Optically induced solitons give directionality to random laser output

The design combats the poor quality of the output of random lasers, paving the way for lower-cost devices.

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IMAGE: Shown is a random laser in a soliton waveguide. The waveguide is formed by focusing a red laser beam to a liquid crystal (blue ellipses). The laser is created when dye molecules are pumped with green light. This generates yellow light, which the soliton channels to the output. The direction of the soliton and the output can be controlled via external voltage. (Image credit: Tampere University of Technology)

Random lasers consist of a random medium with gain and scattering centers, where recurrent multiple scattering of light along random paths can provide sufficient feedback to support cavityless lasing. However, due to their disordered character, random lasers tend to emit light into unspecified directions and with spiky transverse beam profiles. Such poor quality of the output limits the use of random lasers in a number of crucial applications, where one must be able to deliver light at the desired location.

A team of researchers from Tampere University of Technology (TUT; Finland), University "Roma Tre" (Italy), the University of Southampton (UK), and Case Western Reserve University (USA) has demonstrated a new design for random lasers, where an optically induced soliton waveguide improves the lasing characteristics and guides the laser output into a well-defined direction with good profile. Such lasers, defined in their Nature Communications paper, exhibit transistor-like behavior as the soliton aids bringing the system into operation and, most remarkably, emit light in a direction controllable by external voltage.

To overcome the limitations of random lasers, the researchers combined a nematic liquid-crystal all-optical waveguide with random lasing. The liquid crystal, doped with dye molecules to provide gain, plays two different roles. By relying on the nonlinear response to a weak non-resonant continuous-wave laser beam, a light-induced soliton waveguide, known as a nematicon, is induced. The scattering properties provide sufficient feedback for random lasing when the dye molecules are pumped by a pulsed laser resonant with them. The scientists demonstrated that such nematicon-assisted random laser exhibits typical characteristics, such as pump energy threshold for lasing and narrow spectral peaks emerging from a spontaneously emitted background. Moreover, the soliton waveguide collects the generated light and channels it to the output with a smooth spatial structure.

"Besides these basic properties, novel random lasers offer several advantages over conventional lasers," says Sreekanth Perumbilavil, PhD student responsible for the experiments conducted at TUT. "For example, we found that the near-infrared nematicon can reduce the threshold for lasing. In other words, by turning the soliton on or off, the system could be brought to lasing, allowing transistor-like action where a weak signal (nematicon) controls a strong one (laser output)."

"By acting on the gain medium itself, we were able to steer the output direction of the laser emission by controlling the nematicon by an external voltage, providing the first voltage-routed random laser," says visiting professor Gaetano Assanto, who supervised the work.

The researchers believe that their results will bring random lasers closer to practical applications, for example, by allowing spectroscopy over a range of directions or scanning of the environment. In addition, the findings increase our understanding of random lasers in general.

SOURCE: Tampere University of Technology; http://www.tut.fi/en/about-tut/news-and-events/arti?art_id=534

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