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 (SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) resonators is well-known [3]. Here, we engineer the microresonator's geometry to optimize the optical field overlap between the core (ShN4) and cladding (SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) 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.