless because the light is essentially trapped at the nanoparticle array and cannot leave”, adds staff scientist Tommi Hakala. “But by utilising the small size of the array, we found an escape route for the light.” EE


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