Excitonic absorption spectra and Sommerfeld factors of one-dimensional systems

We investigate theoretically optical absorption spectra near a hand edge for one-dimensional (1D) semiconductors with a direct band gap. taking 1D Wannier-type exciton effects into account. The Sommerfeld factor, which is the absorption intensity ratio of the continuum exciton to the free electron-hole pair above the hand edge, is found to be smaller than unity for an allowed transition in striking contrast to the three- and two-dimensional cases. This peculiar feature is interpreted in terms of the anomalously strong concentration of the oscillator strength on the lowest discrete exciton state.