Periodic pulses of light forming a comb in the frequency domain are widely used for sensing and ranging. The key to the minia­turisation of this techno­logy towards chip-integrated solutions is the genera­tion of dissi­pative solitons in ring-shaped micro­resonators. Dissipative solitons are stable pulses circu­lating around the circumference of a nonlinear resonator. Since their first demons­tration, the process of dissipative soliton formation has been extensively studied and today it is rather consi­dered as textbook knowledge. Several directions of further develop­ment are actively inves­tigated by different research groups worldwide. One of these directions is the generation of solitons in coupled resonators. The collec­tive effect of many resonators promises better perfor­mance and control over the frequency combs, exploiting another (spatial) dimension.

But how does the coupling of addi­tional resonators change the soliton generation process? Identical oscillators of any kind, affecting each other, can no longer be considered as a set of distinct elements. Due to the hybri­disation pheno­menon, the excitation of such a system influences all its elements, and the system has to be treated as a whole. The simplest case when the hybri­disation takes place is two coupled oscillators or, in molecular termi­nology, a dimer. As well as coupled pendulums and atoms forming a molecule, modes of coupled optical micro­resonators experience hybridi­sation but, in contrast to other systems, the number of involved modes is large – typically from tens to hundreds. Therefore, solitons in a photonic dimer are generated in hybri­dised modes involving both resonators, which adds another degree of control if one has access to hybridisation parameters.

Now, researchers from the laboratory of Tobias J. Kippen­berg at EPFL, and IBM Research Europe led by Paul Seidler, demons­trated the generation of dissi­pative solitons and, therefore, coherent frequency combs in a photonic molecule made of two micro­resonators. The generation of a soliton in the dimer implies two counter-propa­gating solitons in both resonator rings. The underlying electric field behind every mode of the dimer resembles two gears turning in opposite directions, which is why solitons in the photonic dimer are called Gear Solitons. Imprinting heaters on both resonators, and thereby controlling the hybridi­sation, authors demonstrated the real-time tuning of the soliton-based frequency comb.

Even the simple dimer arrangement, besides the hybri­dised (gear) soliton generation, has demons­trated a variety of emergent phenomena, i.e. phenomena not present at the single-particle (resonator) level. For instance, researchers predicted the effect of soliton hopping: periodic energy exchange between the resonators forming the dimer while maintaining the solitonic state. This pheno­menon is the result of simultaneous generation of solitons in both hybri­dised mode families whose inter­action leads to energy oscillation. Soliton hopping, for example, can be used for the generation of confi­gurable combs in the radio-frequency domain.

“The physics of soliton generation in a single resonator is relatively wellunder­stood today,” says Alexey Tikan a researcher at the Labora­tory of Photonics and Quantum Measure­ments, EPFL. “The field is probing other directions of development and improve­ment. Coupled resonators are one of a few such per­spectives. This approach will allow for the employment of concepts from adjacent fields of Physics. For example, one can form a topo­logical insulator by coupling resonators in a lattice, which will lead to the generation of robust frequency combs immune to the defects of the lattice, and at the same time profiting from the enhanced effi­ciency and addi­tional degrees of control. Our work makes a step towards these fascinating ideas!” (Source: EPFL)

Reference: A. Tikan et al.: Emergent nonlinear phenomena in a driven dissipative photonic dimer, Nat. Phys., online 15 February 2021; DOI: 10.1038/s41567-020-01159-y

Link: Laboratory of Photonics and Quantum Measurements, Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland