Electrically assisted self-confinement and waveguiding in planar nematic
liquid crystal cells
M. Peccianti, A. De Rossi, and G. Assanto
a)
National Institute for the Physics of Matter and Department of Electronic Engineering, Terza University
of Rome, Via della Vasca Navale 84, 00146 Rome, Italy
A. De Luca and C. Umeton
National Institute of the Physics of Matter and Department of Physics, University of Calabria, 87036 Rende
(CS), Italy
I. C. Khoo
Department of Electrical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802
Received 15 February 2000; accepted for publication 8 May 2000
We report on spatial soliton formation and self/cross waveguiding in planar cells containing a
nematic liquid crystal in the presence of an externally applied voltage. Self-confinement and
cross-induced guidance are demonstrated with an Argon ion laser 514 nm and a helium–neon
probe 633 nm, respectively, over millimeter lengths and with milliwatt pump powers. © 2000
American Institute of Physics. S0003-69510001427-3
Spatial solitary waves or solitons have been investigated
in a variety of configurations and material systems because
of their fundamental interest and potential applications in all
optically reconfigurable interconnects and light-controlled
switching.
1,2
Very recently, after the pioneering work by
Braun et al.,
3
spatial self-confinement has been experimen-
tally investigated in nematic liquid crystals NLC by
Warenghem et al. in capillaries filled with dye-doped NLC,
4
and by Karpierz et al. in planar cells with homeotropically
aligned NLC.
5
Such attention to NLC stems from their large
10
9
times greater than in CS
2
and polarization dependent
nonlinearity which, reorientational in nature,
6
allows the ob-
servation of a rich phenomenology despite the relatively
slow response.
7
In experiments for the observation of self-
focusing and spatial solitons in NLC, significant attention
has been devoted to a reduce thermal contributions to the
nonlinear phenomena and b lower the required optical
power. To such extents, while external cooling was em-
ployed in Ref. 3 in the presence of excitations as high as
10 W, dye doping was used in Ref. 4 to further enhance the
nonlinear optical response, while a hybrid field polarization
was launched in homeotropic NLC cells in Ref. 5. In all
cases, nevertheless, self-confinement was observed over
short distances hundreds of micrometers and with nonneg-
ligible thermo-optic effects.
In this letter we undertake an approach towards the ob-
servation of spatial solitons in planar NLC cells, applying an
external voltage to eliminate the threshold inherent to the
Fre
´
edericks transition and defining an input interface to con-
trol the field polarization. The latter was aimed at ensuring
experimental repeatability while providing a nondepolarized
input beam; the former allowed an initial nonzero tilt of the
molecular directors with respect to the propagation wave-
vector, thereby permitting strong reorientational effects at
intensities 50 W/cm.
5,7,8
In such a configuration, we have
observed diffractionless propagation of an Ar-ion beam over
millimeter distances with milliwatt powers, as well as the
all-optical formation of a channel guiding a weak He–Ne
probe.
The NLC can be modeled as a birefringent medium with
orientation locally described by a unit vector or director, the
spatial distribution of which is governed by elastic forces.
When a linearly polarized optical beam propagates in an
NLC with a positive optical anisotropy it determines a torque
which tends to realign the director parallel to the electric
field. Assuming equal Frank constants K for splay, bend, and
twist of the molecules,
6,7
a beam of slowly-varying ampli-
tude A propagating along z, and directors rotating in the
plane x – z defined by the optic axis and the electric field
vector, the tilt
ˆ
= ( A ) -
rest
with respect to the director
orientation profile at rest
rest
is described by the elliptic
equation
4 K
2
ˆ
x
2
+
2
ˆ
y
2
+
0
a
| A |
2
sin 2
ˆ
+
rest
=0, 1
with
a
=n
e
2
-n
o
2
the birefringence. The rest distribution in
the presence of a low-frequency electric field and in the nar-
row region traversed by the optical beam can be modeled
heuristically by
rest
z , V =
0
V +
in
-
0
V exp -z / z
¯
, 2
with
0
( V ) the orientation distribution due to applied voltage
far from the input interface,
in
the director orientation at the
boundary z =0, and z
¯
a relaxation distance. Finally, the beam
amplitude will obey
7,9
2 ik
A
z
+
2
A
x
2
+
2
A
y
2
+k
0
2
a
sin
2
-sin
2
rest
A =0,
3
with k =k
0
n
o
2
+
a
sin
rest
the wave vector. For simplicity,
we assumed k k
0
n
o
2
+
a
sin
2
o
.
a
Electronic mail: assanto@ele.uniroma3.it
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 1 3 JULY 2000
7 0003-6951/2000/77(1)/7/3/$17.00 © 2000 American Institute of Physics