Freedericksz transitions optical

OPTICAL-INDUCED FREEDERICKSZ TRANSITION OF NEMATIC LIQUID CRYSTAL DOPED WITH PORPHYRINATOZINC( II)... 

The Optical-Induced Freedericksz Transition of Nematic Liquid Crystal (5CB) doped with l%(w/w) of 5, 10, 15, 20-tetraphenylporphyrinatozinc( II) (ZnTPP) were studied. Excited by Ti Sappire laser with the 82MHz repetition rate and lOOfs pulse duration, the optical Freedericksz threshold of a 23.6pm-thickness planar alignment sample occurred at an intensity level of0.35mW/mm in contrast to the normally observed 83mW/mm value for pure 5CB. The coordination-bonding interaction between 5CB and ZnTPP were discussed by UV-vis and fluorescence spectra. We attribute the reduction of the optical Freedericksz threshold to the coordination-bonding interaction. 

Due to their large optical anisotropies, liquid crystals (LCs) have a large optical nonlinearity which is the result of molecular reorientation (Freedericksz transition) in an external field which exceeds the critical field [1], The high external field inhibits the application of LCs, and decreasing the threshold as low as possible is a difficult task [2], LCs doped with a small amount of absorbing dyes that could decrease the needed optical field intensity have been reported [3]. The basic assumption is that the anomalous reorientation of the director results from the interaction between the excited dye molecules and the host. However, this sample would easily degrade under the influence of laser radiation. 

In this paper, we first observed the optical-induced Freedericksz transition of nematic liquid crystal (5CB) doped with 5, 10, 15, 20-tetraphenylporphyrinatozinc(ll) (ZnTPP)... 

A. J. Karn, S. M. Arakelian, Y. R. Shen and H. L. Ong, "Observation of Magnetic-field-induced First-order Optical Freedericksz Transition in... 

The dependence of the merit figure on the dielectric anisotropy is a rather sensitive function of the choice of the threshold voltage. Had we selected the Freedericksz transition (capacitance) threshold voltage Instead of an optical threshold V9o(40 ), we would observe a decrease in figure of merit with increasing such as reported elsewhere.15 However, for an optical reponse, the latter seems more relevant. 

J. J. Wu, G. S. Oug et ah, Obseriuition of Optical Field Induced First-Order Electric Freedericksz Transition and Electric Bistability in a parallel Aligned Nematic Liquid-Crystal film, Appl. Phys. Lett. (1988) 152... 

E. Santamato, G. Abbate et ai, Optically Induced Twist Freedericksz transitions in Planar-Aligned Nematic Liquid Crystal, Phys. Rev. A (1987) 174... 

Detailed theories of the optical Freedericksz transition were provided by Zel dovich et and Csillag et In these works the influence of the finite... 

The optical reorientation processes discussed up to now were qualitatively similar to the corresponding low-frequency field effects. As mentioned earlier this is not always the case. A breakdown of the analogy with static fields was first reported by Zolotko et al. who observed in a homeotropic layer a drastic increase of the Freedericksz threshold power for an o-ray as the angle of incidence was increased. Durbin et al. mentioned that in a planar cell Freedericksz transition cannot be induced by a light beam polarized perpendicularly to the director. From a simple analogy one would expect for these cases a threshold not deviating significantly... 

Optical-Field-Induced Birefringence and Freedericksz Transition in a Nematic Liquid Crystal... 

Optical-field—induced birefringence in nematic 4-cyano-4 -pentylbiphenyl was measured with cw pump and probe beams, and the optical-field-induced Freedericksz transition was observed for the first time. The results are in quantitative agreement with the theoretical prediction. 

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. 

To observe the optical-field-induced Freedericksz transition, the angle of incidence of the pump beam was set at 0°. The observed birefringence versus pump intensity at Tffj - T = 9.2 K is shown in Fig. 1. It exhibits a threshold intensity at 155 W/cm. According to Eq. (5),... 

As is shown in Fig. 5, the experimentally observed threshold field dependence on wo/d also follows the theoretical prediction. For E46, a 100-Axm sample has a Freedericksz transition field intensity of 200 W/cm (using the values = 0.3, K lO" , d = 0.01 cm, and n no 1.5). For large values of Wo/d (for example, wo/d > 5), the observed threshold field approaches this value. However, as the incident laser beam size decreases to a value comparable with the thickness d or less, the threshold optical intensities increase dramatically. At wq d, the threshold intensity increases by almost an order of magnitude. In general, the experimentally observed relative increase of the threshold field is slightly larger than the theoretical value, probably because of a systematic difference between the experimental observation of the onset of reorientation (by the appearance of the self-focusing effect on the exit Ar+ laser beam) and also... 

Observation of optical field induced first-order electric Freedericksz transition and electric bistability in a parallel aligned nematic liquid-crystal film... 

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. 

Recently, particular attention has been devoted to the first-order Freedericksz transition (FT), molecular reorientation, and bistability in nematic liquid crystals (NLC s). In the last few years, Ong has shown that all of the electric and magnetic FT s and most of the optical FT s in a nonconducting homogeneous or homeotropic NLC are second-order transitions. However, applying a suitable static field, the second-order optical FT can be converted to first order in all existing NLC s. Similarly, applying a suitable optical field, the second-order electric or magnetic FT can also be converted to first order ... 

In summary, we have demonstrated that via the optical Freedericksz transition, a circularly polarized input beam can activate a precession of the director in a homeotropic nematic film by transferring part of its angular momentum to the medium. This yields an output beam with a... 

An experimental investigation was made of aberration self-focusing due to orientational deformation of a nematic with a hybrid orientation subjected to the held of an incident ordinary optical wave. When the radiation was incident on a sample from the homeotropic substrate side, the reorientation occurred at intensities higher than the threshold for a photoinduced Freedericksz transition in a homeotropic cell of the same thickness, and it depended strongly on the angle of incidence of the ordinary wave on the sample. The effect was not observed when light was incident from the planar substrate side. 

Reorientation of a nematic liquid crystal above a certain threshold value of the electric held of an optical wave, known as the photoinduced Freedericksz transition (PFT)—recently discovered and investigated both experimentally and theoretically -—exhibits a certain number of special features which are absent in the case of the rf analog of the Freedericksz effect. 

Optically induced twist Freedericksz transitions in planar-aligned nematic liquid crystals... 

The optical-field-induced Freedericksz transition for a twist deformation by a normally incident laser beam in a planar-aligned nematic liquid crystal is studied. The Euler equation for the molecular director and the equations describing the evolution of the beam polarization in the birefringent medium are solved simultaneously in the small-perturbation limit. The stability of the undistorted state is investigated. An alternate series of stable and unstable bifurcations is found. This phenomenon has no analog in the Freedericksz transition induced by dc electric and magnetic external fields. 

When the intensity 7 gradually increases from zero, the system remains undistorted with =( —1,0,0) and =0 until the first branching point 7=7q is reached. At this point, a second-order optical twist Freedericksz transition occurs and the output ellipticity begins to deviate from zero, following the solid line in Fig. 4. To reach the second steady-state branch in Fig. 4, the intensity 7 should be suddenly switched to the stable region and then... 

When a light beam linearly polarized in the x-y plane with the elastic field directed at an angle to they axis is normally incident on the SmC layer, there is an additional term in the total free energy density of the system due to the optical field. For the discussion of the optical-field-induced Freedericksz transition, the total free energy density F (erg/cm ) can be written as... 

S. D. Durbin, S. M. Arakelian et al.. Optical-Field-Induced Birefringence and Freedericksz Transition in a Nematic Liquid Crystal, Phys. Rev. Lett. (1981) 107... 

In 1990, Janossy showed that a small amount of dye added to a nematic liquid crystal dramatically reduces the threshold intensity of the optical Freedericksz transition [68]. Subsequently, it was demonstrated that the underlying process is an optically driven Brownian ratchet mechanism [69-71]. Here, energy, but not momentum, from the radiation field causes unidirectional continuous rotation of dye molecules in the nematic, exerting a torque on the director that exceeds the direct optical torque by orders of magnitude. Similar mechanisms could, in principle, be realized in LCEs. Whether such processes are viable in overcoming the orienting effect of the network is not clear the viability of such Brownian motor processes in LCEs is an intriguing open problem. 

Janossy I, Lloyd AD, Wherrett BS (1990) Anomalous optical Freedericksz transition in an absorbing liquid-crystal. Mol Cryst Liq Ciyst 179 1-12... 

From the point of view of physics, LCs are partially oriented fluids that exhibit anisotropic optical, dielectric, magnetic, and mechanical properties. The most important property of LCs is the reorganization of their supramolecular structures on external stimuli such as electric and magnetic fields, temperatnre, and mechanical stress, which lead to changes in their optical properties. In particular, electric tiled-induced control of optical properties of LCs (electro-optical effects based on the Freedericksz transition ) is at the heart of the multi-billion dollar liquid crystal display (LCD) industry. Most current LCD technologies rely on nematic " and to a lesser extent on ferroelectric LCs, while the recently discovered bent-core and orthoconic LCs still require significant investment into fundamental research and development. These and other applications and technologies continne to drive the search for new liquid crystal materials, and provide impetus to continue fundamental studies on new, often exotic, classes of compounds. 

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