  Making use of a droplet-epitaxial technique, we realize nanometer-sized
quantum ring complexes, consisting of a well-defined inner ring and an outer
ring. Electronic structure inherent in the unique quantum system is analyzed
using a micro-photoluminescence technique. One advantage of our growth method
is that it presents the possibility of varying the ring geometry. Two samples
are prepared and studied: a single-wall ring and a concentric double-ring. For
both samples, highly efficient photoluminescence emitted from a single quantum
structure is detected. The spectra show discrete resonance lines, which reflect
the quantized nature of the ring-type electronic states. In the concentric
double--ring, the carrier confinement in the inner ring and that in the outer
ring are identified distinctly as split lines. The observed spectra are
interpreted on the basis of single electron effective mass calculations.
