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A. Mazaheri Tehrani et al.
are excited and the anti-Stokes signal is generated, which finally couples out of the
surface.
Recently, we have demonstrated that a combination of femtosecond time-resolved
tr-CARS spectroscopy and SE-CARS can be used to access ultrafast vibrational
dynamics with nanometer spatial resolution beyond the diffraction-limit of an optical
microscope [41]. While a standard optical microscope was used to focus the light of
the pump, Stokes, and probe laser pulses onto the nano-structured sample surface,
a scanning near-field optical microscope was used to detect the anti-Stokes signal.
Using a gold-coated optical fiber tip with an aperture size of e.g. 100 nm, resulted in
the detection of a locally enhanced (“tip-enhanced”) CARS signal.
4 Conclusions
In this chapter, we have given a short introduction to coherent anti-Stokes Raman scattering (CARS) spectroscopy. We have introduced basics, which explain the nature of
the nonlinear optical process and demonstrated the fundamental properties explained
by energy and momentum conservation. From this, the requirements for the realization of an experimental setup have been deduced like phase-matching geometries, etc.
Advantages of the nonlinear Raman technique over the linear spontaneous Raman
scattering have been pointed out. Among others, there are the avoidance of fluorescence background, the considerable signal intensity due to the generation of a
laser-like coherent signal, the achievable high spatial resolution of the technique,
and the relatively simple use of CARS for time-resolved vibrational spectroscopy.
We have not discussed the more obvious application of CARS for frequencydomain vibrational spectroscopy as a counterpart to linear spontaneous Raman spectroscopy. However, we have selected two specific fields of applications, which on
the one hand demonstrate the usefulness of the four-wave mixing approach and on
the other hand also show that there are still open questions, which require further
research.
As first application, we have introduced time-resolved CARS (tr-CARS). Using
femtosecond laser pulses, the observation of coherent vibrational excitations is
possible giving access to ultrafast vibrational dynamics. Vibrational energy transfer
and the coupling between different vibrational modes can thus be investigated on an
elementary time scale. The usefulness of the tr-CARS technique has already been
demonstrated for different molecular systems, however, there are still many possible
applications, which have not been tackled up to now.
The combination of the surface-enhancement (SE) effect occurring when electromagnetic fields interact with molecules in contact or in close vicinity of nanostructured metal surfaces can also enhance CARS. However, the SE-CARS effect
is less efficient compared to SE-Raman scattering (SERS) when using standard
substrates like colloidal silver or gold. The required phase-matching and coherence conditions can to some extent be locally fulfilled when very regular SE-CARS
substrates are used. Besides the enhancement of CARS signals also extremely high
A. Mazaheri Tehrani et al.
are excited and the anti-Stokes signal is generated, which finally couples out of the
surface.
Recently, we have demonstrated that a combination of femtosecond time-resolved
tr-CARS spectroscopy and SE-CARS can be used to access ultrafast vibrational
dynamics with nanometer spatial resolution beyond the diffraction-limit of an optical
microscope [41]. While a standard optical microscope was used to focus the light of
the pump, Stokes, and probe laser pulses onto the nano-structured sample surface,
a scanning near-field optical microscope was used to detect the anti-Stokes signal.
Using a gold-coated optical fiber tip with an aperture size of e.g. 100 nm, resulted in
the detection of a locally enhanced (“tip-enhanced”) CARS signal.
4 Conclusions
In this chapter, we have given a short introduction to coherent anti-Stokes Raman scattering (CARS) spectroscopy. We have introduced basics, which explain the nature of
the nonlinear optical process and demonstrated the fundamental properties explained
by energy and momentum conservation. From this, the requirements for the realization of an experimental setup have been deduced like phase-matching geometries, etc.
Advantages of the nonlinear Raman technique over the linear spontaneous Raman
scattering have been pointed out. Among others, there are the avoidance of fluorescence background, the considerable signal intensity due to the generation of a
laser-like coherent signal, the achievable high spatial resolution of the technique,
and the relatively simple use of CARS for time-resolved vibrational spectroscopy.
We have not discussed the more obvious application of CARS for frequencydomain vibrational spectroscopy as a counterpart to linear spontaneous Raman spectroscopy. However, we have selected two specific fields of applications, which on
the one hand demonstrate the usefulness of the four-wave mixing approach and on
the other hand also show that there are still open questions, which require further
research.
As first application, we have introduced time-resolved CARS (tr-CARS). Using
femtosecond laser pulses, the observation of coherent vibrational excitations is
possible giving access to ultrafast vibrational dynamics. Vibrational energy transfer
and the coupling between different vibrational modes can thus be investigated on an
elementary time scale. The usefulness of the tr-CARS technique has already been
demonstrated for different molecular systems, however, there are still many possible
applications, which have not been tackled up to now.
The combination of the surface-enhancement (SE) effect occurring when electromagnetic fields interact with molecules in contact or in close vicinity of nanostructured metal surfaces can also enhance CARS. However, the SE-CARS effect
is less efficient compared to SE-Raman scattering (SERS) when using standard
substrates like colloidal silver or gold. The required phase-matching and coherence conditions can to some extent be locally fulfilled when very regular SE-CARS
substrates are used. Besides the enhancement of CARS signals also extremely high