TU Vienna develops 'ordered' laser light using turbulence

25 Jun 2020

Team from Vienna, Milan, Harvard and Yale universities create frequency combs with quantum cascade lasers.

So-called frequency combs are playing an increasingly important role in laser research today. While the light of an ordinary laser only has one single, well-defined wavelength, a frequency comb comprises different light frequencies, precisely arranged at regular distances, much like the teeth of a comb – hence their name.

Such frequency combs can be difficult to generate. However, an international research team from TU Wien, Vienna, Austria, Harvard and Yale universities, USA, and Milan and Turin universities, Italy has succeeded in producing turbulent light, using simple circular quantum cascade lasers, a result that they say seems to contradict conventional laser theories.

Turbulences, as they are known from aerodynamics or water waves, are responsible for this particularly ordered type of laser light. The work is described in Nature.

"Actually, we were first looking for something very different in our experiments," commented Benedikt Schwarz, who researches frequency combs at TU Wien (Vienna) and Harvard University, and was last year awarded an ERC Starting Grant for his research.

"We were investigating circular quantum cascade lasers, which have been manufactured in our laboratories at the Institute of Solid State Electronics for several years. We wanted to investigate how certain defects affect the laser light."

But much to the scientists’ surprise, they found out that these circular mini-lasers can be used in a simple way to produce frequency combs, which are composed of several light frequencies, arranged at equal distances.

"This is great for us, because this is exactly the kind of light we are looking for. Only we didn't expect to find it in this particular experiment - the success seemed to contradict current laser theory," said Schwarz.

If the light from a laser is to consist of different frequencies at the same time, then the light cannot be constant – it must vary in time. An oscillation is required, repeating itself in a regular pattern. Only then, a frequency comb is created.

Turbulence creates chaos – or order

"When we thought about how this oscillation could be explained, we looked for similar phenomena in other scientific fields. Eventually we came across turbulence as the driving force that causes the oscillation leading to our frequency combs," said Schwarz.

Turbulence is a phenomenon that arises in different areas: In the smoke that emerges from an extinguished candle, turbulence can be seen that leads to chaotic, unpredictable patterns. But so-called wave instabilities can be found in all types of waves. A small disturbance gets bigger and bigger and eventually dominates the dynamics of the system.

The mathematical connection between such turbulence effects and the novel laser light could finally be found by a laser theory that Nikola Opačak from the Vienna University of Technology had recently published in November 2019.

"We found that this laser theory can be connected to the same mathematical equation that also describes turbulence in other scientific disciplines," said Schwarz. In a ring-shaped laser, wave instabilities can cause a stable frequency comb to form. In addition, there is a strong connection between different light frequencies - different frequencies are firmly coupled to each other.

Artificial nose

Frequency combs play an important role in research mainly because they can be used to build tiny chemical sensors. Many molecules absorb light in the infrared range in a very characteristic way. By measuring which wavelengths are being absorbed, it is possible to determine which molecules are present. To do this, however, it is necessary to have as many different light frequencies in the infrared range available as possible and this is exactly what an optical frequency comb provides in an ideal way.