THURSDAY AFTERNOON / lQEC'98 / 177

model in Ref. 10. We have shown that this model gives the same result as the polariton- scattering approach providing that the polari- ton nature of the microcavity electric field is

- 80

S .& 6o v) properly accounted for.14 C

In NMC-SMC, the cavity polariton fre- 2 - 40 quencies are well separated from the exciton

excitonic states are suppressed. This reduces $ 2o resonance; therefore, the populations of the

0

Im - $

';y r . r .r I I%*

c 1 ~

.*CSm#% *- 1 < = -% i Ak<

' " ' ' " " ~ ' " ' ' "

QThGl analytical results (Fig. 5 ) . Data from a series of numerical simulations, which use this new al-

Electric-field-induced second-harmonic gorithm and explore a range of initial value generation in polymeric light-emitting problems, will also be presented (Fig. 3). diodes solitons Both our analytical and numerical results

QThG2

Exact analytical nonparaxial optical

Ralf Hildebrandt, Gerd Marowsky, Wolfgang Bruetting,* Thomas Fehn,* Markus Schwoerer,* Laser-Laboratorium Goettingen e. V., Hans-Adolf-Krebs- Weg 1, 37077 Goettingen, Germany; E-mail: gmarows@ gwdg. de Polymeric light-emitting diodes play an im- portant role in scientific and industrial re-

P. Chamorro-Posada, G.S. McDonald,* G.H.C. New,* Departamento de Teoria de la Serial y Comunicaciones e I. T., E. T.S.I. Telecomunicacidn, Universidad de Valladolid, 4701 1 Valladolid, Spain The potential applications of nonlinear optical devices in the field of information technology has motivated an enormous research effort in

explicitly uncove; novel features that will also need to be incorporated into any approximate or numerical investigations of more general nonparaxial nonlinear systems. *Laser Optics &Spectroscopy Group, The Black- ett Laboratory, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 ZBZ, United Kingdom; E-mail: g. mcdonald@ic.ac. uk

178 / lQEC'98 / THURSDAY AFTERNOON

1.06

1.04

1.02

1

0.98 a, 2

5 Z 0.96

5 0.94

a 0.92

m

0.9

0.88

0 86

0 84

QThG2 Longitudinal evolution of peak amplitude when an exact nonparaxial soliton is launched. The correct solution does not display oscillatory features. However, standard numerical approaches can show large oscillatory behavior (the Feit and Fleck method is shown). Progressive inclusion of new higher-order terms corrects this aspect.

Fig. 2.

1 2 , , , , , , , I I , i

0 9

0 8

0 7

0 6

U 6 10 15 20 2; 30 35 40 41 < QThG2 Fig. 3. Longitudinal evolution of peak amplitude, luIm, when a paraxial soliton, U(< ,< = 0) = sech(t) exp(-is,<), is launched under nonparaxial conditions. In each case, the beam evolves toward the appropriate nonparaxial soliton solution. Solid horizontal lines are the predicted asymptotic values from analysis.

1.

2.

3.

4.

G.S. McDonald, W.J. Firth, J. Mod. Opt. 37,613 (1990). G.S. McDonald, W.J. Firth, J. Opt. Soc. Am. B 7,1328 (1990). G.S. McDonald, W.J. Firth, J. Opt. Soc. Am. B 10,1081 (1993). Sien Chi, Qi Guo, Opt. Lett. 20, 1598 (1995).

QThGB

Orientational phenomena in liquid crystals on photorefractive substrates

Nelson V. Tabiryan, Cesare Umeton,* CREOL/UCF, P.O. Box 162700, Orlando, Florida 3281 6-2700; E-mail: nelson@creol. ucjedu We study the opportunities for optical-infor- mation-processing applications provided by photorefractive liquid crystals (LC) and by LC on photorefractive substrates. We reveal unique possibilities of surface-activated new photorefractive phenomena in LC.

Consider a homogeneously orientated

nematic LC (NLC) sandwiched in a cell with hard anchoring ofthe director at the boundary layers z = 0 and z = L (Fig. 1). Application of an electric field E, = E,exp-az) (a is the attenuation constant) leads to reorientation of the NLC if E, is stronger than a threshold value E,. At the limit of small attenuation, u L << 1, the solution of variational equations for the NLC director shows that ESF - EF( 1 + 0.5 aL) , where E, is the threshold for the FrCed- ericksz transition induced by a homogeneous electric field. In the case of strong attenuation, aL >> 1, one gets ESF - E+L.

Thus, the threshold value of the surface- localized electric field is increasing moderately, even if the field is decreasing inside the NLC exponentially. Namely, this circumstance makes possible the electro-optical phenomena in NLC under surface-localized influences.

* =

QThG3 Fig. 1. NLC on aphotorefractive sub- strate. The initially homogeneous orientation of the NLC is distorted by the space-charge field generated by interfering light beams in a photore- fractive substrate.

When NLC is on a photorefractive substrate in which the bulk of the density of charges is modulated with a spatial frequency q of inter- fering light beams, the electric-field strength acting on the NLC overlay has the form

E:' = E~ exp ( - qz) sin qx,

E:' = E, exp ( - qz) cos qx ( 1 )

The expression for E, is rather simple when the dielectric constant of the NLC equals the dielectric constant of the photorefractive sub- strate E: E, - 2np,/q&.

To induce reorientation at spatial frequency q, an external electric field of the strength B >> ELc has to be applied to the NLC. Then, the magnitude of the NLC reorientation at the spatial frequency q is

sinqx, sinhqz e =s z - 4 z - q L e P 7

2E; e sinhqL 1 BE I where

where p, is the amplitude of the charge-den- sity modulation, and d is the thickness of the photorefractive substrate.

Numerical evaluations show that the charge density required to ensure E, - E,, can easily be reached even with low-power laser radia- tion.

The modulation of the space-charge density in the plane of a thin photorefractive medium leads also to an electric field that is normal to the boundaries of the material. The normal component of the field reaches its maximum value at the interface z = 0. The density of charges necessary to get a field comparable to E, can be evaluated as p, - (e/L2)(nK/ IAEI)"', where K is the elastic constant of the NLC, and AE is the anisotropy of the dielectric constant of the NLC.

The normal component of the photorefrac- tive field can deposit the ions on the substrate, resulting in permanent modification of sur- face-anchoring conditions. Evaluations show that this effect may provide explanation for several recent experimental observations of re- cording of permanent orientational gratings in LC.*,Z *Department of Physics, University of Calabria, 87036 Rende (CS), Italy 1. I.C. Khoo, Opt. Lett. 20,2137 (1995). 2. D. Voloshchenko et al., Jpn. J. Appl. Phys.

34,566 (1995).

QThG4

Theory of mixed-polarization spatial solitons in anisotropic cubic media

D.C. Hutchings, J.M. Arnold, D.F. Parker,* Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, United Kingdom; E-mail: dch@elec. gla.ac.uk AlGaAs is an attractive material system for the study of the dynamics of Kerr spatial solitons, having a nonlinear coefficient -1000 times