First Proof of a Transistor Random Laser

Prototype of a random laser: It is able to control the direction of a laser’s output beam by applying external voltage. (Source: Case Western Reserve U.)

In the last half-century, laser tech­nology has grown into a multi-billion-dollar global industry and has been used in every­thing from optical-disk drives and barcode scanners to surgical and welding equipment. Now, lasers are poised to take another step forward: Researchers at Case Western Reserve Univer­sity, in colla­boration with a finnish group and partners around the world, have been able to control the direction of a laser’s output beam by applying external voltage.

It is a historic first among scientists who have been experi­menting with “random lasers” over the last 15 years or so. “There’s still a lot of work to do, but this is a clear first proof of a tran­sistor random laser, where the laser emission can be routed and steered by applying an external voltage,” said Giuseppe Strangi, Ohio Research Scholar in Surfaces of Advanced Materials at Case Western Reserve Univer­sity. The project was aimed at over­coming certain physical limi­tations intrinsic to that second gene­rations of lasers.

Laser tech­nology has been hampered by signi­ficant short­comings: Not only do users have to physi­cally mani­pulate the device projec­ting the light to move a laser, but to function, they require a precise alignment of components, making them expensive to produce. Those limi­tations could soon be eliminated: Strangi and research partners in Italy, Finland and the United Kingdom have recently demon­strated a new way to both generate and mani­pulate random laser light, including at nano-scale.

Even­tually, this could lead to a medical proce­dure being conducted more accurately and less inva­sively or re-routing a fiber optic communi­cation line with the flip of a dial, Strangi said. Conven­tional lasers consist of an optical cavity, or opening, in a given device. Inside that cavity is a photo­luminescent material which emits and amplifies light and a pair of mirrors. The mirrors force the photons, or light particles, to bounce back and forth at a specific frequency to produce the red laser beam we see emitting from the laser.

“But what if we wanted to minia­turize it and get rid of the mirrors and make a laser with no cavity and go down to the nano­scale?” he asked. “That was a problem in the real world and why we could not go further until the turn of this century with random lasers.” So random lasers, which have been researched in earnest for about the last 15 years, differ from the original tech­nology first unveiled in 1960 mostly in that they do not rely on that mirrored cavity.

In random lasers, the photons emitted in many direc­tions are instead wrangled by shining light into a liquid-crystal medium, guiding the resulting particles with that beam of light. Therefore, there is no need for the large, mirrored structure required in traditional appli­cations. The resulting soliton functions as a channel for the scattered photons to follow out, now in an orderly, concen­trated path.

One way to under­stand how this works is by envi­sioning a light-particle version of the solitary waves that surfers can ride when rivers and ocean tide collide in certain estuaries, Strangi said. Finally, the researches hit the liquid crystal with an elec­trical signal, which allows the user to steer the laser with a dial, as opposed to moving the entire structure. “That’s the big develop­ment by this team”, Strangi said.

“That’s why we call it “tran­sistor,” because a weak soliton signal controls a strong one – the laser output.” Strangi said. “Lasers and transistors have been the two leading tech­nologies that have revo­lutionized the last century, and we have dis­covered that they are both inter­twined in the same physical system.” The researchers believe that their results will bring random lasers closer to practical appli­cations in spectro­scopy, various forms of scanning and bio­medical procedures. (Source: Case Western Res. U.)

Reference: S. Perumbilavil et al.: Beaming random lasers with soliton control, Nat. Commun. 9, 3863 (2018); DOI: 10.1038/s41467-018-06170-9

Link: Laboratory of Photonics, Tampere University of Technology, Tampere, Finland • Nanoplasm Laboratory (G. Strangi), Dept. of Physics, Case Western Reserve University, Cleveland, USA

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