Researchers manage to control the optical spectrum of random lasers, expanding their applications

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The Materials Science Institute of Madrid (ICMM-CSIC), part of the Spanish National Research Council (CSIC), has developed a method to control the persistence of the spectrum of pulsed random lasers in a cheap and precise way, expanding the potential applications of these devices for optical microscopy and spectroscopies. The results of the work have just been published in the journal Optica.

Conventional lasers emit coherent light beams in a well-defined direction and frequency, thanks to the orderly and precise arrangement of a light source and several mirrors, where the light is amplified after multiple reflections. Stochastic lasers, on the other hand, are characterized by allowing broader spectra — and therefore less coherence — with rays in multiple directions, due to disordered mirrors with rough surfaces. "In these lasers, broad ranges of wavelengths are possible, which expands their potential uses," says Cefe López, a researcher at ICMM-CSIC and leader of the study.

"Since the amplification of light — the basis of a laser's operation — only occurs while there is pumping — that is, while the laser is receiving energy — the definition of the spectrum will only occur if the light has time to explore the cavity, the space between the mirrors where it must reflect," the scientist explains. He adds that if the light beam does not have time to reach the end of a cavity, each time a pulse of light is emitted, its spectrum will be different. However, if the cavity length allows the beam enough time to go back and forth multiple times, the laser will generate a persistent spectrum that repeats with each pulse.

"These results suggest an easy and robust way to control stability in stochastic lasers and pave the way for creating miniaturized systems," López argues, noting that this method has "great applicative advantage" because it allows choosing between repetitive or unpredictable spectra, depending on the need: "For certain spectroscopies, you need a repetitive spectrum, but in others, just the opposite; like in random number generation, one of the great current challenges, where unpredictability is needed. It's also essential when using lasers as illumination in optical microscopy," López exemplifies.

In the scientist's words: "This can be used in any application where you need control over variability or persistence,". "We have done our experiment in a very cheap way: our sample can be made by anyone with just a film of a biopolymer cut carelessly," concludes the researcher, who already succeeded two years ago in making portable and affordable stochastic lasers.