All optical poling

The main differences between various poling methods are presented in Table 11.1 The only purely optical method is all optical poling (AOP). 

Orientation Method Corona Poling Photo-Assisted Poling All Opdcal Poling 

Excitation mechanism Thermal activation Optical activation  

Orient.ition mechanism Electric field Optical field 

The interference between a fundamental wave, and its SH, leads to the presence of a polar field, E t) = + El J t), inside the optical medium. 

Orientation Method Corona Poiing Photo-Assisted Poiing Ali Opticai Poiing 

The all optical poling experiment consists of two periods the writing period (so-called seeding) and the readout period of the induced second-order susceptibility x. During the seeding period, the mutually coherent fundamental (O-beam and second harmonic 2(0-beam simultaneously irradiate the sample, and due to similar one- and two photon absorptions, the x grating is printed 

Charra et al. [137] and Kajzar et al. [159] observed formation of a polar orientation of chromophores in nondegenerate (pump beams at 1064 nm, probe beam at 532 nm) phase conjugate experiments performed on a copolymer of PMMA with DR 1 chromophore. The observed signal at 532 nm rose slowly with time, up to a saturation value. A spontaneous SHG was observed after switching off the probe beam, with a fast relaxation component at the beginning [159]. The maximum second-order nonlinear optical susceptibility value obtained in these experiments was 3 pm [137]. This experiment showed that by using purely optical fields one could obtain a polar orientation of chromophores in a functionalized or a doped polymer film. 

The mechanism responsible for the creation of x grating in glass fibers was proposed originally by Baranova and Zeldovich [165] in terms of a polychromatic interference of input fields at co and 2co frequencies, leading to a non-zero temporal average poling field  

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Unfortunately, electric-field poling is not applicable to octopolar systems, because of the absence of a dipole moment. However, the recently developed all-optical poling might be useful in obtaining the required non-centrosymmetry. All-optical... 

FIG. 11.4 Induced molecular orientation in the material after the all optical poling process. The difference in the amount of molecules ori ted upward or downward is the largest in the direction parallel to the polarization direction of the seeding beams. In the direction perpendicular to the polarization of the seeding beams the number of molecutes oriented In positive or negative sense remains the same, i.e.,it is centrosymmetric. ... 

In comparison to other poling methods, all optical poling has several advantages It does not require the use of any electrodes, and the phase matching... 

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