  Most of superconductors in a magnetic field are penetrated by a lattice of
quantized flux vortices. In the presence of a transport current causing the
vortices to cross sample edges, emission of electromagnetic waves is expected
due to the continuity of tangential components of the fields at the surface.
Yet, such a radiation has not been observed so far due to low radiated power
levels and lacking coherence in the vortex motion. Here, we report emission of
electromagnetic waves from vortices crossing the layers of a
superconductor/insulator Mo/Si superlattice. The emission spectra consist of
narrow harmonically related peaks which can be finely tuned in the GHz range by
the dc bias current and, coarsely, by the in-plane magnetic field value. Our
findings show that superconductor/insulator superlattices can act as dc-tunable
microwave generators bridging the frequency gap between conventional
radiofrequency oscillators and (sub-)terahertz generators relying upon the
Josephson effect.
