Periodic Reporting for period 2 - SOLIRING (Solitons and frequency combs in micro-resonators) Reporting period: 2018-01-01 to 2019-02-28 Summary of the context and overall objectives of the project The project joins efforts of the institutions from the UK, Sweden, South Africa and Russian Federation and aims to combine and share cross-disciplinary expertise of the participating groups in the fields of nonlinear photonics, optical microresonators, and nonlinear dynamics, to advance understanding of the frequency comb generation, parametric frequency conversion and fabrication of the surface nanoscale axial photonics (SNAP) microresonators. The collaboration between scientists from photonics and applied mathematics communities in this project plays a critical role in the development of conceptually new theoretical approaches to the description of the frequency comb generation and microresonator designs and facilitates the interdisciplinary knowledge exchange. The work is organized in three work packages (WPs):- Frequency combs in microring resonators (WP1)- Nonlinear SNAP microresonators (WP2)- Lugiato-Lefever microring model through the prism of soliton theory (WP3) Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Within the WP1 we have found that the Lugiato-Lefever theory of the frequency comb generation in ring resonators can be generalized to describe the simultaneous nonlinear resonances leading to the multistability of nonlinear modes and coexisting breathing solitons that are associated with the spectrally distinct frequency combs. We have studied impact of Raman scattering on comb formation in the normal GVD range and identified existence and destabilization scenarios of Raman combs. We have discovered locking mechanisms of the comb free-spectral range due to emission of dispersive Cherenkov radiation.Within the WP2 we have demonstrated the subangstrom precise fabrication of individual and coupled SNAP microresonators having the effective radius variation of several nanometers using the femtosecond laser inscription. We proposed a new type of broadband and low repetition rate frequency comb generator that has the shape of an elongated and nanoscale-shallow optical bottle microresonator created at the surface of an optical fiber. We demonstrated the two coupled bottle microresonators fabricated at the fiber surface with resonances that are matched with a better than 0.16 GHz precision. We have studied numerically and analytically comb generation in SNAP microresonator and predicted effects of multistability of the SNAP modes.Within the WP3 we have studied nonlinear dynamics of ring resonators with non-Kerr nonlinearities. Considering the exciton-polariton nonlinearities we have studied the existence and stability of the vectorial vortex and half-vortex states and taking the Raman nonlinearity we have identified threshold conditions for the soliton comb states. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The action has fully achieved its research, networking and dissemination objectives for the period. Our discovery of frequency comb stabilisation mechanism due backaction of the Cherenkov radiation on its parent soliton and predictions of frequency comb generation in SNAP resonators push frontiers of knowledge in understanding of frequency combs that are vital for high precision measurements and signal processing applications.
