Four-wave mixing degenerate

The arrangement employed for the VPC experiment is described in Reference 4. A cw argon-ion laser at 488 nm was used in a standard DFWM geometry. The s-polarized output beam was first split by a beam-splitter to provide the pump and the probe beams. The transmitted beam from the beam-splitter was then divided into the two s-polarized pump beams each with a power of approximately 0.35 mW. The reflected beam from the beamsplitter was used as the probe beam, whose intensity was about 7% of the total intensity in both pump beams. The forward pump beam and the probe, which constituted writing beams, were overlapped at the sample. Their optical path length difference was much smaller than the laser coherence length, so that they were coherent at the sample. The backward pump beam was 

18 (a) Con%iration I. geometry for the intensity gn ng formation witll s-pobrteed pump and probe beams (b) Configurabon 2 geometry for the polarization grating formation with the pump beams s-polarized and p-polarized probe beam. After Ref. 4. 

19 The reflectivity of the intensity grating. Rmt. and of the polarization grating,, as a function of sample temperature. At about T=S0°C, the reflectivities are equal, and this is the VPC temperature Type. After Ref. 4. 

Now we are going to give a physical insight into an origin of the mechanisms responsible for experimental results mentioned above. The physics of the temperature tuning of the VPC can be qualitatively understood in the framework of the model presented in the Section 12.1. The amplitude of the DFWM signal can be written as  

The X3333 component is then responsible for intensity grating efficiency, and X jW, the component is responsible for the polarization grating efficiency. Therefore, the conjugation efficiencies for two different DFWM configurations are 

---

Joo T and Albrecht A C 1993 Electronic dephasing studies of molecules in solution at room temperature by femtosecond degenerate four wave mixing Chem. Phys. 176 233—47... 

Table 1 Coefficients for 7[ (a ) for third harmonic generation (THG), degenerate four wave mixing (DFWM), electric field induced second harmonic generation (ESHG), and Kerr effect in methane at the experimental geometry rcH = 2.052 a.u. A CCSD wavefunction and the t-aug-cc-pVDZ basis were used. (Results given in atomic units, the number in parentheses indicate powers of ten.)...

In a separate study, the cross section of diphenylbutadiene in chloroform has been measured at 532 nm by two different methods and reported to be 40 8GM by degenerate four-wave mixing, and 34 12GM by nonlinear transmission [66]. It should be pointed out that both results were obtained using ns excitation pulses, but that the cross sections obtained by the two methods are comparable one to the other and on the same order of the cross sections listed earher (although those were obtained in a different wavelength range). 

Absorption spectroscopy, quartz crystal microbalance and electrical measurements and femtosecond degenerate four-wave mixing studies of third-order optical non-linearity... 

TEM and absorption spectrophotometry a large third order non-linearity was observed by degenerate four-wave mixing... 

The structure of carboxylic acid dimers results by time-resolved femtosecond degenerate four-wave mixing spectroscopy... 

Fig. 2. RCS of cyclohexylbenzene. Experimental data a) from pump-probe photoionization in a molecular beam (T 10 K) [2], b) from time-resolved degenerate four-wave mixing in a gas cell (T 305 K).

RCS OF THE GROUND STATE TIME-RESOLVED DEGENERATE FOUR-WAVE MIXING... 

Femtosecond degenerate four-wave mixing of cycloalkanes... 

Time resolved femtosecond degenerate four-wave mixing is based on the response of the third order polarisability. With a femtosecond pulse rotational levels are coherently prepared by a... 

Fig. 1. Degenerate four-wave mixing transient of cyclopropane (left) and cyclobutane (right). The upper traces in the insets are simulations. The recurrences in cyclobutane are very weak compared to the coherence peak at time zero.

Fig. 2. Geometry of degenerate four-wave mixing (BOXCARS geometry) for short-pulse, time-resolved measurements of the nonlinear response. Beams 1, 2, and 3 are derived from a single laser beam by the use of beam splitters and the beam paths are adjusted for the pulses to arrive simultaneously at the sample. By delaying one of the beams with respect to the others, the time-resolved measurements can be performed.

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. 

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

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... 

---