Nematicons in azobenzene liquid crystals

Article abstract of DOI:10.1080/15421406.2012.658710

We review our comprehensive investigation of light self-localization phenomena in planarly configured azobenzene liquid crystals. Cis-trans isomerization of azobenzene molecules and related changes in the order parameter of the liquid crystals support a h

Full text of DOI:10.1080/15421406.2012.658710

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Nematicons in Azobenzene Liquid
Crystals
a
a
b
Svetlana V. Serak , Nelson V. Tabiryan & Gaetano Assanto
a
BEAM Engineering for Advanced Measurements Co., 809 S. Orlando
Ave., Suite I, Winter Park, FL, 32789, USA
b
NooEL-Nonlinear Optics and OptoElectronics Lab., Via della Vasca
Navale 84, 00146, Rome, Italy
Available online: 11 May 2012
To cite this article: Svetlana V. Serak, Nelson V. Tabiryan & Gaetano Assanto (2012): Nematicons in
Azobenzene Liquid Crystals, Molecular Crystals and Liquid Crystals, 559:1, 202-213
To link to this article: http://dx.doi.org/10.1080/15421406.2012.658710
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Mol. Cryst. Liq. Cryst., Vol. 559: pp. 202–213, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.658710
Nematicons in Azobenzene Liquid Crystals
1
,∗
1
SVETLANA V. SERAK, NELSON V. TABIRYAN,
2
AND GAETANO ASSANTO
1BEAM Engineering for Advanced Measurements Co., 809 S. Orlando Ave.,
Suite I, Winter Park, FL 32789 USA
2
NooEL-Nonlinear Optics and OptoElectronics Lab., Via della Vasca Navale 84,
00146 Rome, Italy
We review our comprehensive investigation of light self-localization phenomena in
planarly configured azobenzene liquid crystals. Cis-trans isomerization of azobenzene
molecules and related changes in the order parameter of the liquid crystals support
a high nonlinear response and the formation of optical spatial solitons at microwatt
excitations. We describe bright soliton angular steering, deflection, interleaving and
merging, as well as dark solitons.
Keywords Cis-trans isomerization; laser; liquid crystals; solitons
1. Introduction
The propagation of optical spatial solitons in nonlinear media and their perspective use
for signal steering, readdressing, all-optical switching and other photonic applications
have attracted growing interest in the past few years [1–4]. Liquid crystals (LCs) are
excellent optical materials for these investigations, owing to extended spectral transparency,
high electro-optic response and large nonlinearity [5–7]. In the nematic LC phase, the
reorientation of the (elongated) organic molecules in the presence of an electromagnetic
field is the basic mechanism for self-focusing, the latter enabling the generation of robust
spatial optical solitons at low power levels [8–11] down to microwatts [12,13]. In addition,
the nonlocality inherent to the dielectric response of a liquid mediates soliton stability in two
transverse dimensions [14–17] as well as a variety of soliton-soliton [18–22], soliton-beam
[23–25] and soliton-perturbation interactions [26–29].
While most soliton demonstrations were reported in nematic LC, the recent develop-
ment and commercial availability of room temperature azobenzene nematic LC (azo-LC)
12,30] have stimulated the investigation of a nonlinear refraction mechanism related to
[
photoinduced isomerization: rod-shaped azo molecules in the trans-form are bent into the
cis-form when absorbing light in the ultraviolet (UV)-green region, the reverse process tak-
ing place with radiation at longer wavelengths. The accumulation of cis-isomers lowers the
LC order parameter and can change the LC mesophase into the isotropic one. In azo-LCs
containing a mixture of both trans and cis isomers, red light can increase the refractive
∗
Address correspondence to Svetlana V. Serak, BEAM Engineering for Advanced Measurements
Co., 809 S. Orlando Ave., Suite I, Winter Park, FL 32789, USA. Phone: (407) 629-1282; Fax: (407)
6
29-0460. E-mail: svetlana@beamco.com
202

Nematicons in Azobenzene Liquid Crystals
203
index for extraordinary waves and produce optical self-focusing owing to cis-trans photoi-
somerization of the azobenzene [12,31]. The resulting dielectric constant ε of the mixture
can be expressed as:
ε = εtransNtrans + εcisNcis
(1)
with Ncis and Ntrans the fractions of cis and trans isomers, respectively, the weights being
the corresponding dielectric constants at optical frequency [32]. Since both the dielectric
properties and the order parameter of the azo-LC depend on the concentration of trans and
cis isomers, photoisomerization modifies the refractive index of the mixture and mediates
a novel type of optical nonlinearity which is efficient and relatively fast as compared to
reorientational and thermal responses of nematic liquid crystals [33,34].
In this paper we show that the photoisomerization-induced nonlinear response of azo-
LC can support self-focusing and the formation of optical spatial solitons or nematicons
at wavelengths in the red even at µW power levels, offering a wealth of opportunities for
their efficient steering and all-optical control with extra beams of various wavelengths.
Furthermore, exploiting the photoinduced reduction of the order parameter, the resulting
self-defocusing response allows generating optical dark spatial solitons.
2. Experiments and Discussion
2.1 Bright Solitons in Nematic Azo-LC
We carried out our experiments using a standard setup as sketched in Fig. 1(a), with a
continuous wave Helium-Neon laser oscillating at λ = 632.8 nm. The beam was linearly
polarized – in either an extraordinary (e−) or ordinary (o−) wave with the aid of a twisted
LC cell, and then focused into a planar glass cell with either a 20 X microscope objective or
a 35 mm lens. Light propagation in the sample was monitored with an inverted microscope
and a high-resolution Silicon CCD camera. The azo-LC layer in between the glass walls
was 80-µm thick and planar oriented with molecular director n parallel to the x-axis, i.e.
Figure 1. (a) Setup for generation and observation of spatial solitons and (b) photo of a planar
azo-LC sample. The LC sample was 23 mm wide, 10 mm long and 0.08 mm thick. NDF stands for
neutral density filters.

204
S. V. Serak et al.
orthogonal to the wavevector of the incident beam (s). Fig. 1(b) displays a photograph of
the sample.
In our measurements we employed a variety of azo-LC materials (available from
beamco.com). For the study in the visible spectrum presented hereby we used either the
◦
azobenzene LC no. 1005 or azo-LC no. 1205, with clearing temperatures of 49 and 59 C,
◦
respectively. At 22 C and for light at 632.8 nm the refractive indices of birefringent azo-LC
1005 (1205) in the trans state are n|| = 1.726 (n|| = 1.751) and n⊥ = 1.540 (n⊥ = 1.541) and
the value in the saturated cis state is ncis = 1.631 [35]. These two azo-LC materials exhibit
similar linear and nonlinear properties in the visible; hence, all the results presented hereby
can be considered characteristic of the entire class of azo-LC with a positive birefringence
n|| − n⊥ > 0. For extraordinarily-polarized incident light with electric field parallel to the
LC director n it is εcis − εtrans = εcis − ε|| < 0; hence, cis-trans light-induced isomerization
increases the e-refractive index, Fig. 2(a). Therefore, an intense enough e-polarized red
laser beam can reduce the concentration of cis isomers and cause self-focusing and the
formation of a spatial soliton, as sketched in Fig. 2(b). Conversely, if the incident beam is
polarized orthogonal to the LC director (ordinary polarization), then εcis−εtrans = εcis−ε⊥ >
0
and light undergoes diffraction, as illustrated in Fig. 2(c).
Spatial solitons were readily generated at power levels even below milliWatt, as shown
in Fig. 3(a): the diffraction of the extraordinary-wave beam starts to decrease at microwatts
powers, giving rise to stable solitons from around 25 µW up to 3.5 mW. Above the latter
value the beam divergence increases again, probably due to the interplay between saturation
Figure 2. Light beam propagating in the azo-LC planar sample. (a) The beam transforms cis isomers
into trans isomers, aligned along the rubbing direction x. (b) An intense extraordinary beam with
E| |n gets self-focused and can form a soliton, as shown in the photograph for He-Ne excitation. The
transverse size of the soliton is 22 µm. (c) An ordinary polarized beam diffracts (photograph). In
both experiments ((b) and (c)) the red laser beam is launched from the left at a power of 43 µW and
propagated for about 1 mm.

Nematicons in Azobenzene Liquid Crystals
205
Figure 3. (a) xz evolution of an extraordinary beam for various excitations: 1–10 µW; 2–33 µW,
3–72 µW, 4–340 µW; 5–0.98 mW; 6–2.8 mW; 7–5.2 mW; 8–8 mW. Here the medium is azo-LC
1005, and the (shown) propagation length is 0.8 mm. (b) Divergence of the He-Ne beam versus power
measured at 0.69 mm from the input.
in the reorientational response and the insurgence of thermal effects. Fig. 3(b) plots the
measured divergence (i.e. size) versus excitation. The time dynamics of soliton formation
is faster at higher excitations, well below 100 ms for sub-mW powers, as visible in Fig. 4.
Despite the nature of the nonlinearity in azo-LC, nonlocality plays an important role in
photoisomerization, as in reorientational and thermal responses [15,16,36]. Owing to the
interplay of nonlocality and reorientational effects at higher powers, breathing [16] (Fig. 5)
and instabilities could be observed as well, with the formation of multiple solitons from
a single input beam, as in Fig. 5. At variance with a previous report in nematic LC [37],

206
S. V. Serak et al.
Figure 4. (a) Temporal dynamics of self-focusing (decreasing divergence) of an extraordinary beam
in azo-LC 1205 for various input powers. (b) Response time versus excitation.
no special beam focusing was required to yield the latter result, relying on transverse
modulational instability in order to seed the effect in the relatively wide input beam.
Due to long lifetime of cis isomers in the sample, a (relatively) fast transverse dis-
placement of the cell with respect to the input laser beam resulted in the lateral bending
deflection of the soliton, progressively reducing in time as shown by the photo sequence
of Fig. 6(a). Here the initial shift is of the order of the beam size. The redistribution of the
isomers around the beam restored the straight propagation of the soliton with typical times
of the order of a few seconds, as visible in Fig. 6(b) [12].
2.2 Bright Soliton Merging in Nematic Azo LC
LCs exhibit a highly nonlocal behavior associated to their fluid state and the elastic forces
linking their molecules [5–7]. As stated above, a nonlocal response also characterizes azo-
LC owing to diffusion of isomers generated by and along the laser beam. Therefore, even
spatial solitons in azo-LC can sense the presence of extra solitons or beams nearby and
can interact as elastic or inelastic particles [18–20]. Typically, the index gradient resulting
from the nonlocal index perturbation gives rise to an attractive force bending the soliton
trajectory(ies) and substantially insensitive to coherent effects associated to their relative
phase. Figure 7 shows formation of two solitons and their interleaving in azo-LC for
different launching conditions that could not be precisely controlled in the experiment. The
power of the beam was 3.2 mW in both cases.
For two equi-power beams of 0.63 mW launched in an 80 µm-thick cell with azo-
LC 1005, we investigated the dynamics of soliton attraction, interleaving and, eventually,
Figure 5. Examples of spatial solitons generated by a red beam of 3.2 mW power. The beams are
injected in azo-LC 1205 from the left; the propagation length is 0.8 mm.

Nematicons in Azobenzene Liquid Crystals
207
Figure 6. (a) Photographs (inverted images) of soliton deflection produced by a cell translation.
Snapshots taken at: 1 − t = 0.4 s; 2 – t = 0.9 s; 3 – t = 1.08 s; 4 – t = 1.7 s, 5 – t = 1.7 s; 6 – t =
2
.14 s; 7 – t = 2.6 s; 8 – t = 3.9 s. The propagation length is 0.8 mm. (b) Time dynamics of soliton
“straightening” after deflection (Media 1). Here we employed the azo-LC 1205 and a 3.2 mW He-Ne
beam.
◦
merging. Typical results for two input beams injected at a relative angle of 4.3 are shown in
Fig. 8. The merging process took a few seconds after mutual oscillations and interleaving,
and was evaluated in time by measuring the separation between the self-localized beams at
a given propagation distance in the material, as graphed in Fig. 9.
Figure 7. Examples of mutual interactions of spatial solitons in azo-LC with input beams launched
from left to right and propagating for 0.8 mm.

208
S. V. Serak et al.
Figure 8. Temporal dynamics of merging solitons in azo-LC 1005 (Media 2). The time sequence is:
1
4
– 0 s; 2 – 1.12 s; 3 – 1.48 s; 4 – 2.08 s; 5 – 2.9 s; 6 – 3.58 s. The initial angle between the beams is
.3 inside the medium. The visible area in the photographs is 0.8 mm × 0.16 mm. The beams carry
◦
equal powers of 0.63 mW at the input.
2.3. Bright Channels in the Photoinduced Isotropic State of Azo LC
It has been shown that the photoinduced isotropic (PI) state of azo LCs exhibits one of
the highest known nonlinear optical refraction among isotropic materials [35]. The optical
nonlinearity of these media is large in a wide band of the visible spectrum and it possesses
high, over 90%, transmission for wavelengths in the red. The PI state of azo LCs is
Figure 9. Separation between solitons in azo-LC 1005 versus propagation distance at various times
following the excitation.

Nematicons in Azobenzene Liquid Crystals
209
obtained by exposing it to UV-blue radiation; the accumulation of a critical concentration
of the non-mesogenic cis form of the azo molecules drives the material into the isotropic
phase. Noteworthy, the PI state of an azo LC is substantially different from the thermally
induced isotropic (TI) phase. The TI state consists primarily of trans molecules whereas
the PI state of azo LCs is a mixture of trans and cis isomers with a substantial concentration
of cis molecules. Removal of the heat source results in fast (ms to seconds) relaxation of
the TI phase back to the mesophase. The PI state of azo LCs can last tens of hours, since
the dark relaxation is governed by the lifetime of cis isomers at a fixed temperature. Most
importantly for optical applications, the absorption spectrum and the refractive index of the
PI state differs from the TI state: nTI = 1.66 and nPI = 1.63 at wavelength λ = 632.8 nm [35].
The interaction of a He-Ne laser beam with the PI state of azo LCs revealed a few
interesting peculiarities. The beam, initially nearly invisible due to low scattering losses in
the isotropic phase, became visible first at the end of the cell, opposite to the input. This
is attributed to an initially weak self-focusing, with higher power densities near the end of
the propagation distance and, therefore, enhanced efficiency of the cis-trans isomerization
towards the critical region of photoinduced isotropic-nematic phase transition. Thus, the
enhanced fluctuations resulted in augmented light scattering making the beam visible [38].
The extent of the isomerized region increased in time revealing the self-induced channel,
Fig. 10, which shifted closer and closer to the cell entrance. Such rather complex and not en-
tirely explained space-time dynamics resulted in “fireworks” launched from the output to the
input. Eventually, cis-trans isomerization made the beam visible by the cell input, Fig. 11.
2.4. Dark Solitons Via Light-Induced Reduction of the Order Parameter
Dark solitons are supported by a self-defocusing response and are intensity notches with
a π phase jump in a uniform background. [1] Even these self-localized waves can act as
self-induced waveguides .
Figure 10. Light propagating away from the input interface. Snapshots at: 1 – t = s, 2 – t = 0.07 s;
3
0
– t = 0.1 s; 4 − t = 0.13 s, 5 − t = 0.16 s; 6 − t = 0.2 s; 7 − t = 0.26 s; 8 − t = 0.32 s; 9 − t =
.39 s, 10 − t = 0.45 s, 11 − t = 0.55 s; 12 − t = 0.64 s, 13 − t = 0.77 s, 14 − t = 0.96 s, 15 −
t = 1.09 s, 16 − t = 1.22 s; 17 − t = 1.35 s, 18 − t = 1.48 s; 19 − t = 1.6 s; 20 − t = 1.73 s; 21 −
t = 1.89 s; 22 − t = 2.05 s; 23 − t = 2.18 s. The used azo LC 1205 was pre-exposed for 38 min using
2
an UV lamp (λ = 365 nm, I = 10 mW/cm ). The 5.2 mW red laser beam was focused at the input
with a 35-mm focal length lens. The propagation distance is 0.8 mm. The white arrow indicates the
direction of the launch beam.

210
S. V. Serak et al.
Figure 11. Dynamics of self-focused bright spots in isotropic azo-LC. Time is: 1 – t = 0 s; 2 – t =
0
1
3
.09 s; 3 – t = 0.13 s; 4 – t = 0.32 s; 5 – t = 1.2 s; 6 – t = 1.8 s; 7 – t = 2.4 s; 8 – t = 2.8 s; 9 – t = 3.14 s;
0 – t = 3.84 s; 11 – t = 4.77 s; 12 – t = 5.65 s; 13 – t = 6.26 s; 14 – t = 8.75 s; 15 – t = 13.4 s (Media
2
). The azo LC 1205 was pre-exposed for 30 minutes using a UV lamp (λ = 365 nm, I = 10 mW/cm ).
The 3.4 mW red laser beam was focused at the cell entrance with a 20X microscope objective. The
propagation length was 0.8 mm. The arrow on the left indicates the incidence beam direction.
To obtain a self-defocusing nonlinearity, we used a mixture of the commercial nematic
LC 5CB with the mesogenic azo dye CPND [39,40] (10% in weight), in a standard planar
cell of thickness 9 µm and with inner interfaces treated to obtain homeotropic anchoring,
i. e. with molecular director perpendicular to the confining surfaces.
By illuminating the sample with green light (λ = 532 nm) of wavelength within
the absorption band we could lower the order parameter as the LCs rearranged around the
excited-state dye molecules. First we focused a 2 µm waist beam and observed that the
extraordinary wave (electric field along x) self-defocused, whereas the ordinary wave (field
along y) self-focused (Fig. 12).
Figure 12. (a) Sketch of the sample. The blue ellipses represent the LC molecules, the red ones
the azo dye. (b) Waist versus power of an input Gaussian beam, for ordinary (bottom line) and
extraordinary (top line) polarizations. (c–h) Photographs of ordinary (c–e) and extraordinary (f–h)
green beams launched at 0.1, 1 and 2 mW respectively.

Nematicons in Azobenzene Liquid Crystals
211
Figure 13. Dark spatial solitons in azo nematic LCs. Photographs (left), input (center row) and output
right) profiles for P = 1 (first row), 2 (second row) and 4 mW (third row), respectively. The bottom
(
graphs display the FWHM of the notch versus propagation (left) for P = 1 (squares), 2 (circles), 3
(triangles) 4 mW (dots) and in z = 0.5 mm versus excitation (right).
Then we launched a bell-shaped beam with a one-dimensional notch on a wide bright
background along y. By increasing the launch power of an extraordinarily polarized beam
Fig. 13), self-defocusing progressively narrowed the notch. At 4 mW we observed a nearly
(
constant FWHM, consistent with the propagation of a dark soliton [41]. To validate the
generation of a dark spatial soliton, we coupled a low power near-infrared probe (λ =
1064 nm) (transmitted by the mixture) and studied its confinement in the dark soliton
excited in the green. As shown in Fig. 14, when the weak probe was ordinarily polarized
it sensed a refractive index dip (due to change in order parameter) and diffracted; when
Figure 14. Collinear near-infrared probe: (a) [(d)] Photographs of the extraordinary [ordinary] wave
when the 532 nm beam is in the linear regime and (b) [(e)] when it forms a dark spatial soliton. (c) and
(f) show the waist of extraordinary and ordinary weak signals versus green excitation, respectively.

212
S. V. Serak et al.
it was extraordinarily polarized (co-polarized with the green soliton), it was guided in the
channel and propagated in a confined fashion.
3. Conclusions
Using commercially available materials we have shown that azobenzene nematic liquid
crystals support efficient nonlinear optical localization based on the induced transformation
of cis to trans isomers in the presence of visible light beams at microwatt power levels. In
analogy to “standard” nematic liquid crystals, this nonlinearity is nonlocal and saturable,
provides transverse stability and response times well below one second and adjustable with
input power. Spatial solitons in azo-LC propagate with moderate attenuation and can be
angularly steered and deflected; they undergo long-range attractive interactions with other
solitons in the medium, the latter property being incoherent in nature and permitting beam
merging independently of the reciprocal phase. Specific mixtures exhibit a self-defocusing
nonlinearity and support dark spatial solitons, with waveguiding properties for optical
signals outside the absorption band.
“Azo-nematicons” can find interesting applications for auto-aligning fiber couplers,
as well as switchable polarizers, optical gates and beam combiners. A detailed model of
the photoisomerization-mediated nonlinear response of azo-LC is underway and will be
published elsewhere.
Acknowledgments
The authors are grateful to T. J. Bunning (AFOSR) for interest in and support of this study.
GA thanks M. Peccianti, A. Alberucci and A. Piccardi.
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