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A tiny laser using


he science of nanotechnology holds the same allure that microelectronics once held. Entrepreneurs see new generations ofmicroelectronic

devices, optics, pharmaceutical delivery systems,medical diagnostic devices and an array ofmolecular level products about to break through. Attend a nanotechnology forumor trade

show and the exhibition floor is crammed with great ideas available for licensing or investment or partnering opportunities. What is rarely found is themuch needed attention to detail regarding how the device is to be produced in a cost effectivemanner, to insure that initial investments are protected by profitable production. Contaminants that negatively affect

product yield can be particles or gases. The purpose of the cleanroomis to isolate the product fromthe contaminants that cause its rejection.

A SENSE OF SCALE Nanotechnology product research and development eventually calls for a pilot line that permits the production process to be examined, fine-tuned and scaled up as volume increases.Often the facility serves to generate cash as small lots of product are manufactured for introduction to the marketplace. The tight budget of today’s nanotechnology facility start-upsmakes the need for small-scale cleanrooms important. A small-scale cleanroomoffers the

environment of a full-scale production facility, provides the stringent environment commonly required for nanotechnology

32 /// Environmental Engineering /// February 2017

 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


Researchers at Aalto University in Finland are the first to develop a plasmonic nanolaser that operates at visible light frequencies using so-called dark latticemodes

productmanufacture, enables the process to be evaluated in a “real” environment and offers the opportunity to establish real yields upon which financial projections can be based, all this at a reasonable cost so important to a start-up operation. As an example, a new Finnish laser

works at length scales 1,000 times smaller than the thickness of a human hair. In their development, the teamused the nanofabrication facilities and cleanrooms of the national OtaNano research infrastructure. The lifetimes of light captured in such

small dimensions are so short that the light wave has time to wiggle up and down only a few tens or hundreds of times. The results open new prospects 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 ordinarymirrors, this nanolaser utilises radiative coupling between silver nanoparticles. These 100nm sized particles act as tiny antennas. To

produce high-intensity laser light, the interparticle distance wasmatched 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.

LIGHT FROM DARK Amajor challenge in achieving lasing this way was that lightmay 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 darkmodes. “A darkmode 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 darkmode in a nanoparticle array induces similar opposite-phase currents in each nanoparticle, but now with visible light frequencies,” she continues. Staff scientist TommiHakala adds: “Dark

modes are attractive for applications where low power consumption is needed. But without any tricks, darkmode lasing would be quite useless because the light is essentially trapped at the nanoparticle array and cannot leave.” “But by utilising the small size of the

array,” says PhDstudentHeikkiRekola, “we found an escape route for the light. Towards the edges of the array, the nanoparticles start to behavemore andmore like regular antennas that radiate to the outerworld.” EE

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