3.3 Strong Exciton–Photon Coupling and Polariton Bose–Einstein Condensation
85
THz emitters [74], with important physics still not studied well enough to exploit
many-particle effects in novel device concepts and quantum information schemes.
Nevertheless, many terahertz-emitting polariton systems have been proposed so far
[71–73, 75, 76].
Promises of Light–Matter Coupled Systems
Exploring this domain of light–matter interaction thus has still the potential to unravel
new applications of quantum gases and microcavity systems, such as the development
of polariton qubits [78], ultrafast optical switches based on coherent control [34],
electronic bias [244] or operation bistability [251], manipulation [35] and cooling of
quantum gases [252], and generation of coherent light in nonlinear regimes [71].
Polariton States Exposed to Terahertz Pulses
Within the past decade, a couple of independent efforts have been made to combine THz radiation and polariton systems in experiments. For instance, Tomaino et
al. used terahertz radiation to depopulate the upper/higher-energy polariton branch
(UP/HEP) by a THz-induced transition to the excitonic 2 p state as was predicted
by theory [253], while Ménard et al. combined a THz-probe experiment with a
polariton condensation study, “revealing the dark side of a bright exciton–polariton
condensate”—in other words, showing the pronounced existence of an uncondensed
high-energy fraction of “hot” excitons, which do not couple to the light field and thus
coexist with a condensate [172]. Such uncondensed fraction typically contributes to
intensity noise and is considered parasitic for coherent condensates (see [54, 209,
254, 255]). In addition, a single THz-pulse protocol was proposed and demonstrated
as a polarisation “reset” in a transient reflection experiment on a microcavity, which
was disturbed by the THz pulse at certain times in order to probe light–matter interaction time scales [36].
Hunt for Terahertz-Generating Polaritons
On the other hand, a few experimentalist groups worldwide tried to obtain THz
emission as proposed by expert theoreticians from polariton condensates [71–73, 75],
demonstrating a strong interest in the field of THz-related polariton physics. On such
a path, Rojan et al. found that in CdTe microcavities the Rabi oscillation resonates
with the phonon–polariton, rendering the Rabi oscillation as a potentially active
source of THz radiation, whereas the LP–UP transition itself is else dipole forbidden
[76]. Nevertheless, the wider lack of success with polariton-based THz-generation
schemes can be attributed to different technical aspects such as THz-detection sensitivity, design parameters, and condensate sizes, as summarised in [243].
Filling the Knowledge Gaps on Terahertz Interactions
In order to gain a better understanding of how THz radiation and polaritonic gases
interact with each other, still further studies are needed both in the linear regime, such
as performed by Pietka et al. on doubly-dressed bosons [256], and, more importantly,
in the nonlinear regime. It can be expected that practical generation of THz waves
from polariton systems will strongly benefit from such endeavours by the polariton
85
THz emitters [74], with important physics still not studied well enough to exploit
many-particle effects in novel device concepts and quantum information schemes.
Nevertheless, many terahertz-emitting polariton systems have been proposed so far
[71–73, 75, 76].
Promises of Light–Matter Coupled Systems
Exploring this domain of light–matter interaction thus has still the potential to unravel
new applications of quantum gases and microcavity systems, such as the development
of polariton qubits [78], ultrafast optical switches based on coherent control [34],
electronic bias [244] or operation bistability [251], manipulation [35] and cooling of
quantum gases [252], and generation of coherent light in nonlinear regimes [71].
Polariton States Exposed to Terahertz Pulses
Within the past decade, a couple of independent efforts have been made to combine THz radiation and polariton systems in experiments. For instance, Tomaino et
al. used terahertz radiation to depopulate the upper/higher-energy polariton branch
(UP/HEP) by a THz-induced transition to the excitonic 2 p state as was predicted
by theory [253], while Ménard et al. combined a THz-probe experiment with a
polariton condensation study, “revealing the dark side of a bright exciton–polariton
condensate”—in other words, showing the pronounced existence of an uncondensed
high-energy fraction of “hot” excitons, which do not couple to the light field and thus
coexist with a condensate [172]. Such uncondensed fraction typically contributes to
intensity noise and is considered parasitic for coherent condensates (see [54, 209,
254, 255]). In addition, a single THz-pulse protocol was proposed and demonstrated
as a polarisation “reset” in a transient reflection experiment on a microcavity, which
was disturbed by the THz pulse at certain times in order to probe light–matter interaction time scales [36].
Hunt for Terahertz-Generating Polaritons
On the other hand, a few experimentalist groups worldwide tried to obtain THz
emission as proposed by expert theoreticians from polariton condensates [71–73, 75],
demonstrating a strong interest in the field of THz-related polariton physics. On such
a path, Rojan et al. found that in CdTe microcavities the Rabi oscillation resonates
with the phonon–polariton, rendering the Rabi oscillation as a potentially active
source of THz radiation, whereas the LP–UP transition itself is else dipole forbidden
[76]. Nevertheless, the wider lack of success with polariton-based THz-generation
schemes can be attributed to different technical aspects such as THz-detection sensitivity, design parameters, and condensate sizes, as summarised in [243].
Filling the Knowledge Gaps on Terahertz Interactions
In order to gain a better understanding of how THz radiation and polaritonic gases
interact with each other, still further studies are needed both in the linear regime, such
as performed by Pietka et al. on doubly-dressed bosons [256], and, more importantly,
in the nonlinear regime. It can be expected that practical generation of THz waves
from polariton systems will strongly benefit from such endeavours by the polariton