Broadband Visible Wavelength Microcomb Generation In Silicon Nitride Microrings Through Air-Clad Dispersion Engineering

The development of broadband microresonator frequency combs at visible wavelengths is pivotal for the advancement of compact and fieldable optical atomic clocks and spectroscopy systems. Yet, their realization necessitates resonators with anomalous dispersion, an arduous task due to the prevailing normal dispersion regime of materials within the visible spectrum. In this work, we evince that silicon nitride microring resonators with air cladding on top and sides -- a deviation from the frequently employed silica-embedded resonators -- allows for the direct generation of broadband microcombs in the visible range. We experimentally demonstrate combs pumped at 1060~nm (283~THz) that reach wavelengths as short as 680~nm (440 THz), and combs pumped at 780~nm (384 THz) that reach wavelengths as short as 630 nm (475 THz). We further show through simulations that microcombs extending to wavelengths as low as 461 nm (650 THz) should be accessible in this platform.