Laser-Induced Pretransitional Ordering and Nonlinear Optical Properties of Rigid-Rod Polymer Suspensions
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Boris E. Vugmeister, Michelle S. Malcuit, John C. Kralik and Colleen Stevens* Physics Department, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015 *Physics Department, Georgetown University, Washington, DC 20057
ABSTRACT We investigate the pretransitional behavior in laser-induced alignment of rigid rod-like polytetraflouroethylene (PTFE) suspensions. Using a laser-induced birefringence experiment, we measure both the orientational order parameter and the orientational relaxation time. We find that both increase as the volume fraction of colloidal particles approaches the critical value for the isotropic-nematic phase transition. Experimental results are compared with theory which takes into account the possibility of a first-order phase transition induced by a laser electric field. INTRODUCTION When optically anisotropic dielectric microparticles in colloidal suspension are subjected to a uniform electric field a torque is generated which tends to align the particles along the field direction. This orentational effect gives rise to large nonlinear susceptibilities for these materials [1, 2, 3]. For example, the nonlinear index of refraction for dilute aqueous dispersions of rod-like polytetrafiouroethylene (PTFE) is n 2 = 5 x 1010 can 2/W [1], some four orders of magnitude larger than the reorientational nonlinearity of carbon disulfide. On the other hand, the orientational ordering of microparticles is significantly affected by excluded -volume effects when the colloidal dispersions are not necessarily dilute [4]. Such a system of shaped microparticles is classified a lyotropic liquid crystal. In order to elucidate the physics of a laser-electric-field-induced isotropic-nematic phase transition in lyotropic liquid crystals and investigate its implications for nonlinear optics we present the results of experimental and theoretical work on lyotropic systems consisting of aqueous dispersions of PTFE. THEORY We assume that the optical axis coincides with the long axis of the particles [1], in which case the laser-induced birefringence is proportional to the orientational order parameter S. The orientational order parameter for suspensions of microparticles at arbitrary concentrations and in an applied electric field can be written in the form
S
=
J
dPA'(o)(3f•
-
1)
(1)
where T1(0) is the distribution function of the particle alignment, z is the direction of polarization of the laser electric field and 4. is the z-component of the unit vector I parallel with the long axis of the particle. Due to the particle's anisotropic polarizability 619 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
.7
.6 m4
Sm=4.3 'Do
o .2 ' .1m= 0 0
.4
.2
.6
.8
1.0
Intensity Parameter J Figure 1: Orientational order parameter versus intensity parameter from the solution of Eq. (2). Dashed lines indicate the solution is unstable.
S= all - aj1 , the interaction energy of the particle with the electric field depends on the particle's orientation, and has the form U5 = -(1/2)03(1. - 1/3)T2 wh
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