Raman Frequency Combs in Integrated Silicon Nitride Microresonators

Raman effects [1], [2] have been extensively studied in integrated photonics for the generation of new optical frequencies at low powers. Kerr microresonators are efficient tools for observing various optical nonlinearities [3], [4] due to their small mode volumes and high-quality (Q) factors. In microresonators, the interplay of Raman and Kerr effects leads to the formation of Raman-Kerr optical frequency combs and has been reported for different materials like silica, lithium niobate and silicon carbide [5]–[7]. The Raman gain is very low in silicon nitride (ShN4) microresonators and thus Raman effects have not been directly observed. On the other hand, the Raman effect in silica (SiO2) resonators is well-known [3]. Here, we engineer the microresonator's geometry to optimize the optical field overlap between the core (ShN4) and cladding (SiO2) regions. The cladding material initiates the Raman effect, thereby producing a strong Raman signal. The high Kerr-coefficient of the core material facilitates the generation of additional frequency sidebands due to nondegenerate four-wave mixing between the pump and the Stokes waves. Our approach enables comb generation by triggering the different nonlinearities in both the core and the cladding of the microresonator.