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Investigation of relaxor PLZT thin films as resonant optical waveguides and the temperature dependence of their refractive index


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Relaxor lead lanthanum zirconate titanate (PLZT) thin films, with compositions of (7/65/35), (8/65/35), and (9/65/35), have been investigated as optical waveguides. Resonant structures were observed in the reflected light beam that passes through these thin films after coupling with a laser-inscribed azo polymer surface relief diffraction grating. The temperature was then varied on the PLZT thin films between -20 and 70 degrees C, and a shift in the above resonance peaks was observed that is due to a change in the refractive index of the samples. The temperature dependence of the refractive index of the tested PLZT thin films was subsequently plotted and was found to decrease linearly with an increase in temperature at different rates for all the thin-film compositions tested.
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Investigation of relaxor PLZT thin films as resonant
optical waveguides and the temperature
dependence of their refractive index
Ribal Georges Sabat* and Paul Rochon
Department of Physics, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston,
Ontario K7K 7B4, Canada
*Corresponding author:
Received 27 January 2009; revised 1 April 2009; accepted 3 April 2009;
posted 9 April 2009 (Doc. ID 106876); published 4 May 2009
Relaxor lead lanthanum zirconate titanate (PLZT) thin films, with compositions of (7=65=35), (8=65=35),
and (9=65=35), have been investigated as optical waveguides. Resonant structures were observed in the
reflected light beam that passes through these thin films after coupling with a laser-inscribed azo poly-
mer surface relief diffraction grating. The temperature was then varied on the PLZT thin films between
20 and 70 °C, and a shift in the above resonance peaks was observed that is due to a change in the
refractive index of the samples. The temperature dependence of the refractive index of the tested PLZT
thin films was subsequently plotted and was found to decrease linearly with an increase in temperature
at different rates for all the thin-film compositions tested. © 2009 Optical Society of America
OCIS codes: 310.2785, 310.3840, 310.5448, 310.6860.
1. Introduction
Perovskite ferroelectric lead lanthanum zirconate ti-
tanate (PLZT) thin films are usually identified by
their atomic composition such as (La=Zr=Ti), where
La is the percentage of lead atoms that have been re-
placed by lanthanum in the perovskite structure A-
sites, and Zr and Ti are the respective percentage of
zirconium and titanium atoms in the B-sites. In gen-
eral, PLZT ceramics with compositions of ða=65=
35Þ, where 7<a<12, are known as relaxor ferro-
electrics because they exhibit a frequency relaxation
in their thermal dielectric response near their Curie
transition temperature. Relaxor PLZT thin films are
prime candidates for applications in integrated op-
tics, such as light waveguides and modulators, be-
cause they possess excellent transparency in the
visible and the IR, a relatively high index of refrac-
tion and dielectric permittivity, large electro-optic
coefficients, and a fairly slim ferroelectric hysteresis
around room temperature.
PLZT thin films were first RF sputtered and char-
acterized by Ishida et al. [1,2] in 1977, and they were
first investigated as light waveguides by Kawaguchi
et al. [3] in 1984. After improving the fabrication pro-
cess of such films, they were successfully used as
light modulators [4] and other optical applications [5]
ensued in the 1990s. More recently, further advances
have been made on the fabrication methods of good
quality PLZT thin films, and their characteristics as
light modulators have been improved [6,7]. There
has also been an increased interest in gaining a bet-
ter understanding of the physical material properties
of PLZT thin films, particularly the influence of the
substrate orientation on their electro-optic response
[8] as well as their nonlinear optical behavior [9]. In
general, relaxor PLZT bulk ceramics are known to
undergo a thermally or electrically dependent ferro-
electricparaelectric phase transition just below
room temperature [10]. Reports on the temperature
dependence of other ferroelectric materials have in-
dicated that there is a change in the sign of the
temperature coefficient of the refractive index of
both SbSI [11]andPb
1xGexTe [12] around their
© 2009 Optical Society of America
10 May 2009 / Vol. 48, No. 14 / APPLIED OPTICS 2649
transition temperature between the ferroelectric and
the paraelectric phases. Even the application of a
large dc bias on ferroelectric Ba0:5Sr0:5Nb2O6thin
films proved to change the sign of the thermal coeffi-
cient of the refractive index compared to a zero field
condition [13]. These results indicate that the varia-
tion of the refractive index with temperature in ferro-
electric materials can be affected by the phase in
which they are in.
We used azo polymer surface relief diffraction grat-
ings to couple a laser beam into relaxor PLZT thin-
film waveguides. After varying the temperature of
the samples, a shift was then observed in the reso-
nance peaks of the reflected light that is due to a
change in the refractive index of the films, and the
temperature coefficients of their refractive index
were subsequently found.
2. Experiment
Relaxor PLZT thin films, prepared with the chemical
solution deposition method, were acquired from
Inostek (Gyeonggi-do, South Korea). These films,
with compositions of ð7=65=35Þand ð8=65=35Þ, were
grown on Si wafers and had a thickness of 500 nm;
composition ð9=65=35Þfilms were grown on c-cut
h0001isapphire substrates and had a thickness of
1500 nm. All the films had a 150 nm Pt bottom elec-
trode. A 3% solution of an azo polymer compound,
poly (MEA-co-DR1M) 2:8 [14], dissolved in dichloro-
methane, was prepared and spin coated with a thick-
ness of 450 nm on top of the PLZT thin films. This
polymer has a refractive index of 1.659. Here we take
advantage of the fact that surface relief diffraction
gratings can be written on azo polymers using an in-
terference pattern generated by a laser [15].
The spacing of the diffraction gratings written on
azo polymers can easily be modified by changing the
interference pattern. A 1 min exposure to the writing
laser usually yields gratings with a depth between
100 and 150 nm. After inscribing the azo polymer
gratings on our PLZT thin films, a separate appara-
tus was used to measure precisely the positive and
negative first-order diffraction angles and an accu-
rate value of the grating spacing was calculated. Sub-
sequently, the light from a spectrometer, with a 1nm
resolution, passed through a mechanical chopper, a
quarter-wave plate, a converging mirror, a polarizer,
and onto the PLZT films, at which point it was re-
flected by the Pt bottom electrode and its intensity
was measured by a photodetector connected to a
lock-in amplifier. The quarter-wave plate was used
to render the laser light circularly polarized, and,
hence, the incident light beam polarization on a
tested sample could easily be chosen by rotating
the polarizer. The PLZT thin films were positioned
on a computer-controlled turntable located inside a
liquid-nitrogen-cooled Delta Design 9023 tempera-
ture chamber with a temperature uncertainty
of 2°C.
3. Results and Discussion
Figure 1is a picture of an azo polymer surface relief
diffraction grating with 750 nm spacing taken with
an atomic force microscope. As illustrated in Fig. 2,
when a light beam in the air is incident at an angle θi
on a diffraction grating located on a PLZT film with a
refractive index of n, the transmitted light will be
concentrated in discrete diffraction orders numbered
as m¼0,1,2, etc., according to the well-known
grating equation
k0nsin θm¼k0sin θiþ2πm
where k0is the free-space wavenumber ðk0¼2π=λ0Þ,
and Λis the grating spacing. For clarity, Fig. 2shows
only the first-order forward (positive) and backward
(negative) diffracted beams.
The modal theory of light propagation in slab op-
tical waveguides has been explained in many texts
[16,17]. Since the refractive index of the PLZT film
is higher than that of the azo polymer [9], a diffracted
light beam can be trapped inside the PLZT film if the
longitudinal propagation constant of that particular
diffraction order matches that of a discrete allowed
guided mode in the PLZT film [18]. If one sets κand
βas the x-axis and z-axis components, respectively, of
a discrete guided mode propagating in a thin film, κ
and βmust be (for m¼1)
κ¼k0ncos θ1;ð2Þ
β¼k0nsin θ1¼k0sin θi2π
It then follows that
Several discrete propagation modes, hence different
values of κand β, can be allowed in a particular film
Fig. 1. (Color online) Atomic force microscope picture of a surface
relief diffraction grating with 750 nm spacing.
2650 APPLIED OPTICS / Vol. 48, No. 14 / 10 May 2009
depending on its refractive index and thickness, as
well as the index of the cladding and the substrate
[18]. First we consider light incident on a PLZT film
at normal incidence, θi¼0, as one varies the wave-
length, the 0th-order reflected intensity will have ne-
gative resonance peaks when forward and backward
guided modes are simultaneously excited in the film.
Each mode will have a unique κvalue, but all will
have the same jβ2π=Λ. On the other hand, when
θi0,κremains the same for each corresponding for-
ward and backward mode, but two different solutions
of βare possible for every mode depending on the
sign of the second term on the right-hand side of
Eq. (3). Therefore, forward and backward resonance
peaks will occur at different wavelengths. Using
Eqs. (3) and (4) it can be shown that, for the same
guided mode, λbackward >λforward. Figure 3shows
three distinct guided modes in a 2 μm PLZT
ð8=65=35Þthin film with Λ¼900 nm and θi¼2°.
In Fig. 3, the higher the θithe greater the separation
between the forward and the backward coupling of
each guided mode.
Since each guided mode has a unique value of κ,by
subtracting Eq. (4) for a forward guided coupling
from a backward guided coupling for the same mode,
we obtain
forward β2
backward ¼ðk2
0forward k2
Hence, by finding the forward and backward cou-
pling wavelength for each mode at various incident
angles, a graph of β2
forward β2
backward as a function
of k2
0forward k2
0backward can be plotted and the refrac-
tive index of the film can be calculated. An example
of such a graph is shown in Fig. 4as well as an ap-
proximate value of the refractive index of each tested
PLZT thin-film composition at an average wave-
length. The refractive index of a PLZT ð9=65=35Þthin
film was measured independently with a HeNe la-
ser ellipsometer at room temperature, and it was
found to be 2:24 0:20, which is in good agreement
with the value found in Fig. 4. The refractive index of
a thin-film PLZT ð8=65=35Þwas also measured using
aZ-scan technique [9] and was found to be approxi-
mately 2.12 at 700 nm, again showing good agree-
ment with the value found in Fig. 4. For a fixed
wavelength of 632:8nm, scanning the incident angle
would also exhibit similar resonance peaks in the
0th-order reflected intensity on the PLZT thin films,
as seen in Fig. 5. A second grating was written in the
azo polymer on each film with a slightly larger spa-
cing. This allowed the identification of each peak as a
forward or backward coupling, since the value of β
must remain unchanged for each resonance peak
at a fixed wavelength. From Eq. (3), if Λincreases,
θiforward must increase and θibackward must decrease.
Now, if the temperature of a PLZT thin film is
varied for an angular intensity scan at a fixed
λ¼632:8nm, we expect a change in its refractive in-
dex n, and, subsequently, a change in both κand βfor
each resonance peak. From Eq. (4) we have
where subscripts 1 and 2 denote the values of n,κ,
and βat a temperature of 20 °C and T, respectively.
Fig. 2. Side view of the tested PLZT samples.
Fig. 3. Normalized intensity as a function of wavelength for
PLZT ð8=65=35Þthin film.
Fig. 4. (Color online) β2
forward β2
backward as a function of k2
0backward for various composition PLZT thin films.
10 May 2009 / Vol. 48, No. 14 / APPLIED OPTICS 2651
Using Eq. (3) and subtracting Eq. (7) from Eq. (6),
we obtain an expression for the refractive-index
change that is due to a temperature variation of
ΔT¼T20 °C:
2nave ðsin2θi2sin2θi1Þ
Λð sin θi2sin θi1Þþ λ2
where nave is the average refractive index of the film,
using the values of nave from Fig. 4. As expected,
Fig. 6shows a significant angular shift in the reso-
nance peaks for all the thin-film compositions. After
increasing the temperature on the samples from 20
to 60 °C, forward resonance peaks occurred at smal-
ler incident angles, whereas backward resonance
peaks occurred at larger incident angles. The
temperature range used is well below the glass tran-
sition temperature of the azo polymer (130 °C), and
diffraction gratings are known to be stable up to
120 °C. From the data of Fig. 4, we can obtain an ap-
proximate value of κfor every mode, and it can be
readily seen that the last term on the right-hand side
of Eq. (8) is very small compared with the other two.
Hence, Δncan be calculated directly for a corre-
sponding ΔTfrom the angular shift of the resonance
peaks. Figure 7shows that the refractive index of the
various PLZT thin films decreased as a function of
increased temperature ranging from 20 to 70 °C
with minimal hysteresis.
A model of our PLZT thin films was constructed in
the electromagnetic numerical analysis software pro-
gram GSOLVER to simulate the effects of changes in
grating spacing, thickness of the PLZT films and
their index of refraction, for a 0th-order reflectivity
angular scan. The simulation results were closely
matched to the experimental results. For example,
a simulation of Δn¼0:005 resulted in a shift in
the incident angle for a forward resonance mode from
16:6°to16:2° and for a backward mode from 3:9°to
4:1°, as modeled in GSOLVER. Inputting these an-
gles into Eq. (8) results in Δn¼0:0042. The effect
of linear thermal expansion was also taken into con-
sideration by modeling its effect on the angular reso-
nance peaks using GSOLVER. Haertling [19] found
that most relaxor PLZT bulk ceramics have an aver-
age linear thermal expansion coefficient of approxi-
mately 3:4×106=°C. A much larger hypothetical
change of 0.1% in the PLZT thin-film thickness over
the entire tested temperature range was modeled
and resulted in only negligible changes in the angu-
lar position of the resonances because of the thick-
ness change alone. Therefore, the majority of the
angular shift in the experimental resonance peaks
must come from the refractive-index change alone.
From Fig. 7, the temperature coefficients of the
refractive index of thin-film PLZT ð7=65=35Þ,ð8=
65=35Þ, and ð9=65=35Þwere ð23:20:2Þ×105=°C,
ð13:30:1Þ×105=°C, and ð10:51:4Þ×105=°C,
respectively. These temperature coefficients seem
to increase with an increase in La content and clearly
indicate a strong dependence of the refractive index
on the composition and, hence, their crystalline
phase [20]. The results of Fig. 7also contradict
Fig. 5. (Color online) Measured intensity as a function of incident
angle θiat different grating spacing for thin-film PLZT:
(a) ð7=65=35Þ, (b) ð8=65=35Þ, (c) ð9=65=35Þ.
2652 APPLIED OPTICS / Vol. 48, No. 14 / 10 May 2009
previously reported results that indicate positive
temperature coefficients of the refractive index for
bulk PLZT ð9=65=35Þand ð10=65=35Þ[21,22]. This
inconsistency mirrors that seen in the dielectric mea-
surements of similar composition bulk and thin-film
PLZT samples. For example, bulk PLZT ð9=65=35Þ
was found to have a Curie transition temperature
around 70 °C at 100 kHz [23], whereas PLZT thin
films with the same composition have a Curie tem-
perature around 170 °C at the same frequency [5].
This indicates that, at a set temperature, the crystal
structure of bulk and thin-film PLZT with identical
composition could differ. Even though several studies
Fig. 7. Refractive-index change as a function of temperature for
thin-film PLZT: (a) ð7=65=35Þ, (b) ð8=65=35Þ, (c) ð9=65=35Þ.
Fig. 6. (Color online) Measured intensity as a function of incident
angle θiat different temperatures for thin-film PLZT:
(a) ð7=65=35Þ, (b) ð8=65=35Þ, (c) ð9=65=35Þ.
10 May 2009 / Vol. 48, No. 14 / APPLIED OPTICS 2653
[10,24,25] have concentrated on understanding the
phase transitions of bulk PLZT ceramics in the
range between 125 and 125 °C, the ferroelectric
paraelectric phase transition in PLZT ceramics in
general is still not fully understood. As mentioned
in Section 1, this phase transition might affect the
temperature coefficient of the refractive index of fer-
roelectric materials in general [1113]. Therefore,
the difference in the results of thin-film and bulk
PLZT is probably associated with differences in their
phase structure in the tested temperature range.
4. Conclusion
Relaxor thin-film PLZT compositions were investi-
gated as optical waveguides. A light beam was
coupled inside these films by laser-inscribed azo
polymer surface relief diffraction gratings. Relatively
deep resonance peaks were observed in the 0th-order
reflected light from the thin films. After varying the
temperature on the PLZT samples, a shift was ob-
served in the angular position of the guided modes
inside the films that is due to a change in the refrac-
tive index of the PLZT. The temperature coefficients
of the refractive index of all tested compositions were
calculated, and they were found to increase with an
increase in La content. However, a significant differ-
ence was observed between those coefficients of
similar composition PLZT thin films and previously
reported bulk PLZT. This inconsistency was
associated with differences in the temperature-
dependent changes in the crystal lattice structure
between thin-film and bulk PLZT materials, mainly
because of the fabrication process.
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Conference Paper
We will report ultrafast optical response of lead lanthanum zirconium titanate ceramics. The photo-induced birefringence was by approximately 20 times larger than that of SiO2, while its optical response was shorter than 70 fs.
The real-time thermo-optical properties of thin-film relaxor ferroelectric (9/65/35) lead lanthanum zirconate titanate (PLZT) films (∼2 μm) were investigated and compared to identical composition of thick PLZT ceramics (∼1 mm). Since PLZT absorbs strongly in the far-infrared, a CO2 laser beam (λ = 10.6 μm) was used to create a Gaussian temperature distribution at the surface of the material, emulating a lens. Subsequently, a low power visible He–Ne laser, travelling along the same path as the CO2 laser, probed the optical properties of the induced PLZT thermal lens. It was found that identical composition thin-film and thick PLZT ceramics have opposite thermal lensing behaviors. An explanation of this phenomenon is provided from basic physical concepts.
Full-text available
Going beyond the usual orientation of azobenzene groups within amorphous polymers, an increased amount of incident light may produce physical gratings of the order of magnitude of the wavelength. The diffraction efficiency of such gratings can reach 28% or even more. The surface profile of the grating can be observed with an atomic force microscope. Such gratings can be used to couple beams optically into the film and out of the film, as well as for waveguides. Diffraction gratings, Efficiency, Azo polymers.
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Transition lines between various phases in the electric-field-temperature phase diagram of 9/65/35 lanthanum-modified lead zirconate titanate ceramics were determined by measurements of the temperature and electric-field-dependent dielectric constant. Above a critical field (E-C) the dc bias electric field induces a transition from the relaxer (R) to the long-range ferroelectric (FE) phase. In the temperature direction of the approach to the FE phase the R-FE; transition line was determined from the field-cooled-field-heated dielectric susceptibilities, while depolarization temperatures were obtained from the field-cooled-zero-field-heated dielectric susceptibilities. A considerably large shift was found for the above two R-FE transition lines demonstrating the strong impact of the electric field on the stability of the FE phase with increasing temperature. It was found that below E-C ergodicity is broken due to the divergence of the longest relaxation time at the freezing temperature T-0 = 259 K. Hence the system exhibits a transition line between the ergodic (ER) and nonergodic (NR) relaxor state. In the de bias field direction of the approach to the FE phase, the temperature dependence of E-C, i.e., the transition lines between ER or NR and FE phases were studied by measurements of the complex dielectric constant as a function of a de bias field at several fixed temperatures. The experimental results are compared with the results of a spherical random bond-random field model of relaxer ferroelectrics.
Thin ferroelectric interferometers (TFI's) for use as light-modulating devices were fabricated entirely with thin-film techniques on sapphire substrates. The ferroelectric layer in the TFI devices was a lead lanthanum zirconated titanate thin-film material, which can be formed from a chemical solution on highly reflective dielectric mirror surfaces. Light intensity modulation in both transmission and reflection modes was demonstrated with the fabricated devices. Experimental data and simulations show that TFI devices possess tremendous potential in spatial light modulators because of their fast-switching, low-driving voltage and readiness for integration with a variety of substrates, including silicon.
Lanthanum-modified lead zirconate titanate (Pb,La)(Zr,Ti)O3 (PLZT) ferroelectric thin films with perovskite structure were fabricated on quartz substrates by rf magnetron sputtering at 650 °C. Their optical fundamental constants (the band gap, linear refractive index, and absorption coefficient) were obtained through optical transmittance measurements with the envelope method. The nonlinear optical properties of the PLZT films were investigated by the Z-scan technique. The films display strong nonlinear optical effects. A negative nonlinear refractive index n2 is determined to be 1.21× 10-6 esu in the films. These results show that the PLZT ferroelectric thin films are promising materials for nonlinear optics.
In this work thermal lens spectrometry, thermal relaxation calorometry and optical interferometry were employed to determine the thermo-optical properties of transparent PLZT 10/65/35 ceramics. The thermal diffusivity, the specific heat and the temperature coefficient of the optical path length change were determined. These parameters are important to obtain the figure of merit of this material in terms of its application as optical components used under high power laser excitation.
It is demonstrated that thin films of PLZT prepared by rf sputtering exhibit an electro-optic effect. The effects are measured using the reflection of the light polarized with the electric vector parallel to the incidence plane of the light. The reflected light intensity changes with the electric field and shows a decrease of about 33% at 12.5 kV/cm, when the light is incident at 70°. This technique turns out to be useful for the measurements of the electro-optic effects of thin films, and the results suggest the possibility of a reflection-type light modulator consisting of a thin film.
Lanthanum-modified lead zirconate titanate epitaxial films with (100), (101), and (111) orientations were grown on (100), (101), and (111) niobium lending conductivity strontium titanate using chemical solution deposition, respectively. We investigated the changes in the refractive index induced by the electric field in these films using the prism coupling method. Birefringence was observed in the (101)- and (111)-oriented epitaxial films induced by the electric field. However, no birefringence was observed in the (100) epitaxial film. This crystal orientation dependence of the electrooptic effect can be explained by the rotation of the refractive indicatrix that accompanies the switching of the polar clusters.
Electrooptical properties of Ba0.5Sr0.5Nb2O6 thin films produced by RF-sputtering have been investigated. It is shown that the films investigated are ferroelectric with a diffused phase transition. The connection between electrooptical properties and the substrate temperature is shown.
A series of copolymers with two structural units1-(4-nitrophenyl)-2-(4-{[2-(methacryloyloxy)ethyl]ethylamino}phenyl)diazene (DR1M), which contains donor−acceptor substituents in the azobenzene group, and 4-[2-(methacryloyloxy)ethyl]azobenzene (MEA), which has no donor−acceptor substituentswas prepared. The maximum obtainable photoinduced birefringence and the fraction of birefringence conserved after relaxation increase with increasing DR1M concentration in the copolymer. The rate of inducing birefringence is independent on the copolymer composition. In terms of the relaxation of the photoinduced birefringence, copolymers with higher DR1M content relax faster. The β-parameter of the stretched exponential functions describing the growth and relaxation of the photoinduced birefringence also appears to be independent of copolymer composition. The electronic spectra, as well as the birefringence studies, reveal that there is no significant interaction between DR1M and MEA structural units in the copolymers.