  Photonic integrated Raman microlasers, particularly discrete multi-color
lasers which are crucial for extending the emission wavelength range of
chip-scale laser sources to much shorter wavelength, are highly in demand for
various spectroscopy, microscopy analysis, and biological detection. However,
integrated multi-color Raman microlasers have yet to be demonstrated because of
the requirement of high-Q microresonators possessing large second-order
nonlinearity and strong Raman phonon branches and the challenging in
cavity-enhanced multi-photon hyper-Raman scattering parametric process. In this
work, integrated multi-color Raman lasers have been demonstrated for the first
time at weak pump levels, via the excitation of high-Q (>6 X 10^6)
phase-matched modes in single thin-film lithium niobate (TFLN) microresonators
by dispersion engineering. Raman lasing was observed at 1712 nm for a 1546-nm
pump threshold power of only 620 uW. Furthermore, multi-color Raman lasers were
realized at discrete wavelengths of 1712 nm, 813 nm, 533 nm and 406 nm with
pump levels as low as 1.60 mW, which is more than two order of magnitude lower
than the current records (i.e., 200 mW) in bulk resonators, allowed by the
fulfillment of the requisite conditions consisting of broadband natural phase
match, multiple-resonance and high Q-factors.
