Poling field limit

Since the dipoles of chromophore molecules are randomly distributed in an inert organic matrix in amorphous PR materials, the material is centrosymmet-ric and no second-order optical nonlinearity can be observed. However, in the presence of a dc external field, the dipole molecules tend to be aligned along the direction of the field and the bulk properties become asymmetric. Under the assumption that the interaction between the molecular dipoles is negligible compared to the interaction between the dipoles and the external poling field (oriented gas model), the linear anisotropy induced by the external field along Z axis at weak poling field limit (pE/ksT <[Pg.276]

Fig. 3.25. The degree of order induced by poling can be determined using the results above to analyse the variation in second harmonic intensity as the poled sample is rotated about an axis normal to the fundamental beam. Generally, the low field limit, given by Equation (3.106), is found to apply and the degree of orientation is low. However, for molecules with large dipole moments th,e divergence from the low field limit shown in Fig. 3.25 can be observed, anil much higher orientation is obtained. The experimentally determined non-linearities are usually the electro-optical (r) and the second harmonic generation (d) coefficients. These are related to the hyperpolarisabilities by ...

The effect of a limited mobility of holes in the material may be accounted for in Eq. (1) by using a parameter called the mobility field, E, which is the electric field necessary for a hole to drift 1 radian across a sinusoidal intensity pattern in one hole recombination lifetime. The assumptions of long lifetime and high mobility make the mobility field small in comparison with the projection of the poling field along... 

Materials (lwt.%1) Poling field, Kpoi [Vpm- l Estimated quantum efficiency, r Intensity contrast, / Wcm-] Fast time constant observed, rfast [s] Charge generation limit obtained [%] Reference... 

Another problem is that switching of the SHG signal is possible only for a limited number of cycles the higher SHG value reached after UV irradiation decreases with the performed number of cycles. This decrease is interpreted as an effect of disorientation. Indeed, in the absence of a poling field, the chromophores have no reason to recover exactly the same orientation after one isomerization cycle. In the worst cases, the poled order is completely lost after one switching attempt. Usually, this type of disorientation occurs faster in side-chain polymers than in doped ones. ... 

In all cases the use of an electrode is required, with all possible negative aspects such as charge injection and light absorption. As a consequence, it implies the necessity of using buffer layers, in such applications such as frequency conversion in periodically poled systems [149], in which otherwise it is unnecessary. Moreover, the poling fields are limited due to the microcircuits connected with the point effect. This leads also to unwanted and prohibitory increase of the optical propagation losses. 

The sum of the poling saturation limit values for different processes (P = Pi + P2 + P3) represents the overall poling efficiency under the experimental conditions temperature, geometry, poling field, atmosphere. 

In this limit, the two coefficients have a constant ratio of df d i 3- Furthermore, both nonlinear coefficients increase in proportion to the applied poling field. This is an advantage of poled polymers, since the sign and magnitude of the d coefficient can be tailored by applying an appropriate poling field. 

The poles con espond to excitation energies, and the residues (numerator at the poles) to transition moments between the reference and excited states. In the limit where cj —> 0 (i.e. where the perturbation is time independent), the propagator is identical to the second-order perturbation formula for a constant electric field (eq. (10.57)), i.e. the ((r r))Q propagator determines the static polarizability. 

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