We report a quantum ringlike toroidal cavity naturally formed in a vertical-cavity-like active microdisk plane due to Rayleigh's band of whispering gallery modes. The $\sqrt{T}$-dependent redshift and a square-law property of microampere-range threshold currents down to $2\mu \mathrm{A}$ are consistent with a photonic quantum wire view, due to whispering gallery mode-induced dimensional reduction.

• Received 8 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.536