The Enigma of Ultrashort Soliton Molecules is Solved by Atomic Terahertz-Vibrations

In ultrashort-pulse lasers, stable packets of light waves called optical solitons are emitted as a chain of light flashes. These solitons frequently form pairs with very little temporal separation.

Researchers from the Universities of Bayreuth and Wrocaw have solved the mystery of how these temporal linkages are generated by using atomic vibrations in the terahertz band. They publish their findings in the journal Nature Communications.

The dynamics of linked light packets can be utilized to measure atomic vibrations as material’s characteristic “fingerprints” in a very short time.

Optical solitons can create extremely tight spatial and temporal connections in ultrashort-pulse lasers. Because they are permanently attached to each other, like chemically bonded atoms in a molecule, these are also known as ultrashort “soliton molecules.”

To investigate how this coupling happens, a Bayreuth research team employed a commonly used solid-state laser composed of a sapphire crystal doped with titanium atoms. The atoms in the sapphire’s crystal lattice are first stimulated to vibrate by a single leading flash of light.

The mechanism we discovered is based on the physical effects of Raman scattering and self-focusing. It explains a variety of phenomena that have puzzled science since the invention of titanium-sapphire lasers over 30 years ago. What is particularly exciting about the discovery is that we can now exploit the dynamics of solitons during their generation in the laser cavity to scan atomic bonds in materials extremely rapidly.

Dr. Georg Herink

This distinctive motion oscillates in the terahertz band and decays in a matter of picoseconds (a picosecond corresponds to a trillionth of a second). The refractive index of the crystal changes in such a brief period of time.

It detects this change when a second flash of light follows the first and catches up with it: it is not just marginally impacted by the atomic vibrations, but it can also be stably connected to the previous soliton. There is the birth of a “soliton molecule.”

“The mechanism we discovered is based on the physical effects of Raman scattering and self-focusing. It explains a variety of phenomena that have puzzled science since the invention of titanium-sapphire lasers over 30 years ago. What is particularly exciting about the discovery is that we can now exploit the dynamics of solitons during their generation in the laser cavity to scan atomic bonds in materials extremely rapidly,” explains junior professor Dr. Georg Herink, head of the study and junior professor of ultrafast dynamics at the University of Bayreuth.

“The entire measurement of a so-called intracavity Raman spectrum now takes less than a thousandth of a second. These findings may help to develop particularly fast chemically sensitive microscopes that can be used to identify materials. In addition, the coupling mechanism opens up new strategies to control light pulses by atomic motions and, conversely, to generate unique material states by light pulses.”

Parallel to the experimental data analysis, the researchers were able to create a theoretical model for soliton dynamics. The model can be used to explain experimental results and to predict unexpected effects of atomic vibrations on solitons behavior.

The DFG research project FINTEC at the University of Bayreuth is currently investigating the interactions of solitons in optical systems and their applications for high-speed spectroscopy.