6.3 Benefits and Applications
201
Fig. 6.4 Sketch of a polariton system’s phase diagram, at the example of a GaAs-based microcavity
device. Here polariton condensation, a BEC-like effect in a quantum-well microcavity structure in
the strong light–matter coupling regime, is indicated for low temperatures and intermediate polariton
densities when crossing the thick solid phase line in the directions indicated by red (cooling) and blue
arrows (density increase). In fact, the dilute Bose gas of neutral excitons is only provided well below
the Mott transition for that material system, where Coulomb binding is not spoiled by high chargecarrier densities and interparticle interactions remain little. At too high e–h densities, conventional
lasing takes place, indicated by the horizontal dashed line. Owing to their light effective mass,
polaritons promise observation of superfluid behaviour and condensation at elevated temperatures
up to room temperature. Nevertheless, for a given material platform, at too high temperatures the
excitons break up and the structure operates in a weak light–matter coupling regime, indicated by
the vertical dashed line. For high temperatures and densities, conventional lasing occurs (loosely
dotted line), corresponding to VCSEL operation. Drawn freely after [41] with additional markings
and labels
alternatively, a quantum-dot-based single-photon source has been recently triggered
by a mode-locked VECSEL, exhibiting a record-high single-photon flux of 143 MHz
[99].
The use of quantum-well microcavity systems has not only enabled access to
Bose condensation studies at elevated temperatures owing to the light effective mass
of polaritons—hybrid light–matter quasi-particles [39, 47, 100, 101]. It has also
brought up a new scheme of coherent light generation that does not rely on population inversion in the classical sense to obtain stimulated emission of radiation as in
conventional lasers, but on stimulated scattering of bosons into a macroscopicallyoccupied ground state, in which coherence is established due to the indistinguishability of particles when described by the same macroscopic wave-function [40]. With
the phase transition to occur in the excitonic regime well below a Mott transition
in the semiconductor system, i.e. the transition from an insulator to a charge-carrier
plasma state, the polariton condensation threshold naturally lies below that of conventional lasing [41, 102–104] (cf. Fig.6.4). This motivates the development and
study of a coherent light source referred to as polariton laser [40, 41, 48, 50, 105–
201
Fig. 6.4 Sketch of a polariton system’s phase diagram, at the example of a GaAs-based microcavity
device. Here polariton condensation, a BEC-like effect in a quantum-well microcavity structure in
the strong light–matter coupling regime, is indicated for low temperatures and intermediate polariton
densities when crossing the thick solid phase line in the directions indicated by red (cooling) and blue
arrows (density increase). In fact, the dilute Bose gas of neutral excitons is only provided well below
the Mott transition for that material system, where Coulomb binding is not spoiled by high chargecarrier densities and interparticle interactions remain little. At too high e–h densities, conventional
lasing takes place, indicated by the horizontal dashed line. Owing to their light effective mass,
polaritons promise observation of superfluid behaviour and condensation at elevated temperatures
up to room temperature. Nevertheless, for a given material platform, at too high temperatures the
excitons break up and the structure operates in a weak light–matter coupling regime, indicated by
the vertical dashed line. For high temperatures and densities, conventional lasing occurs (loosely
dotted line), corresponding to VCSEL operation. Drawn freely after [41] with additional markings
and labels
alternatively, a quantum-dot-based single-photon source has been recently triggered
by a mode-locked VECSEL, exhibiting a record-high single-photon flux of 143 MHz
[99].
The use of quantum-well microcavity systems has not only enabled access to
Bose condensation studies at elevated temperatures owing to the light effective mass
of polaritons—hybrid light–matter quasi-particles [39, 47, 100, 101]. It has also
brought up a new scheme of coherent light generation that does not rely on population inversion in the classical sense to obtain stimulated emission of radiation as in
conventional lasers, but on stimulated scattering of bosons into a macroscopicallyoccupied ground state, in which coherence is established due to the indistinguishability of particles when described by the same macroscopic wave-function [40]. With
the phase transition to occur in the excitonic regime well below a Mott transition
in the semiconductor system, i.e. the transition from an insulator to a charge-carrier
plasma state, the polariton condensation threshold naturally lies below that of conventional lasing [41, 102–104] (cf. Fig.6.4). This motivates the development and
study of a coherent light source referred to as polariton laser [40, 41, 48, 50, 105–