Coherent Anti-Stokes Raman Scattering: Basics, Theoretical …
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vibrations, the 1223 and 1120 cm
−1 modes belong to the asymmetric stretch and the
symmetric stretch vibrations of SO 2 , respectively. A dephasing time T 2 of 0.72 ps
for the Raman signal at 1215 cm
−1 can be determined.
Among other interesting findings, the tr-CARS experiments revealed that also
beatings with modes could be detected, which have not been directly excited by the
pump-Stokes laser pair. This pointed to an efficient interionic vibrational energy
transfer, which again helped to better understand coupling mechanisms in the
molecular IL system.
There are many more examples where tr-CARS has been applied to obtain detailed
information about vibrational or rotational dynamics of molecules. In this short
chapter, we will not be able to discuss further applications. However, as last comment,
we would like to add that besides getting information about dynamics, the delay
between the laser pulses can also be used to suppress the non-resonant background
signal mentioned above (see e.g. Fig. 1). Since this two-photon absorption process
is purely electronic and therefore has an extremely short lifetime, it vanishes as soon
as a short time-delay is introduced between the laser pulses. Using pulses in the
picosecond time range still allows for a good spectral resolution, which now yields
nonlinear Raman spectra free of the undesirable background.
3.2 Surface-Enhanced CARS (SE-CARS)
CARS spectroscopy in frequency and time domain has interesting applications. The
use in frequency domain as alternative for linear (spontaneous) Raman spectroscopy
has not been further discussed in the chapter since it appears to be obvious. From
the examples given in Sect. 3.1, the attractiveness for time-resolved vibrational spectroscopy has been demonstrated. However, there are also more “specialized” developments. In our contribution, we would like to only mention one of them, “Surfaceenhanced CARS” (SE-CARS). This is an example, which demonstrates that on the
one hand an interesting CARS technique exists, which is attractive for certain applications, but on the other hand the mechanisms involved are not completely understood
yet.
Surface-enhanced Raman scattering (SERS) [26, 27] is a well-known and still
rapidly growing technique, in which the relatively weak spontaneous Raman scattering is drastically enhanced by several orders of magnitude when the molecules
are placed in close proximity of a metallic (typically nano-structured) surface. There
are two main mechanisms, which result in the signal enhancement. When the light
couples to the surface electrons of the metal, plasmons (quanta of the electron oscillation) are excited resulting in “surface plasmon polaritons” (SPPs), which in principle concentrate the energy of the field in a very close proximity of the surface. This
results in an “electro-magnetic enhancement” effect where both exciting laser field
and signal field are affected. Additionally, the coupling of the metal electrons and the
molecular electronic system can result in resonance effects, which contribute to the
SERS process as “chemical enhancement” mechanism. The enhancement can be so
247
vibrations, the 1223 and 1120 cm
−1 modes belong to the asymmetric stretch and the
symmetric stretch vibrations of SO 2 , respectively. A dephasing time T 2 of 0.72 ps
for the Raman signal at 1215 cm
−1 can be determined.
Among other interesting findings, the tr-CARS experiments revealed that also
beatings with modes could be detected, which have not been directly excited by the
pump-Stokes laser pair. This pointed to an efficient interionic vibrational energy
transfer, which again helped to better understand coupling mechanisms in the
molecular IL system.
There are many more examples where tr-CARS has been applied to obtain detailed
information about vibrational or rotational dynamics of molecules. In this short
chapter, we will not be able to discuss further applications. However, as last comment,
we would like to add that besides getting information about dynamics, the delay
between the laser pulses can also be used to suppress the non-resonant background
signal mentioned above (see e.g. Fig. 1). Since this two-photon absorption process
is purely electronic and therefore has an extremely short lifetime, it vanishes as soon
as a short time-delay is introduced between the laser pulses. Using pulses in the
picosecond time range still allows for a good spectral resolution, which now yields
nonlinear Raman spectra free of the undesirable background.
3.2 Surface-Enhanced CARS (SE-CARS)
CARS spectroscopy in frequency and time domain has interesting applications. The
use in frequency domain as alternative for linear (spontaneous) Raman spectroscopy
has not been further discussed in the chapter since it appears to be obvious. From
the examples given in Sect. 3.1, the attractiveness for time-resolved vibrational spectroscopy has been demonstrated. However, there are also more “specialized” developments. In our contribution, we would like to only mention one of them, “Surfaceenhanced CARS” (SE-CARS). This is an example, which demonstrates that on the
one hand an interesting CARS technique exists, which is attractive for certain applications, but on the other hand the mechanisms involved are not completely understood
yet.
Surface-enhanced Raman scattering (SERS) [26, 27] is a well-known and still
rapidly growing technique, in which the relatively weak spontaneous Raman scattering is drastically enhanced by several orders of magnitude when the molecules
are placed in close proximity of a metallic (typically nano-structured) surface. There
are two main mechanisms, which result in the signal enhancement. When the light
couples to the surface electrons of the metal, plasmons (quanta of the electron oscillation) are excited resulting in “surface plasmon polaritons” (SPPs), which in principle concentrate the energy of the field in a very close proximity of the surface. This
results in an “electro-magnetic enhancement” effect where both exciting laser field
and signal field are affected. Additionally, the coupling of the metal electrons and the
molecular electronic system can result in resonance effects, which contribute to the
SERS process as “chemical enhancement” mechanism. The enhancement can be so