Coherent Anti-Stokes Raman Scattering: Basics, Theoretical …
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spectrum on the higher energy side of the excitation, and a high spectral resolution,
which in principle eliminates the need for a monochromator when lasers with narrow
bandwidth are used [8].
The advance of the laser technology in the following years made high-power
tunable laser systems available, which resulted in an increased research activity on
CARS spectroscopy. The CARS technique rapidly became one of the most popular
nonlinear spectroscopic methods to study fluorescing samples, gases in discharges,
plasmas, combustion and atmospheric chemistry [9, 10]. The elimination of the
fluorescence background in particular opens a wide range of applications to study
biological samples, which previously was not possible using spontaneous Raman
spectroscopy due to the strong emission background typical for many bio-chemical
compounds. Also for CARS, limitations have to be overcome like e.g. the existence
of a third-order nonlinear “non-resonant” background, which will be discussed later
in this chapter. However, the aforementioned advantages have made CARS a great
non-destructive tool featuring a chemical contrast without the use of labels (like e.g.
dyes) to investigate and rapidly image biological samples and living cells in their
natural environment. Applications of CARS in microscopy have been intensively
studied and reviewed elsewhere [11–13] and are not the focus of this chapter.
Our contribution will not be able to cover the full range of CARS theory, instrumentation, and applications, which easily would fill a book on its own. However,
we would like to give the reader an idea of the usefulness of this nonlinear optical
spectroscopy technique. In this chapter, we will first briefly present the theoretical background of the nonlinear CARS process, which is also of relevance for
understanding the required experimental setup. Then, we will focus on only two
recently introduced CARS techniques, femtosecond-time-resolved CARS (tr-CARS)
and surface-enhanced CARS (SE-CARS).
2 Theoretical Background
In this section, a brief description of the underlying theory behind the CARS process is
presented, which will provide the background required for understanding the method
and is important for designing and performing an experiment successfully and for
interpreting the results correctly. For a more in-depth theoretical description, we refer
the interested reader to more detailed publications [14–17].
To put it simple, CARS is only one of the possibilities that nonlinear optics would
offer, in which three incoming photons nonlinearly interact with a medium and a new
photon is generated. These are the so-called “four-wave-mixing” (FWM) processes.
Since with increasing order of nonlinearity, the resulting signals become smaller,
FWM does not play a role unless the sample is irradiated with a very high intensity of
light or generally electro-magnetic radiation. In this case, the well-known principle of
“superposition” does not describe this process any more. This also explains why such
nonlinear optical phenomena—while theoretically predicted already in 1931 [18]—
have not been observed until about thirty years later [19, 20] after the Ruby laser
237
spectrum on the higher energy side of the excitation, and a high spectral resolution,
which in principle eliminates the need for a monochromator when lasers with narrow
bandwidth are used [8].
The advance of the laser technology in the following years made high-power
tunable laser systems available, which resulted in an increased research activity on
CARS spectroscopy. The CARS technique rapidly became one of the most popular
nonlinear spectroscopic methods to study fluorescing samples, gases in discharges,
plasmas, combustion and atmospheric chemistry [9, 10]. The elimination of the
fluorescence background in particular opens a wide range of applications to study
biological samples, which previously was not possible using spontaneous Raman
spectroscopy due to the strong emission background typical for many bio-chemical
compounds. Also for CARS, limitations have to be overcome like e.g. the existence
of a third-order nonlinear “non-resonant” background, which will be discussed later
in this chapter. However, the aforementioned advantages have made CARS a great
non-destructive tool featuring a chemical contrast without the use of labels (like e.g.
dyes) to investigate and rapidly image biological samples and living cells in their
natural environment. Applications of CARS in microscopy have been intensively
studied and reviewed elsewhere [11–13] and are not the focus of this chapter.
Our contribution will not be able to cover the full range of CARS theory, instrumentation, and applications, which easily would fill a book on its own. However,
we would like to give the reader an idea of the usefulness of this nonlinear optical
spectroscopy technique. In this chapter, we will first briefly present the theoretical background of the nonlinear CARS process, which is also of relevance for
understanding the required experimental setup. Then, we will focus on only two
recently introduced CARS techniques, femtosecond-time-resolved CARS (tr-CARS)
and surface-enhanced CARS (SE-CARS).
2 Theoretical Background
In this section, a brief description of the underlying theory behind the CARS process is
presented, which will provide the background required for understanding the method
and is important for designing and performing an experiment successfully and for
interpreting the results correctly. For a more in-depth theoretical description, we refer
the interested reader to more detailed publications [14–17].
To put it simple, CARS is only one of the possibilities that nonlinear optics would
offer, in which three incoming photons nonlinearly interact with a medium and a new
photon is generated. These are the so-called “four-wave-mixing” (FWM) processes.
Since with increasing order of nonlinearity, the resulting signals become smaller,
FWM does not play a role unless the sample is irradiated with a very high intensity of
light or generally electro-magnetic radiation. In this case, the well-known principle of
“superposition” does not describe this process any more. This also explains why such
nonlinear optical phenomena—while theoretically predicted already in 1931 [18]—
have not been observed until about thirty years later [19, 20] after the Ruby laser