The ability to confine and store optical energy in small volumes has implications in fields ranging from cavity quantum electrodynamics to photonics. Of all cavity geometries, micrometre-sized dielectric spherical resonators are the best in terms of their ability to store energy for long periods of time within small volumes. In the sphere, light orbits near the surface, where long confinement times (high Q) effectively wrap a large interaction distance into a tiny volume. This characteristic makes such resonators uniquely suited for studies of nonlinear coupling of light with matter. Early work recognized these attributes through Raman excitation in microdroplets-but microdroplets have not been used in practical applications. Here we demonstrate a micrometre-scale, nonlinear Raman source that has a highly efficient pump-signal conversion (higher than 35%) and pump thresholds nearly 1,000 times lower than shown before. This represents a route to compact, ultralow-threshold sources for numerous wavelength bands that are usually difficult to access. Equally important, this system can provide a compact and simple building block for studying nonlinear optical effects and the quantum aspects of light.