Four-wave mixing experiment

To date, results have only been acquired for the ethoxy-substituted poly(ester) with a pentamethylene spacer. A 2.5 pm film produced a third-harmonic signal at 632.8 nm 29 times the intensity of a quartz signal, corresponding to a of approximately 8 x 10 13 esu. Degenerate four-wave mixing experiments are underway on this and other materials, and preliminary results are comparable to the values from third harmonic generation. 

The photorefractive rise (t r) and decay (tpd) times versus A, as measured by the four-wave mixing experiments, are shown in Fig. 14a and b for the polymerized and unpolymerized samples, respectively. The decay times are... 

Fig. 7. Setup for the degenerate four wave mixing experiments. The input beam is split in three beams. The beam splitter BS3 deflects a part of one of the pump beams to a power meter, which detects laser power fluctuations. The delay line with the retro reflector R adjusts the temporal overlap of the two pump beams coming from the front side on the sample. The long delay line with retro reflector R2 is moved to probe the temporal behavior of the nonlinearity in the sample. The phase conjugated signal beam propagates from the sample back to BSj and is then deflected through a stack of attenuation filters on a second power meter. An iris in front of the power meter increases the signal to noise ratio by removing scattered light...

Fig. 8. Temporal scans of the phase conjugated output beam in degenerate four wave mixing experiments. On the left side, the nonlinearity is instantaneous with the DFWM signal being the autocorrelation of the laser pulse. On the right side, the DFWM signal does not vanish after the writing beams have left the sample. Two photon absorption has induced a grating of excited and ground state molecules which is read by the delayed beam and decays slowly...

In a four-wave mixing experiment, the induced polarization, to third order in the driving field, is given by (18) ... 

Picosecond Degenerate Four-Wave Mixing Experiments... 

Figure 2. A schematic of the four-wave mixing experiment The microscope objective (M.O.) and TV camera combination is used to observe spatial overlap of the three incident beams on the same spot on the sample during intial beam alignment...

Fig. 15. a Geometry for four-wave mixing experiments, b Geometry for two-beam coupling experiments... 

Early theoretical work on FAD was concerned with the dimer equilibrium geometry and the electronic structure (see for example Refs. [32-36]). Ah initio molecular orbital studies on the structure of formic acid dimer in 1984 agreed very well with the experimental structures as determined by electron diffraction [37]. Due to the importance of the double proton transfer of FAD as a key prototype for multiple proton transfer reactions several theoretical studies have been reviewed in the literature [38]. Rotational constants for formic acid dimer were obtained by high resolution spectroscopy of (DCOOH)2 [39] and by femtosecond degenerate four wave mixing experiments in the gas cell at room temperature and under supersonic jet experiments by Matylisky et al. [40]. 

The discussion of the influence of bimolecular quenching and reabsorption of exciton fluorescence on the decay of the exciton gratings in organic crystals may be found in (26). The microscopic formulation of TG and four-wave-mixing experiments in molecular crystals in terms of polaritons may be found in (27). 

The third order nonlinear optical properties of the functionalized polymers have been studied by optical third harmonic generation and by four wave mixing. The four wave mixing experiments performed at 1. 064 pm fundamental wavelength reveal index grating writing through trans-cls isomerization via a two photon process. This process can be used for all optical orientation of chromophores, as it will be discussed later. 

The four wave mixing experiments have been performed in the configuration shown in Fig. 4. Two counter propagating beams (pump beams) 1 and 2 with intensities, respectively, l and 1 make an angle 9 with probe beam 3 with intensity I. The signal beam 4, counter propagating to beam 3 has space part... 

Fig. 4. Schematic representation of four wave mixing experiment. Beams 1 and 2 are pump, beam 3 is probe and beam 4 is signal, respectively...

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