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A. Mazaheri Tehrani et al.
There are in principle two main vibrational spectroscopy methods. IR absorption spectroscopy makes use of vibrational resonances in the infrared energy range.
Raman spectroscopy is based on inelastic scattering of higher-energy photons (generally in the range from near IR to UV) from the molecules of the sample. In this process,
the photons loose (“Stokes scattering”) or gain (“anti-Stokes scattering”) energy by
an excitation or de-excitation of molecular vibrations or rotations (in the following,
we will mainly refer to vibrational spectra), respectively. Both methods provide a
relatively fast, label-free, and accurate measurement of a huge variety of samples,
like gases, liquids, powders, thin films, nanostructures, etc. The application ranges
from a quick identification of the species to an in-depth investigation of the sample
structural state and changes. Despite having great advantages, both of these methods
suffer from certain disadvantages, which limit the range of their applications. For
example, low spatial resolution of few microns due to longer infrared wavelength
makes IR absorption spectroscopy less attractive for microscopic investigations [3,
4] and absorption of infrared radiation by water molecules in the sample is a hurdle
for using this technique in aqueous environment or for living species [3]. Raman
spectroscopy, in contrast, using a shorter excitation wavelength, features a higher
spatial resolution and can be easily applied in aqueous solutions due to transparency
of water in the visible wavelength range. However, systems emitting fluorescence in
many cases let Raman signals vanish in the intense emission background. In addition, Raman signals are relatively weak due to the very low scattering cross section
– roughly a single inelastic scattering event per million photons. This limits the access
to low concentrations of sample molecules and in many cases results in relatively
long measurement times due to the need for longer integration [5]. Without special
techniques, which allow for a suppression of the fluorescence and/or an enhancement
of the scattered signal, a number of samples cannot be investigated.
While researchers have heavily utilized these methods for several decades (since
the discovery of the “Raman Effect” in 1928) to advance the scientific research, the
discovery of the laser in 1960 [6] has drastically improved the successful application
of this method, by providing a well-defined, coherent, and intense optical excitation
source. At the same time, this has opened a completely new front in optics, the socalled “nonlinear optics”, which only plays a role when very high field intensities
are applied as they for example are found in tightly focused laser beams or for very
short laser pulses.
A few years later, in 1965, two scientists, Maker and Terhune, working in a
research laboratory of the Ford Motor company in Michigan [7], did research in
pursuit of a so-called “Raman Laser”—a laser system based on light amplification of
a stimulated Raman signal in a Raman active medium. They found that when exciting
with a laser beam together with a second beam red-shifted (“Stokes shifted”) by the
energy of a vibrational mode, a coherent laser-like signal at the anti-Stokes energy,
i.e. the laser energy increased by the mode energy, could be observed. This laid the
foundation for a technique, which in 1974 was named “coherent anti-Stokes Raman
spectroscopy” (CARS) by R. F. Begley et al. [8]. It could be demonstrated that CARS
has significant advantages compared to normal spontaneous Raman spectroscopy,
which include five order of magnitude higher conversion efficiency, fluorescence-free
A. Mazaheri Tehrani et al.
There are in principle two main vibrational spectroscopy methods. IR absorption spectroscopy makes use of vibrational resonances in the infrared energy range.
Raman spectroscopy is based on inelastic scattering of higher-energy photons (generally in the range from near IR to UV) from the molecules of the sample. In this process,
the photons loose (“Stokes scattering”) or gain (“anti-Stokes scattering”) energy by
an excitation or de-excitation of molecular vibrations or rotations (in the following,
we will mainly refer to vibrational spectra), respectively. Both methods provide a
relatively fast, label-free, and accurate measurement of a huge variety of samples,
like gases, liquids, powders, thin films, nanostructures, etc. The application ranges
from a quick identification of the species to an in-depth investigation of the sample
structural state and changes. Despite having great advantages, both of these methods
suffer from certain disadvantages, which limit the range of their applications. For
example, low spatial resolution of few microns due to longer infrared wavelength
makes IR absorption spectroscopy less attractive for microscopic investigations [3,
4] and absorption of infrared radiation by water molecules in the sample is a hurdle
for using this technique in aqueous environment or for living species [3]. Raman
spectroscopy, in contrast, using a shorter excitation wavelength, features a higher
spatial resolution and can be easily applied in aqueous solutions due to transparency
of water in the visible wavelength range. However, systems emitting fluorescence in
many cases let Raman signals vanish in the intense emission background. In addition, Raman signals are relatively weak due to the very low scattering cross section
– roughly a single inelastic scattering event per million photons. This limits the access
to low concentrations of sample molecules and in many cases results in relatively
long measurement times due to the need for longer integration [5]. Without special
techniques, which allow for a suppression of the fluorescence and/or an enhancement
of the scattered signal, a number of samples cannot be investigated.
While researchers have heavily utilized these methods for several decades (since
the discovery of the “Raman Effect” in 1928) to advance the scientific research, the
discovery of the laser in 1960 [6] has drastically improved the successful application
of this method, by providing a well-defined, coherent, and intense optical excitation
source. At the same time, this has opened a completely new front in optics, the socalled “nonlinear optics”, which only plays a role when very high field intensities
are applied as they for example are found in tightly focused laser beams or for very
short laser pulses.
A few years later, in 1965, two scientists, Maker and Terhune, working in a
research laboratory of the Ford Motor company in Michigan [7], did research in
pursuit of a so-called “Raman Laser”—a laser system based on light amplification of
a stimulated Raman signal in a Raman active medium. They found that when exciting
with a laser beam together with a second beam red-shifted (“Stokes shifted”) by the
energy of a vibrational mode, a coherent laser-like signal at the anti-Stokes energy,
i.e. the laser energy increased by the mode energy, could be observed. This laid the
foundation for a technique, which in 1974 was named “coherent anti-Stokes Raman
spectroscopy” (CARS) by R. F. Begley et al. [8]. It could be demonstrated that CARS
has significant advantages compared to normal spontaneous Raman spectroscopy,
which include five order of magnitude higher conversion efficiency, fluorescence-free