Four-wave mixing techniques

The description of pump-probe signals presented in the preceding section can be immediately generalized to heterodyne-detected transient grating spectroscopy as well as to other four-wave mixing techniques. Heterodyne detection involves mixing the scattered field with an additional heterodyne field 4(r). The signal in the ks direction can then be written in terms of the polarization Ts(t) as... 

The four-wave mixing technique serves to measure the diffraction efficiency t] during the writing process as a function of time and as a function of the strength of the external electric field. Figure 4.4 shows a schematic representation of a typical set-up employed in four-wave mixing experiments. 

Amorphous PDAs. Yu et al.(2i) prepared poly(hexa-2,4-diynylene terephthalate), which is not photosensitive, but does polymerize by heating at 150 C. A value of 3.2 X 10 ° esu (determined the degenerate four wave mixing technique at 532 nm) has been reported for this material The polymers 3 and 4 (Chart 4) are not photosensitive, but underwent cross-polymerization when heated at 180°C (in the molten state) for 2.5 hours with simultaneous UV irradiation, giving red transparent materials. The x values for these materials were found to be 1.9 - 3.5 x 10 ° esu for polymers 3 and 2.7 -2.9 x 10 esu for polymers 4. Absorption spectra of one of the polymers 4 are shown in Figure 3. The films have an absorption maximum at 400 nm and a trough at 340-350 nm, but absorption tails down towards 700 nm due to their amorphous nature. 

In solution, the polymers show a bluish-green fluorescence. The nonlinear optical properties of the polymers were investigated at 532 nm using single beam Z-scan and degenerate four-wave mixing techniques... 

There are several four-wave mixing techniques that can be used to evaluate Xotl components of a material [3]. It should be noted that there are numerous third-order processes that are described by different ) ukl terms. Hence, a comparison of derived from one technique with evaluated with another technique should be made cautiously. 

The forward folded boxcar four-wave mixing technique is illustrated in Fig. la and lb. Three pulsed laser beams are directed, through variable delay lines, toward the sample. Two time-coincident short excitation pulses and a third delayed probe beam cross in the sample. Figure 1 shows the beam geometries on the surface of the sample and at the aperture plane. The three incident beams are labeled 1, 2, and 3. Beam 4 is the signal beam, generated by the input beams via The... 

A novel pump-probe degenerate four-wave mixing technique was introduced to study the excited state enhanced nonlinear optical response in polymers [7]. In this technique (see Fig, 3) a picosecond laser pulse at w, is divided into three beams... 

Myers, A,B, and Hochstrasser, R.M. (1986) Comparison of four-wave mixing technique for studying orientational relaxation. IEEE J. Quantum Electron., QE-22, 1482. 

Ultrafast, time-resolved OKE measurements (which involve another type of four-wave mixing process) may also occasionally be used to determine x(3) and 7 values.5 7 In each of the THG, DFWM, Z-scan, and OKE techniques, 7 is derived from the measured x values by using the solute number density of the solution. In a few cases, x(3) values have also been determined by using Stark spectroscpy. It should be remembered that comparisons of 7 or x values obtained using different techniques with different experimental conditions are generally of little utility. 

Optical detection offers the most conventional technique to time-resolve the coherent phonons. It includes four-wave mixing [8], transient reflectivity [9,10] and transmission [7] measurements, as well as second harmonic generation (SHG) [15,32]. Coherent nuclear displacement Q induces a change in the optical properties (e.g., reflectivity R) of the crystal through the refractive index n and the susceptibility y,... 

Degenerate four-wave mixing has been widely used for the study of organometallics. At present, it forms a complementary technique to the technically less difficult Z-scan, in that is can be used to verify that the origin of the observed nonlinearity is electronic in nature. 

Third-Order NLO Techniques. There is a wider range of third-order techniques commonly used to characterize materials, including electric field induced second harmonic generation (EFISH) (15, 16), third harmonic generation (THG) (17) and degenerate four wave mixing (DFWM) (18). EFISH and DFWM will be discussed briefly then... 

In the degenerate four wave mixing (DFWM) experiment the third-order susceptibility 3)(-tt>,tt>,-CL>,CL>) with degenerate frequencies can be determined [22]. This nonlinear susceptibility is directly proportional to the nonlinear refractive index n2, which is used to describe optically induced refractive index changes. An advantage of this technique is the possibility to record the temporal shape of the third-order nonlinear optical signal. 

Degenerate four-wave mixing (DFWM) Degenerate four-wave mixing (DFWM) is another well-known nonlinear technique based on third-order nonlinear susceptibility similar to CARS... 

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