  We investigate, numerically and experimentally, the effect of thermo-optical
(TO) chaos on direct soliton generation (DSG) in microresonators. When the pump
laser is scanned from blue to red and then stopped at a fixed wavelength, we
find that the solitons generated sometimes remain (survive) and sometimes
annihilate subsequent to the end of the scan. We refer to the possibility of
these different outcomes arising under identical laser scan conditions as
coexistence of soliton annihilation and survival. Numerical simulations that
include the thermal dynamics show that the coexistence of soliton annihilation
and survival is explained by TO chaos accompanying comb generation. The random
fluctuations of the cavity resonance occurring under TO chaos are also found to
trigger spontaneous soliton generation after the laser scan ends. The
coexistence of soliton annihilation and survival as well as spontaneous soliton
generation are observed experimentally in a silicon-nitride microresonator. The
elucidation of the role of TO chaos provides important insights into the
soliton generation dynamics in microresonators, which may eventually facilitate
straightforward soliton generation in fully-integrated microresonators.
