Ultralow-loss silicon nitride waveguides for nonlinear optics
Abstract: The field of nonlinear optics relies on the interaction between high-intensity optical waves and nonlinear media. An integrated waveguide with large refractive index contrast allows to highly confine optical waves in a sub-μm^2 area, thus enhancing the optical intensity. However, such a high optical confinement increases the susceptibility to scattering losses induced from nanometer-level inhomogeneities. Silicon nitride is a dielectric material featuring a relatively large nonlinear-index coefficient and a broadband transparency window, from ultraviolet to mid-infrared. Its refractive index contrast to silica allows high confinement and controlling the dispersion with the waveguide geometry. This material platform has emerged in the past years as a workhorse for nonlinear optics applications that rely on the Kerr effect, from microcomb generation to parametric amplification. In this thesis work, we focused on the development of advanced manufacturing techniques for the realization of ultralow-loss silicon nitride waveguides. Meter long high-confinement waveguides with record low losses in the order of 1.4 dB/m and dispersion-engineered microresonators with quality factors of 19 million are reported. Based on this technology, we demonstrated octave-spanning coherent microcombs and microcombs with photodetectable repetition rates occupying a device area less than 1 mm^2, i.e., one order of magnitude smaller than state of the art. The high yield and ultralow-loss silicon nitride waveguides also allowed us to achieve, for the first time, continuous-wave parametric amplification in an integrated waveguide, with a demonstrated gain of 9.5 dB and noise figure of 1.2 dB when operated in phase-sensitive mode.
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