23 research outputs found
Comment on "Photoluminescence ring formation in coupled quantum wells: Excitonic versus ambipolar diffusion"
This is a comment on M. Stern, V. Garmider, E. Segre, M. Rappaport, V.
Umansky, Y. Levinson, and I. Bar-Joseph, Phys. Rev. Lett. 101, 257402 (2008)
Kinetics of the inner ring in the exciton emission pattern in GaAs coupled quantum wells
We report on the kinetics of the inner ring in the exciton emission pattern.
The formation time of the inner ring following the onset of the laser
excitation is found to be about 30 ns. The inner ring was also found to
disappear within 4 ns after the laser termination. The latter process is
accompanied by a jump in the photoluminescence (PL) intensity. The spatial
dependence of the PL-jump indicates that the excitons outside of the region of
laser excitation, including the inner ring region, are efficiently cooled to
the lattice temperature even during the laser excitation. The ring formation
and disappearance are explained in terms of exciton transport and cooling.Comment: 19 pages, 6 figure
Kinetics of indirect excitons in the optically-induced exciton trap
We report on the kinetics of a low-temperature gas of indirect excitons in
the optically-induced exciton trap. The excitons in the region of laser
excitation are found to rapidly -- within 4 ns -- cool to the lattice
temperature T = 1.4 K, while the excitons at the trap center are found to be
cold -- essentially at the lattice temperature -- even during the excitation
pulse. The loading time of excitons to the trap center is found to be about 40
ns, longer than the cooling time yet shorter than the lifetime of the indirect
excitons. The observed time hierarchy is favorable for creating a dense and
cold exciton gas in optically-induced traps and for in situ control of the gas
by varying the excitation profile in space and time before the excitons
recombine.Comment: 4 pages, 3 figure
Microcavity polariton-like dispersion doublet in resonant Bragg gratings
Periodic structures resonantly coupled to excitonic media allow the existence
of extra intragap modes ('Braggoritons'), due to the coupling between Bragg
photon modes and 3D bulk excitons. This induces unique and unexplored
dispersive features, which can be tailored by properly designing the photonic
bandgap around the exciton resonance. We report that one-dimensional
Braggoritons realized with semiconductor gratings have the ability to mimic the
dispersion of quantum-well microcavity polaritons. This will allow the
observation of new nonlinear phenomena, such as slow-light-enhanced nonlinear
propagation and an efficient parametric scattering at two 'magic frequencies'
Kinetics of indirect excitons in the optically-induced exciton trap
We report on the kinetics of a low-temperature gas of indirect excitons in
the optically-induced exciton trap. The excitons in the region of laser
excitation are found to rapidly -- within 4 ns -- cool to the lattice
temperature T = 1.4 K, while the excitons at the trap center are found to be
cold -- essentially at the lattice temperature -- even during the excitation
pulse. The loading time of excitons to the trap center is found to be about 40
ns, longer than the cooling time yet shorter than the lifetime of the indirect
excitons. The observed time hierarchy is favorable for creating a dense and
cold exciton gas in optically-induced traps and for in situ control of the gas
by varying the excitation profile in space and time before the excitons
recombine.Comment: 4 pages, 3 figure
Direct and indirect excitons in semiconductor coupled quantum wells in an applied electric field
An accurate calculation of the exciton ground and excited states in AlGaAs
and InGaAs coupled quantum wells (CQWs) in an external electric field is
presented. An efficient and straightforward algorithm of solving the
Schrodinger equation in real space has been developed and exciton binding
energies, oscillator strengths, lifetimes, and absorption spectra are
calculated for applied electric fields up to 100 kV/cm. It is found that in
symmetric 8-4-8 nm GaAs/Al(0.33)Ga(0.67)As CQW structure, the ground state of
the system switches from direct to indirect exciton at approximately 5 kV/cm
with dramatic changes of its binding energy and oscillator strength while the
bright excited direct-exciton state remains almost unaffected. It is shown that
the excitonic lifetime is dominated either by the radiative recombination or by
tunneling processes at small/large values of the electric field, respectively.
The calculated lifetime of the exciton ground state as a function of the bias
voltage is in a quantitative agreement with low-temperature photoluminescence
measurements. We have also made freely available a numerical code for
calculation of the optical properties of direct and indirect excitons in CQWs
in an electric field.Comment: 12 pages, 11 figure
Bragg Polaritons: Strong Coupling and Amplification in an Unfolded Microcavity
Periodic incorporation of quantum wells inside a one--dimensional Bragg
structure is shown to enhance coherent coupling of excitons to the
electromagnetic Bloch waves. We demonstrate strong coupling of quantum well
excitons to photonic crystal Bragg modes at the edge of the photonic bandgap,
which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg
polariton branches are in good agreement with the theory and allow
demonstration of Bragg polariton parametric amplification.Comment: 4 pages, 4 figure