Biomedical Engineering Reference
In-Depth Information
which the thermal quasi-equilibrium of occupations of the dark and bright states is
maintained. This transfer of occupation to the short-living lower energy state makes
the dark state unstable and precludes occupation trapping.
The only way to avoid the phonon-induced quenching of exciton occupation is
to adjust the system parameters in such a way that the thermal processes become
slow compared to the radiative emission. This is possible due to the oscillating form
of the spectral density (Fig.
9.23
). If the energy parameters
Δ
and
V
are adjusted in
such a way that the energy splitting
corresponds to a sufficiently low minimum of
the spectral density, the phonon effects become weak. As can be seen by comparing
Fig.
9.30
c and d, the phonon-induced processes at the third minimum of the spectral
density (Fig.
9.30
c) are still to strong for the vacuum-induced coherence to be
restored but for the energy
E
corresponding to the fifth minimum phonon effects
become very weak and thus the slow decay of the final occupation is observed again.
Interestingly, the character of temperature dependence of the evolution is dif-
ferent for strong and weak phonon coupling. In the former case, the decay rate
decreases with increasing temperatures, while in the latter case the dependence
is opposite. This can be explained as follows. If the phonon dynamic is fast, the
occupation of the two single-exciton states remains in a quasi-equilibrium, which
means that the occupation of the higher energy (darker) eigenstate increases with
temperature. On the contrary, if the phonon dynamic is slow compared to the
radiative processes, the long-living tail is present, due to the excitation trapping in
the darker eigenstate. Since the probability of phonon-assisted escape to the brighter
state grows with temperature so does the emission intensity.
E
9.6
Conclusions
The presented overview of phonon-related and radiative effects in DQD structures
shows that the physics of these systems is much richer than that of a single QD.
Already the coupling to phonons alone can lead to new phenomena, including
phonon-assisted tunneling of carriers or Coulomb-mediated dissipative excitation
transfer. Also the processes that are known from QD systems attain new aspects.
In particular, new effects appear in the pure dephasing process, where, e.g., the
mutual impact of phonon packets emitted by the other dot plays a role and the
decay of spatial coherence occurs. Moreover, due to the presence of an energetically
close doublet of levels, the broadening of the fundamental line is determined by
an interplay of real and virtual phonon scattering. Phonon-induced dephasing of
exciton states in DQDs leads also to strong suppression of entanglement between
the two dots.
The linear and nonlinear optical response of DQDs also show features character-
istic of these systems. The essential effect here is the formation of bright and dark
states and the interplay of the coupling and transition energy mismatch in the two
dots. It turns out that sufficiently strong coupling can stabilize the superradiance-
like and subradiance-like effects even for DQD systems composed of very different
