  We measured the absorption spectrum of a single T-shaped, 14x6 nm
lateral-sized quantum wire embedded in an optical waveguide using
waveguide-transmission spectroscopy at 5 K. In spite of its small volume, the
one-dimensional-exciton ground state shows a large absorption coefficient of 80
/cm, or a 98 % absorption probability for a single pass of the 500-um-long
waveguide.
