Dynamic process of soliton generation in CaF₂ crystalline whispering gallery mode resonators with negative TO effects

Calcium fluoride (CaF 2 ) crystalline whispering gallery mode resonators (WGMRs) are emerging as promising platforms for the generation of temporal dissipative Kerr soliton (DKS) or soliton microcomb, attributed to their ultrahigh intrinsic quality factors and favorable anomalous dispersion properties at low power, particularly within the communication and mid-infrared bands. However, their unique negative thermo-optic (TO) effects present in CaF 2 WGMRs pose challenges in achieving thermal equilibrium, a critical aspect for stable soliton comb generation. Here, we report the successful demonstration of soliton microcombs in CaF 2 WGMRs. Our experiments have revealed a variety of soliton formation dynamics. These encompass the generation of single and multiple solitons, as well as the observation of breathing solitons and vibrational solitons. Particularly noteworthy within our discoveries are the soliton bidirectional switching and soliton self-starting, which are induced by the negative TO effect. We also conducted theoretical analyses using the Lugiato-Lefever Equation (LLE) which incorporates the negative TO effect. The consistency between the theoretical simulations and our experimental results substantiates the authenticity of our observations. The outcomes derived from this study enrich the soliton generation platform and contribute to understanding the formation of solitons in WGMRs composed of various materials exhibiting negative TO effects.