New fast-relaxed liquid crystal materials for optical communication networks
Recording the dynamic holograms with microsecond relaxation times under action of laser pulses was obtained in composites based on the novel class of liquid crystals (LC), namely in ionic metal-alkanoates. Holographic parameters of the recording and relaxation characteristics were studied for doped...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2006
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| Zitieren: | New fast-relaxed liquid crystal materials for optical communication networks / Yu. Garbovskiy, A. Sadovenko, A. Koval'chuk, G. Klimusheva, S. Bugaychuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 1. — С. 63-67. — Бібліогр.: 10 назв. — англ. |
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irk-123456789-1215942017-06-15T03:04:08Z New fast-relaxed liquid crystal materials for optical communication networks Garbovskiy, Yu. Sadovenko, A. Koval'chuk, A. Klimusheva, G. Bugaychuk, S. Recording the dynamic holograms with microsecond relaxation times under action of laser pulses was obtained in composites based on the novel class of liquid crystals (LC), namely in ionic metal-alkanoates. Holographic parameters of the recording and relaxation characteristics were studied for doped lyotropic ionic LC and sandwich-like cells with photo-sensitive impurities for purposes of real-time dynamic holography applications. The thin cells demonstrate high-velocity dynamic grating recording under laser pulses both of nanosecond and picosecond durations at the visible wavelengths. The cells exhibit a fast temperature relaxation time (with the time constant 30 μs for the store heat density more than 50 kJ/s). Ionic lyotropic smectic LCs possess a high intrinsic anisotropic conductivity as compared with other LCs – dielectrics. To explain the relaxation mechanisms in ionic smectic LC matrix, the temperature dependences of the electro-conductivity have been investigated. The charge currier mobility and activation energy in cells were estimated. The mechanism of high-velocity resonance nonlinearity due to the saturation of excited states in photosensitive centers and mechanisms of the grating erasure connected with charge transport in the ionic LC matrix were discussed. 2006 Article New fast-relaxed liquid crystal materials for optical communication networks / Yu. Garbovskiy, A. Sadovenko, A. Koval'chuk, G. Klimusheva, S. Bugaychuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 1. — С. 63-67. — Бібліогр.: 10 назв. — англ. 1560-8034 PACS 42.70.Df http://dspace.nbuv.gov.ua/handle/123456789/121594 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Recording the dynamic holograms with microsecond relaxation times under action of laser pulses was obtained in composites based on the novel class of liquid crystals (LC), namely in ionic metal-alkanoates. Holographic parameters of the recording and relaxation characteristics were studied for doped lyotropic ionic LC and sandwich-like cells with photo-sensitive impurities for purposes of real-time dynamic holography applications. The thin cells demonstrate high-velocity dynamic grating recording under laser pulses both of nanosecond and picosecond durations at the visible wavelengths. The cells exhibit a fast temperature relaxation time (with the time constant 30 μs for the store heat density more than 50 kJ/s). Ionic lyotropic smectic LCs possess a high intrinsic anisotropic conductivity as compared with other LCs – dielectrics. To explain the relaxation mechanisms in ionic smectic LC matrix, the temperature dependences of the electro-conductivity have been investigated. The charge currier mobility and activation energy in cells were estimated. The mechanism of high-velocity resonance nonlinearity due to the saturation of excited states in photosensitive centers and mechanisms of the grating erasure connected with charge transport in the ionic LC matrix were discussed. |
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Garbovskiy, Yu. Sadovenko, A. Koval'chuk, A. Klimusheva, G. Bugaychuk, S. |
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Garbovskiy, Yu. Sadovenko, A. Koval'chuk, A. Klimusheva, G. Bugaychuk, S. New fast-relaxed liquid crystal materials for optical communication networks Semiconductor Physics Quantum Electronics & Optoelectronics |
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Garbovskiy, Yu. Sadovenko, A. Koval'chuk, A. Klimusheva, G. Bugaychuk, S. |
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Garbovskiy, Yu. |
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New fast-relaxed liquid crystal materials for optical communication networks |
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New fast-relaxed liquid crystal materials for optical communication networks |
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New fast-relaxed liquid crystal materials for optical communication networks |
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New fast-relaxed liquid crystal materials for optical communication networks |
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New fast-relaxed liquid crystal materials for optical communication networks |
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new fast-relaxed liquid crystal materials for optical communication networks |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2006 |
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http://dspace.nbuv.gov.ua/handle/123456789/121594 |
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New fast-relaxed liquid crystal materials for optical communication networks / Yu. Garbovskiy, A. Sadovenko, A. Koval'chuk, G. Klimusheva, S. Bugaychuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 1. — С. 63-67. — Бібліогр.: 10 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-07-08T20:11:23Z |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 1. P. 63-67.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
63
PACS 42.70.Df
New fast-relaxed liquid crystal materials
for optical communication networks
Yu. Garbovskiy, A. Sadovenko, A. Koval’chuk, G. Klimusheva, S. Bugaychuk
Institute of Physics, NAS of Ukraine, 46, prospect Nauky, 03028 Kyiv, Ukraine
Phone: 525-78-41; e-mail: klimush@iop.kiev.ua
Abstract. Recording the dynamic holograms with microsecond relaxation times under
action of laser pulses was obtained in composites based on the novel class of liquid
crystals (LC), namely in ionic metal-alkanoates. Holographic parameters of the recording
and relaxation characteristics were studied for doped lyotropic ionic LC and sandwich-
like cells with photo-sensitive impurities for purposes of real-time dynamic holography
applications. The thin cells demonstrate high-velocity dynamic grating recording under
laser pulses both of nanosecond and picosecond durations at the visible wavelengths. The
cells exhibit a fast temperature relaxation time (with the time constant 30 μs for the store
heat density more than 50 kJ/s). Ionic lyotropic smectic LCs possess a high intrinsic
anisotropic conductivity as compared with other LCs – dielectrics. To explain the
relaxation mechanisms in ionic smectic LC matrix, the temperature dependences of the
electro-conductivity have been investigated. The charge currier mobility and activation
energy in cells were estimated. The mechanism of high-velocity resonance nonlinearity
due to the saturation of excited states in photosensitive centers and mechanisms of the
grating erasure connected with charge transport in the ionic LC matrix were discussed.
Keywords: ionic smectic liquid crystals, dynamic holographic recording, liquid crystal
electrolyte, high-velocity process.
Manuscript received 23.11.05; accepted for publication 15.12.05.
1. Introduction
Nowadays, known is a lot of different nonlinear optical
materials used for the holographic recording [1]. The
media with resonance nonlinearity has much interest for
fast and high-efficient recording and processing the
information. The liquid [2], the liquid crystal (LC) [3]
and the rigid [4] dye solutions belong to them. However,
strong thermal and long-time orientation effects are
often observed in these matrixes, and they lead to
restrictions in the spatial resolution and to increased
relaxation times of the holographic recording.
The recording of dynamic holographic gratings with
fast relaxation (erasure) under nanocesond laser pulses is
obtained in the novel class of ionic LCs, namely, in the
ionic mesogenes of metal alkanoate with photosensitive
impurities. Researches of nonlinear-optical properties of
the sandwich-like cells and particular features of the
electric conductivity in pure and impurity ionic lyotropic
liquid crystals (ILLC) are presented in the paper.
To explain the role of the ILLC matrix in the grating
relaxation process, we investigated the electric
conductivity of the ILLC in the temperature range of 292
to 333 K. We obtained the high value of the conductivity
for comparison with other LC dielectrics. We
determined the activation energy of the ILLC. An
estimation of the diffusion characteristics for charge
carriers was made. The mechanisms of the holographic
recording in sandwich-like cells as well as the
mechanism of the conductivity in new materials (pure
and impurity ILLC) were discussed.
2. Materials
ILLC are formed by water solution of metal alkanoates
and they have the spatial structure similar to smectic A.
The molecules in the ILLC have the bi-layer packing of
the hydrophobic alkanoate chains consisting of the metal
cations, carboxyl anions and water molecules.
Polymethine dyes are easily solved in ILLC.
In this work, we investigate the sandwich cells
consisting of a thin film of a dye (a polymethine dye
relating to either an anion or cation types) and a layer of
the planar oriented ILLC. The ILLC is the water solution
of the potassium caproate in the proportion 1:1. The
sandwich cells are prepared in the following way. We
take two glass plates. A thin layer of a dye solution is
placed onto one plate. After evaporation of the solvent,
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 1. P. 63-67.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
64
Fig. 1. The changes of the optical density versus the energy
density in one laser pulse in a sandwich-like cell ILSLC with
the anion dye.
Fig. 2. The measured energy density of the first diffracted
order of the self-diffraction versus the energy density in one
laser pulse in a sandwich-like cell with the anion dye.
the dye polycrystalline plate is covered by an ILLC
layer, covered with the second plate, and all edges are
glued to avoid evaporation of water. The cells have the
thickness of 20 to 30 μm.
To prepare the samples for investigation of electric
properties, an ILLC layer was placed between glass
plates. As spacers, we used two metal strips (made of Cu
and Ni) that served as electrodes and preset the thickness
of the ILLC layer. The edges of this cell were glued. The
thickness of these cells varied from 25 μm up to 1.5 mm.
3. Holographic properties of ILLC
The method of two-beam mixing by pulse Nd:YAP laser
operated at the double frequency is used to investigate
the holographic grating recording. The parameters of the
laser were as follows: the wavelength λ = 539.8 nm, the
pulse duration τ = 20 ns, the pulse repletion frequency
ν = 3 Hz. The output laser radiation was linearly
polarized in the horizontal plane. The laser beam went
through a diaphragm of the diameter 1 mm and a
splitting system, so that two laser beams were converged
on an investigated cell. The intensities of both the beam
incident on the cell and the diffracted orders were
detected by photodiodes in the self-diffraction regime.
To investigate the time relaxation characteristics of the
dynamic gratings, we use a probe non-polarized beam
from a continuous He-Ne laser.
In sandwich cells, the self-diffraction is revealed
already after one laser pulse. Several diffraction orders
are observed during this process. This fact proves that a
thin grating is formed (the Raman-Nath regime). We
have found that solid dye films provide the recording of
permanent gratings, and only covering them with the
ILLC layer allows one to obtain the dynamic grating
recording with fast erasure time.
To determine the dependence of the absorption in the
cells on the radiation energy density E at λ = 539.8 nm,
we investigated the changes of the optical density D in
the cells versus E during increase of E and, in the reverse
direction, during decrease of E (Fig. 1). We found the
brightening effect in the sandwich cells: the optical
density drops on the average from the value 2 to 1 in the
range of 0.1 to 1 mJ/mm2. But after action of the laser
irradiation, the optical density of the cells does not return
to its initial value. This fact shows that in addition to
evident reversible changes in the absorption, there are
also some irreversible ones connected with photo-
chemical transformation of dye molecules under
powerful laser irradiation. In this process, the sandwich
cells with the anion dye turn out to be stable to the laser
irradiation in comparison with the cells containing the
cation one.
In Fig. 2, shown is the dependence of the energy
density in the first diffracted order of the self-diffraction
(E+1) versus the input energy density (E) in the sandwich
cell with the anion dye. The grating period was fixed and
equaled to 16 μm. One can see that the increasing E+1
with growing E obey the cubic law. The similar
dependence is observed for the cell with the cation dye
(Fig. 3). The experimental results testify that our cells
have the cubic nonlinearity that is typical for resonance
media. We investigated the diffraction efficiency
depending on the grating period. The efficiency of the
self-diffraction demonstrates a slight increase in the
range from 3 to 30 μm versus the grating period. In
contrast to impurity nematics [5], the sandwich cells
provide a more effective recording for small periods,
which leads to increasing the spatial resolution of the
holographic recording in the medium.
The kinetics of dynamic grating relaxation starting
after 5 μs of nanosecond recording laser pulses is
investigated for the energy density E = 1 mJ/mm2 and
grating period Λ = 16 μm. The experiment allows us to
define the real time of the complete erasure of gratings.
The typical behavior of the observed relaxation curve is
shown in Fig. 4. One can see that vanishing the
secondary thermal grating obeys the exponential law
with the characteristic time of the order of 30 μs.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 1. P. 63-67.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
65
Fig. 3. The measured energy density of the first diffracted
order of the self-diffraction versus the energy density in one
laser pulse in sandwich-like cell with the cation dye.
Fig. 4. The kinetics of the probe beam intensity in the first
diffracted order for a sandwich-like cell with the cation dye.
The obtained main holographic characteristics in
sandwich cells (consisted of a thin dye film and an ILLC
layer) prove that the primary mechanism of the grating
recording is the resonance nonlinearity caused by the
effect of the saturation absorption in the dye molecules.
The ILLC matrix provides a fast erasure of the
secondary heat grating, which is created after non-
radiative relaxation of excited impurity molecules. Local
heating in the cells is a result of dye molecule relaxation
from the excited state to the ground one. This process
lasts within nanoseconds. In the case when there is a
solid dye film alone in the cell, the fast temperature
increase leads to melting and evaporation of
photosensitive molecules what can be seen from the
interference peaks of the light pattern. A permanent
relief grating is formed as the result of this process. Just
the ILLC layer covered the dye film in sandwich cells
provides fast heat dissipation and makes the recording to
be the dynamical one.
4. Electrical properties of ILLC
To find the explanation of grating recording relaxation,
we study the dependence of conductive properties of
ILLC versus the temperature. The electric conductivity
was measured using the oscilloscopic technique [6, 7]. A
voltage signal had a triangular shape with the pick
amplitude 0.10 to 0.25 V. The signal frequency was
changed within the range of 0.1 to 106 Hz. The electro-
conductivity was measured in the range of 104 to 106 Hz,
where the resistance of the samples does not depend on
the frequency. To make the temperature measurements,
the samples were placed into a thermostat that was
especially designed for these experiments. The
temperature values were determined using a
thermocouple with the accuracy 0.5 K.
The phase transition of ILLC is absent in the
investigated temperature range (292 – 333 K). The ILLC
in our cells have a planar domain structure. The direction
of electric field is mainly along alkyl chains to be
perpendicular to the electrostatic layers of ILLC. In the
experiments, we measured a geometric parameter of the
cell )/( zdk ⋅= l by varying its thickness d, the length of
metal electrode-spacers z and the distance between them
l . The conductivity σa.c. was calculated from the
measured value of the volume resistance R and the
known geometric parameter k of the cell by using the
formula: Rk /a.c. =σ . We obtained the value of the
conductivity at the room temperature σa.c. =
10…11 Ohm−1⋅m−1. A typical temperature dependence
of the volume resistance for the sample R is shown in
Fig. 5. One can see that the experimental points are very
well approximated by Arrhenius’ equation [8]. From the
slope of the line in Fig. 5, we determine the conductivity
activation energy Ea (Ea = 0.21 eV).
Earlier it was established [9] that the anisotropy of
the conductivity is the distinguishing feature for
thermotropic ionic LCs of metal alkanoates. One has to
expect a similar anisotropic conductivity for ILLC that
has the same smectic structure as the thermotropic
mesophases. To prove this statement, the conductivity in
thick samples (1.5 mm) with random domain structure in
the volume has been investigated. We found that any
changes of the electric field direction do not change the
conductivity of the volume samples, i.e., the “thick”
sample possesses isotropic electric properties. The
conductivity of such samples is 32 to 36 Ohm−1⋅m−1 at
the room temperature what is higher as compared with
the planar orientated thin cells. We determined the
conductivity activation energy Ea = 0.16…0.17 eV from
the temperature dependences of the volume resistance in
non-oriented samples. Nevertheless, the fact of the
significant difference in the conductivity (by 3 to
3.5 orders) between non-oriented samples
(32…36 Ohm−1⋅m−1) and planar oriented cells
(10…11 Ohm−1⋅m−1) (where the electric field is applied
on the average not along the electrostatic layer but
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 1. P. 63-67.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
66
Fig. 5. The volume resistance of pure ILSLC versus an inverse
temperature.
perpendicular to them) is indicative of the existence of
the anisotropic conductivity in ILLC.
Similar experimental data for the conductivity and
the activation energy have been obtained for impurity
ILLC with anion dye. Since the high own intrinsic
conductivity of ILLC exists, doping by the anion dye
does not lead to essential changes of those electrical
properties.
We would like to note that one has to use only
metallic electrodes to measure conductivity in ILLC.
Applying the ITO electrodes that are usually used to
study electric properties in dielectrics gives values
several orders smaller for well conductive media.
We investigated also the charge carrier mobility in
ILLC. For this purpose, we use the method of the inverse
polarity [10]. The sample was connected with a source
of the constant voltage (2 В) that polarize the sample.
When the current passing through the sample reaches a
constant value, this source is disconnected and other
source gives inverse pulling voltage (1.5 В) to the
sample. The signal from the sample input to a grapher
that fixed the dependence with a maximum for the
current value. Being based on the position of the
maximum and the relation µ = d2/(U·t), where d is the
sample thickness, U is the voltage, t is the time of the
maximum current, we found the value of the charge
carrier mobility µ in ILLC. We determined the charge
carrier mobility µ ≈ 2·10−7 m2/V·s.
Our experimental researches and estimations permits
to make some conclusions about a possible mechanism
of the conductivity in ILLC as well as its role in the
process of the dynamic grating relaxation. The high
value σa.c. (10 to 36 Ohm−1⋅m−1) and absence of the
effect of the electrolytic treatment of ILLC prove the
existence of mainly intrinsic ionic conductivity. Taking
the ILLC structure into consideration, one can assume
that the main contribution to the conductivity is provided
by potassium cations that can easy move along
electrostatic layers of ILLC in contrast to carboxyl-
anions because they have a size that is tens times greater
than the potassium cation one.
The obtained results about the small activation
energy of the conductivity in ILLC are important to
understand a role of the ILLC matrix in the process of
the dynamic grating erasure. As it was noted above, the
local heating occurs during dynamic grating recording
on the interference peaks as the result of relaxation of
excited dye molecules. During this process, a thermal
activation of potassium cations in ILLC takes place. The
activated charge can move over the distance of fractions
of a micrometer over the time of several microseconds,
to provide in such a manner a partial heat removal from
the places of local heating.
5. Conclusions
For the first time, the holographic recording of dynamic
grating was obtained in materials based on conductive
ILLC that provide fast relaxation of recorded holograms
in the microsecond range. Our experimental results
prove that the primary mechanism of grating recording
in sandwich cells (consisted of a thin dye film covered
with the ILLC layer) is caused by a resonance
nonlinearity of the saturation absorption in two-level
electronic transitions in dye molecules. Under intensive
laser irradiation with the frequency close to the
frequency of the electronic transition, the occupance of
the excited level takes place. Also the brightening effect
is observed as the result of this process.
From our experimental researches of electric
properties, the activation character of the temperature
dependences of the conductivity in ILLC is ascertained.
We found that the ILLC belongs to systems that are
sensitive to low electric fields and possesses anisotropic
high-valued intrinsic ionic conductivity (from 10 to
36 Ohm−1⋅m−1). The conductivity of the investigated
ILLC is 5-10 orders higher as compared with the
traditional molecular LCs. Obtained values of the
conductivity are close to those in liquid or solid
electrolytes having the similar compounds, but in
contrast to them ILLC are a LC electrolyte.
Investigated specific properties of ILLC show that
these materials belong to media that provide fast erasure
of thermal gratings being formed after relaxation of
excited molecules of dyes. This fact follows from the
possibility of thermal activation of potassium cations
due to existence of a close contact between an ILLC
layer and a dye film. Optimization of the compound of
ILLC based cells permits to obtain new perspective
materials possessing high-velocity effective dynamical
recording with fast erasure time.
References
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