04.10.2018
An international team of researchers has demonstrated a new design for random lasers that paves the way for cheaper laser sources. For the first time in the world, researchers were able to control the direction of a laser’s output beam by applying external voltage. The results of the project coordinated by Tampere University of Technology (TUT) were recently published in the prestigious Nature Communications.
Lasers are unique sources of light, which are revolutionising essentially all areas of modern life, including telecommunications, biomedicine and measurement technology. Conventional lasers consist of a gain medium and an optical cavity. The gain medium is able to spontaneously emit light and further amplify it through stimulated emission. The simplest cavity typically has two mirrors facing each other, enclosing the gain medium and thereby forcing emitted light to travel back and forth between the mirrors. Such feedback through the gain medium leads to self-sustained oscillations of light at the cavity resonance frequencies, part of which is extracted as the laser output beam. While extremely useful and versatile, such designs require precise alignment of the components and are relatively expensive to construct.
Random lasers consist of a random medium with gain and scattering centres, 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 (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 exhibit transistor-like behaviour 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 waveguiding 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.
The research was funded by the Academy of Finland through the Finland Distinguished Professor Programme and by Tampere University of Technology.
The research paper was published in Nature Communications:
Picture:
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.
News submitted by: Tuuli Laukkanen
