Photobleaching and reorientational dynamics of dyes in a nematic liquid crystal
M. No
¨
llmann and D. Shalo
´
m
Instituto Balseiro, Universidad Nacional de Cuyo, 8400 San Carlos de Bariloche, Rı ´
Negro, Argentina
P. Etchegoin and J. Sereni
Centro Ato ´mico Bariloche and Instituto Balseiro, Comisio ´n Nacional de Energı ´a Ato ´mica and Universidad Nacional de Cuyo,
8400 San Carlos de Bariloche, Rı ´
Negro, Argentina
Received 25 August 1998
The polarized fluorescence of excited dyes in a prototype nematic liquid crystal is studied as a function of
temperature, polarization of the light, and laser wavelength. We show explicitly the coexistence of pho-
tobleaching and dye diffusion through the nematic host as two mechanisms influencing the magnitude of the
fluorescence signal. In addition, we exhibit clear evidence of the presence of a torque at low input laser powers
that twist the dye molecules with respect to the director orientation of the liquid crystal, if the dyes are
resonantly excited. The fluorescence emission in this latter case is able to perceive the birefringence of the
nematic liquid crystal host, and this is shown as clear oscillations in the polarized fluorescence as a function of
temperature. Extensive qualitative comparisons of the experimental results with the mean-field Maier-Saupe
theory of the nematic state are presented, and a model is proposed to account for the observations.
S1063-651X9910802-X
PACS numbers: 61.30.Gd, 87.64.Ni, 78.20.Fm, 32.80.Lg
I. INTRODUCTION AND OVERVIEW
Dye doped liquid crystals exhibit a myriad of optical ef-
fects which are intrinsic to their unique nature. Originally,
the orientation of dichroic dyes by the cooperative alignment
of the liquid crystal LC molecules in the nematic phase was
discovered by Heilmeier, Castellano, and Zanoni 1, who
designated the effect as guest-host interaction. By measuring
the polarized fluorescence of the dyes, the method was im-
mediately recognized as a powerful tool to gain microscopic
information on the different liquid crystalline phases 2. The
guest-host effect has also been used for tailor-made electro-
optical LC devices with improved contrast ratios at specific
wavelengths in the visible 3,4. More recently, enhanced
light-induced molecular reorientations in dye doped LC’s
were observed, and their understanding in terms of a micro-
scopic interaction between the excited dyes and the LC mol-
ecules was developed by the pioneering work of Ja
´
nossy 5.
It turns out that a light beam acting on a dye doped LC may
display an optical torque which is 2–3 orders of magnitude
larger than the usual one, which is proportional to the dielec-
tric anisotropy of the LC molecules. This effect has dramatic
consequences for the threshold of the optical Fre
´
edericksz
transition 5. The enhanced nonlinear optical properties and
the variety of phenomena observed in dye doped LC’s have
recently triggered several studies. Notwithstanding, many of
the details of the observed phenomena, as well as their rela-
tionships, are not fully worked out. Moreover, as pointed out
by Ja
´
nossy 6, the connection between the microscopic mo-
lecular properties of the dyes and LC’s and the resulting
macroscopic optical nonlinearities is not yet understood.
Among the most salient results reported in the literature
of dye doped LC’s we highlight the following: Khoo and
co-workers 7 reported the existence of a negative reorien-
tational effect in the transient regime of a dye doped nematic
LC after a short laser pulse illumination. Fundamental as-
pects of the order parameter in the nematic and smectic-A
phases using the guest-host effect were reported by Bauman
and Wolarz 8 using polarized fluorescence and absorption,
and by Wu 9 using second harmonic generation. The pho-
tochromism of azo dyes and their effect on liquid crystalline
ordering was reported by Blinov et al. 10, while the funda-
mental aspects of the optical torque enhancement in bulk dye
doped LC’s were principally studied by Ja
´
nossy and co-
workers 5,6,11 and Shen and co-workers 12. From the
standpoint of applications, dye doped liquid crystals were
investigated as possible candidates for optical cavities 13
as well as spatial filtering 14,15, photothermal self-phase
modulation 16, holograms 17,19, and optical recording
18,20. Surface-mediated alignment in dye doped LC’s un-
der laser light illumination was reported by Gibbons et al.
21. In the latter work, the alignment of the LC becomes
perpendicular on a macroscopic scale to the electric field
polarization of the laser, presumably by the indirect action of
the dye and the aid of the surface interaction. It is worth
noting at this stage that several of the dye-assisted reorienta-
tional mechanisms delineated in the literature are not neces-
sarily equivalent or related to each other. The transient reori-
entation observed in Ref. 7 is, in the simplest possible
approach, not related to the enhanced optical torque seen
under constant wave laser illumination in Ref. 5. Likewise,
the surface-assisted reorientation used for pattern recording
by optical means in Ref. 21 seems to be specifically related
to the presence of a boundary. In fact, the reversible surface-
mediated optical alignment in a dye doped LC reported in
Ref. 21 is such that the molecules orient themselves in a
direction perpendicular to both the electric field of the light
E
and the director field n
in a planar aligned LC cell. Con-
versely, the enhanced bulk optical torque influencing the op-
tical Fre
´
edericksz transition in Ref. 5 is found to be pro-
portional, and to have the same sign as the normal optical
torque exerted by the laser, implying that the molecules will
PHYSICAL REVIEW E FEBRUARY 1999 VOLUME 59, NUMBER 2
PRE 59 1063-651X/99/592/185010/$15.00 1850 ©1999 The American Physical Society