Dynamic ring resonator offers new opportunity in synthetic frequency dimension

Synthetic dimensions in photonics offer exciting new ways to manipulate light, study physical phenomena with exotic connections, and explore higher-dimensional physics. Dynamically modulated ring resonator systems, in which resonance modes are coupled to construct a synthetic frequency dimension, can provide great experimental flexibility and reconfigurability.

The construction of complex synthetic lattices, such as Lieb lattices and multi-ring honeycomb lattices, will open up rich possibilities for exploring exotic physical phenomena that currently exist only in the theoretical realm, such as parity-time-phase transition in non-Hermitian systems and higher order topologies. Towards experimental construction of more complicated multi-ring lattices, allowing synthetic frequency space systems in two rings of different lengths is an important step.

As reported in Advanced Photonics, a team of researchers from Shanghai Jiao Tong University recently constructed a synthetic stub grid along the frequency dimension. They used two linked rings of different lengths, while the larger ring underwent dynamic modulation. Their study, which was the first such experimental demonstration, observed and verified the intrinsic physical properties of such lattices, especially the natural existence of the flat (dispersionless) band. They also observed the mode localization near the flat band. Such flat tires in the synthetic space can be further modified by introducing long-range couplings into the modulation, which allow transitions from flat to non-flat tires, for dynamic control of light.

In addition, by selectively choosing the input and output ports for excitations and transmission measurements, they were able to observe different band structure patterns. Such results differ markedly from previous work in the field of flat band physics. They reveal that signals in the system can carry optical information from superposition modes in synthetic frequency dimensions.

This demonstration of exotic light manipulation could enable essential applications of optical communications in fiber optic or on-chip resonators. The work may also be a important milestone: Construction of the stub schedule in two linked rings of different lengths proves the experimental feasibility of connecting multiple rings of different types to construct complex lattices beyond line or square geometry in synthetic space. The authors expect their results to pave the way for future experimental realization of previous theoretical proposals.