  For its many useful properties, including second and third-order optical
nonlinearity as well as electro-optic control, lithium niobate is considered an
important potential microcomb material. Here, a soliton microcomb is
demonstrated in a monolithic high-Q lithium niobate resonator. Besides the
demonstration of soliton mode locking, the photorefractive effect enables mode
locking to self-start and soliton switching to occur bi-directionally.
Second-harmonic generation of the soliton spectrum is also observed, an
essential step for comb self-referencing. The Raman shock time constant of
lithium niobate is also determined by measurement of soliton
self-frequency-shift. Besides the considerable technical simplification
provided by a self-starting soliton system, these demonstrations, together with
the electro-optic and piezoelectric properties of lithium niobate, open the
door to a multi-functional microcomb providing f-2f generation and fast
electrical control of optical frequency and repetition rate, all of which are
critical in applications including time keeping, frequency synthesis/division,
spectroscopy and signal generation.
