278
M. B. Raschke et al.
A possible implementation of adiabatic nanofocusing for imaging and quantum
coherent control is shown in Fig. 7.20. Nanofocusing in combination with a pulse
shaper can be used for linear, nonlinear, and ultrafast imaging of individual quantum
systems or mesoscale variations in bulk and dense media. The ability to control
pulse shape and phase enables pump-probe techniques including collinear FWM
and 2D spectroscopy. With feedback on the nonlinear response nω, Δω, etc. of
the coupled tip-sample system, the extension to quantum coherent control of the
excitation pathways and evolution of the quantum system can also be realized.
7.6 Outlook
In spite of the importance of the nonlinear and ultrafast properties of metals in general, and metal surfaces and nanostructures in particular, for the understanding of
phenomena such as enhancement and dephasing associated with plasmonic excitations, considerably less work has focused on these aspects compared to linear and
continuous wave spectroscopies. Here we have attempted to summarize the fundamental properties of metals and their plasmonic excitations and their effect on
nonlinear behavior in order to provide a guide for future extensions of plasmonic
studies. Optimization and control of these properties will be important for increasing
sensitivity and efficiency in a wide variety of sensing and optical switching applications, chemical spectroscopy, nano-scale imaging, or coherent control on the single
quantum limit. Thus far little nanoscale imaging using a nonlinear response of a
material system has been demonstrated [72], though there has been interest in further increasing the nonlinear response through engineered nanostructures with a bulk
nonlinear material at the tip, which in combination with field enhancement could provide high wave-mixing conversion efficiencies and provide access to the additional
spectroscopic and symmetry degrees of freedom enabled by nonlinear techniques.
In the ultrafast regime, recent work has demonstrated the control of mode coherence in plasmonic systems [73], in spite of the extremely short plasmon dephasing
times. Control of coupled plasmonic-photonic modes, which have longer dephasing
times and are therefore easier to direct, has been demonstrated, as well as nonlinear
wavemixing in plasmonic-photonic waveguides [57]. Furthermore, taking advantage
of SPP properties such as nanofocusing provides one of the most promising routes to
achieving full spatio-temporal control of individual nanostructures and nanosystems.
This would enable the investigation of ultrafast dynamics on the characteristic time
scales of the elementary electronic and vibrational excitations in matter, and with
direct and selective spectroscopic access to the corresponding energy levels.
Plasmonic field enhancement has also been proposed as a means to achieve
the necessary peak intensities for high harmonic generation (HHG). However, the
nanoscopic interaction volume for plasmonic structures in comparison to conventional intracavity HHG suggests that the HHG yield would be small [74]. Multiphoton
or high field fluorescence processes can however be effective below the ionization
threshold [75], in addition to electron emission [76]. Strong field gradients could
M. B. Raschke et al.
A possible implementation of adiabatic nanofocusing for imaging and quantum
coherent control is shown in Fig. 7.20. Nanofocusing in combination with a pulse
shaper can be used for linear, nonlinear, and ultrafast imaging of individual quantum
systems or mesoscale variations in bulk and dense media. The ability to control
pulse shape and phase enables pump-probe techniques including collinear FWM
and 2D spectroscopy. With feedback on the nonlinear response nω, Δω, etc. of
the coupled tip-sample system, the extension to quantum coherent control of the
excitation pathways and evolution of the quantum system can also be realized.
7.6 Outlook
In spite of the importance of the nonlinear and ultrafast properties of metals in general, and metal surfaces and nanostructures in particular, for the understanding of
phenomena such as enhancement and dephasing associated with plasmonic excitations, considerably less work has focused on these aspects compared to linear and
continuous wave spectroscopies. Here we have attempted to summarize the fundamental properties of metals and their plasmonic excitations and their effect on
nonlinear behavior in order to provide a guide for future extensions of plasmonic
studies. Optimization and control of these properties will be important for increasing
sensitivity and efficiency in a wide variety of sensing and optical switching applications, chemical spectroscopy, nano-scale imaging, or coherent control on the single
quantum limit. Thus far little nanoscale imaging using a nonlinear response of a
material system has been demonstrated [72], though there has been interest in further increasing the nonlinear response through engineered nanostructures with a bulk
nonlinear material at the tip, which in combination with field enhancement could provide high wave-mixing conversion efficiencies and provide access to the additional
spectroscopic and symmetry degrees of freedom enabled by nonlinear techniques.
In the ultrafast regime, recent work has demonstrated the control of mode coherence in plasmonic systems [73], in spite of the extremely short plasmon dephasing
times. Control of coupled plasmonic-photonic modes, which have longer dephasing
times and are therefore easier to direct, has been demonstrated, as well as nonlinear
wavemixing in plasmonic-photonic waveguides [57]. Furthermore, taking advantage
of SPP properties such as nanofocusing provides one of the most promising routes to
achieving full spatio-temporal control of individual nanostructures and nanosystems.
This would enable the investigation of ultrafast dynamics on the characteristic time
scales of the elementary electronic and vibrational excitations in matter, and with
direct and selective spectroscopic access to the corresponding energy levels.
Plasmonic field enhancement has also been proposed as a means to achieve
the necessary peak intensities for high harmonic generation (HHG). However, the
nanoscopic interaction volume for plasmonic structures in comparison to conventional intracavity HHG suggests that the HHG yield would be small [74]. Multiphoton
or high field fluorescence processes can however be effective below the ionization
threshold [75], in addition to electron emission [76]. Strong field gradients could