Stimulated Raman effect

Figure 9.21 Transitions in the stimulated Raman effect in benzene...

In the stimulated Raman effect it is only the vibration that gives the most intense Raman scattering that is involved this is the case for Vj in benzene. 

This Raman shifted radiation is obtained using the stimulated Raman effect (Section 9.3.2)... 

A similar calculation will show that the stimulated Raman effect applied to frequency tripled radiation from a Nd YAG laser, with a fundamental wavelength of 1064.8 nm, produces wavelengths of 299 nm, with H2, and 289 nm, with H2. 

Draw a diagram similar to that in Figure 9.21 to illustrate the stimulated Raman effect in FI2. Fligh-pressure FI2 is used to Raman shift radiation from a KrF laser. Calculate the two wavelengths of the shifted radiation which are closest to that of the KrF laser. 

With the available high-power lasers the nonlinear response of matter to incident radiation can be studied. We will briefly discuss as examples the stimulated Raman effect, which can be used to investigate induced vibrational and rotational Raman spectra in solids, liquids or gases, and the inverse Raman effect which allows rapid analysis of a total Raman spectrum. A review of the applications of these and other nonlinear effects to Raman spectroscopy has been given by Schrotter2i4)... 

The first observation of the stimulated Raman effect was reported by Woodbury and Ng 215) j e effect was then thoroughly studied by several authors 216-218) and its theoretical background developed 219.220) (see also the review articles by Zubov et a/.22D). The stimulated Raman effect can be described as a parametric process where the coupling between a light wave at the Stokes frequency (Os and an optical phonon (vibrational wave) at cOy is produced by a pump field at col = (Oj + ojy. 

This phenomenon is known as the inverse Raman effect and was first observed by Stoicheff The inverse Raman spectrum is the analogue of the stimulated Raman emission spectrum and therefore theories of the stimulated Raman effect apply to both emission and absorption. There are, however, significant differences in the corresponding spectra ... 

When the power of the exciting radiation is raised into the megawatt range, nonlinear Raman effects are observed, namely the stimulated Raman effect, the inverse Raman effect (Stoicheff absorption), and the hyper-Raman effect. The results of such experiments with single crystals will be discussed in the last chapter, with special emphasis on stimulated Raman scattering from polaritons. 

Not long after the discovery of the stimulated Raman effect in liquids 63> it was also detected in single crystals 64), namely diamond, calcite, and a-sulfur. Only much later could it be shown that the effect can also be observed in crystal powders 651. The stimulated Raman effect 99 > is excited by giant-pulse lasers with a power of several MW. The strongest Raman lines of a substance are amplified until their intensity is of the same order of magnitude as that of the exciting line furthermore second, third, etc. Stokes lines of the fundamentals in question are observed with twice, thrice, etc. the frequency shift. 

The inverse Raman effect was detected in liquids 93> soon after the discovery of the stimulated Raman effect. When a medium is irradiated simultaneously by intense monochromatic light from a giant-pulse laser and by a continuum, sharp absorption lines are observed on the anti-Stokes side of the laser line, and under special conditions also on the Stokes side 94 >. McLaren and Stoicheff 95) used the intense fluorescence from a dye solution excited by frequency-... 

The optical Kerr and stimulated Raman effects cause a local modification of the optical susceptibility... 

Here, / is the fraction of the delayed nonlinear response, and 7Z is the memory function of the stimulated Raman effect. Parameterization by 7Z(t) sin Qr)e rT is often sufficient for ultrashort pulses [7], This simple formula has the advantage of easy implementation that avoids explicit calculation of the convolution integral. Often, an even simpler, exponential memory function is used, 7Z(r) e rr in simulations (see e.g. [28]). If the real memory function is sufficiently complex, a numerical convolution approach must be used to calculate the convolution. This is e.g. the case in silica [29],... 

Here, E is the strength of the applied electric field (laser beam), a the polarizability and / and y the first and second hyper-polarizabilities, respectively. In the case of conventional Raman spectroscopy with CW lasers (E, 104 V cm-1), the contributions of the / and y terms to P are insignificant since a fi y. Their contributions become significant, however, when the sample is irradiated with extremely strong laser pulses ( 109 V cm-1) created by Q-switched ruby or Nd-YAG lasers (10-100 MW peak power). These giant pulses lead to novel spectroscopic phenomena such as the hyper-Raman effect, stimulated Raman effect, inverse Raman effect, coherent anti-Stokes Raman scattering (CARS), and photoacoustic Raman spectroscopy (PARS). Figure 3-40 shows transition schemes involved in each type of nonlinear Raman spectroscopy. (See Refs. 104-110.)... 

Nonlinear Raman processes 163 Spontaneous scattering hyper Raman effect 163 Stimulated Raman effect 164... 

As a further class of continuously tunable IR lasers, we will discuss methods to change a given laser frequency by the stimulated Raman effect. 

There is a different approach using the stimulated Raman effect by Raman shifting the radiation of a tunable laser by a fixed frequency. The laser source is a dye laser and the Raman shift is produced by the H2-vibration. There are Raman shifted lines of the 1st, 2nd and even higher orders thus shifting the visible output of a dye laser into the near and medium infrared. Also antistokes lines can be observed [1,15]. 

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