5 Coherent Nonlinear Processes in Metal-Semiconductor …
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wavepackets are localized in different quantum system, there out-of phase dynamics
reduces the nonlinear signal [25]. Thus a stronger differential reflectivity signal is
observed at delays corresponding to the integer multiples of the Rabi period. The
polariton dynamics discussed here is strongly supported by optical Bloch equations.
On increasing the pump pulse energy, the transient suppression in NMS is sufficiently
strong to manipulate the Rabi period on ∼ 10 fs timescale.
These results present the first real-time observation of coherent exciton-SPP Rabi
oscillations in hybrid nanostructures. The oscillations observed in the nonlinear
response reflect the modulation in SPP population due to the coherent transfer of
energy between SPP excitations of the metallic grating and excitons in the coupled
J-aggretate. Remarkably, the dominant nonlinearity governing the nonlinear response
under incoherent as well coherent excitation is not the saturation of exciton but the
reduction in NMS due to saturation of exciton density. Such a fundamentally different nonlinear response of the hybrid structures can be important for developing
novel all-optical ultrafast plasmonic circuits and devices [41–44].
5.5 Optical Starck Effects in Hybrid Systems
The optical Stark effect (OSE) is another fundamental, coherent nonlinear interaction [55–60]. It involves transient shifts of energy levels in the presence of a slightly
off-resonant light field. Since it involves coherent response of the system, it is potentially useful for implementing quantum logic and information processing [61, 62].
At least in atomic and some of the semiconductor systems, it can be satisfactorily
described by a two-level system interacting with a coherent, transient light field.
Recently, OSE has also been explored in metal-semiconductor hybrid nanostructures [56–60]. Along with the transient shift of the resonance, OSE in these hybrid
structures is also found to be strongly affected by exciton-SPP interaction [8, 9, 27].
The ultrafast excitation conditions used by Vasa et al. for investigating OSE in
metal-Jaggregate hybrid nanostructures [8, 27], is shown in Fig. 5.6a. Under these
excitation conditions, three different physical mechanisms are expected to contribute
to the transient nonlinearity. First, there may be a saturation of exciton (SPPs do not
exhibit significant nonlinearity under these pumping conditions). This saturation
should result in saturation of absorption at polariton resonance without affecting
its spectral position. However, as in earlier cases, the observed nonlinear response
(Fig. 5.6b) is dispersive (transient shift of resonance), These investigations again
confirm that saturation of absorption is not the dominant nonlinearity in metal-Jaggregate hybrid structures [8, 23, 26]. Second, the finite exciton population may
induce transient reduction in normal mode splitting as discussed earlier. This reduction results in the red-shift of the UP. Also, it results in a red-shift of the UP and
thus in a dispersive lineshape with opposite sign of that seen in Fig. 5.6b [8, 23,
25–27]. The third contribution can arise due to the transient Stark-shift of the resonances induced by the off-resonant pump. However, in this case both LP and UP will
be blue-shifted, in turn blue-shifting both polariton resonances [8, 27]. The eigen
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wavepackets are localized in different quantum system, there out-of phase dynamics
reduces the nonlinear signal [25]. Thus a stronger differential reflectivity signal is
observed at delays corresponding to the integer multiples of the Rabi period. The
polariton dynamics discussed here is strongly supported by optical Bloch equations.
On increasing the pump pulse energy, the transient suppression in NMS is sufficiently
strong to manipulate the Rabi period on ∼ 10 fs timescale.
These results present the first real-time observation of coherent exciton-SPP Rabi
oscillations in hybrid nanostructures. The oscillations observed in the nonlinear
response reflect the modulation in SPP population due to the coherent transfer of
energy between SPP excitations of the metallic grating and excitons in the coupled
J-aggretate. Remarkably, the dominant nonlinearity governing the nonlinear response
under incoherent as well coherent excitation is not the saturation of exciton but the
reduction in NMS due to saturation of exciton density. Such a fundamentally different nonlinear response of the hybrid structures can be important for developing
novel all-optical ultrafast plasmonic circuits and devices [41–44].
5.5 Optical Starck Effects in Hybrid Systems
The optical Stark effect (OSE) is another fundamental, coherent nonlinear interaction [55–60]. It involves transient shifts of energy levels in the presence of a slightly
off-resonant light field. Since it involves coherent response of the system, it is potentially useful for implementing quantum logic and information processing [61, 62].
At least in atomic and some of the semiconductor systems, it can be satisfactorily
described by a two-level system interacting with a coherent, transient light field.
Recently, OSE has also been explored in metal-semiconductor hybrid nanostructures [56–60]. Along with the transient shift of the resonance, OSE in these hybrid
structures is also found to be strongly affected by exciton-SPP interaction [8, 9, 27].
The ultrafast excitation conditions used by Vasa et al. for investigating OSE in
metal-Jaggregate hybrid nanostructures [8, 27], is shown in Fig. 5.6a. Under these
excitation conditions, three different physical mechanisms are expected to contribute
to the transient nonlinearity. First, there may be a saturation of exciton (SPPs do not
exhibit significant nonlinearity under these pumping conditions). This saturation
should result in saturation of absorption at polariton resonance without affecting
its spectral position. However, as in earlier cases, the observed nonlinear response
(Fig. 5.6b) is dispersive (transient shift of resonance), These investigations again
confirm that saturation of absorption is not the dominant nonlinearity in metal-Jaggregate hybrid structures [8, 23, 26]. Second, the finite exciton population may
induce transient reduction in normal mode splitting as discussed earlier. This reduction results in the red-shift of the UP. Also, it results in a red-shift of the UP and
thus in a dispersive lineshape with opposite sign of that seen in Fig. 5.6b [8, 23,
25–27]. The third contribution can arise due to the transient Stark-shift of the resonances induced by the off-resonant pump. However, in this case both LP and UP will
be blue-shifted, in turn blue-shifting both polariton resonances [8, 27]. The eigen