Jun 19 2020
The so-called frequency combs play a significant role in current laser research. They can be described as a very unique kind of light that can be used for crucial measurements.
Quantum Cascade Lasers create a very special kind of light. Image Credit: Second Bay Studios/Harvard SEAS.
A standard laser emits light that has a single, well-defined wavelength. By contrast, a frequency comb contains different frequencies of light, which are accurately organized at regular distances, similar to the teeth of a comb.
It is very difficult to produce such frequency combs. An international team of researchers from Austria (Vienna University of Technology [TU Wien, Vienna]), the United States (Harvard, Yale), and Italy (Milan, Turin), has effectively generated this unique kind of light with the help of simple circular quantum cascade lasers. This breakthrough appeared to fully contradict traditional laser concepts.
But it turned out that turbulences contribute to this specifically ordered type of light. These turbulences are known from water waves or aerodynamics. The findings have been recently published in Nature—a scientific journal.
Better than Physics Permits?
Actually, we were first looking for something very different in our experiments. We were investigating circular quantum cascade lasers, which is a special type of laser that has been manufactured in our laboratories at the Institute of Solid State Electronics for years. We wanted to investigate how certain defects affect the laser light.
Benedikt Schwarz, Postdoctoral Fellow, Vienna University of Technology
Schwarz focuses on frequency combs at TU Wien (Vienna) and Harvard University. He received an ERC Starting Grant for his research performed in 2019.
But to their amazement, the researchers discovered that such circular mini-lasers can be employed in an extremely simple way to create frequency combs. These frequency combs are made up of many light frequencies, organized at equivalent 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,” Schwarz explained.
If the light emitted by a laser is to contain different frequencies simultaneously, then the light should differ in time and cannot remain constant. There is a need for an oscillation that repeats itself in a consistent pattern. Only this phenomenon creates a frequency comb.
Turbulence can Cause 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.
Benedikt Schwarz, Postdoctoral Fellow, Vienna University of Technology
Turbulence can be described as a kind of phenomenon that emerges in several highly different regions: In the smoke that arises from an extinguished candle, turbulence can be observed that results in unpredictable, chaotic patterns.
But all types of waves exhibit the so-called wave instabilities. A slight disturbance becomes increasingly bigger and ultimately governs the dynamics of the system.
The mathematical link between the innovative laser light and these turbulence effects could eventually be detected by a laser theory that was recently published in November 2019 by Nikola Opačak from TU Wien, Vienna.
We found that this laser theory can be connected to the same mathematical equation that also describes turbulence in other scientific disciplines.
Benedikt Schwarz, Postdoctoral Fellow, Vienna University of Technology
Wave instabilities in a ring-shaped laser can result in the formation of a stable frequency comb. There is also a powerful link between different frequencies of light—different frequencies are solidly coupled to one another.
The Comb as an Artificial Nose
Frequency combs have a crucial role to play in research, predominantly because they can be used for making miniature chemical sensors. A majority of the molecules absorb light in the infrared range in a highly typical manner. By quantifying the type of wavelengths being absorbed, researchers can find out the type of molecules that are present.
But to do this, the infrared range should have as many different light frequencies as possible. An optical frequency comb offers these different light frequencies in an ideal way.
Journal Reference:
Piccardo, M., et al. (2020) Frequency combs induced by phase turbulence. Nature. doi.org/10.1038/s41586-020-2386-6.