Optical Materials 33 (2011) 1737–1740

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Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Far-infrared dispersion of complex dielectric constant in the ferroelectricnear-stoichiometric LiNbO3:Fe

Liang Wu a, Furi Ling a,⇑, Xiaoguang Tian a, Haitao Zhao a, Jinsong Liu a, Jianquan Yao a,b

a Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Chinab College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 April 2011Received in revised form 31 May 2011Accepted 3 June 2011Available online 23 July 2011

Keywords:Near-SLN:FeTHzPhotorefractionDomain-reversal

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.06.006

⇑ Corresponding author.E-mail address: lingfuri@163.com (F. Ling).

The dielectric properties of near-stoichiometric LiNbO3:Fe single crystal have been investigated by usinga terahertz time domain spectroscopy (THz–TDS) in a frequency range of 0.7–1.6 THz at room tempera-ture. When coupled with an applied external optical field, an obvious photorefractive effect wasobserved, resulting in the modulation of the complex dielectric constant for near-SLN:Fe. The variationof refractive index |Dn| has a linear relationship on scale with the applied light intensity accompaniedwith a steplike decrease. These findings were attributed to the internal space charge field of photorefrac-tion and the light-induced domain reversal in the crystal.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ferroelectric lithium niobate (LiNbO3, abbreviated as LN) crystalis one of the most widely used materials because of its remarkableelectro-optical, acoustooptical, ferroelectric and nonlinear opticalproperties [1]. LN is extensively exploited in various aspects, suchas holographic volume storage, optical image and signal process-ing, phase conjugation, real-time interferometry, beam deflection,and novelty filters. It is also a typical polar material that exhibitsa large pyroelectric effect. Kitamura et al. [2] reported the largepyroelectric effect in LN:Fe induced by a high-power short pulselaser. Periodically poled LiNbO3 (PPLN) has been widely investi-gated as an important nonlinear optical waveguide [3–8]. Kah-mann et al. [9] proposed a light-induced poling inverse methodin 1994, which has been developed in resent years to a promisingmethod for domain engineering. Wang et al. [10] reported that thecoercive field of different doped LN crystals could be reduced in theilluminated area, which is important for the light pattern to betransferred into a domain pattern with a homogeneous electricfield.

Photorefractive properties of LiNbO3 crystals have beenoptimized by doping with transition-metal impurities, especiallyiron in the past. A lot of work has been carried out to investigatethe transport processes in the crystal at the usual cw laser inten-sities (<100 mW/cm2), which are rather well understood now[1,11–14]. However, additional effects could also be observed in

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the crystal at high light intensities, which cannot be describedby the conventional model. For practical applications, it is neces-sary for the permittivity to be modulated over as wide a range aspossible around room temperature. In this letter, we report theTHz spectroscopy of near-stoichiometric LN:Fe single crystal, aswell as the variation of its dielectric properties under an externaloptical field. A light-induced domain reversal in the crystal wasalso observed in the measurement.

2. Experiment

The starting materials used for crystal growth were Li2CO3 (4 Npurity), Nb2O5 (4 N purity), K2CO3 (spectrum purity), and Fe2O3

(spectrum purity). The Fe-doped near-SLN crystals were grownby Czochralski and TSSG (top-seeded-solution-growth) techniquesfrom a melt of congruent composition (cLi = 48.6 mol%) containing6 wt.% K2O, Fe2O3 was added to the melt at Fe concentrations of0.1 wt.%. Fe-doped near-SLN crystals were grown along Z-axis withCLN seeds in Pt crucibles with the size of u73 � 22 mm2. The crys-tals were grown under the optimum conditions: the temperaturegradient above melt was 25 �C mm�1, pull speed was 0.2 mm h�1,and seed rotation rate was 15 rpm. The ratio of Li to Nb, whichwas estimated from the Curie temperature measurement, washigher than 49.8/50.2. The crystal was cut and polished to form aplate with polar faces perpendicular to the Z axis and with dimen-sions of 10 � 10 � 0.125 mm3.

We use a terahertz time domain spectroscopy (THz–TDS) [15]to measure the transmission spectra and dielectric constant ofthe samples at room temperature, as shown in Fig. 1. In the THz–

M1

M2

M5

M3

6M4M

HWP1

HWP2

P2

P1

fs laser

Delay line

L1 L2sample

532nm laser

Fig. 1. The installation diagram of TDS. A green laser was obliquely incident uponthe surface of the sample at an angle of 45� with the polar axis.

1738 L. Wu et al. / Optical Materials 33 (2011) 1737–1740

TDS system, a model-locked Ti:sapphire femtosecond laser beam(center wavelength of 800 nm, repetition 80 MHz) was dividedinto two beams via a beam splitter. The pump beam was focusedon a GaAs photoconductive antenna in a 20 V DC bias for the gen-eration of THz waves, and the detecting beam was focused into aGaAs crystal for the detection of the THz wave by electro-opticsampling (EO sampling). Two polyethylene lenses were used tocollimate and focus the emitted THz radiation onto the sample.The transmitted THz wave was collected and focused on the GaAscrystal with two polyethylene lenses. The detectable spectral rangewas from 0.7 to 1.6 THz (23–53 cm�1) with a sensitivity of S/N1000 in the electric field amplitude. The time resolution for theTHz measurement was 66.6 fs, and the spot size was 5 mm diam-eter. The polarization of the THz beam was perpendicular to the

Fig. 2. (a) Time domain transmission waveform of near-SLN:Fe crystal and (b) its time shtime shift (relative to the curve belongs to zero intensity) in the function of the applied

optical axis of the crystal. An all-solid-state green laser (centerwavelength of 532 nm) was employed in the experiment to pro-vide an external optical field. The light was obliquely incident uponthe surface of the samples at an angle of 45� with the polar axis,and the spot size was 8 mm diameter. To make sure that the sam-ple restored to its previous dynamic behavior, we performed athermal process after each measurement. The sample was placedin a furnace at 220 �C for 3 h, and no external field was appliedto it in this recovery process.

3. Results and discussion

The time domain waveforms were obtained by experiment, asshown in Fig. 2. The time lag between reference (air) and the sam-ple is around 2.5 ps, as visible in Fig. 2a. The second and thirdwaveforms at 7.8 ps and 13 ps were supposed to be introducedby the multiple internal reflections of the sample. By applying anexternal optical field, a time shift could be observed in the time do-main. For example, the transmission waveform shifted about0.05 ps when the light intensity was 200 mW/cm2, compared tothat without light excitation, as shown in Fig. 2b.

Through a Fourier transform, we got the transmission spectraand the intrinsic phase shift of the samples. Fig. 3 shows the trans-mission spectra of near-SLN:Fe crystal under different externaloptical fields at room temperature. The oscillation in the low fre-quency was supposed to be the influence of noises. A narrowabsorption line exists at 1.1 THz, which is caused by the absorptionof vapor. The complex refractive index N�f) and permittivity e�(f)were calculated from the THz transmission spectra. Fig. 4a showsthe frequency dependence of the real part of dielectric constant

ift under different external optical fields at room temperature. The inset shows theintensity.

Fig. 4. Frequency dependence of (a) real part and (b) imaginary part of complexdielectric constant of near-SLN:Fe crystal with different external optical fields atroom temperature (the values belong to ordinary THz polarization).

Fig. 5. Frequency dependence of absorption coefficient of near-SLN:Fe crystal withdifferent external optical fields at room temperature.

Fig. 3. Transmission spectra of near-SLN:Fe crystal under different external opticalfields at room temperature.

L. Wu et al. / Optical Materials 33 (2011) 1737–1740 1739

e0(f) and its variation under different external optical fields of near-SLN:Fe (The oscillation in the low frequency was supposed to bethe influence of noises and the absorption of vapor). In the fre-quency range of 0.7–1.6 THz, the dielectric constant of near-SLN:Feis about 42–46. The measured permittivity was found to be tunableby up to 3% via the application of an external optical field. Whenthe light intensity is lower than 120 mW/cm2, the dielectric con-stant nonlinearly decreases with increasing the power of green la-ser. When the light intensity is higher than 120 mW/cm2, the

dielectric constant increases suddenly and then decreases againwith increasing the power of light.

In contrast, Fig. 4b demonstrates that the imaginary part ofdielectric constant do not show obvious difference, indicating thatthe dielectric loss do not change appreciably with external opticalfield. This may be attributed to the fact that two loss mechanismsare responsible for the dielectric loss under external field: a con-duction loss tan dR and an intrinsic loss tan dC [16,17]. The intrinsicloss factor was reduced when applying an external optical field,while the conduction loss increased with the optical field strength.Since the influence of the optical field on the loss factor of the spec-imens is minor, as shown in Fig. 4b, it indicates that the effect ofthe intrinsic loss counteracts the effect of conduction effect.

Fig. 5 shows the frequency dependence of absorption coefficientof near-SLN:Fe crystal with different external optical fields at roomtemperature. In the frequency range of 0.7–1.6 THz, the absorptioncoefficient of near-SLN:Fe is about 60–110, which is higher thanthat of undoped CLN [18]. The absorption coefficient increasesobviously with the frequency increasing, while it do not changeappreciably with the applied light intensity increasing, which isin agreement with the analysis above.

In order to illustrate the microscopic process in near-SLN:Feclearly, it is worthwhile to study the variation of its refractive index.Fig. 6 shows the modulation of refractive index as a function of theapplied light intensity and its fit. When the light intensity is lessthan 120 mW/cm2, the variation of refractive index |Dn| is foundlinearly proportional to the intensity of applied light. The resultscould be described by using a relationship proposed by Chen [20],

Dn ¼ 12ðn3

0c13 � n3ec33ÞE3 / I ð1Þ

where n0 and ne are the refractive index of the ordinary light andextraordinary light, respectively, cij is the linear optoelectronic coef-ficient, E3 is the internal space charge field along the polar axis, andI is the applied light intensity. This result indicates that the changeof the refractive index dispersion of near-SLN:Fe may be attributedto the internal space charge field in the crystal caused by the spacedisplacement of excited free carriers.

When the light intensity is higher than 120 mW/cm2, the vari-ation of refractive index |Dn| shows a steplike decrease. For exam-ple, at the frequency of 1.0 THz, |Dn| decreased to 0.08 when thelight intensity is 160 mW/cm2, and then increases sightly againwith the light intensity increasing. This phenomenon is supposedto be introduced by the light-induced domain reversal in the crys-tal. The light intensity at which the steplike decrease of photore-

Fig. 6. Light intensity dependence of variation of refractive index at (a) 1.0 THz and(b) 1.2 THz of near-SLN:Fe. The data points are our measured values, and the solidlines are the fit.

1740 L. Wu et al. / Optical Materials 33 (2011) 1737–1740

fraction takes place is supposed to has some relationship with themicro structure of the crystal.

The micro mechanisms behind the observed effect here are dis-cussed as follows: (1) Both Fe2+ and Fe3+ ions in near-SLN:Fe crys-tals are octahedrally coordinated by six oxygen atoms and occupythe Li1+ site [19], resulting in lots of free electrons in the crystal.When applying an external optical field, the free electrons migratein a preferred direction. The dominant charge driving force is sup-posed to be the photovoltaic effect. Drift and diffusion could alsohave contribution to the movement of the electrons, and thermalexcitations are negligible here. The space displacement of carriersleads to an internal space charge field, shielding the macroscopicspontaneous polarization of near-SLN:Fe. The internal field causedthe variation of refractive index via a linear electro-optic effect[20], as shown in Fig. 6. (2) When the light intensity is higher than120 mw/cm2, the internal space charge field is enhanced suffi-ciently that a light-induced domain reversal occurs in the crystal,resulting in the electric neutralization of some space charges. An-other electric field which is induced by the screening charge onthe surface could have an additional contribution to the domainreversal [10]. Then the domain walls of the reversed domain donot have plenty of negative bound charges, which induce the wea-king of the space charge field, leading to the corresponding de-crease of photorefraction.

4. Conclusion

In summary, we have investigated the frequency dependence ofdielectric spectra of iron doped lithium niobate single crystal at

room temperature. An obvious photorefractive effect was demon-strated in near-SLN:Fe with different level of external optical fields.This property originates from the internal space charge fieldcaused by free carriers apart from the light-induced domain rever-sal which leads to a steplike decrease of photorefraction in thecrystal. Thus it is presumed that these two mechanisms wouldcomplement with each other, resulting in the observed photo-fer-roelectric behaviors of near-SLN:Fe.

Acknowledgments

This work is supported by the National Natural Science Founda-tion of China under Grant No. 10974063, the Research Foundationof Wuhan National Laboratory under Grant No. P080008, and theNational ‘‘973’’ Project under Grant No. 2007CB310403.

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