Nonlinear optical properties of lead-bismuth-gallium glasses

Materials

Research Bulletin,

Vol. 31, No. 9, pp. 1057-1065, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in the USA. All rights resewed 0025-5408/96 $15.00 +.OO

PI1 SOOZS-5408(96)00104-3

NONLINEAR

OPTICAL

PROPERTIES OF LEAD-BISMUTH-GALLIUM GLASSES

E. Golis’, I.V. Kityk’, J. Wasylak’ and J. Kasperczyk’* I Physics Institute, Pedagogical University,

Al. Armii Krajowej 13/15, PL-42201 Czestochowa, Poland ‘University of Mining and Metallurgy, Al. Mickiewicza 30, PL-30059 Cracow, Poland

(Received December 6, 1995; Accepted June 13, 1996)

ABSTRACT

Low-temperature optical properties of PbO-Bi203-Ga203 glasses were investigated for the first time. Nonlinear optical methods, including photoinduced second harmonic generation (SHG) and piezooptics, were used as the main tools in our investigations. A ruby laser beam (h = 694 run) was applied as a source of photoinduced changes. A temperature split of the SHG signal for wavelength h = 1.06 pm was observed, depending on light polarization. Dielectric and dilatometry measurements were performed to investigate contributions of spontaneous polarization and deformation, respectively. We have observed the appearance of maxima in the corresponding optical susceptibilities between 40 and 60 K. KEYWORDS: A. glasses, A. oxides, D. elastic properties, D. optical properties INTRODUCTION

An increasing interest in the lead-bismuth-gallium glasses of PbG-Bi20&a20s type [l] has been observed in recent years because of a possibility to use them for d&rent optoelectronic devices. These glasses are characterized by a very short light switching time and, therefore, they can be applied in short-time switching devices. These materials are also

*To whom correspondence

should be addressed 1057

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very promising for the quantum electronics because of high values of the nonlinear optical susceptibilities (two times greater than corresponding parameters for quartz) and r&action indexes (3.6-3.7) [ 11. It is also necessary to note that the presence of heavy ions always leads to an enhancement of the nonlinear optical susceptibilities. One of the main advantages of the heavy metal gallates in comparison to the traditional lead-bismuth glasses consists in a greater homogeneity (an inhomogeneity degree is smaller than 0.1%) [2]. Typical volume densities of the lead-bismuth-gallium glasses lie between 8205 and 8215 kg/m3 [3]. It is necessary to emphasize that the mentioned materials show exceptionally high magnetooptical Verdet constants, which are at least six times greater than corresponding constants for the traditional flint glasses in a visible spectrai range (570-710 mn) [41. The PbG-BiZ03-GaZ03 glasses are potentially very promising because they show good transparency in an infrared spectral region, compared to other oxide glasses, and they should compete with halogenide and nonoxide chalcogenides in a spectral region (up to 8 pm) due to their better technological, physical, and chemical properties (higher microsoftening, smaller thermal expansion coefficients and higher chemical resistivities) [3] . We have recently observed the appearance of the photoinduced second harmonic generation (SHG), using a nitrogen laser light (h = 337 nm) [4]. Dependencies of the SHG intensity on the nitrogen laser flux intensity and temperature were found. The temperature behavior of the SHG intensity shows a maximum at low temperatures (40-60 K) for the leadbismuth-gallium glasses. No such maxima were observed in relative glass materials (without the Ga-G chemical bonds). Therefore, we will use the notion of anomaly to describe the unusual presence of such a temperature maximum for the SHG dependence in the lead-bismuth-gallium glasses in contrast to the typical temperature behaviors. Most CE the photoinduced light, however, is absorbed by the samples because of a narrow energy gap of about 2.1 eV that determines the observed red color of the investigated materials. In fact, our measurements deal with the surface effects. To prevent any infhrence of the surface perturbations, we have applied a ruby laser as a source of the stimulating light, because the photon energies are sufficiently smaller than the electronic energy gap. Therefore, the photoinduced fhrx is not absorbed by the sample bulk and polarization measurements can be additionally performed in this case. It would be interesting to investigate the occurrence of phase transitions connected with the glass and chemical structure of the PbO-BizO&azO3 glasses. The traditional dilatometry methods, sound velocity and piezooptical measurements, should be very helpful to explain a nature OE the observed temperature anomalies. EXPERIMENTAL

The lead-bismuth-gallium glasses were synthesized in platinum crucibles at temperatures ranging from 1323 to 1525 K. Melting time at the highest temperature was approximately 30 minutes. The glasses were mixed during melting. The glasses under investigation had the following content: PbO (25 mol%), BiOr.5 (50 mol%) and GaOr.5 (25 mol%) [5]. The specimens were formed as parallelepipeds with typical sizes of 5 mm x 5 mm x 15 mm and had a red-orange color. We carried out the measurements of the structure and optical parameters within a temperature range of 4.2 to 500 K, using a helium optical cryostat with a temperature variation rate changing from 0.1 to 0.2 Wmin and temperature stabilization with an accuracy

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of 0.03 K. The piezooptical constants were measured by the Senarmon method using a helium-neon laser (wavelength h = 0.6328 pm) in a longitudinal and transversal geometry [6]. A quartz crystal was used as an etalon in order to calibrate the measurements. The measurements were performed in more than 30 points of sample surfaces to prevent any influence of specimen nonuniformity. Changes of sample thickness were simultaneously taken into account during calculations of the induced birefiingence. Estimations of errors were obtained using Student tests, and resulting values were less than 2.5 x lo-l6 m’/N. Relative spontaneous deformation U was measured using a dilatometer DT-23. A high stability of high &quency generators allowed us to perform the measurements with a precision of about lOa. The measurements were carried out after 5 min of temperature stabilization, in more than 20 different sample points; the results were then averaged. An extrapolation of thermal expansion coefficients from the temperature region near 80 K was made, using a polynomial-exponent method to eliminate any contribution of thermal expansion. Experimental investigations of the photoinduced changes caused by the ruby laser irradiation (h = 694 nm) were performed in the present work. The photoinduced changes were caused by focused laser light with a photon flux of 10” to 102? photon/m2. The upper power limit is necessary to avoid overheating of the sample. The laser light intensity was varied using neutral density filters and was checked using a commercial fast-response joulemeter (Genetic, Inc., model ED-200). An apparatus for SHG was set up with an unfocused light beam from a single-mode picosecond YAG: Nd laser (W = 30 MW, h = 1.06 pm). The measurements were carried out for polarized light of the ruby laser. A separation between the SHG and pump light was achieved using a grating monochromator. The SHG intensity was measured using a FEU-79 photomultiplier. Measurements were carried out in a single-pulse regime with a pulse frequency repetition of 12 Hz. A quartz single crystal was used as an intensity etalon in plane of the optical axis. The sample was placed in a temperature-regulated cryostat, allowing a smooth variation of temperature from 4.2 to 300 K. The spontaneous polarization was measured by the Sawyer-Tower method (f = 10 kHz, E, < lo6 V/m) [6]. Elastic constant component C ,, was determined from longitudinal ultrasound velocities using the Papadakis echo-pulse method [7] at a fresuency of 12 MHz. The transversal elastic component Cd4 showed no essential change. Therefore, we will present a behavior of the Cl1 tensor component only. Corresponding values of the elastic constants were estimated using expressions of the pv2 type. RESULTS AND DISCUSSION A dependence of the elastic constant C, I (in arbitrary units) on temperature is depicted in Figure 1 for the temperature range of 4.2 to 150 K. One can clearly see two sharp maxima near 55 K. The asymmetry of the temperature behavior can be seen. Moreover, a few changes in the curve slope at temperatures between 50 and 60 K and in the vicinity of 62-67 K were observed during cooling. A temperature hysteresis of about 4 K was found during the heating-cooling processes. The temperature behavior was similar for all three directions cf the parallelepiped specimens, which suggests isotropic features of the observed anomaly. The hysteresis indicates a similarity of the presented phenomena to first-order phase transitions according to Landau’s classification.

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FIG. 1 Temperature hysteresis of elastic constant component Cl1 (in arbitrary units). The temperature behavior of the piezooptical constants xl1 is even more interesting (see Fig. 2). The temperature hysteresis in this case is practically two times greater than for the dilatometry measurements and is equal to about 10 K . Corresponding temperatures of the maxima during the cooling-heating process are equal to about 40 K and 50 K, respectively. Moreover, one can observe slope changes at temperatures of 60 K and 68 K that might indicate the possible occurrence of other peculiarities. A great contribution of the electronic subsystem to the nonlinear optical susceptibilities is therefore expected, in agreement with the temperature dependencies of the piezooptical constants. For a deeper understanding of the observed low-temperature behavior, it is necessary to take into account special features of the lead-bismuth-gallium glasses. Structural analysis CE the glasses and their infrared spectra have unambiguously shown that all cations participate in creation of a glass network by formation of Ga04 pyramids and Bi06 distorted octahedra. It is possible that coordination numbers of the cations in such complex compounds change with decreasing temperature, leading to the appearance of different structural rearrangements. Results obtained for the spontaneous deformation U are presented in Figure 3. We have observed no temperature hysteresis, contrary to the previous cases. Nevertheless, one can suggest a good correlation between temperature positions of the slope changes in the U temperature dependence with corresponding points for the elastic constant Cl 1. It is very interesting that the behavior of corresponding parameters shows no maximum with increasing temperature. The C!,I and U temperature dependences point at the ferroelastic nature of these slope changes. The nonlinear-optical phenomena, particularly SHG, described by polar third-rank tensors does not usually occur in amorphous media (including glasses), according to general

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1.0

FIG. 2 Temperature dependence of piezooptical constant component RII. symmetry considerations. Therefore, it would be very interesting to investigate the photoinduced SHG, which is caused by the light-induced noncentrosymmetry. It is well known that the SHG is a very effective tool to detect possible low-temperature phase transitions [8]. The measurements of the photoinduced SHG were performed for two difherent ruby laser light polarizations: perpendicular and parallel to a direction of the YAG-Nd light beam propagation. A temperature split of the photoinduced SHG was observed (see Fig. 4) . An observation of the polarization phenomena was very difficult in the case of the nitrogen laser, due to the influence of surface effects on the observed results, as reported previously [4]. The ruby laser light is not absorbed at all and the photoinduced intensity increases with increasing intensity of the ruby laser, as in the case of the nitrogen laser. It is well known that the temperature behavior of photoinduced SHG is fast of all determined by the influence of the phonons and the electron subsystem [9]. It is necessary to induce noncentrosymmetry in the electron charge distribution. The temperature behavior of the SHG is determined mainly by the phonon subsystem. Therefore, one can predict an essential role of an electron-phonon anharmonicity that usually leads to noncentrosymmetric charge densities. The very great value of the temperature split should be noticed. A major role in appearance of such a split is possibly played by the gallium ions and it is a result of essential interactions of the parallel as well as perpendicular light polarizations. The temperature dependencies of the SHG show additional slope changes, which, of course, reflect the rather complicated nature of this transition. An incident light interacts with the intramolecular, as well as the intermolecular, dipole assemblies, resulting in the appearance of the photostimulated polarization. It is well known

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Temperature

FIG. 3 behavior of relative spontaneous

deformation

U.

An incident light interacts with the intramolecular, as well as the intermolecular, dipole assemblies, resulting in the appearance of the photostimulated polarization. It is well known that the weak SHG signals occur in the glass fibers due to formation of a grating-like structure or a spatially periodic electric field that destroys the centrosymmetry of the bulk [ 10,111. The up-to-date investigations have reported on three main structural clusters of the Bi06, GaO+ and Pb04 types, the contributions of which dominate the optical hyperpolarizabilities [l]. The aforementioned structural fragments are responsible for the light emission of the higher harmonics, and the results should be averaged over the randomly distributed molecular dipole directions [ 121. Let us consider a process of the transformation of two incident photons (with wavevector k) into a single photon (with wavevector k'and double h-equency). An anisotropy of the electron as well as the quasi-phonon subsystems may be assumed to be a source of the temperature SHG dependencies. Therefore, one should introduce contributions of the electron and the quasi-phonon (vibrational) subsystems originating from different clusters into the output SHG intensity. The total output SHG intensity of the assembly of N clusters (labeled by subscript i) can be written in the form IsHG= 16~~1,*A*A,~ (I, + 12+ Ix + I,)/(32h4K2&&&

(1)

where I, is an incident ruby laser intensity, A denotes a cluster coherence length, Apt, is the photoinduced power, E denotes a dielectric constant and Ih (h = 1, 2, 3, and 4) are contributions from the electronic intramolecular, electronic intermolecular, vibronic

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FIG. 4 Temperature behavior of SHG (in arbitrary units) induced by ruby laser light for two light polarizations. intramolecular and vibronic intermolecular electric dipoles, respectively. The detailed expressions for the above multi-photon contributions can be obtained on the ground of the nonrelativistic quantum electrodynamics [ 131:

11= Ci=,Nl<(d*;e,h(e,e,)e*~~<

d*ie*ph(eoeo)e20><(d*ieph(eoe,)*e2,>


di e ph(eoe,)*e*20>exp(iAkZi)lav,

(2)

where Ak = 2k - k’ is a wavevector mismatch. The intramolecular electric dipole moment di has complex values. The polarization vectors of the photoinduced, pump and SHG beams are denoted by eph, e,, and el,, respectively, and they are complex quantities. The asterisk means usual complex conjugation. The notion “av.” denotes an average over all dipole directions and the sum is performed over all cluster positions described by position vectors Zi. Eq 2 describes an influence of the intramolecular electronic subsystem on the output SHG intensity that can explain appearance of the photoinduced SHG anisotropy. This contribution is nearly independent on temperature. The corresponding electronic intermolecular term can be expressed as follows : 11= &I”

Zj*=lN


1<

m*ii.e*ph(e,e,)e2,,,>eXp(iAk~jj,)lav,

(3)

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where j + j’ label different ionic clusters, rnp is the hyperpolarizability between j-th and j’-th clusters, and Zjj*is a vector connecting the mentioned clusters. The indirect influence CE temperature via the cluster rotations is more pronounced in this intermolecular contribution than in the case of the previous intramolecular one. The vibrational effects can be described by the following intramolecular term:

I3 =

&=INRN(Q,m)

I
e ph(e&,,)*e&

~me*hp(e,e,)*e2,>eXp(iAk~~)l~“,,

(4)

where the intramolecular vibronic electric dipole moment A,,, has complex values and the cluster position vectors are denoted by 2,. N(Qm) is the temperature dependent BoseEinstein distribution for the phonon mode with frequency R for the m-th structural fragment. Therefore, this term strongly contributes to the temperature behavior of the ISHo.A similar influence is given by the following intermolecular vibronic contribution.

(5) where 1 # 1’ label ditferent ionic clusters, rnip is a vibronic hyperpolarizability between j-th and j’-th clusters, and 211,is a vector connecting the mentioned clusters. The latter contribution to the SHG intensity is mainly responsible for appearance of the so-called “soft vibrational modes” (at least one zero fi-equency, fi = 0). The frozen mode leads to a longrange structural modulation that suppresses the “parasitic” influence of the random cluster orientations and increases simultaneously the anisotropy degree. Therefore, the noncentrosymmetry of the glass system is more pronounced that allows to explain the origin of the SHG intensity maximum in the case of heavy metal gallates. The increase of the nonlinear optical polarizability observed between 20 and 80 K can be explained by the aforementioned effects. To explain the differences in the temperature behaviors of the piezooptical constants and SHG, it is necessary to point out the essential influence of the electron-phonon interaction on the piezooptical constants. The latter is expressed by the following term. K = C {Xi=,” N(Q,i)l
(eoeo)*>Jrv. +

?$=I”Zj *=INN(C2,j ) N(i2.j’) I
(enem)*>lav.} ,

(6)

where Ui and Ujp are the relative intramolecular and intermolecular displacements, respectively, e, denotes the phonon polarization vector, and C is the temperature independent constant which can be calculated using the quantum electrodynamics approach. Comparing the expression given by eq 6, on one hand, and by the eqs l-5, on the other hand, one can easily understand the observed di&rence between the SHG and piezooptical constant temperature dependencies. This diffamnce is, therefore, caused by different physical mechanisms responsible for both quantities.

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Nevertheless, data obtained unambiguously show that the observed anomaly can result in behavior of the surface as well as bulk excited electron states. It is necessary to perform additional measurements using other methods in order to clarify the observed features. The traditional measurements include a study of the spontaneous polarization and deformation. The occurrence of the low-temperature peculiarities can thus be preliminarily detected by the photoinduced SHG. On one hand, the existence of a fenoelectric phase is excluded, because the spontaneous polarization is less than 2 pC!/m’. On the other hand, the small values cf the spontaneous deformation may indicate the possible appearance of a ferroelastic improper phase. The obtained data also show the essential role of the Ga5 chemical bonds in the formation of the nonlinear optical coefficients and corresponding temperature behaviors, because the experiments performed on the relative glasses (without gallium) show no essential changes of the same physical parameters with changing temperature. The presence of heavy cations leads to an increase of the refractive index and modifies the nonlinear optical properties. The latter offers us new possible applications of the gallium glasses in nonlinear optics and optoelectronics. However, the nature of the observed phenomena is not yet clear in full and requires further experimental as well as theoretical investigations.

CONCLUSIONS The low-temperature optical features (between 40 and 60 K) in the behavior of the photoinduced nonlinear optical, piezooptics and elastic properties were first time observed in the lead-bismuth-gallium glasses. Their exposure to polarized light usually leads to a temperature split of the second harmonic generation intensity that can serve as a preliminary tool to detect mainly structural changes. The occurrence of weak spontaneous deformation and the absence of spontaneous electric polarization show a similarity of the observed phenomena to the phase transitions of the first order. Additional arguments supporting such a point of view are supplied by the temperature hysteresis observed during cooling-heating cycles. The lead-bismuth-galium glasses seem to be promising materials for optoelectronics.

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