Self-diffraction

Eichler and Macdonald carried out experiments with 80 psec pulses in a nematic. The energy was a few mJ per pulse. They produced an intensity grating and detected the self-diffraction of the laser beam. The diffraction efficiency was measured both in the nematic and in the isotropic phases. In the nematic phase the diffracted intensity depended on the angle between the grating and the director in a characteristic way. This dependence corresponded to a collective reorientation... 

In R25, R26 and R27 the reader flnds detailed descriptions of phase-conjugation, self-diffraction and optical bistability realized with the help of thermal effects in nematics. The absorption of the cells were controlled by adding dyes to the nematic or coating metallic layers onto the substrates. Due to the large thermo-optic coefficients very low operating power-levels could be used. As an example, Lloyd and Wherrett observed optical bistability at an input power as low as 20/iiy. ... 

In these experiments, self diffractions from the two incident Nd YAG lasers are also observed. This shows that the rise time of the thermal grating buildup is therefore on the order of the laser pulse duration, as mentioned earlier. To investigate further the rise and fall dynamics, we again monitor the diffraction from a CW He-Ne beam. Fig. 3(b) and (c) show the typical dynamics of the thermal buildup and decay for the grating wave vector q perpendicular and parallel to the nematic axis, respectively. The rise time is on the order of the incident laser pulses, while... 

The radiation intensity 7 1 in first-order self-diffraction is uniquely connected with the optically induced change An c of refractive index, viz.,... 

The experimental dependence of the self-diffraction intensity of the extraordinary polarized radiation on the normalized temperature r = (T— 7V)/r is shown in Fig. 1. The same figure shows the temperature dependence of the characteristic hologram-erasure time tff, which is equal to the writing time as well as to half the characteristic relaxation time of the nonlinearity. The value of, which is proportional to (J ), increases as the phase-transition point is approached, even though in this case the hologram-erasure time, which determines the stationary value of (Ref. 2), decreases. The dependence of the refraction intensity of the ordinary polarized radiation is similar in form. 

FIG. 1. Dependences of the self-diffraction intensity / , and of the effective time of erasure of the holographic grating on the normalized temperature. 

If it is assumed that the shift At of the phase transition is small compared with the values of r, the stationary value of the self-diffraction intensity will be proportional to the square of the first term of the expansion (5) in r ... 

In 1981, self-diffraction was observed in nematic MBBA [64] under circumstances where director reorientation could not occur. The measured nonlinear susceptibility was as large as the giant nonlinearity associated with director reorientation the mechanism proposed was photostimulated conformational changes of the liquid crystal molecules. Since the linear polarizability of the excited conformer differs from that of the molecule in the ground state, there is a direct contribution to the bulk susceptibility. The metastable conformers also act as... 

Figure 8.14. (a) Typical dependence of the photorefractive self-diffraction efficiency as a function of applied dc voltage, (b) Dependence of photorefractive self-diffraction efficiency on the grating constant. Sample is 25 pm thick homeotropic C60-doped 5CB cell. Beam power is 10 mW (beam size 2 mm), (c) Two beam coupling effect (unidirectional transfer of energy from one beam to the other) via photorefractive effect in nematic liquid crystals. Upper plot depicts dependence on direction of dc field lower plot shows the dependence on the tilt of the sample w.r.t. the incidence beams. Sample is 25 pm thick homeotropic 5CB cell. Beam power is 1.5 mW (beam size 2 mm). 

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