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PHOTONICS

An unconventional laser

Variable-intensity light reveals unexpected connections between areas of physics and promises clearer microscopic images

A conventional laser (green) strikes a material that converts the light into a random laser (red), able to propagate in a variety of directions

Diederik Wiersma / LENS – Florence A conventional laser (green) strikes a material that converts the light into a random laser (red), able to propagate in a variety of directionsDiederik Wiersma / LENS – Florence

Physicist Cid Araújo of the Federal University of Pernambuco (UFPE) likes to say in his talks that he carries out astrophysics experiments on his workbench. On the tables in his optics laboratory, he and his colleagues explore the properties of some materials, making them emit an unconventional type of laser light. It is what is known as random laser light, which can be emitted naturally by objects in interstellar space and whose production is now beginning to be better understood due to the recent results obtained by groups such as that at UFPE.

A conventional laser is generated by imprisoning a beam of light between two mirrors and forcing it to pass through a crystal that emits light efficiently. The back-and-forth of the light between the mirrors stimulates the crystal to emit even more light (see infographic). When escaping through one of the mirrors, which is semi-transparent, this light is high-intensity. Its electromagnetic waves oscillate in synchrony, at the same frequency, and propagate in the same direction.  It is completely ordered light, very different from that emitted by an incandescent bulb, whose waves have different frequencies and travel in different directions.

Random lasers, however, are not produced using mirrors. To create them, the UFPE researchers use a normal laser to illuminate a material with special optical properties, generally a powder or colloid containing particles capable of absorbing, emitting and scattering light in a disordered way. Nanoparticles containing ions of neodymium or titanium oxide are among the materials used, similar to those prepared and described by physicist Lauro Maia of the Federal University of Goiás, a member of the UFPE team. Illuminated by a normal laser, these nanoparticles, mixed with other materials in a solid or liquid state, can generate a random laser.

“The random laser is a source of light with intermediate properties between that of an incandescent bulb, whose electromagnetic waves are emitted randomly, and a conventional laser, whose waves are synchronized and condensed into a unidirectional beam,” explains Araújo.

In a conventional laser, the light waves are ordered and acquire extremely well-organized behavior. In contrast, in a random laser, the radiation oscillates in a disordered way, like in one of the rare natural sources of laser light already discovered in the universe: the clouds around the giant star Eta Carinae, located 7,500 light years from Earth. In addition to their similarity to laser emissions from interstellar space, random lasers have just revealed unexpected similarities to completely different natural phenomena.

One of the properties of random lasers is that, each time a pulse is emitted, its intensity varies randomly. In an article published in August 2016 in the journal Optics Letters, and in another published in June 2016 in Scientific Reports, the team coordinated by Araújo and physicists Anderson Gomes and Ernesto Raposo, both at UFPE, showed how to explain and describe the statistical properties of this random intensity based on the properties of the materials emitting the laser light.

Raposo, who is a theoretical physicist, explains that, in general, the intensity of the different types of light varies according to two patterns: one governed by what is known as the Gaussian statistical distribution, and the other by the Lévy statistical distribution. In the former case, the statistical fluctuations are much less intense than in the latter. This is what happens with the intensity of normal light and that from conventional lasers, which have Gaussian fluctuations. The intensity of the random laser light also follows a Gaussian distribution. However, under certain conditions, it can broaden and behave like the Lévy distribution.

“They found a simple way to investigate the transition from the more common Gaussian distribution to the Lévy distribution in the fluctuations,” says physicist Diederik Wiersma, a specialist in random laser emissions at the University of Florence, Italy.

The Lévy statistical distribution is the same that Raposo and his colleagues had already shown to be behind the fluctuation in the distances that sea birds, like the albatross, travel when searching for food. In an article published in Nature in 1999, they had demonstrated that the albatross flies in random directions when it is fishing, mostly staying close to its starting point. Sometimes, however, it covers much longer distances.

Laser_247In the experiments carried out at UFPE, researchers found that the sudden increase in the variation of the intensity of the laser depends on an abrupt, complex change in optical behavior, influenced by the nanoparticles. As the light passes through the material, the electromagnetic waves interact, doubling in some sections and annihilating each other in others. When the fluctuations increase, the properties of the electromagnetic waves change from a more ordered pattern to another, more disordered one, that remind physicists of the microscopic structure of certain materials like glass.

This change follows a behavior predicted by a mathematical model known as the theory of spin glasses, used to understand various phenomena that involve many parts interacting among each other in a complex manner: from the collective behavior of atoms in a disordered magnetic material to the dynamics of the neuron networks in the brain. “One of the advantages of random lasers is that they serve as a platform for studying multidisciplinary problems,” comments physicist Anderson Gomes, who took part in the experiments together with André Moura, a physicist currently at the Federal University of Alagoas, and PhD students in the UFPE Physics Department.

The connection between the laser’s intensity fluctuations and spin glass theory, investigated by UFPE researchers, might also lead to the development of laser sources with variable randomness that could be employed in different technological applications. One possible application is the transmission of data over optical fiber. In 2007, Araújo, Gomes and others filled optical fibers with a liquid capable of emitting random laser radiation. The filled optical fiber transmitted light signals with an efficiency 100 times greater than conventional fibers.

The physicists at UFPE and other institutions are also investigating the properties of random lasers because of their potential application in the detection of chemical substances or in medical diagnostics. In 2004, the group led by physicist Randal Polson of the University of Utah, found that, when illuminated by a conventional laser, tissues containing cancer cells stained with special dyes emit light more random than that produced by healthy cells. More recently, in 2012, a study coordinated by physicist Hui Cao of Yale University demonstrated that microscopic objects illuminated by a random laser produce clearer images (with greater spatial resolution) than those produced by an object illuminated with LED light or a conventional laser. This is because, when the conventional laser light hits an object, it creates optical illusions in the form of a cloud of luminous points that blur the image captured by a camera. The disorder of the random laser waves attenuates this effect. In principle, explains Araújo, the more the laser amplitudes vary randomly, the clearer the image generated.

Research using random lasers took off in 1994 when physicists from Brown University reported the development of the first high-efficiency random laser in the journal Nature. Gomes participated in this study, which unequivocally demonstrated for the first time that one can generate laser light from the emission and scattering of disordered light inside a material, as proposed in 1966 by Russian physicists led by Nicolay Basov, one of the winners of the Nobel Prize for Physics in 1964 for inventing the conventional laser.

One of Basov’s colleagues, physicist Vladilen Letokhov, proposed shortly afterwards that the emission of random laser light could explain why light emitted at certain frequencies by some interstellar clouds was more intense than expected theoretically. The theory helped Brazilian astronomer Augusto Damineli of the University of São Paulo (USP) explain the origin of some infrared light emissions from a nebulous region near the star Eta Carinae. Sometimes these emissions cease, a phenomenon that the theory of lasers helps us understand. Damineli and Letokhov worked together on this problem in 2005.

Scientific articles
GOMES, A. S. L. et al. Observation of Lévy distribution and replica symmetry breaking in random lasers from a single set of measurements. Scientific Reports. June 13, 2016.
PINCHEIRA, P. I. R. et al. Observation of photonic paramagnetic to spin-glass transition in a specially designed TiO2 particle-based dye-colloidal random laser. Optics Letters. V. 41 (15). August 1, 2016.

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