Optical Field-Induced Reorientation

A well-known nonlinear process taking place in the liquid state of anisotropic molecules is the optical-field induced birefringence (optical Kerr effect ). This nonlinearity results from the reorientation of the molecules in the electric field of a light beam. In the isotropic phase the optical field perturbs the orientational distribution of the molecules. In the perturbed state more molecules are aligned parallel to the electric field than perpendicularly to it and as a consequence the medium becomes birefringent. On the other hand in liquid crystals the orientational distribution of the molecules is inherently anisotropic. The optical field, just as a d.c. electric or magnetic field, induces a collective rotation of the molecules. This process can be described as a reorientation of the director. 

Optical field induced first-order electric Freedericksz transition and electric bistability in a homogeneously aligned nematic film is first observed. It is experimentally demonstrated that an applied optical field can transform the electric Freedericksz transition from second order to first order. The molecular reorientation as a function of electric field is then characterized by a hysteresis loop which exhibits the electric bistability. The results are in good agreement with theoretical predictions. 

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

The superiority of using lasers for material studies often lies in its spatial and temporal flexibilities, that is, the material can selectively excited and probed in space and time. These qualities may allow us to elucidate fundamental material properties not accessible to conventional techniques. The location, dimension, direction, and duration of the material excitation can be readily controlled through adjustment of the beam spot, direction, polarization, and pulse width of the exciting laser field. The flexibilities can be further enhanced when two or more light waves are used to induce excitations. Such a technique, however, has not yet been fully explored in liquid-crystal research. Although the recent studies of optical-field-induced molecular reorientation in nematic liquid-crystal films have demonstrated the ability of the technique to resolve spatial variation of excitations, corresponding transient phenomena induced by pulsed optical fields have not yet been reported in the literature. Because of the possibility of using lasers to induce excitations on a very short time scale, such studies could provide rare opportunities to test the applicability of the continuum theory in the extreme cases. 

A. Transient optical-field induced molecular reorientation... 

The response time r given by Eq. (12) is a function of the pump intensity, but in our case, r (of the order of 10—ICX) fistc during the pump pulse) is always much longer than the pump pulse width. The solution of Eq. (14) is then trivial. Immediately after the pump pulse is over, the optical-field-induced reorientation is simply... 

Optical field-induced director reorientation is responsible for the largest nonlinear optical susceptibility observed in liquid crystals, the largest in any known material. Although the process is slow, the nonlinearity is about 10 times greater than that of CS2. Because of its magnitude, the orientational nonlinearity of liquid crystals has been termed giant optical nonlinearity (GON). 

Simplified Treatment of Optical Field-Induced Director Axis Reorientation... 

As one can see from the preceding discussions on optical field induced director axis reorientation in hquid crystals, the torque exerted by the optical field on the director axis is basically quadratic in the field amplitude. Except for its dispersion influence on the optical dielectric constant 8(co), the frequency of the electric field is basically not involved. Furthermore, if two or more fields are acting on the director axis, the resulting torque exerted on the director axis is simply proportional to the square amplitude of the total fields. Accordingly, it is possible to enhance the optical field induced effect by application of a low-frequency ac or dc electric field, much as the optically addressed liquid crystal spatial light modulator discussed in Chapter 6. In the latter, the responsible mechanism is the photoconduction generated by the incident optical field in the semicondnctor layer adjacent to the liquid crystals. 

Kitzerow et al. recently demonstrated that temperature-induced phase transitions (Iso-N) and electric field-induced reorientation of a nematic liquid crystal (5CB in this case) can be used to tune photonic modes of a microdisc resonator, in which embedded InAs quantum dots serve as emitters feeding the optical modes of the GaAs-based photonic cavity [332],... 

Fig. 34. Light-induced reorientation of the DR 1 molecule in the presence of the static field and photoisomerization process. From almost parallel orientation to the exciting optical field the molecule dipole moments reorient to an almost perpendicular direction...

The sketch of the experimental set-up is shown in Figure 1. A Q-switched Nd-YAG laser, operating at 1.06 ixm and a pulse repetition 2-12.5 Hz was used to provide the fundamental (pump) beam. The peak power was 200-300 kW. The beam was focused with a 43 cm lens so that the power density on the sample placed in a thermostate was about 100-200 MW-cm. " For investigation the field-induced SHG, short pulses (tp = 20 fxs) of high voltage Up = 4kV) were provided by an electrical generator. The pulse duration was chosen from the condition Trelaxation time for dipolar (Debye) polarization, and T is the director reorientation time. Under such a condition, molecular dipoles are oriented by the field but the Fredericks transition does not take place. The sensitivity of our set-up was about 30 photons of the optical second harmonic per single laser pulse. The cell temperature was stabilized with an accuracy of 0.1° K. 

The dynamic behavior of the optical-field-in-duced Freedericksz transition is also analogous to the dc case. The initial response of the induced molecular reorientation to the laser switch-on and the long-time response to the laser switch-off are both exponential with relaxation times Tqjj and Toff, respectively. 

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