Anomalous variation of the exciton Fano-resonance spectra in strongly biased Wannier-Stark ladder

Effects of the Wannier-Stark ladder (WSL) resonance on optical absorption spectra in strongly biased superlattices are theoretically investigated by solving the multichannel scattering problem relevant to the WSL-exciton Fano-resonance. When the bias of an electric field F is applied such that a WSL subband state is energetically aligned with adjacent ones, resulting in strong repulsion (anticrossing) due to Zener resonance, an onset of exciton absorption notably shifts toward the lower energy side. However, just a slight change of F away from the anticrossing leads to a peculiar suppression, lowering the absorption edge. According to a qualitative analytic model, such an anomalous variance is found ascribable to delocalization of WSL subband wave functions across several periods through a mixing of an exciton reduced mass in the region of the potential well with that in the region of the potential barrier.