  We introduce a monolithic Brillouin generator based on a semiconductor
micropillar cavity embedding a high frequency nanoacoustic resonator operating
in the hundreds of GHz range. The concept of two nested resonators allows an
independent design of the ultrahigh frequency Brillouin spectrum and of the
optical device. We develop an optical free-space technique to characterize
spontaneous Brillouin scattering in this monolithic device and propose a
measurement protocol that maximizes the Brillouin generation efficiency in the
presence of optically induced thermal effects. The compact and versatile
Brillouin generator studied here could be readily integrated into fibered and
on-chip architectures.
