Coherent anti-Stokes Raman spectroscopy

Coherent anti-Stokes Raman spectroscopy, also called Coherent anti-Stokes Raman scattering spectroscopy (CARS), is a form of spectroscopy used primarily in chemistry, physics and related fields. It is sensitive to the same vibrational signatures of molecules as seen in Raman spectroscopy, typically the nuclear vibrations of chemical bonds. Unlike Raman spectroscopy, CARS employs multiple photons to address the molecular vibrations, and produces a signal in which the emitted waves are coherent with one another. As a result, CARS is orders of magnitude stronger than spontaneous Raman emission. CARS is a third-order nonlinear optical process involving three laser beams: a pump beam of frequency ω_p, a Stokes beam of frequency ω_S and a probe beam at frequency ω_pr. These beams interact with the sample and generate a coherent optical signal at the anti-Stokes frequency (ω_p-ω_S+ω_pr). The latter is resonantly enhanced when the frequency difference between the pump and the Stokes beams (ω_p-ω_S) coincides with the frequency of a Raman resonance, which is the basis of the technique's intrinsic vibrational contrast mechanism.^[1][2]

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History

The acronym CARS, which invokes a seemingly inadvertent relation to automobiles, is actually closely related to the birth story of the technique. In 1965, a paper was published by two researchers of the Scientific Laboratory at the Ford Motor Company, P. D. Maker and R. W. Terhune, in which the CARS phenomenon was reported for the first time.^[3] Maker and Terhune used a pulsed ruby laser to investigate the third order response of several materials. They first passed the ruby beam of frequency ω through a Raman shifter to create a second beam at ω-ω_v, and then directed the two beams simultaneously onto the sample. When the pulses from both beams overlapped in space and time, the Ford researchers observed a signal at ω+ω_v, which is the blue-shifted CARS signal. They also demonstrated that the signal increases significantly when the difference frequency ω_v between the incident beams matches a Raman frequency of sample. Maker and Terhune called their technique simply 'three wave mixing experiments'. The name coherent anti-Stokes Raman spectroscopy was assigned almost ten years later, by Begley et al at Stanford University in 1974.^[4] Since then, this vibrationally sensitive nonlinear optical technique is commonly known as CARS.

Principle

  The CARS process can be physically explained by using either a classical oscillator model or by using a quantum mechanical model that incorporates the energy levels of the molecule. Classically, the Raman active vibrator is modeled as a (damped) harmonic oscillator with a characteristic frequency of ω_v. In CARS, this oscillator is not driven by a single optical wave, but by the difference frequency (ω_p-ω_S) between the pump and the Stokes beams instead. This driving mechanism is similar to hearing the low combination tone when striking two different high tone piano keys: your ear is sensitive to the difference frequency of the high tones. Similarly, the Raman oscillator is susceptible to the difference frequency of two optical waves. When the difference frequency ω_p-ω_S approaches ω_v, the oscillator is driven very efficiently. On a molecular level, this implies that the electron cloud surrounding the chemical bond is vigorously oscillating with the frequency ω_p-ω_S. These electron motions alter the optical properties of the sample, i.e. there is a periodic modulation of the refractive index of the material. This periodic modulation can be probed by a third laser beam, the probe beam. When the probe beam is propagating through the periodically altered medium, it acquires the same modulation. Part of the probe, originally at ω_pr will now get modified to ω_pr+ω_p-ω_S, which is the observed anti-Stokes emission. Under certain beam geometries, the anti-Stokes emission may diffract away from the probe beam, and can be detected in a separate direction.

While intuitive, this classical picture does not take into account the quantum mechanical energy levels of the molecule. Quantum mechanically, the CARS process can be understood as follows. Our molecule is initially in the ground state, the lowest energy state of the molecule. The pump beam excites the molecule to a virtual state. A virtual state is not an eigenstate of the molecule, rather it exhibits an infinitely short lifetime, and thus the molecule cannot remain in this state. If a Stokes beam is simultaneously present along with the pump, the virtual state can be used as an instantaneous gateway to address a vibrational eigenstate of the molecule. The joint action of the pump and the Stokes has effectively established a coupling between the ground state and the vibrationally excited state of the molecule. The molecule is now in two states at the same time: it resides in a coherent superposition of states. This coherence between the states can be probed by the probe beam, which promotes the system to a virtual state. Again, the molecule cannot stay in the virtual state and will fall back instantaneously to the ground state under the emission of a photon at the anti-Stokes frequency. The molecule is no longer in a superposition, as it resides again in one state, the ground state. In the quantum mechanical model, no energy is deposited in the molecule during the CARS process. Instead, the molecule acts like a medium for converting the frequencies of the three incoming waves into a CARS signal. There are, however, related coherent Raman process that occur simultaneously which do deposit energy into the molecule.

Properties

The CARS technique exhibits a number of interesting properties, which make it an attractive alternative to spontaneous Raman spectroscopy:

1) It is sensitive to Raman-active molecular vibrations. Therefore, CARS can be used as an analytical tool for identifying chemical samples.

2) CARS produces generally much stronger signals than spontaneous Raman, typically by a factor of 10⁵. The reason for the stronger signals is the active driving of the Raman mode (as opposed to a spontaneous process) and the coherent addition of the CARS waves (as opposed to incoherent addition). How much stronger depends very much on the details of the sample. It is unclear at the moment whether CARS from a single molecule is stronger than the corresponding spontaneous Raman signal, because in the case of a single emitter the gain obtained from coherently adding waves is no longer relevant.

3) The CARS emission is blue-shifted relative to the incident laser beams. Contrary, in Raman spectroscopy, the signal is red-shifted. A blue-shifted signal is preferred when dealing with samples that fluoresce, because the red-shifted fluorescence does not overlap with the CARS emission.

4) The CARS signal is temperature dependent. The strength of the signal scales with the difference in the ground state population and the vibrationally excited state population. Since the population of states follows the temperature dependent Boltzmann distribution, the CARS signal carries an intrinsic temperature dependence as well. This temperature dependence makes CARS a popular technique for monitoring the temperature of hot gases and flames.

Applications

CARS is used for species selective microscopy and combustion diagnostics.

See also

• Coherent Stokes Raman spectroscopy
• Raman spectroscopy
• Four-wave mixing

References

1. ^ A Review of the Theory and Application of Coherent Anti-Stokes Raman Spectroscopy (CARS) Applied Spectroscopy, Volume 31, Number 4, July/August 1977, pp. 253-271(19)
2. ^ Coherent anti-Stokes Raman scattering: from proof-of-the-principle experiments to femtosecond CARS and higher order wave-mixing generalizations Journal of Raman Spectroscopy, Volume 31, Issue 8-9 , pp. 653 - 667
3. ^ Study of Optical Effects Due to an Induced Polarization Third Order in the Electric Field Strength Physical Review, Volume 137, Issue 3A, pp. 801-818
4. ^ Coherent anti-Stokes Raman spectroscopy Applied Physics Letters, Volume 25, Issue 7 , pp. 387-390