Download Optical Parameters of Paratellurite Crystals

ISSN 10637745, Crystallography Reports, 2012, Vol. 57, No. 7, pp. 909–911. © Pleiades Publishing, Inc., 2012.
PHYSICAL PROPERTIES
OF CRYSTALS
Optical Parameters of Paratellurite Crystals
A. I. Kolesnikova, I. A. Kaplunova, S. E. Il’yashenkob, V. Ya. Molchanovc, R. M. Grechishkina,
M. A. Arkhipovaa, and S. A. Tret’yakova
a
Tver State University, Tver, 170002 Russia
email: [email protected]
b Tver State Technical University, Tver, 170026 Russia
c
National University of Science and Technology MISiS, Moscow, 119049 Russia
Received December 20, 2010
Abstract—Refractive indices and transmittances of paratellurite single crystals have been measured. The spe
cific optical rotation is determined and the Verdet constants are calculated for a radiation wavelength of 355 nm.
DOI: 10.1134/S1063774512070115
INTRODUCTION
Unique acoustooptic, nonlinear, and piezoelec
tric properties of paratellurite (αTeO2) crystals make
this material very promising for applications in many
fields of science and technology. To date, the applica
tion of paratellurite in devices using its optical proper
ties has been limited by its intrinsic material absorp
tion at a wavelength of 0.35 μm, which corresponds to
the short UV range [1]. It was even believed that para
tellurite crystals barely transmit 0.36μm light.
Recently it was reported that, to a great extent, the
weak light transmission at this wavelength is related,
not to absorption, but to the sharp increase in reflec
tance, which is in turn caused by the sharp increase in
the refractive index [2]. It was also found that, in the
range of 0.35–0.36 μm, there is a sharp increase in not
only the refractive indices, but also in the specific optical
rotation to values of more than 700 deg/mm [2], which
are larger those predicted by the approximation depen
dences obtained in the visible range [2–5]. The afore
mentioned range includes a working wavelength of mod
ern commercial lasers of 0.355 µm (the third harmonic of
radiation of lasers with active media containing Nd3+
ions) [6]. Therefore, the optical properties of paratellu
rite at this wavelength are of particular interest.
MEASUREMENTS OF OPTICAL PARAMETERS
OF PARATELLURITE CRYSTALS
We measured the refractive indices of paratellurite
crystals (αTeO2) for ordinary and extraordinary rays
using an optical scheme that included a frequency
doubled YAG:Nd3+ laser (λ = 355 nm), a digital cam
era, a Laser Power Meter, and a large transparent sam
ple with sizes of 60 × 45 × 40 mm along the [001],
[110], and [ 110] axes, respectively, cut from a paratel
lurite single crystal; all sample faces were polished.
Since the laser partially emits the second harmonic
with a wavelength of 533 nm, two beams were recorded
at the output face for both ordinary and extraordinary
rays: violet (UV induced fluorescence) and green. The
distance between the centers of the corresponding
spots on the crystal face, the angle of incidence, and
the crystal thickness were used to calculate the No and
Ne values. They were found to be No = 2.688 and Ne =
2.891, i.e., somewhat larger than those measured in [7]
(No = 2.568 and Ne = 2.778). This means that the effi
ciency of paratelluritebased acoustooptic devices
that operate at a wavelength of 355 nm should exceed
the value expected based on the previous data by 31–
32% because the optical quality factor of material Ì2 is
proportional to the sixth power of the refractive index.
The transmittances T of polished cubes with sides
of 1 cm cut from paratellurite crystals were measured
by the directed transmission method in [8, 9]. The
corresponding T values for the wavelength of 355 nm
in the [001] and [110] directions were found to be 0.66,
and 0.62. These values are 5–8% larger than those
established previously in [4]. This circumstance
expands the range of applications of paratellurite in
deflectors, as well as in the acoustooptic devices of
multispectral spectroscopy in the part of the UV range
that is very important for medical diagnostics, biolog
ical studies, and astrophysics. Another possible appli
cation of paratellurite is the fabrication of polarization
and birefringent prisms for light in this range.
Exact data on the gyrotropy of material are also
necessary for designing acoustooptic waveguides and
calculating the frequency characteristics of acousto
optic devices because the diffraction of light by ultra
sonic waves depends on the polarization state of trans
mitted light [10, 11].
It should be emphasized that inaccurate values of
specific rotation lead to an error in determining the
component G33 = ρλ / π N 03 of the gyration pseudoten
sor, which is used to calculate the acoustooptic inter
actions [11]. Experimental data on the specific rota
tion were obtained for a limited number of wave
909
910
KOLESNIKOV et al.
ρ, deg/mm
700
600
500
400
300
200
100
400
lengths. Therefore, the corresponding curves ρ(λ)
plotted using approximations [3] may yield inaccurate
values of G33, especially for the spectral ranges where
ρ(λ) values were not measured. Specifically, this prob
lem was encountered in [11], where attempts were
made to calculate the parameters of a UV acousto
optic deflector based on TeO2 designed for operation
at a wavelength λ = 0.355 μm (the third harmonic of
YAG:Nd3+ laser radiation.
An analysis of the studies on the gyrotropy of para
tellurite shows that the data on the specific rotations
ρ(λ) were obtained by measuring the intensity of light
transmitted through a polarizer, sample, and analyzer
[2, 3, 12]. The desired rotation of plane of polarization
caused by the gyrotropy of the material was compen
sated for by the analyzer rotation that was necessary to
make to recover the initial intensity value. The resulting
error included the errors related to the measurements of
light intensities and angles. In addition, because of very
large ρ(λ) values, especially in the spectral range of
0.35–0.5 μm (which is close to the fundamental
absorption edge), the samples must have fairly small
thickness of less than 1 mm. If not, the rotation of
plane of polarization could be multiple. These diffi
culties were noted in [3], where circular dichroism was
λ, nm
ρ(λ), deg/mm [14]
ρ(λ), deg/mm
633
531
488
438
87
143
184
271
84.7 ± 0.3
146.0 ± 0.3
186.3 ± 0.3
600
700
λ, nm
Fig. 2. Dependences ρ(λ), plotted according to data of [14]
and this study.
Fig. 1. Manifestation of gyrotropy and its dispersion at
scattering of laser beams with different wavelengths, prop
agating in a paratellurite single crystal 41.0 mm thick along
the [001] optical axis: (upper) red beam, λ = 633 nm;
(medium) green beam, λ = 533 nm; and (lower) blue
beam, λ = 488 nm.
Values of specific optical rotation in paratellurite crystals
500
measured: along with circular dichroism bands, oscil
lations in the form sin(2ρl) (l is the sample thickness)
were recorded, and it was necessary to use a special
method to eliminate them. This led to additional
errors caused by the errors in measuring the sample
thickness and errors in the orientation of the sample.
In this study, we measured the optical rotation on a
paratellurite single crystal with sizes of 41.0 × 20.1 ×
20.0 mm along the [001], [110], and [1 10] directions,
respectively. All sample faces were polished. In the first
stage, measurements were performed at laser wave
lengths of 633, 533, and 488 nm, according to the
technique described in [13].
Figure 1 shows patterns of laser beam scattering in
paratellurite, which were used to determine the spe
cific rotation by the aforementioned method. The cal
culated values of specific rotation ρ(λ) and the values
found in [14] are listed in the table. These data were
used to perform approximations in which the best cor
relation coefficients were obtained with the function
(1)
ρ −1 = a + bλ 2 ln ( λ),
where λ and ρ(λ) are measured in angstroms and
deg/mm, respectively. The corresponding coefficients
were found to be a = –0.00283 and b = 4.06 × 10–11.
The table contains the comparative values of specific
rotation, which were determined experimentally in
this study and in [14].
As follows from the table, at large (small) wave
lengths, the known values [14] of specific optical rota
tion are larger (smaller) than those found in this study.
Figure 2 shows the dependences ρ(λ), which were
plotted by approximating the values obtained in [14]
and in this study. In the latter case, the best approxi
mation is given by the formula
ρ −1 = a + bλ 2 ln ( λ),
where a = –0.00310 and b = 4.17 × 10–11.
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OPTICAL PARAMETERS OF PARATELLURITE CRYSTALS
According to both dependences, the specific rota
tion for a wavelength of 355 nm should not exceed
750–800 deg/mm. However, direct measurements of
the rotation of the plane of polarization of the UV
beam in the sample (Fig. 1) after its thinning by 40 μm
and repolishing yielded another value of 840 ± 8 deg/mm.
Thus, the value of the gyration pseudotensor G33 in
paratellurite at 355 nm was also larger than previously
expected.
Then, we measured the Faraday rotation in paratel
lurite for a wavelength of 355 nm. The Faraday effect
implies the rotation of the plane of polarization of a
beam that propagates coaxially with the vector H of a dc
magnetic field applied to the crystal. The rotation of
plane of polarization by an angle θ is related to the crystal
length l in the field direction by the relation [13]
(3)
θ = VHl ,
where V is the Verdet constant. The following V values
were obtained in [5] for two radiation wavelengths:
V(λ = 533 nm) = 6.11 × 10–5 rad A–1 and V (λ =
633 nm) = 3.78 × 10–5 rad A–1. In this study the Verdet
constant was measured for UV laser radiation with a
wavelength λ = 355 nm. To measure the Faraday rota
tion, we cut a cubic sample from a paratellurite single
crystal; this cube had sizes of 1 × 1 × 1 cm along the
[001], [110], and [ 1 10 ] axes, respectively, and pol
ished (001) faces. The cube was placed between the
polarizer and analyzer so as to orient the laser beam be
perpendicularly to the input (001) face and make it
pass through the face center. A photodetector (Laser
Power Meter) was placed behind the analyzer; the
photodetector signal was amplified and applied to
the measurement scheme. An annular permanent
Nd–Fe–B magnet with an inner diameter of 2.0 cm
was used to generate a dc magnetic field. The magnetic
field strength on the magnet axis was 1.43 × 105 A m–1.
The magnet could be moved along the optical scheme
and coaxially approach the crystal. First, in the
absence of a magnet, the laser beam was suppressed by
the analyzer to minimum. Then, the magnet was
moved toward the crystal. The rotation of plane of
polarization was detected by an increase in the photo
current. Then, the beam outgoing from the crystal was
suppressed again to minimum by rotating the ana
lyzer; the latter was equipped with an angle counter
(having an error of ±0.5′). The angle of rotation of the
plane of polarization θ was found to be 14°40′ ± 0.5′.
According to (3), this yields the Verdet constant V =
1.7 × 10–4 rad A–1 for the wavelength λ = 355 nm. The
dispersion of the Faraday effect is approximately
described by the formula [13]
(4)
V = A λ2 + B λ4 .
The dispersion constants were calculated in [1]: A =
3.37 × 10–14 m2 rad A–1 and B = 2.06 × 10–30 m4 rad A–1.
According to these values, the angle of rotation of plane of
polarization under the conditions of our experiment
should not exceed 12°. Thus, paratellurite exhibits
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911
also a sharp enhancement of magnetooptical proper
ties near the fundamental absorption edge. In princi
ple, the large value of the Verdet constant can be used
to design paratellurite modulators of 355nm laser
radiation, whose operation is based on the Faraday
effect rather than on acoustooptic interactions.
CONCLUSIONS
The values of the refractive indices, transmittances,
specific optical rotation, and Verdet constants mea
sured in paratellurite single crystals for 355nm light
turned out to be larger than the previously published
values. This circumstance presents new prospects for
the application of paratellurite as a material for
acoustooptic waveguides in acoustooptic deflectors
and acoustooptic filters for a multispectral analysis of
microscopic and telescopic images, as well as for bire
fringent prisms and magnetooptical modulators
operating in the UV range.
ACKNOWLEDGMENTS
This study was supported by the Federal Target
Program “Scientific and ScientificPedagogical Per
sonnel of Innovative Russia” for 2009–2013.
REFERENCES
1. A. I. Kolesnikov, R. M. Grechishkin, V. Ya. Molchanov,
et al., Physics of Crystallization (TvGU, Tver, 1999),
p. 69 [in Russian].
2. K. A. McCarthy, A. P. Goutzoulis, M. Gottlieb, and
N. B. Singh, Opt. Commun. 47, 157 (1987).
3. V. A. Kizel’ and V. I. Burkov, Gyrotropy of Crystals
(Nauka, Moscow, 1980), p. 304 [in Russian].
4. A. A. Blistanov, V. S. Bondarenko, N. V. Perelomova,
et al., Acoustic Crystals. A Handbook (Nauka, Moscow,
1982), p. 252 [in Russian].
5. W. Kaminsky and E. Hartmann, Z. Phys. B 90, 47
(1993).
6. V. Ya. Molchanov, O. Yu. Makarov, A. I. Kolesnikov, and
Yu. M. Smirnov, Vestn. TvGU, Ser. Fiz. 6 (4), 88 (2004).
7. K. Takizawa, M. Okada, and S. Ieiri, Opt. Commun. 23
(2), 279 (1977).
8. I. A. Kaplunov, A. I. Kolesnikov, I. V. Talyzin, et al.,
Opt. Zh. 72 (7), 85–89 (2005).
9. A. I. Kolesnikov, I. A. Kaplunov, I. A. Terent’ev, and
S. L. Shaiovich, Proc. X Nat. Conf. on Crystal Growth
(Izdvo IK RAN, Moscow, 2002), p. 188.
10. L. N. Magdich and V. Ya. Molchanov, Acoustooptic
Devices and Their Applications (Gordon and Breach,
New York, 1989).
11. V. Ya. Molchanov, O. Yu. Makarov, A. I. Kolesnikov,
and Yu. M. Smirnov, Vestn. TvGU, Ser. Fiz., No. 4(6),
88–93 (2004).
12. N. Uchida, Phys. Rev. B 4 (10), 3736 (1971).
13. V. Yu. Vorontsova, R. M. Grechishkin, I. A. Kaplunov,
et al., Opt. Spektrosk. 104 (5), 822 (2008).
14. A. Yariv and P. Yeh, Optical Waves in Crystals: Propaga
tion and Control of Laser Radiation (Wiley, New York,
1984; Mir, Moscow, 1987).
2012
Translated by Yu. Sin’kov