JOURNAL OF MATERIALS SCIENCE35 (2000 ) 215– 219

Photoinduced nonlinear optical phenomena

in Sb2Se3-BaCl2-SnCl2 glasses

J. WASYLAKUniversity of Mining & Metallurgy, Al. Mickiewicza 30, PL-30059 Krakow, Poland

A. MEFLEHLaboratoire de Physique Applique et Automatique, Universitaire de Perpignan,bld. Villeneuve 58, F-66860 Perpignan, France

I. V. KITYK, J. KASPERCZYKPhysics Institute, Pedagogical University, Al. Armii Krajowej 13/15,PL-42201 Czestochowa, PolandE-mail: i.kityk@wsp.czest.pl

Photoinduced nonlinear optical phenomena have been studied in amorphousSb2Te3-CaCl2-SnCl2 glasses using photoinduced optical second harmonic generation(SHG). The photoinduced SHG signal was measured for double frequency of a CO2 laser(λ = 5.3 µm) using a photoinducing CO laser (λ = 5.5 µm). We have found that the SHGsignal intensity increases with increase of the CO laser photoinducing exposure andachieves its maximum value after 1.5 h. Absolute values of the SHG signals were morethan one order in magnitude smaller comparing to a χ222 tensor component for the ZnSesingle crystals. The SHG signal strongly increases if temperature decreases from 39 to 16 K.Femtosecond probe-pump measurements indicate existence of the SHG maximum atpump-probe time delay of about 43 ps. It is suggested that Sb-Se tetrahedra play a key rolein the observed photoinduced nonlinear optical effects. Degree of noncentrosymmetry ofcorresponding bonds depends on time of the CO-laser illumination, temperature andmechanical stresses. Simultaneously, the investigated glasses can serve as promisingmaterials for a femtosecond IR quantum electronics. C© 2000 Kluwer Academic Publishers

1. IntroductionOxygen-free glasses were subject of increasing inter-est in last few years [1–3] because of their possibleuse in different branches of IR optoelectronics, holog-raphy and IR quantum electronics materials engineer-ing. Light transmission coefficients between 36% and57% have been observed for wavelengths from 0.65 to49 µm. A main purpose of present paper consists ininvestigations of the new synthesized glasses as ma-terials for nonlinear optical devices in IR spectral re-gion. An original IR photoinduced setup (see Fig. 1)have been proposed for the mentioned measurements.More details of the measurement setup as well the spec-imen preparation is presented in the Section 2. In theSection 3 are given experimental results for the pho-toinduced SHG versus the CO-laser photoinducing ex-posure, delaying time between the photoinducing andprobing beams, pressure and temperature.

2. Experimental detailsA glass formation region of the Sb2Se3-BaCl2-SnCl2system is shown in Fig. 2. The above mentioned regionis situated near a point of binary Sb2Se3-SnCl2 system.An upper and lower limit of Sb2Se3 concentration for

the bulk glass formation is 75 and 46 mol %, respec-tively. A third compound of BaCl2 enhances this glassformation tendency of the binary system. The binarysystem of Sb2Se3-BaCl2 cannot form itself a bulk glassbecause of larger field strengths of Ba2+ ions.

The Sb2Se3-SnCl2-BaCl2 glass was prepared using5 parts of NbSb powder, 5 parts of NSe beads and an-hydrous halides of BaCl2 and SnCl2. The raw materialswere weighed, mixed and placed into a silica glass tubewith a sealed end. The silica glass tube was then evac-uated to a vacuum of about 10−2–10−3 Pa and sealed.The tube was heated at about 650–670◦C for 5 h inan electric furnace. During melting the silica glass tubewas shaken to mix a glass melt. The glass melt wasquenched in water at temperature of 20–25◦C togetherwith the silica glass tube. The glass formation and thestructural investigations of the obtained glasses wereconducted by the X-ray diffraction method, includinga phase analysis, an analysis of the radial distributionfunction (RDF) and infrared absorption spectroscopy(IR method).

DTA measurements have given the following param-eters a verification temperatureTg= 238◦C; crystal-lization temperaturesTc1= 304◦C andTc2= 388◦C; amelting temperature of a crystalline phaseTm= 398◦C.

0022–2461 C© 2000 Kluwer Academic Publishers 215

Figure 1 An apparatus for experimental investigations of photoin-duced SHG.

Figure 2 A glass formation region of the Sb2Se3-SnCl2-BaCl2 system.

Figure 3 IR transparency spectra of the investigated glasses.

The IR spectra show a wide transparency band (Fig. 3).A typical radial distribution function (RDF) of a sam-ple glass shows two main peaks located at 0.282 nmand 0.403 nm. The position of the former peak can beattributed to bond lengths of Sb-Se and Sb-Cl atomicpairs, while the position of the latter peak can be at-tributed to interatomic distances within Sb-Sb, Se-Se,Sn-Sn, Ba-Ba, Sn-Cl and Cl-Cl pairs. Unfortunately, itis difficult to determine the atomic distances of eachpair and coordination numbers of the cations becauseof such complicated composition of the investigatedglass.

The glass formation region of Sb2Se3-SnCl2-BaCl2system has been obtained showing a better glassformation ability than other Sb2Se3-based glasses with

halogen univalent cations. Light transmittance forwavelengths between 0.65 and 50µm and other tech-nological properties of these glasses support a state-ment that they represent suitable materials for infraredoptic fibers. The halide-chalcogenide glasses on the ba-sis of Sb2Se3 are important from application point ofview.

Since structural rearrangements induce changesin localized-state distributions of the material, themeasurements of the transient SHG signals are of con-siderable importance to understand the photoinducedmetastable states in amorphous glasses.

Results of the optical transient second harmonic gen-eration (SHG) study have been obtained for IR-lightinduced non-centrosymmetry structural fragments inthe amorphous Sb2Se3-SnCl2-BaCl2 glasses. We havealso compared metastable states for systems of differentcontents and analyzed their high-resolved time depen-dencies.

Experimental investigations of the photoinducedSHG have been performed. A typical sample has a formof plate with thickness of 5 mm and surface area ofabout 1.7 cm2. As a source of the photoinduced changesa CO-laser (λ = 5.5 µm) was used with a photon fluxof 1018–1019 photon/cm2. An upper power restrictionis necessary to avoid sample overheating. Intensity ata specimen position was checked using a commercialfast-response joulemeter (Genetic, Inc, model ED-200).

Using a ZnSe splitter and proustite delaying cell, thelight beam is splitted in two beams of wavelength of10.6µm. Our apparatus (Fig. 1) allows us to changethe delay time between the pump and probe beam. It isalso possible to modify intensity of the light flux. Themeasurements have been carried out for polarized lightobtained by use of a KBr-NaCl polarizer. A maximalintensity of the output SHG signal was achieved forparallel directions of the probe and the input CO2-laserbeams.

A separation between the output SHG (λ = 5.3µm),photoinducing CO-laser (λ = 5.5µm) and pump beamwas achieved using an IR grating monochromator ofIRG-8.96-M type. The SHG intensity was detected us-ing a RIR-34 IR bolometer. The measurements werecarried out in a single-pulse regime (with a pulse repeti-tion frequency of 7 Hz). A pyrargiryt single-crystal cell(cut in a plane of optic axis) was applied as an inten-sity standard. The time-dependent SHG signals weremeasured using a high-time-resolved spectroanalyzerSA-165.

3. Results and discussionWe have measured dependences of the photoinducedSHG intensities versus the photoinducing CO-laser il-lumination time (Fig. 4). The SHG intensity increaseswith increase of the CO-laser illumination time andachieves its maximum after approximately 1.5 h forthe flux of 14× 1013 photon/cm2. In our opinion, sucha behaviour reflects occurrence of noncentrosymme-try metastable structural components due to the CO-laser induced photo-generation. The observed phenom-ena are caused by the photo-generation of electrons

216

Figure 4 Dependence of PISHG as a function of time of CO-laser photonflux at different photoinducing flux (in photon/cm2): ¤, 12× 1019; ♦,12× 1011; +, 14× 1013.

(and appropriate vibrations, i.e., quasi-phonon modes)from occupied states to unoccupied (conduction)states.

If the photoinducing CO-laser power increases moreand more, then the third harmonic generation (THG) be-comes more probable. Such phenomena are describedtheoretically by fourth rank tensors and they dominatefor a photon flux of about 11× 1016 photon/cm2 thatcan be caused by destruction of the lower excited statesby CO-laser light. A separate paper will be devoted tosuch investigations. The most typical examples of thedependences for the Sb2Se3-SnCl2-BaCl2 specimensare presented in Fig. 4 and other samples show a simi-lar behaviour. It should be pointed out that the obtaineddependencies are asymmetric.

Values of the SHG intensity are more than one or-der in magnitude smaller comparing to the ZnSe singlecrystals in the case of itsχ222 tensor components. Timeof the photoinduced SHG signal did not exceed 1.5 ps. Amaximal signal was achieved for a delay time betweenthe pump and probe beams of about 43 ps (Fig. 5). Thisreflects an essential role of the electronic subsystem inappearance of the photoinduced SHG.

An important aspect of the investigated phenomenaconsists in temperature dependencies of the photoin-duced SHG (Fig. 6). The investigations of all sam-ples have shown an enhancement of the SHG signalwith decreasing temperature below 38 K. Accordingto this behavior, one can suggest occurrence of thelow-temperature structural rearrangements of the Sb-Sepyramidal bridges caused by low-temperature thermo-activation processes. The results have unambiguouslyconfirmed essential role of the Sb-Se vibrations in for-mation of the metastable non-centrosymmetry statesbecause experiments performed for samples withoutthe Sb-Se groups have shown a vanishing effect on theoutput SHG signal. Therefore, one can expect that thesebonds mainly contribute to the observed SHG. The es-

Figure 5 Dependence of output SHG as a function of time delay betweenthe probe and pump beam.

Figure 6 Temperature dependence of the photoinduced SHG at delayingtime of about 50 ps.

sential role of the electron-quasiphonon interactions inthe nonlinear optical effects will be subject of a sepa-rate work. Special theoretical as well as experimentalinvestigations are required for this purpose.

Dependencies of the photoinduced SHG intensitiesare presented in Fig. 7 for the above mentioned glasssamples as functions of duration time of a mechanicalstress. It is clearly seen that a maximal value of theSHG intensity is obtained for glasses treated by the ex-ternal mechanical stresses of 5 N/cm2 during 20 min(and applied in parallel to a surface plane). The up-per limit is avoided by a mechanical reliability of thespecimen. The PISHG signal saturates after 14 min andthen slowly decreases.

A correlation between the time of mechanical treat-ment and the output SHG signal has been unambigu-ously found. A saturation of the SHG signal wasachieved after 10 min of such treatment. One can expect

217

Figure 7 Dependence of the photoinduced SHG on time after a mechan-ical treatment for pressure of 5 N/cm2.

Figure 8 Time-resolved dependence of the photoinduced SHG signalaveraged within a low-temperature range.

that the increasing mechanical treatment stimulates ap-pearance of additional non-centrosymmetry due to dif-ferent elastic constants of particular chemical bonds inthe glass system. In the result, we have observed theSHG intensity that is caused by the mechanically in-duced local structural noncetrosymmetry.

The appearance of the PISHG saturation is mainlyconnected with a saturation of corresponding photo-orientational processes. The similar behavior has beenobserved under influence of the mechanical uniaxialstresses. This indicates important role of the chemicalbonds which are sensitive to influence of these mechan-ical fields.

Time-resolved dependencies of the SHG maxima at-tenuation are presented in Fig. 8 for these samples. Wehave observed no slopes within a time region between4.1 and 6.5 ps that indicates presence of, at least, threedifferent physical mechanisms, contributing to the out-put PISHG. It is expected that an essential role belongs

to the electron decay of exciting states below 4.1 ps,while the quasi-phonons interacting with the electronicsubsystem take part for times above 4.1 ps.

Our investigations have unambiguously shown theappearance of the photoinduced SHG signals describedby the third-rank polar tensors. Several mechanismscan serve to explain this phenomenon. The first oneconsists in photo-orientational (or piezo-orientational)deformations resulting from photo-chemical (piezo-induced) reactions that leads to appearance of thenon- centrosymmetric structural components (and cor-responding polar tensors). The second possibility maybe an electric quadrupolar interaction which is allowedin the centrosymmetric systems but it is usually weak.These two mechanisms are able to produce (in a co-herent way) a light beam of double frequency beingcollinear with the pump laser beam.

However, an incoherent localized SHG random irra-diation (propagating in all directions and caused mainlyby a second-order Rayleigh scattering hyperpolariz-ability) can not be in full excluded. To avoid such influ-ence of the hyper-Raman scattering, we have performedadditional investigations of light scatterring for wave-lengths from 5500 to 10600 nm. We have detected twohyper-Raman maxima at 3300 and 7600 nm which in-tensities were, at least, 7 times weaker than the orienta-tional SHG intensity. It was also found that the intensityof this light was two orders in magnitude weaker thanthe SHG intensity.

4. ConclusionsWe have revealed, for the first time, the essential influ-ence of IR CO-laser light on the nonlinear optical sus-ceptibilities in Sb2Te3-CaCl2-SnCl2 glasses. We havefound that the SHG intensity increases with increase ofthe CO-laser exposure time and achieves its maximumafter 1.5 h for a light flux of 14× 1013 photon/cm2

and a pump-probe delay time of about 43 ps. The ab-solute SHG values were more than one order in magni-tude smaller comparing with the ZnS single crystals inthe case of itsχ222 (λ = 10.6µm) tensor components.The relaxation time of the photoinduced SHG signaldid not exceed 1.5 ps.

Special investigations have shown that one canobserve the enhancement of the SHG below 38 K.According to this behaviour, one can say about oc-currence of the low-temperature structural rearrange-ments of the Sb-Se pyramidal bridges caused by low-temperature thermo-activation processes. The essentialrole of the Sb-Se vibrations can be observed in for-mation of the metastable non-centrosymmetry states.Therefore, it is expected that these bonds give a maincontribution to the observed PISHG. One can also ex-pect an essential role of the electron-phonon contribu-tion to the observed nonlinear optical effects.

We have revealed that the maximum value of thephoto-induced SHG signal can be obtained in the caseof the investigated glasses if one treats them during20 min by external mechanical stresses (applied in par-allel to a surface plane) of 5 N/cm2. The upper limit isavoided by the mechanical reliability of the specimen.The PISHG signal saturation should be underlined.

218

References1. Z . L I N G , H. L I N andZ. C H E N G S H A N, in Proc. of 9th Int.

Symposium on Non-Oxide Glasses, Hangzhou, 1994, p. 44.2. Y . M E R E S S E, G. F O N T E N A U andI . L U C A S, in Proc. of 9th

Int. Symposium on Non-Oxide Glasses, Hangzhou, 1994, p. 395.3. E. R A B I N E L , J. Non-Cryst. Solids57 (1983) 49.4. X . Z H A O, L . X U , H. Y I N andS. S A K K A , ibid. 167 (1994)

70.

5. I . V . K I T Y K , J. Phys.: Cond. Matter6 (1994) 4119.6. I . V . K I T Y K , E. G O L I S, J. F I L I P E C K I , J. W A S Y L A K and

V . M . Z A C H A R K O, J. Mater. Sci. Lett.14 (1995) 1292.

Received 6 October 1997and accepted 21 June 1999

219