Photoinduced nonlinear optical phenomena in PbO-BiO1.5-GaO1.5 glass

J O U R N A L OF M A T E R I A L S S C I E N C E L E T T E R S 14 (1995) 1 2 9 2 - 1 2 9 3

Photoinduced nonlinear optical phenomena in PbO-BiOl.s-Ga01.5 glass I. V. KITYK, E. GOLIS, J. FILIPECKI

Pedagogical University, Institute of Physics, AI. Armii Krajowej 13/15 42 201 Czestochowa, Poland J. W A S Y L A K

University of Mining and Metallurgy, AI. Mickiewicza 30, 30 059 Cracow, Poland V. M. Z A C H A R K O

Lviv University, Lviv, Dragomanova 50, UA-290005, Ukraine

In the last few years increasing interest has been shown in lead-bismuth glasses due to the possibility of their use in different branches of optoelectronics. Investigation of new compositions of glasses are intended to obtain the greatest possible light transmittance in the infrared region. Due to their high values of nonlinear refractive index these glasses are possible candidates for nonlinear optics [1-3]. This letter reports an investigation of nonlinear optical properties describable by third-rank polar tensors. According to general symmetry considerations, nonlinear-optics phenomena describable by polar third-rank tensors cannot occur in amorphous media (including glasses). But in previous work devoted to nonlinear optical phenomena in high-T~ superconducting ceramics [4], photopolymers [5] and centrosymmetric crystalline media [6], the possibility of the occurrence of the mentioned phenomena, which are stimulated by external light or are photoinduced, was unambiguously shown. We investigate photoinduced second-harmonic generation (SHG), caused by light induced structural non-centrosymmetry components. SHG methods are very sensitive to possible phase transitions. In the present work experimental investigations of the photoinduced changes caused by nitrogen laser (it=337nm) UV irradiation were made. Photoinduced changes were caused by focused nitrogen laser UV-light with a photon flux between 1017 and 1023 photons/m2. The upper power restriction is necessary to avoid sample heating. The intensity of the nitrogen laser was varied using neutral density filters. The laser intensity at the sample position was checked using a commercial fast-response joulemeter (Genetic, Inc, model ED-200). An apparatus for SHG was set up with an unfocused beam from a single-mode picosecond YAG:Nd laser (power = 30 MW, it = 1.06 ].tin). Measurements were performed for both polarized and unpolarized light. It was revealed that the practical intensity of SHG signals did not depend on light polarization. Separation between the SHG and the pump light was achieved using a grating monochromator. SHG intensity was measured using a FEU-79 photomultiplier. The measurements were carried out 1292

in the single-pulse regime, with a pulse repetition frequency of 12 Hz. As an intensity standard a quartz single-crystal cell was used in the plane of the optics axis. The sample was mounted in a temperatureregulated cryostat for smooth variations of temperature in the range 4.2-300 K. Glasses in the system PbO-Bi203-Ga203 were synthesized in platinum crucibles at temperatures ranging from 1323 to 1525 K. In Fig. 1 are presented the dependencies of SHG intensity as a function of the photon fluxes of the nitrogen laser. One can clearly see that with an increase of a nitrogen laser photon flux the SHG intensity increases and reaches a maximum at fluxes of 24 × 1019 photons/m2. In our opinion this reflects the occurrence of acentrical structural components due to laser-induced photogeneration. The observed phenomena are caused by photogeneration of electrons from the occupied states into the unoccupied conducting states. Due to the change of excited dipole matrix elements, SHG occurs [6]. Simultaneously with the increase of intensity the processes called cubic susceptibilities occur, which dominate at the higher photon fluxes. Therefore there exist processes which are described by four rank tensors. A photon fluxes higher than 24 x 1019 photordm2 the acentrical dipole matrix elements are destroyed by the intensity of UV-light which leads to the processes

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rence of low-temperature phase transitions which can be detected by UV-photoinduced SHG. The obtained data show an essential role for the Ga-O chemical bonds in the formation of the nonlinear-optical coefficients, because the experiments performed on glasses without gallia show an absence of any essential changes with temperature. Therefore one can say that these bonds cause lowtemperature phase transitions. The presence of heavy cations leads also to an increase of refractive index and modifies the nonlinear optical properties, thus offering the possibility of applying these glasses in nonlinear optics and optoelectronics. The nature of the observed phenomena is not yet clear fully and requires further special experimental and theoretical investigations.

References of energy migration. The asymmetrical behaviour of the obtained dependence is very interesting, and suggests an essential role for the structural reorientation. Another very important aspect of the investigated phenomena is the temperature dependencies of the possible low-temperature phase transitions. These investigations have shown that with a decrease of temperature the SHG maximum appears at temperatures around 25 K. According to the behaviour of the obtained dependencies one can specify the occur-

1.

M. J A N E W I C Z ,

2. 3. 4. 5.

Proc. SPIE 179 (1992) 8. g. M. V O G E L Phys. Chem. Glasses 32 (1991) 231. N. J. K R E I D L J. Non-Ct~Fst. Solids 123 (1990) 377. I. V. K I T Y K J. Phys.: Condensed Matter 6 (1994) 4119. R. [. M E R V 1 N S K I I and I. v . K I T Y K Ukr. Polymer J. 3

6.

K. K O P C Z Y S K I

and Z. M I E R C Z Y K

(1994) (in press). M. I. K O L I N K O , A. S. K R O T C H U K and I. V. K I T Y K Phys. Status Solidi 180B (1992) K87.

Received 22 December 1994 and accepted 1 May 1995

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