  Silica nanofibers fabricated by tapering optical fibers have attracted
considerable interest as an ultimate platform for high-efficiency light-matter
interactions. While previously demonstrated applications relied exclusively on
the low-loss transmission of only the fundamental mode, the implementation of
multimode tapers that adiabatically transmit several modes has remained very
challenging, hindering their use in various emerging applications in multimode
nonlinear optics and quantum optics. Here, we report the first realization of
multimode submicron tapers that permit the simultaneous adiabatic transmission
of multiple higher-order modes including the LP02 mode, through introducing
deep wet-etching of conventional fiber before fiber tapering. Furthermore, as a
critical application, we demonstrate 'fundamental-to-fundamental' all-fiber
third-harmonic generation with high conversion efficiencies. Our work paves the
way for ultrahigh-efficiency multimode nonlinear and quantum optics,
facilitating nonclassical light generation in the multimode regime, multimode
soliton interactions and photonic quantum gates, and manipulation of the
evanescent-field-induced optical trapping potentials of atoms and
nanoparticles.
