The phase matching

Using now the phase matching condition, it can be seen that besides the quasi shear wave (qSV) which is obtained as usual, a second quasi shear wave (qSV(2)) results from the upper quasi shear wave part. Since the direction of the group velocity vector points downwards this wave is able to propagate and can be seen in the snapshot (see Fig. 10) if a is properly adjusted, i.e. is pointing upwards as in Fig. 2. 

The latter condition corresponds to the phase matching requirement already mentioned—the wavelength and direction of the material polarization wave must match those of the new EM wave as closely as possible. However, for all Class I spectroscopies, this condition is automatically achieved because of quadrature. In fact, this is tnie for all quadrature spectroscopies—the Class I spectroscopies being the principal such, but, as noted, it is a nontrivial requirement in the nonquadrature Class II spectroscopies, particularly in optically dispersive media. 

Biaxial crystals offer the possibiUty of coincidence of the phase matching direction with one of the optic axes. This highly desirable situation, called non-critical phase matching, is quite tolerant of divergence of the incident beam from the most efficient phase matching direction. 

In the solid state, the phase matched second harmonic signal increases with increased polymeri zation(7 ). Figure 14 shows preliminary data for the second harmonic intensity of NTDA microcrystals polymerized under x-ray radiation. As the polymer forms, the second harmonic intensity... 

The phase-matching techniques for non-linear interactions in these fibres are discussed and optical second harmonic generation is demonstrated in monomode benzil crystal cored fibres. 

The phase-matching for three wave mixing process can be achieved in an optical fibre for modes which satisfy the condition 3 = i where 3, 3g and 3, are "the propagation constants for the three modes. For opticaX second harmonic generation this condition becomes 3 = 23, where 3 and 3... 

Fig. 4.7. The measured harmonic intensities from (a) H2 and (b) D2 gases as functions of harmonic order q and gas pressure p. The solid lines indicate the phase matching pressures, which maximize the harmonic intensities...

A further improvement and more freedom in the choice of laser wavelengths can be expected with the use of dye vapors. In liquids, the phase-matching concentration is set by the requirement that the anomalous dispersion of the dye compensates for the normal dispersion of the solvent. The latter is a new parameter that can be varied at will in the gas phase by changing the nature and partial pressure of the buffer gas. The broader resonances of dyes as opposed to metal vapors, which are sometimes used for this purpose, is an advantage for tunable frequency tripling of dye lasers. Another advantage results from the possibility of working at much lower temperatures than with metal vapors. 

There are several pathways to reach the fifth-order nonlinearity with k) - 2k2 + 2k3. Are there any interference effects observed among these processes Even with the phase matching to observe the k) -2k2 + 2k3 component there are several paths contributing. 

Two types of phase-matching exist for SHG Type I, where the two fundamental photons are of the same polarization, and Type II, where they are orthogonally polarized. The phase- matching direction and one of the principal vibration directions of the crystal may be coincident in biaxial and uniaxial (birefringent) crystals. This situation, called noncritical phase-matching, is quite... 

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