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fast relaxation of polariton is expected due to the extremely fast relaxation rate of
SPPs [8, 14, 37, 38]. Near the anti-crossing, polaritons are expected to have the
average relaxation rate of exciton and SPP. Since exciton relaxation rate is much
slower, polariton relaxation should be comparable to that of SPPs. However, the
observed relaxation dynamics is similar to that of the uncoupled excitons. The relatively slower polariton dynamics is presumably due to the incoherent excitation
conditions used in these experiments [8, 47–50]. There is continuous population
transfer occurring from higher energy states to polariton states within 1 ps, slowing
down the polariton relaxation. In contrast to the faster polariton relaxation observed
in these experiments, Schwartz et al. have reported longer relaxation times for LP
mode in J-aggregate/metallic planar cavity hybrid structure under nonresonant excitation [51]. Such slower dynamics can also arise due to motional narrowing [52],
and incoherent population transfer from other states or dark mode formation. These
results suggest that to get the correct estimate of the polariton dynamics, it is essential to use coherent excitation conditions [47–49]. Nevertheless, the observation of a
pronounced, externally-induced, fast and reversible change in reflectivity of a metalmolecular aggregate nanostructure suggests that these hybrid structures can indeed
act and potentially find applications as externally controllable, ultrafast switchable
mirrors [42, 44].
Recently, Wang et al. have investigated the relation between strong coupling and
the radiative damping of the coupled modes in hybrid structures both in the spectral and temporal domains [8, 9, 14, 26]. Under resonant excitation conditions, in
which the pump wavelength is resonant with the coupled modes, the LP nonlinear
response (Fig. 5.4a) mainly reflects the saturation of the polariton exciton-fraction
and a weak transient reduction in the NMS . The dynamics of R/R 0 corresponding
to the J-aggregate (exciton), LP and the UP resonances along with the bi-exponential
fits are depicted in Fig. 5.4b. The exciton population relaxation times in J-aggregate
are ∼ 550 fs [28]. The relatively short-lived component observed near zero-delay
corresponds to the fast decay of the coherent scattering signal within the exciton
polarization relaxation time. Much shorter polariton response times compared to the
J-aggregate reflect the pronounced radiative damping of the SPP-fraction [8, 14, 37,
38]. Here, the coherent scattering and the population relaxation times are indistinguishable. For the LP mode, the rapid exponential decay of R with a lifetime of
∼ 55 fs is observed. A weak signal remaining for τ > 200 fs results from probing a
small fraction of uncoupled excitons. An even faster decay of the optical nonlinearity
is seen when resonantly exciting the UP mode. Here, the R signal decays to less than
0.1 of its maximum value with a decay time of only ∼ 22 fs [8, 26]. The faintly visible non-exponential decay is again attributed to a weak contribution from uncoupled
excitons. These observations provide unambiguous evidence for distinctly different
population lifetimes of UP and LP modes at zero detuning and resonant excitation [8,
26, 35, 36, 40]. There is no major contribution from the disorder or the incoherent
population transfer discussed earlier. They independently confirm the predictions
of the coupled oscillator model and thus establish the formation of sub- and superradiant polariton states in J-aggregate/metal hybrid nanostructures via co-operative
damping. Taken together, these investigations strongly suggest that fluctuating vac-
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fast relaxation of polariton is expected due to the extremely fast relaxation rate of
SPPs [8, 14, 37, 38]. Near the anti-crossing, polaritons are expected to have the
average relaxation rate of exciton and SPP. Since exciton relaxation rate is much
slower, polariton relaxation should be comparable to that of SPPs. However, the
observed relaxation dynamics is similar to that of the uncoupled excitons. The relatively slower polariton dynamics is presumably due to the incoherent excitation
conditions used in these experiments [8, 47–50]. There is continuous population
transfer occurring from higher energy states to polariton states within 1 ps, slowing
down the polariton relaxation. In contrast to the faster polariton relaxation observed
in these experiments, Schwartz et al. have reported longer relaxation times for LP
mode in J-aggregate/metallic planar cavity hybrid structure under nonresonant excitation [51]. Such slower dynamics can also arise due to motional narrowing [52],
and incoherent population transfer from other states or dark mode formation. These
results suggest that to get the correct estimate of the polariton dynamics, it is essential to use coherent excitation conditions [47–49]. Nevertheless, the observation of a
pronounced, externally-induced, fast and reversible change in reflectivity of a metalmolecular aggregate nanostructure suggests that these hybrid structures can indeed
act and potentially find applications as externally controllable, ultrafast switchable
mirrors [42, 44].
Recently, Wang et al. have investigated the relation between strong coupling and
the radiative damping of the coupled modes in hybrid structures both in the spectral and temporal domains [8, 9, 14, 26]. Under resonant excitation conditions, in
which the pump wavelength is resonant with the coupled modes, the LP nonlinear
response (Fig. 5.4a) mainly reflects the saturation of the polariton exciton-fraction
and a weak transient reduction in the NMS . The dynamics of R/R 0 corresponding
to the J-aggregate (exciton), LP and the UP resonances along with the bi-exponential
fits are depicted in Fig. 5.4b. The exciton population relaxation times in J-aggregate
are ∼ 550 fs [28]. The relatively short-lived component observed near zero-delay
corresponds to the fast decay of the coherent scattering signal within the exciton
polarization relaxation time. Much shorter polariton response times compared to the
J-aggregate reflect the pronounced radiative damping of the SPP-fraction [8, 14, 37,
38]. Here, the coherent scattering and the population relaxation times are indistinguishable. For the LP mode, the rapid exponential decay of R with a lifetime of
∼ 55 fs is observed. A weak signal remaining for τ > 200 fs results from probing a
small fraction of uncoupled excitons. An even faster decay of the optical nonlinearity
is seen when resonantly exciting the UP mode. Here, the R signal decays to less than
0.1 of its maximum value with a decay time of only ∼ 22 fs [8, 26]. The faintly visible non-exponential decay is again attributed to a weak contribution from uncoupled
excitons. These observations provide unambiguous evidence for distinctly different
population lifetimes of UP and LP modes at zero detuning and resonant excitation [8,
26, 35, 36, 40]. There is no major contribution from the disorder or the incoherent
population transfer discussed earlier. They independently confirm the predictions
of the coupled oscillator model and thus establish the formation of sub- and superradiant polariton states in J-aggregate/metal hybrid nanostructures via co-operative
damping. Taken together, these investigations strongly suggest that fluctuating vac-