ISSN 0012�5008, Doklady Chemistry, 2009, Vol. 428, Part 2, pp. 252–254. © Pleiades Publishing, Ltd., 2009.Original Russian Text © N.V. Sidorov, A.A. Yanichev, P.G. Chufyrev, B.N. Mavrin, M.N. Palatnikov, V.T. Kalinnikov, 2009, published in Doklady Akademii Nauk, 2009, Vol. 428,No. 4, pp. 492–495.

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Photoinduced change in the refractive index (pho�torefractive effect) is generated in illuminated regionsof a ferroelectric crystal as a result of spatial transfer ofelectrons followed by their capture into low�lyingenergy levels to form a nonequilibrium space chargefield [1–3]. In the place where a laser beam passes andin the adjacent region, there is a change in refractiveindex and distortion of the crystal structure, persistingfor some time after the laser beam has been removed.Despite of numerous publications (reviewed in [1, 3]),specific features of this distortion have been inade�quately studied. Of special interest are such studies fora single crystal of lithium niobate LiNbO3, which is animportant nonlinear optical material. Its propertiescan be modified by changing its stoichiometry or dop�ing. Raman spectroscopy is an efficient tool for simul�taneously studying the photorefractive effect, struc�tural distortions, and defects in the crystal [1].

In treatment of the results of some physical impacton a solid, traditional materials science is usuallybased on consideration of the composition–struc�ture–property triad. The actual structure formed by theend of the process is realized as a micro� and nanostruc�ture in the solid state, which can be especially pro�nounced under strongly nonequilibrium conditions.Under these conditions, the composition–fractal(dynamic) structure–property should be considered.

Laser illumination of a photorefractive lithiumniobate single crystal, which has a large number ofdefects, leads to creation of strongly nonequilibriumconditions [1, 3], which makes it possible to describethe results using the self�organization ability of thesystem [4] since there is a prerequisite for self�organi�zation in the form of an energy flow from an externalsource delivered into the system and dissipated by the

latter. Due to this flow, the nonequilibrium systembecomes active, i.e., acquires the ability to autono�mously form micro and nanostructures. The type anddimension of micro� and nanostructures basicallydetermine the physical characteristics of single crystalsof photorefractive optical materials. In particular, laserlight can induce the appearance of a 3D sublattice oflocal micro� and nanoheterogeneities with modifiedphysical parameters in the illuminated single�crystalregion. In this sublattice, the structure is more disor�dered than the single�crystal structure in the absenceof the photorefractive effect. Formation of a similarinduced sublattice should lead to broadening of bandsin Raman spectra. The sublattice of micro� and nano�structures should be most pronounced when a crystalis exposed to visible laser light and should be almostabsent on exposure to IR laser light since excitation ofphotoelectrons drifting across the spontaneous polar�ization field and trapped into deep traps in the bandgap to produce nonequilibrium space charge field and,hence, the photorefractive effect are most pronouncedat wavelengths shorter than 500 nm and almost absentat wavelengths longer than 800 nm [1]. Therefore, thebands in the Raman spectrum of photorefractive singlecrystals excited by visible light should be broader thatthose in the spectra excited by IR radiation. In thiscontext, it is of interest to compare line parameters(frequency, width) in the Raman spectra of photore�fractive crystals of different composition excited byvisible and IR light. Such comparative study has notbeen performed so far.

In this paper, we report the results of studying basicparameters of some lines in the Raman spectra of pho�torefractive crystals of lithium niobate LiNbO3 of sto�ichiometric (R = Li/Nb = 1) and congruent R =0.946) compositions, as well as of the congruent com�position doped with non�photorefractive1 Y3+ cations

1 Photorefractive cations (cations with variable valence) changetheir charge in the illuminated crystal and enhance the photore�fractive effect, whereas non�photorefractive cations in the crys�tal do not change their charge in response to illumination.

CHEMICALTECHNOLOGY

Laser�Induced Sublattice of the Micro� and Nanostructuresin the Photorefractive Singe Crystals of Lithium Niobate

N. V. Sidorova, A. A. Yanicheva, P. G. Chufyreva, B. N. Mavrinb, M. N. Palatnikova, and Academician V. T. Kalinnikova

Received April 2, 2009

DOI: 10.1134/S0012500809100061

a Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Research Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, 184209 Russia

b Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow oblast, 142190 Russia

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LASER�INDUCED SUBLATTICE OF THE MICRO� AND NANOSTRUCTURES 253

when the spectra are excited by visible (λ0 = 514.5 nm)and IR (λ0 = 1064 nm) light. The Y(ZX)Y scatteringgeometry was used, which affords the strongest photo�refractive effect in the crystals under consideration [1].

EXPERIMENTAL

Nominally pure and doped single crystals of con�gruent composition were grown from a congruent meltby the Czochralski method in air on a Kristall�2 setup.Stoichiometric single crystals were grown from a meltcontaining 58.6 mol % Li2O. The procedures of pre�paring initial mixtures and growing single crystals oflithium niobate of different composition weredescribed in detail in [5]. For recording spectra, froma single crystal, specimens as a parallelepiped 5 × 6 ×

7 mm in size were cut so that its edges coincided withthe crystallographic axes. The Raman spectra in thevisible region were excited by an ILA�120 argon laser(λ0 = 514.5 nm, P ≈ 200 mW) and were recorded on aRamanor U�1000 spectrophotometer. In the IRregion, spectra were recorded on an RFS�100/S FTRaman spectrophotometer (λ0 = 1064 nm, P ≈

50 mW). All measurements were taken at room tem�perature. In photorefractive crystals, laser irradiationcan induce time�dependent changes [1], namely, dur�ing exposure of a specimen to laser light, the intensi�ties of phonons forbidden by selection rules for givenscattering geometry increase with time. Therefore,spectra were recorded about 1 h after the onset of laserirradiation when these changes almost vanished. Theerrors of measuring the line frequency and width were±1.0 and ±5 cm–1, respectively.

RESULTS AND DISCUSSION

The figure shows the photograph of the passage ofa laser beam with a wavelength of 514.5 nm in a singlecrystal of stoichiometric lithium niobate. It is clearlyseen that radiation is scattered on the 3D sublattice ofmicro� and nanostructures with altered physicalparameters in the form of local refractive index heter�ogeneities generated in the crystal due to the photore�fractive effect. Induced microstructures are absentwhen the crystal is exposed to IR laser radiation (λ0 =1064 nm) since the photorefractive effect is almostlacking under these conditions. The largest micro�structures are about 0.08 mm is size.

Selected parameters of the lines corresponding tovibrations of NbO6 octahedra in the Raman spectra ofsingle crystals of lithium niobate of different composi�tion are presented in the table. The spectra stronglydepend on the single crystal composition and excitingline frequency. It follows from the table that, in thespectra excited by laser light at both λ0 = 514.5 nm andλ0 = 1064 nm, the frequency of the combination lineat 578 cm–1 corresponding to fundamental vibrations

of the E(TO) symmetry Raman�allowed for the scat�tering geometry Y(ZX)Y [1] remains constant. In theseries of single crystals of stoichiometric, congruent,and doped congruent (doped with 0.46 wt % Y) lith�ium niobate, the width of this line slightly increases inthe spectra excited with both visible and IR laser light.This is likely due to an increase in the degree of disor�der of structural units of the cationic sublattice anddistortion of oxygen octahedra NbO6 in going from themost ordered stoichiometric crystal to the most disor�dered congruent crystal doped with Y3+ [1]. There isonly insignificant difference in the width of the line at578 cm–1 for each crystal in the spectra excited by laserlight at λ0 = 514.5 and 1064 nm.

The width of the line at 632 cm–1 corresponding toA1(ТО) fundamental vibrations of oxygen octahedraexhibits the strongest dependence on the compositionof a crystal in the spectra excited with visible light(table). The line width significantly increases in theabove series of crystals. This line is forbidden by selec�tion rules for the Y(ZX)Y scattering geometry but isobserved due to the manifestation of the photorefrac�tive effect [1]. The line at 632 cm–1 is usually used asanalytical for determining the magnitude of the pho�torefractive effect in a lithium niobate single crystal. Itis worth noting that the line at 632 cm–1 experiencesthe strongest dispersion dependence of frequency ascompared with the other bands of the vibrational spec�trum of lithium niobate [1, 6]. In the above series ofcrystals, the width of the line at 632 cm–1 increasesmuch less significantly in the spectra excited by IRlaser radiation than in the spectra excited by visiblelaser light (table) since the photorefractive effect in theformer case is considerably weaker.

The differences in the behavior of the lines withfrequencies of 614–632 cm–1 in the spectra excited byvisible and IR laser radiation (λ0 = 514.5 and 1064 nm,

Photograph of the illuminated region near the laser beamin a photorefractive single crystal of stoichiometric lithiumniobate. The polar axis Z and the laser beam are directedperpendicular to the plane of the paper.

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SIDOROV et al.

respectively) (table) are of special interest. These linewidths are considerably larger in the spectra excited atλ0 = 514.5 nm than in the spectra excited at λ0 =1064 nm. This is typical of all crystals under consider�ation expect stoichiometric crystals where the differ�ence is insignificant, which is likely due to a highdegree of order of the cationic sublattice of this crystal.

In our opinion, the observed differences in linewidths in the spectra excited by visible and IR light canbe explained as follows. Due to the photorefractiveeffect, exposure of a crystal to visible laser light gener�ates, in the illuminated region of the crystal, an addi�tional 3D sublattice of micro�and nanostructures withmodified physical parameters in the form of heteroge�neities of the refractive index, permittivity, conductiv�ity, and other parameters differing from the corre�sponding parameters of a lithium niobate single crystalin the absence of the photorefractive effect. On thissublattice, photorefractive light scattering occurs,which is a special case of holographic effects [4]. Theformation of the 3D sublattice of micro� and nano�structures with modified structural and physicalparameters leads to additional disorder in the crystalstructure and to additional broadening (in addition tothe broadening caused by disorder in the arrangementof structural units of the basic crystal lattice) of thelines in the visible Raman spectrum. A similar sublat�tice of microstructures is almost lacking when thecrystal is illuminated by IR laser light since the photo�refractive effect is weak, if at all, in this case. It is quiteevident that additional structure disordering inducedby the photorefractive effect and related to the forma�tion of the 3D sublattice of micro� and nanostructureswith altered physical parameters will be much morepronounced in more disordered medium with asuperequilibrium amount of charged structuraldefects.

Formation of the 3D sublattice of micro� andnanostructures with altered physical parameters in aferroelectric crystal illuminated with laser light andconcomitant broadening of lines in the Raman spectraexcited by visible laser radiation was observed by us forthe first time and has not been described so far.

ACKNOWLEDGMENTS

This work was supported by the Russian ScienceSupport Foundation and the Council for Grants of thePresident of the Russian Federation for Support ofLeading Scientific Schools (grant no. NSh�4383.2006.03)

REFERENCES

1. Sidorov, N.V., Volk, T.N., Mavrin, B.N., and Kalinni�kov, V.T., Niobat litiya: defekty, fotorefraktsiya, koleba�tel’nyi spektr, polyaritony (Lithium Niobate: Defects,Photorefraction, Vibrational Spectra, Polaritons),Moscow: Nauka, 2003.

2. Kalinnikov, V.T., Palatnikov, M.N., and Sidorov, N.V.,Niobat i tantalat litiya. Fundamental’nye aspekty tekh�nologii (Lithium Niobate and Tantalate. Basic Aspectsof Technology), Apatity: Izd. KNTs RAN, 2005.

3. Maksimenko, V.A., Syui, A.V., and Karpets, Yu.M.,Fotorefraktivnyi effekt i fotorefraktivnoe rasseyanie svetav kristallakh niobata litiya (Photorefractive Effect andPhotorefractive Light Scattering in Lithium NiobateCrystals), Moscow: Fizmatlit, 2007.

4. Sidorov, N.V., Palatnikov, M.N., and Kalinnikov, V.T.,in Materialy mezhdunarodnoi nauchnoi konferentsii.Optika kristallov i nanostruktur. 12–15 noyabrya 2008(Proceedings of International Conference “Optics ofCrystals and Nanostructures”), Khabarovsk, 2008,pp. 62–66.

5. Biryukova, I.V., Extended Abstract of Cand. Sci. (Chem.)Dissertation, Apatity: IKhTREMS KNTs RAN, 2005.

6. Yang, X., Lan, G., Li, B., and Wang, H., Phys. Stat.Solidi B, 1987, vol. 141, pp. 287–300.

Table 1. Selected parameters of some lines of the Raman spectrum of single crystals of lithium niobate of different compo�sition in the region of vibrations of oxygen octahedra in the spectra excited by visible and IR laser light

LiNbO3 compositionFrequency, cm–1

at λ0 = 514.5 nm at λ0 = 1064 nm

Stoichiometric 578 (17) 632 (27) 578 (18) 632 (22)

Congruent 579 (22) 614 (79) 578 (25) 632 (37)

Congruent doped with 0.46 wt % Y 578 (27) 621 (90) 578 (30) 631 (38)

Note: Line widths (cm–1) are parenthesized.