  Mode tapering, or the gradual manipulation of the size of some mode, is a
requirement for any system that aims to efficiently interface two or more
subsystems of different mode sizes. While high efficiency tapers have been
demonstrated, they often come at the cost of a large device footprint or
challenging fabrication. Topological photonics, offering robustness to certain
types of disorder as well as chirality, has proved to be a well-suited design
principle for numerous applications in recent years. Here we present a new kind
of mode taper realized through topological bandgap engineering. We numerically
demonstrate a sixfold change in mode width over an extremely compact 8$\mu$m
distance with near unity efficiency in the optical domain. With suppressed
backscattering and no excitation of higher-order modes, such a taper could
enable new progress in the development of scalable, multi-component systems in
classical and quantum optics.
