5 Coherent Nonlinear Processes in Metal-Semiconductor …
117
5.6 Other Coherent Interactions
Since the periodic energy transfer in metal-semiconductor nanostructures occur due
to coherent exchange of energy, these structures are promising for exploring coherent phenomena like stimulated emission, lasing and condensation. Apart from periodic energy transfer, Rabi oscillations in a two-level system interacting with light
are also interpreted as the system undergoing periodic stimulated emission [4–9].
Since in strong-coupling regime the resonator and the emitter are already interacting coherently, it is expected that these systems will exhibit stimulated emission
or lasing at much lower pumping threshold compared to the conventional lasers,
which operate in weak coupling regime. The hybrid nature of the polariton mode as
contrast to lasing in weak coupling regime has been demonstrated in semiconductor coupled micro-cavities and plasmonic hybrid nanostructures [43, 65, 66]. Some
experiments on polariton lasing in metal-semiconductor hybrid nanostructures have
recently been reported [67, 68]. Polariton lasing is an example of a collective manybody coherent interaction, which is possible to achieve because of much longer
coherence length of polaritons compared to excitons in semiconductors [8, 9, 14,
69]. Another many-body nonlinear interaction that has been observed in strongly
coupled microcavities is that of polariton scattering [6]. Like other scattering phenomena such as Raman scattering involving light and lattice vibration, polariton
can also interact with other polariton or a different excitation so that they can lose
energy as well as momentum via non-radiative mechanism to reach thermodynamically the most favored distribution. Under such a condition, it is possible to have this
state highly occupied from which amplified emission can occur. As the excitation
and emission are not in the same direction, the emission process here is referred
to as polariton scattering. Other nonlinear frequency conversion processes such as
parametric generation/amplification and condensation of polaritons involving manybody effects and pulsed operation have also been reported in semiconductor coupled
micro-cavities [6]. It would be interesting to explore such coherent interactions in
metal-semiconductor hybrid nanostructures. Due to the longer coherence length,
exciton-SPP polaritons are also expected to be potential candidates to explore other
quantum mechanical many-body interactions like Bose-Einstein condensation that
are challenging to be observed otherwise because of their sensitivity to disorder and
impurities like in case of purely excitonic systems. Their unique properties make
them promising for developing novel types of optical and opto-electronic devices.
5.7 Emerging Trends and Outloook
So far, all coherent as well as incoherent light-matter interactions discussed here
have been performed on large ensembles of excitons and with comparatively high
pulse energies at least in nanojoule regime. The fundamental quantum mechanical
limit in the strong coupling is the dipole interaction between a single exciton and a
117
5.6 Other Coherent Interactions
Since the periodic energy transfer in metal-semiconductor nanostructures occur due
to coherent exchange of energy, these structures are promising for exploring coherent phenomena like stimulated emission, lasing and condensation. Apart from periodic energy transfer, Rabi oscillations in a two-level system interacting with light
are also interpreted as the system undergoing periodic stimulated emission [4–9].
Since in strong-coupling regime the resonator and the emitter are already interacting coherently, it is expected that these systems will exhibit stimulated emission
or lasing at much lower pumping threshold compared to the conventional lasers,
which operate in weak coupling regime. The hybrid nature of the polariton mode as
contrast to lasing in weak coupling regime has been demonstrated in semiconductor coupled micro-cavities and plasmonic hybrid nanostructures [43, 65, 66]. Some
experiments on polariton lasing in metal-semiconductor hybrid nanostructures have
recently been reported [67, 68]. Polariton lasing is an example of a collective manybody coherent interaction, which is possible to achieve because of much longer
coherence length of polaritons compared to excitons in semiconductors [8, 9, 14,
69]. Another many-body nonlinear interaction that has been observed in strongly
coupled microcavities is that of polariton scattering [6]. Like other scattering phenomena such as Raman scattering involving light and lattice vibration, polariton
can also interact with other polariton or a different excitation so that they can lose
energy as well as momentum via non-radiative mechanism to reach thermodynamically the most favored distribution. Under such a condition, it is possible to have this
state highly occupied from which amplified emission can occur. As the excitation
and emission are not in the same direction, the emission process here is referred
to as polariton scattering. Other nonlinear frequency conversion processes such as
parametric generation/amplification and condensation of polaritons involving manybody effects and pulsed operation have also been reported in semiconductor coupled
micro-cavities [6]. It would be interesting to explore such coherent interactions in
metal-semiconductor hybrid nanostructures. Due to the longer coherence length,
exciton-SPP polaritons are also expected to be potential candidates to explore other
quantum mechanical many-body interactions like Bose-Einstein condensation that
are challenging to be observed otherwise because of their sensitivity to disorder and
impurities like in case of purely excitonic systems. Their unique properties make
them promising for developing novel types of optical and opto-electronic devices.
5.7 Emerging Trends and Outloook
So far, all coherent as well as incoherent light-matter interactions discussed here
have been performed on large ensembles of excitons and with comparatively high
pulse energies at least in nanojoule regime. The fundamental quantum mechanical
limit in the strong coupling is the dipole interaction between a single exciton and a