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Electrons and holes in type-II nanostructures are spatially separated. Therefore, both the radiative and nonradiative
recombination rates are reduced. Although the photon conversion efficiency is hence decreased, the lowered
nonradiative recombination such as Auger process benefits photovoltaic applications. Furthermore, if generated
carriers can be rapidly removed from nanostructures through quasi-bound states, the photon absorption may be
designed and enhanced regardless of the concern on nonradiative mechanisms. Here, we model the bound-tocontinuum
absorption of type-II nanostructures in the presence of tunneling using the density-matrix formalism
and convert it into a radiation problem in the multiband space with band mixing. An effective source is derived
from the eight-band momentum operator, and the corresponding field is expressed in terms of the source and
retarded Green’s function of the eight-band Luttinger-Kohn Hamiltonian. On the other hand, the response is
actually calculated without the Green’s function. Perfectly-matched layers in the multiband space are introduced
to model the effect of quasi-bound states in open regions. In this way, the interplay between photon absorption
and tunneling is fully taken into account. We present both the transverse-electric and transverse-magnetic
absorption spectra of type-II GaAs 0:65Sb0:35/GaAs coupled quantum wells. The corresponding lineshape broadening near the resonant energy can be divided into two parts. One comes from various incoherent relaxation
mechanisms, and another well-fitted by the Fano resonance originates from the coherent tunneling. For a 2-nm
potential barrier, the tunneling times of metastable states in nanostructures are around 20 fs, and their degrees
of mixing to the continuum are high.
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