Implementing and studying non Hermitian topological physics using mode-locked lasers

A mode-locked laser is an advanced laser that can generate very short optical pulses with durations ranging from femtoseconds to picoseconds. These lasers are widely used for studying ultrafast and nonlinear optical phenomena, but they have also been proven to be applicable to various technological applications.

Researchers at the California Institute of Technology have recently been exploring the potential of mode-locked lasers as a platform for studying topological phenomena. Their paper was published in the journal Nature Physics, outlining the potential of these lasers in the study and implementation of new non Hermitian topological physics, with various potential applications.

"In the past decade, the idea of utilizing the topological robustness and topological protection of photonic devices has attracted widespread attention, but it is still unclear whether this behavior can provide substantial practical benefits," the main author of the paper, Alireza Marandi, told Phys.org.

We have been exploring this issue, especially for lasers and nonlinear photonic devices, whose functions are essentially nonlinear. By the way, the field of topological physics is also developing around the interaction between topology and nonlinearity, and there are relatively few experimental platforms for such exploration.

Marandi and his colleagues have recently pursued a dual goal in their research. On the one hand, they hope to open up new opportunities for studying nonlinear topological behavior, and on the other hand, they hope to broaden the practical applications of topological physics in mode-locked lasers.

"From an experimental perspective, our platform is a time multiplexed resonator network consisting of many synchronous pulses from long resonators," Marandi explained. Pulses can be coupled to each other in a controllable manner using precise delay lines. This allows us to create a programmable network of large-scale resonators with great flexibility. This is not easy on other platforms.

In an earlier paper published in 2022, researchers explored topological phenomena in large-scale photonic resonators, but particularly in linear states. As part of their new research, they used the same resonator to achieve coupled mode-locked lasers.

The team indicates that the pulse patterns generated by these lasers can benefit from non Hermitian and topological phenomena. Essentially, they created a long cavity, multi pulse, mode-locked laser and introduced a junction inside it.

"The flexibility of our experimental method enables us to study the intersection of topology and laser mode locking, and to achieve non Hermitian topological physics that has not been previously proven in photon systems," Marandi said.

For example, we found that the synergistic effect between non Hermitian topological structures and the nonlinear dynamics of our system spontaneously generates skin patterns in our mode-locked laser. This is in stark contrast to linear non Hermitian topological systems, where external sources must be used to detect skin patterns.

Marandi and his collaborators recently demonstrated the potential of mode-locked lasers in studying topological physics, which has been difficult to obtain experimentally so far. In addition, their research can stimulate mode-locked lasers for the development of new sensing, computing, and communication technologies.

In addition, in their experiment, researchers used their developed laser to confirm the robustness of the mathematical model used to study the behavior of randomly moving particles to the localization induced by disorder. Although this model has been extensively studied before, it has not yet been proven on a mode-locked photon platform.

"In terms of this understanding, we further explored the robustness of the Hatano Nelson model to disorderly induced localization and how it can design robust frequency comb sources," Marandi said. Usually, this robustness to something is followed by sensitivity to other things.
In their next study, Marandi and his colleagues will attempt to use their method to explore the use of the Hatano Nelson model as a sensor with enhanced sensitivity. In addition, they hope that their research can inspire other teams to try using mode-locked lasers to study topological physical phenomena.

"We also believe that our platform can become a fertile ground for exploring a large number of difficult to obtain nonlinear topologies and non Hermitian phenomena," Marandi added. An example that interests us is the interaction between soliton formation and topological behavior.

Source: Laser Net