Optical second harmonic generation in poled MgOZnOTeO2 and B2O3TeO2 glasses

JOURNAL OF

ELSEVIER

Journal of Non-CrystallineSolids 203 (1996) 49-54

Optical second harmonic generation in poled MgO-ZnO-TeO 2 and B203-TeO 2 glasses Katsuhisa Tanaka *, Aiko Narazaki, Kazuyuki Hirao, Naohiro Soga Division of Material Chemist~; Faculty of Engineering, Kyoto Unic,ersity, Sakyo-ku, Kyoto 606-01, Japan

Abstract Optical second harmonic generation has been observed in electrically poled MgO-ZnO-TeO 2 and B203-TeO 2 glasses. The theoretical Maker fringe curve fitted to the experimental data leads to the fact that the poled region is almost identical with the sample thickness. In other words, the poled region is not restricted to the surface of the sample. For the MgO-ZnO-TeO 2 glasses, the second harmonic intensity is higher in the glass with lower glass transition temperature when the poling temperature is constant. It is thought that the orientation of asymmetrical tellurite structural units which possess electric dipole moments causes the optical second harmonic generation and that the orientation takes place as a result of the structural relaxation of the tellurite network around the glass transition temperature. The second harmonic intensity and the second-order non-linear coefficient are larger for the MgO-ZnO-TeO 2 glasses than for the B203-TeO 2 glasses. This phenomenon is explainable in terms of the difference in the glass network structure between these two systems.

1. Introduction The optical second harmonic generation in poled oxide glasses which was originally discovered by Myers et al. [1] for poled silica glass is a very interesting phenomenon, because this phenomenon indicates that a long-range order in orientation of electric dipole moments is realized in a disordered lattice such as oxide glass. Also, attempts are in progress to achieve wave-guides for second harmonic waves using a glass material. In addition to the silica-based glasses [2-9], poled tellurite glasses [10-12] are known to show second-order non-linear optical effects as well. Previous works by Tanaka et al. [10-12] on the optical second harmonic genera-

* Corresponding author. Tel.: +81-75 753 5541; fax: +81-75 751 6456; e-mail: [email protected].

tion in poled tellurite glasses indicate that the second harmonic generation is suppressed in tellurite glasses containing cations with large electron polarizability such as Ba 2÷ ion. It is believed that in these glasses, the poling process mainly causes electron polarization of the Ba 2+ ion, which does not contribute to the freezing of electric dipole orientations because of the rapid relaxation of this electron polarization. Also, it is known that the second harmonic intensity increases as the number of TeO 3 trigonal pyramid a n d / o r non-bridging oxygen increases in the glass network structure. This phenomenon was explained in terms of the flexibility of glass structure. The electric dipole moments whose long-range orientation gives rise to the second harmonic generation are probably ascribed to asymmetrical structural units such as TeO 4 trigonal bipyramids and TeO 3 trigonal pyramids, and the orientation of these structural units

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K. Tanaka et al. / Journal of Non-Crystalline Solids 203 (1996) 49-54

in the direction of the external dc electric field can occur more readily in a more flexible glass network structure. Nonetheless, the detailed relationship between glass structure and second harmonic intensity in tellurite glass as well as the mechanism of orientation of tellurite structural units still remains unclear. In the present investigation, we examine the relation between second harmonic intensity and glass transition temperature of M g O - Z n O - T e O 2 glasses to evaluate the mechanism of orientation of tellurite structural units under an applied dc field. Also, we compare the second harmonic intensity of poled B 2 0 3 - T e O 2 glasses which consist of only network forming oxides with that of M g O - Z n O - T e O 2 glasses which contain network modifying cations. This comparison may lead to evidence for the relation between the flexibility of glass structure and second harmonic intensity.

2. Experimental procedure 2.1. Sample preparation

Glasses were prepared from MgO, ZnO, B203 and TeO 2 starting materials by using the conventional melt-quenching method. The raw materials were mixed thoroughly to make compositions x M g O • (30 - x)ZnO • 70TeO 2 with x = 0, 10 and 15, and y B 2 0 3 • (100 - y)TeO 2 with y = 20 and 25 (mol%). The mixtures were melted in platinum crucibles at 800 to 900°C for 20 to 40 min in air. The

DC voltmeter

insulator

/ electrode (stainless steel)

DC power supply

Fig. 1. Apparatus for poling the glass sample. The insulators sandwiching the glass sample are commercial glass plates for an optical microscope.

Nd:YAG j L j =fla~ge+fsmall pulsed --~ ~-laser 1 12 [ 2 3 4

photomultiplier

4 5 1 2

' Hv vUH V ,UU Vrotatable stage

~

sample

computer • )

trigger pulse

oxc r integrator •

signal

l:poladzer 2:lens 3:VIS cut filter 4:ND filter 5:IR cut filter Fig. 2. Schematic illustration of equipment for measurements of optical second harmonic intensity. melt was poured onto a stainless steel plate and quenched by being pressed with an iron plate. The resulting samples were determined to be amorphous by using X-ray diffraction analysis with C u K a radiation. Glass transition temperatures were determined by differential thermal analysis (DTA). The heating rate was 10 K / m i n . After both sides of the glass samples were polished, the plate-like samples with thicknesses of about 1 mm were placed between two commercial glass plates for an optical microscope and contacted physically to electrodes made of stainless steel as shown in Fig. 1. Then, an external dc electric field of 4 kV was applied, and the sample was heated to a set temperature. After the sample was maintained at this temperature under the applied electric field for 20 min, the temperature was decreased while the applied voltage was held constant. The applied voltage was removed after the sample reached room temperature. The second harmonic intensity was then measured by the method described below. After the measurement, the poled sample was heated at 300°C for 40 min under no external electric fields to depole the sample. The depoled sample was then poled again at different temperatures. In this manner, the effect of poling temperature on the second harmonic intensity was examined. The reason why the glass sample was put between two commercial glass plates is that TeO 2 was reduced via an electrochemical reaction to form metallic Te powder in the glass when the glass sample directly contacted the electrodes.

K. Tanaka et al. / Journal of Non-C~stalline Solids 203 (1996) 49-54 .

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i

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.

.

Table 1 Refractive indices at 532 and 1064 nm (n532 and nl064) of MgO-ZnO-TeO 2 and B203-TeO z glasses

o2.

=o "-t

51

350

E "" 340 ?,E,_ O

t'-

Glass composition (mol%)

n532

nl064

Ic (~m)

30ZnO • 70TeO 2 10MgO. 20ZnO • 70TeO 2 15MgO. 15ZnO. 70TeO 2 20B203 • 80TeO 2 25B203 • 75TeO 2

2.02 1.99 1.98 2.06 2.00

1.97 1.94 1.93 2.00 1.94

5.3 5.3 5.3 4.4 4.4

¢n 330

l

,

,

0

,

I ,

i

i

,

D i

i

I

i

,

i

i

i

10

MgO content (mol%) Fig. 3. Compositional dependence of glass transition temperature of xMgO. ( 3 0 - x)ZnO. 70TeO 2 glasses.

2.2. Measurements The measurements of second harmonic intensity were performed using a pulsed Nd:YAG laser which operated in a Q-switched mode with a 10 Hz repetition rate. The p-excited fundamental wave with 1064 nm wavelength was used as the incident light. The p-polarization component of the second harmonic wave at 532 nm which was generated from the poled glass sample was detected with a monochromator equipped with a photomultiplier. The output signal

60001

. . . . . experimental fringe I .... theoretical fringe /

.

400o

~

2000

I ] ] 1

--

,I~

~ii 'i

,

i

,

E

,,5 0

20 40 60 Angle of incidence ( deg )

Fig. 4. Maker fringe pattern of 15MgO-15ZnO.70TeO 2 glass poled at 280°C. The solid curve denotes the dependence of second harmonic intensity on the angle of incidence obtained experimentally. The fringe pattern is clearly observed. The broken curve represents the theoretical Maker fringe pattern drawn with d 3 3 = 0.13 p m / V , L = 1 mm, n2o, = 1.983 and n,o = 1.930.

was accumulated by using a box-car integrator. A schematic illustration of the equipment for measurements of optical second harmonic intensity used in the present study is shown in Fig. 2. The second harmonic intensity of Y-cut a-quartz was measured as a standard to calculate the second-order non-linear coefficient of poled glasses. In calculations, d~ = 0.34 p m / V was used for the a-quartz. The refractive index was measured for poled glass samples using an ellipsometer. The wavelengths of the incident light were 1064 and 532 nm. The coherence length was determined from the refractive indices. The coherence length, 1c, was evaluated from the relation:

lc = A / [ 4 ( n2,o - n,o) ],

(1)

where A is the wavelength of fundamental wave, n,o is the refractive index at the wavelength of fundamental wave, and n2, o is the refractive index at the wavelength of the second harmonic wave.

3. Results The compositional dependence of the glass transition temperature for the M g O - Z n O - T e O 2 glasses is shown in Fig. 3. The glass transition temperature increases monotonically with the replacement of ZnO by MgO. The refractive indices at 532 and 1064 nm and the coherence length of the M g O - Z n O - T e O 2 and B203-TeO 2 glasses are listed in Table 1. Fig. 4 shows the Maker fringe pattern [13] obtained experimentally for the 15MgO. 15ZnO. 70TeO 2 glass poled at 280°C (solid curve). The fringe pattern is clearly observed. The broken curve in Fig. 4 is the theoretical fringe pattern drawn under the assumption that the poled glass has the C~,,

52

K. Tanaka et aL / Journal of Non-Crystalline Solids 203 (1996) 49-54

Table 2 Optical second-order non-linear coefficient, d33 , of poled MgOZnO-TeO 2 and B203-TeO 2 glasses. The poling temperature is 280°C Glass composition (mol%)

d33 ( p m / V )

30ZnO •70TeO 2 10MgO. 20ZnO. 70TeO 2 15MgO. 15ZnO. 70TeO 2 20B203 • 80TeO 2 25B203 • 75TEO 2

0.22 0.12 0.13 0.11 0.08

qt= (rrL/2)(4/A)(n~

symmetry and that the Kleinman symmetry [14] is satisfied. In addition, it is assumed that the poled glass is described in terms of the isotropic dipole system [15]. Under these assumptions, the relationship between electric field and polarization is expressed by the following equations:

Px= 2dlsEzEx, Py=ZdlsEyE z,

(3)

Pz=d31E~ + d3,e.~ + d33E},

(4)

(2)

where dis = d31 = d33/3. The Maker fringe pattern is generally written as follows:

e"2~= Cd2"'~"'o12to , R( O)p2( O) e2 2

×[1/(n~-n2o,)

where P~o, is the second harmonic power, Po, is the fundamental power, d is the second-order non-linear coefficient, t" and 72'o, are the transmission factors, R(O) is the multiple reflection correction, p(0) is a projection factor, C is a constant which depends on the beam area, and qt is expressed by

2

]sin ~ ,

cos 0' - n2~ cos 0~,o). (6)

Here, L is the sample thickness, A is the wavelength of the fundamental wave, 0" is the refraction angle for the fundamental wave, and 0~,o is the refraction angle for the second harmonic wave. d in Eq. (5) is expressed by d

= d33

sin 0'

(7)

when the relation between electric field and polarization is described in terms of Eqs. (2) to (4). In Fig. 4, the theoretical Maker fringe curve is drawn by assuming that d33--0.13 p m / V , L = 1 mm, n2o~= 1.983 and n o = 1.930. The agreement between theoretical Maker fringe pattern and experimental fringe pattern is rather good. The values of refractive index coincide with those obtained experimentally (see Table 1). In addition, L = 1 mm agrees with the practical sample thickness. The second-order nonlinear coefficients thus obtained are summarized for the present tellurite glasses in Table 2.

(5) 4. Discussion

i

i

i

i

I

r

i

i

r

i

i

°100 ¢-

=

. m

x=O

,.

~

x=15

"1" Of)

Tg(°C) x=O 10 15

0~0, ,

' ' 'a~o' ' ' 350 Poling temperature (°C)

Fig. 5. Variation of second harmonic intensity with poling temperature for 30ZnO. 70TeO 2, 10MgO. 20ZnO- 70TeO 2 and 15MgO15ZnO. 70TeO; glasses. The three arrows indicate the glass transition temperature of these glasses. Lines are drawn as guides for the eye.

The Maker fringe pattern is unambiguous for the present poled tellurite glasses as shown in Fig. 4. Furthermore, the analysis mentioned above indicates that the theoretical fringe curve is fitted well to the experimental data by assuming L = 1 mm, which is coincident with the practical sample thickness. This fact shows that the poled region is identical with the sample thickness. In other words, the poled region is not restricted to the surface of the sample. It has been reported that for poled silica glass, the poled region is only at the surface of the glass sample contacted to the anode [1,6]. In contrast, for the present tellurite glasses, the orientation of electric dipoles extends over the whole bulk glass sample. Fig. 5 shows the variation of second harmonic intensity with poling temperature for 30ZnO. 70TeO 2, 10MgO. 20ZnO- 70TeO 2 and 15MgO. 15ZnO.70TeO 2 glasses. There is a tendency that

K. Tanaka et al./ Journal of Non-Crystalline Solids 203 (1996) 49-54

the second harmonic intensity decreases with an increase in the content of MgO at any poling temperatures. A similar tendency is observed for the second-order non-linear coefficients shown in Table 2. According to a Raman study by Sekiya et al. [16] on M g O - T e O 2 and Z n O - T e O 2 glasses, the number of non-bridging oxygen, TeO 4 trigonal bipyramid and TeO 3 trigonal pyramid in a 30MgO • 70TeO 2 glass is almost identical with that in a 30ZnO • 70TeO z glass. Therefore, the difference in second harmonic intensity among 30ZnO • 70TeO 2, 10MgO. 20ZnO • 70TeO 2 and 1 5 M g O - 1 5 Z n O . 7 0 T e O 2 glasses cannot be attributed to the difference in the number of tellurite structural units. The glass transition temperatures of these glasses are also shown in Fig. 5, as indicated by the three arrows. It is found that the second harmonic intensity is higher for the glass with lower glass transition temperature at any poling temperature. Furthermore, while the second harmonic intensity increases monotonically with increases in the poling temperature for the 10MgO20ZnO • 70TeO 2 and 15MgO • 15ZnO • 70TeO 2 glasses, the poling temperature dependence of second harmonic intensity experiences a maximum for the 30ZnO.70TeO 2 glass which has the lowest glass transition temperature among these three glasses. Such a maximum in the variation of second harmonic intensity with poling temperature was observed for silica glass as well [5]. The decrease in second harmonic intensity with an increase in poling temperature observed for the 30ZnO • 70TeO 2 glass may be explained as follows. The poling temperature, i.e. 300°C, is so high for the 30ZnO. 70TeO 2 glass that the thermal fluctuation of electric dipole moments overcomes the external dc electric field which causes the orientation of electric dipoles. Hence, the poling is less effective at this temperature, leading to the smaller second harmonic intensity. These facts suggest that the orientation of electric dipoles in the direction of the external electric field is presumably ascribed to the orientation of TeO 4 trigonal bipyramids and TeO 3 trigonal pyramids which takes place as a result of structural relaxation of the glass network at around the glass transition temperature. Nonetheless, we cannot rule out the contribution of migration of Mg 2÷ and Zn 2+ ions to the creation of electric dipoles at the present time.

53

Table 2 indicates that the second-order non-linear coefficient is larger for the M g O - Z n O - T e O 2 glasses than for the B203-TeO 2 glasses. A previous Raman study [17] shows that non-bridging oxygens are absent in the B203-TeO 2 glasses. Therefore, the flexibility of glass network structure is larger for the M g O - Z n O - T e O 2 glasses than for the B203-TeO2 glasses. This fact leads to the larger second-order non-linear coefficients for the M g O - Z n O - T e O 2 glasses. This result agrees with that derived from the compositional dependence of second harmonic intensity for binary Z n O - T e O 2 glasses.

5. Conclusions Optical second harmonic generation was observed in poled M g O - Z n O - T e O 2 and B203-TeO 2 glasses. The analysis of the experimental Maker fringe pattern indicates that the poled region is identical to the sample thickness. In other words, the orientation of electric dipoles, which may originate mainly from TeO 4 trigonal bipyramids and TeO 3 trigonal pyramids, extends over the whole glass network structure. For the xMgO • (30 - x)ZnO • 70TeO 2 glasses, the second harmonic intensity increases with an increase in the glass transition temperature when the poling temperature is constant. This fact suggests that the orientation of tellurite structural units takes place as a result of the structural relaxation of the tellurite network around the glass transition temperature. The second harmonic intensity and the secondorder non-linear coefficient are larger for the M g O Z n O - T e O 2 glasses than for the B203-TeO 2 glasses. This fact is ascribed to the difference in glass network structure between these two systems. The number of non-bridging oxygens in the tellurite network is larger in the M g O - Z n O - T e O 2 glasses than in the B203-TeO 2 glasses. Hence, the orientation of tellurite structural units takes place more readily in the M g O - Z n O - T e O 2 glasses, leading to the larger second harmonic intensity and second-order non-linear susceptibility.

Acknowledgements The present authors would like to thank Professor T. Yoko of Institute for Chemical Research, Kyoto University, for the measurements of refractive index.

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K. Tanaka et al. / Journal of Non-Crystalline Solids 203 (1996) 49-54

This work was financially supported by a Grant-inAid for Encouragement of Young Scientists (No. 06750694).

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[7] H. Nasu, K. Kurachi, A. Mito, H. Okamoto, J. Matsuoka and K. Kamiya, J. Non-Cryst. Solids 181 (1995) 83. [8] K. Tanaka, K. Kashima, K. Hirao, N. Soga, S. Yamagata, A. Mito and H. Nasu, Jpn. J. Appl. Phys. 34 (1995) 173. [9] K. Tanaka, K. Kashima, K. Hirao, N. Soga, S. Yamagata, A. Mito and H. Nasu, Jpn. J. Appl. Phys. 34 (1995) 175. [10] K. Tanaka, K. Kashima, K. Hirao, N. Soga, A. Mito and H. Nasu, Jpn. J. Appl. Phys. 32 (1993) L843. [11] K. Tanaka, K. Kashima, K. Kajihara, K. Hirao, N. Soga, A. Mito and H. Nasu, Proc. SPIE 2289 (1994) 167. [12] K. Tanaka, K. Kashima, K. Hirao, N. Soga, A. Mito and H. Nasu, J. Non-Cryst. Solids 185 (1995) 123. [13] P.D. Maker, R.W. Terhune, M. Nisenoff and C.M. Savage, Phys. Rev. Lett. 8 (1962) 21. [14] D.A. Kleinman, Phys. Rev. 126 (1962) 1977. [15] K.D. Singer, M.G. Kuzyk and J.E. Sohn, J. Opt. Soc. Am. 4 (1987) 968. [16] T. Sekiya, N. Mochida and A. Ohtsuka, J. Non-Cryst. Solids 168 (1994) 106. [17] T. Sekiya, N. Mochida, A. Ohtsuka and A. Soejima, J. Non-Cryst. Solids 151 (1992) 222.