Anti-stokes wave

This is not the case for stimulated anti-Stokes radiation. There are two sources of polarization for anti-Stokes radiation [17]. The first is analogous to that in figure B1.3.3(b) where the action of the blackbody (- 2) is replaced by the action of a previously produced anti-Stokes wave, with frequency 03. This radiation actually experiences an attenuation since the value of Im x o3 ) is positive (leading to a negative gam coefficient). This is known as the stimulated Raman loss (SRL) spectroscopy [76]. Flowever the second source of anti-Stokes polarization relies on the presence of Stokes radiation [F7]. This anti-Stokes radiation will emerge from the sample in a direction given by the wavevector algebra = 2k - kg. Since the Stokes radiation is... 

This is not the case for stimulated anti-Stokes radiation. There are two sources of polarization for anti-Stokes radiation [H]. The first is analogous to that in figure B1.3.3(bl where the action of the blackbody (-CO2) is replaced by the action of a previously produced anti-Stokes wave, with frequency co. This radiation actually... 

According to (3.26), the driving term in the wave equation (3.28) for an anti-Stokes wave at 0 = is given by... 

For small amplitudes of the anti-Stokes waves, we can assume that the molecular vibrations are independent of and can replace qy by its solution (3.25). This yields an equation for the amplification of "a that is analogous to (3.29) for Es... 

Despite the enormous intensities of stimulated Stokes and anti-Stokes waves, stimulated Raman spectroscopy has been of little use in molecular spectroscopy. The high threshold, which, according to (3.30), depends on the molecular density N, the incident intensity I a and the square of the small polarizability term (daij/dq) in (3.27), limits stimulated emission to only the strongest Raman lines in materials of high densities N. 

Because of the nonlinear interaction discussed in Sect. 3.3.1, new Stokes and anti-Stokes waves are generated (Fig. 3.16d). The waves co and C02 produce a large population density of vibrationally excited molecules by stimulated Raman scattering. These excited molecules act as the nonlinear medium for the generation of... 

In a similar way, a Stokes wave with frequency cOg = 2co2 — co i is generated by the incident waves at coi and C02 (Fig- 3.16d). Since four waves are involved in the generation of the anti-Stokes wave, CARS is called 3. four-wave parametric mixing process (Fig. 3.17). 

The higher frequency coj, > twi, ci>2 of the anti-Stokes waves allows one to use filters that reject the incident light as well as fluorescence light. 

If the Stokes or anti-Stokes wave becomes sufficiently strong, it can again produce another Stokes or anti-Stokes wave at = cDp — 2cuv... 

If the incident laser intensity /l becomes very large, an appreciable fraction of the molecules in the initial state Ei is excited into the final state Ef and the intensity of the Raman-scattered light is correspondingly large. Under these conditions we have to consider the simultaneous interaction of the molecules with two EM waves the laser wave at the frequency co and the Stokes wave at the frequency ens = n>L or the anti-Stokes wave at Both... 

For small amplitudes a El of the anti-Stokes waves, we can assume that the molecular vibrations are independent of can replace by its... 

In the collinear arrangement the anti-Stokes wave at cua = 2oo — 002 (cua > 0 is detected through filters that reject both incident laser beams as... 

If the Stokes or anti-Stokes wave becomes sufficiently strong, it can again produce another Stokes or anti-Stokes wave at = -Wy = Wp-2Wy and (2) = Wp+2Wy. Therefore, several Stokes and anti-Stokes waves are generated at frequencies = Wp-nWy = Wp + nwy (n = 1,2,3. ..)... 

WL+wy. Both waves are coupled by the molecules vibrating with the frequencies Wy. This parametric interaction leads to an energy exchange between the pump wave and the Stokes or anti-Stokes waves. This phenomenon of stimulated Raman scattering which was first observed by Woodbury et al. [8.32] and then explained by Woodbury and Eckhardt [8.33] can be described in a classical picture [8.34,35]. 

In the collinear arrangement the anti-Stokes wave at = 2o)i-(<)2 (w > Wj ) is detected through filters which reject both incident laser beams and also the fluorescence which may be generated in the sample. Figure 8.12 illustrates a typical experimental setup used for rotational-vibrational spectroscopy of gases by CARS [8.50]. The two incident laser beams are provided by a Q-switched ruby laser and a tunable dye laser, pumped by this ruby laser. Because the gain of the anti-Stokes wave depends quadratically on the molecular density N, see (8.33), high power levels of the incident... 

At the interface befween the silver film and the nonlinear medium (benzene in this case) an anti-Stokes wave with wavevector k, = 2ki — k2 is generated, which is coupled out of the prism. Figure 6.11a shows, with the help of an energy level scheme, the generation of the anti-Stokes wave 0)4 via the Raman resonance. 

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