Preparation and optical properties of cobalt doped lithium tetraborate nanoparticles

Optical Materials 36 (2014) 1598–1603

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Optical Materials

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Preparation and optical properties of cobalt doped lithium tetraboratenanoparticles

http://dx.doi.org/10.1016/j.optmat.2014.04.0410925-3467/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Address: School of Physics, Bharathidasan University,Tiruchirappalli 620024, Tamil Nadu, India. Tel.: +91 0431 2407057; fax: +91 04312407045.

E-mail address: dhanus2k3@yahoo.com (S. Dhanuskodi).

S. Dhanuskodi a,⇑, R. Mohandoss a, G. Vinitha b

a School of Physics, Bharathidasan University, Tiruchirappalli 620024, Indiab Division of Physics, School of Advanced Sciences, V.I.T. University, Chennai 600048, India

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

Article history:Received 15 October 2013Received in revised form 15 April 2014Accepted 23 April 2014Available online 13 May 2014

Keywords:Sol–gel processingStructural characterizationNonlinear optics

Lithium tetraborate doped with cobalt ions of different concentrations were synthesized by chemicalmethod and characterized by X-ray. Field emission scanning electron microscopy reveals the sphericalmorphology of the nanoparticles and the average particle size is estimated to be 50–110 nm. UV–Visstudies show the inclusion of Co2+ and the lithium–vacancy pair production. The samples are completelytransparent in the visible region and having absorption peak (220 nm) in UV region. Nonlinear opticalstudies indicate the suitability of pure and Co2+ doped samples for low power optical limiting and fre-quency doubling applications. From the Z-scan technique, the nonlinear optical parameters such as non-linear refractive index, nonlinear absorption and nonlinear optical susceptibility are estimated as10�8 cm2/W, 10�2 cm/W and 10�5 esu respectively.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

One of the fundamental and important aspects of optical com-munications in optical computing is to control the light intensityin a predetermined and predictable manner. Optical limiting, anincrease of optical absorption with light intensity, is the mostpromising application of nonlinear optical (NLO) activity whichcan be used in broad band laser safety devices and optical sensorprotection [1]. Even after the advent of a group of borate NLOmaterials, lithium tetraborate (LTB) attracts the interest ofresearchers due to its unique physical properties along with lowlost [2]. Li2B4O7 is a tetragonal crystal system which belongs tothe polar point symmetry group 4 mm [3]. It has a transmissionrange of 165–6000 nm [4,5], in which the energy level formationin the upper region of the valence band is due to the contributionof boron–oxygen framework. From the literatures, it is clear thatborates serve as an interesting host for transition metal ions[6,7]. Increase in the transition metal ions concentration will makea shift of the characteristic bands of BO3 and BO4 structural units tothe higher energy regions [8]. Very few reports on optical limitingproperties of Co based systems are found in the literature. The opti-cal limiting properties of Co nanowires, pristine Co nanotubes andcarbon nanotubes with Co were studied using nanosecond laser

pulse of 532 nm [9–11]. It becomes interesting to prepare LTB:Conanoparticles by chemical method and to investigate optical limit-ing behavior of LTB:Co nanoparticles.

2. Experimental

2.1. Sample preparation

Samples were synthesized by dissolving boric acid in doubledistilled water at 323 K till a clear solution was obtained. Lithiumhydroxide monohydrate, cobaltous acetate were then added tothe resultant solution and allowed for continuous stirring for halfan hour. The ratio of boron to lithium was maintained as 1:2 andthe doping concentrations were 0.01, 0.03 and 0.05 M. Then themixture was evaporated to dryness by heating below 323 K. Theresultant white powder was sintered in a SiC furnace at 873 K for1 h. The synthesized compounds were denoted as samples B1, B2and B3 for 0.01, 0.03 and 0.05 M LTB:Co nanoparticles respectively.

2.2. Instrumentation

The structural characterization of the samples were carried outby PANalytical X’Pert Pro powder X-ray diffractometer with Cu Ka(k = 1.5406 Å) over the 2h range of 10–80�. Carl Zeiss field emissionscanning electron microscope (FESEM) was employed to study thesurface morphology of the prepared samples. Optical transmit-tance spectrum was recorded by Shimadzu UV–Vis double beam

Table 1Unit cell constants of pure and cobalt doped LTB.

Sample Unit cell constants (Å)

a c

Pure LTB 9.44 10.28B1 9.47 10.28B2 9.49 10.32B3 9.46 10.43

S. Dhanuskodi et al. / Optical Materials 36 (2014) 1598–1603 1599

spectrophotometer (200–800 nm) after dispersing 2 mg sample in5 ml alcohol and by successive sonication for 20 min at 304 K. Thesolutions were taken in a 1 cm size quartz cuvette. The lumines-cence study was carried out using Fluoromax4 spectrofluorometer.Second harmonic generation efficiency was measured using a Q-switched Nd:YAG laser (1064 nm, 9 ns, 10 Hz).

2.3. Z-scan measurement

Third order nonlinear optical properties were investigated usinga diode pumped cw Nd:YAG laser (532 nm, 50 mW). The beampropagation direction was taken as the Z-axis and the focal pointwas considered as Z = 0. The sample was placed in the beam at aposition (Z) between the lens of focal length 3.5 cm and the focalpoint, and the transmitted energy was measured at each step.The Z-value increases on either side of the focal point, but the signwill be negative on one side and positive on the other. The spot sizeof the laser beam at the focus was calculated as 14.99 lm. At eachof these positions, the sample experienced different laser intensityand the intensity was a maximum at the focus. Similar Z-scangeometry was used for the demonstration of optical limiting undercw laser illumination.

3. Results and discussion

3.1. Structural analysis

Powder X-ray diffraction pattern shows the peak broadeningnature which implies the nanocrystalline nature of the samples(Fig. 1). Lattice parameters (Table 1) and Miller indices are in goodagreement with the JCPDS (Card No. 79-0963). Also it is confirmedthat the prepared nanoparticles crystallize in tetragonal structure.

Fig. 1. XRD pattern of Co doped LTB nanoparticles.

No additional secondary peaks are formed after the addition ofcobalt. The atomic arrangements are prominently oriented along(112) plane, for all the three samples. The synthesize of pure LTBwere reported earlier [12]. From the SEM micrographs (Fig. 2)the particle size is approximately in the range of 50–110 nm. Theparticle size is found to increase with the concentration of the dop-ant. The samples B2 and B3 were observed to be conglomeratedwhen compared with sample B1. Similarly, increase in dopant con-centration also changed the spherical morphology of the sample.The structure of LTB is arranged in such a way that a common oxy-gen atom with lithium atoms localized at interstitial position linksthe boron–oxygen complexes (B4O9) formed by two trigonal (BO3)groups and two tetrahedral (BO4) groups. Due to the extremelysmaller size of boron ion (0.23 Å), the dopant ions occupies primar-ily the Li site [13]. Hence the Co2+ ion occupies the lithium site,which was located interstitially, the lattice distortion leads to thisvariation in the size of sample. Morphology of the doped LTB wasfound to be in spherical shape.

3.2. Linear optical studies

From the UV–Vis transmittance studies (Fig. 3), it is inferredthat there are some absorption peaks in the UV region (220 nm)and the sample is completely transparent to the visible region. Eth-anol is used as solvent and the samples are dispersed and sonicatedfor 20 min before recording the spectrum. El Batal et al. havereported that the cobalt doped lithium tetraborate shows twoprominent absorption peaks at about 210 and 270 nm [14]. Inthe present case, the strong absorption is observed at 220 nmand 271–275 nm. Increasing the dopant concentration enhancesthe intensity of the absorption peak in the UV region (225 nm).To perform nonlinear optics based experiments, the expectedemissive radiations from the samples are to be in visible region,in particular 532 nm. So, the maximum transmittance in the visibleregion is absolutely necessary to investigate the NLO propertiesand the title compound satisfies the requirement very well.

Fluorescence spectrum (Fig. 4) shows the band edge emission ofthe samples B1, B2 and B3 at around 310 nm to 316 nm. An inbuiltxenon lamp is used as an excitation source and the excitationwavelength is selected as 250 nm. If Co2+ ions enter Li+ octahedralpositions, then, due to charge compensation, cation vacanciesarises near Co2+ ions, influencing the symmetry of the surrounding.Excessive increase in the doping level (0.01–0.03 M) of cobalt ionsin distinct manner changes the defect structure of the sample. Inthe present case, cobalt ions enter both cation vacancies and Li+

positions and reduce the total amount of cation defects. The peaksobserved at 369–398 nm are associated with the presence of thelithium–vacancy pair. A broad peak around 414–417 nm is attrib-uted to the Co inclusion in the lattice [15].

3.3. Nonlinear optical studies

3.3.1. Second harmonic generationIn the nonlinear optics regime, the nonlinear polarization serves

as a source for the generation of new waves [16]. In this frequency

Fig. 2. SEM micrograph of LTB:Co nanoparticle.

Fig. 3. UV–Vis transmittance spectrum of LTB:Co nanoparticle.

Fig. 4. Fluorescence spectrum of LTB:Co nanoparticle.

1600 S. Dhanuskodi et al. / Optical Materials 36 (2014) 1598–1603

conversion experiment, the nonlinear polarization is induced in thepowder sample by the optical wave of wavelength 1064 nm gener-ated by the Q-switched Nd:YAG laser (1064 nm, 9 ns and 10 Hz).The incident optical waves induce a phased array of dipoles thatthen radiate waves coherently at new wavelength of 532 nm byharmonic generation. The efficiency of second harmonic genera-tion of the LTB:Co nanoparticles is compared with the standardKDP and undoped LTB. The results show that the pure LTB is capa-ble of generating 532 nm radiation equivalent to KDP. The SHGefficiency after Co doping is relatively lower than the virgin. Therelative SHG efficiency of pure LTB is 0.99 times KDP and for sam-ples B1, B2 and B3 it is 0.94, 0.95 and 0.81 times respectively

(Fig. 5). Therefore, the SHG efficiency of synthesized samples areequivalent to KDP and no vast difference in efficiency wasobserved.

3.3.2. Third order nonlinear optical propertiesZ-scan technique is the modulation of the refractive index by

optical waves via a third order nonlinearity [16]. The self-focusingand self-defocusing are the two basic ideas behind this technique[17]. The open aperture Z-scan is essentially a sample transmissionmeasurement, the data being continuously taken while the sampleis slowly translated from a position before the focus to a position

Fig. 5. SHG efficiency test for LTB:Co nanoparticle.

S. Dhanuskodi et al. / Optical Materials 36 (2014) 1598–1603 1601

after the focus. The nonlinear absorption coefficient b can be esti-mated from the open aperture Z-scan data:

b ¼ 2ffiffiffi

2p

DTI0Leff

ð1Þ

DT = 1 � Tv where Tv is the value at the point of valley at Z = 0obtained from the open aperture graph. Leff = [1 � exp(�aL)/a] isthe effective thickness of the sample, a is the linear absorption coef-ficient and L is the thickness of the sample.

In case of the saturable absorption, the value of b will be nega-tive and for two photon absorption the value is positive. The trans-mission enhancement near the focus indicates the absorptionsaturation at a high intensity (Fig. 6a). From the graph, it can beseen that the open aperture curves of the sample B3 and pureLTB gets coincides. This is due to its nonlinear absorption coeffi-cient, which are closer to each other.

From the closed aperture Z-scan (Fig. 6b), it is observed thatthere is a pre-focal peak followed by a post focal valley which isthe sign of negative nonlinearity i.e. self-defocusing nature of thesamples, a property that has wide applications in the protectionof optical sensors such as night vision devices [18].

The incident intensity (Io) upon the 0.1 M sample is4.85 kW cm�2. The difference between the normalized peak andvalley transmission (DTp–v) is written in terms of the on axis phaseshift jD/j at the focus as

DTp—v ¼ 0:406ð1� SÞ0:25jD/oj ð2Þ

It is observed from the pattern that, the on-axis phase shift isfound to be increasing with doping concentration.

Aperture linear transmittance S can be written as

S ¼ 1� expð�2r2a=x

2aÞ ð3Þ

where ra is the aperture and xa is the beam radius at the aperture.The nonlinear refractive index is obtained from the following

relation

n2 ¼ D/ok=2pI0Leff ð4Þ

where k is the laser wavelength, I0 is the laser intensity at Z = 0,Since cw laser is a probing source for the investigation of third

order nonlinear optical properties (TONLO), the concept of thermallensing is used to explain the nonlinear mechanism. Thermal lens-ing is based upon the influence of thermal change on the opticalproperties of a sample on the absorption of laser energy. This leadsto a temperature rise in the sample and consequently to the forma-

tion of an inhomogeneous spatial profile of the refractive index.The heat released by the nonradiative relaxation processes gener-ates a volume expansion in the sample and a density change withinthe excitation region. The refractive index caused by the heat evo-lution due to the radiationless process turns in most cases the solu-tion into a divergent lens, which defocuses the laser beam [1]. Thepurely effective nonlinear refractive index n2 is obtained by divid-ing the closed aperture transmittance by the corresponding openaperture transmittance. The ratio of closed and open transmissionis shown in Fig. 6c.

The magnitude of the nonlinear refractive index (NLR) is in theorder of 10�8 cm2/W. However, the n2 values are found to increasewith the doping level.

The real and imaginary parts of the third order nonlinear opticalsusceptibility v3 are defined as

Revð3ÞðesuÞ ¼ 10�4e0c2n20n2=p ðcm2=WÞ ð5Þ

Imvð3ÞðesuÞ ¼ 10�2e0c2n20gb=4p2 ðcm2=WÞ ð6Þ

where e0 is the vacuum permittivity and c is the velocity of the lightin vacuum.

The nonlinear absorption (NLA) b, is also found to be increasingwith cobalt concentration. The nonlinear optical susceptibility isestimated to be of the magnitude of 10�5 esu.

Table 2 portrays the measurement details and results obtainedthrough Z-scan for LTB:Co nanoparticles. The order of magnitude ofthe nonlinear refractive index, nonlinear absorption and nonlinearoptical susceptibility is 10�8 cm2/W, 10�2 cm/W and 10�5 esurespectively. From the results, it is inferred that the samples pos-sess a very high third order nonlinear susceptibility.

3.3.3. Optical limiting behaviorThe optical limiting is obtained by varying the input energy of

the laser and measuring the energy transmitted by the sample.At very low laser energies, the transmission obeys the Beer–Lam-bert law [19]. At high input intensity the transmittance decreaseswith input intensity. The variation of the transmitted energy withthe input energy is given in Fig. 6d for different concentrations ofthe samples. From the pattern, it is observed that the samples haveoptical limiting property with saturated or clamped output inten-sity. With the increasing order of nonlinearity, the probability ofthe transition drastically reduces and it takes higher intensitiesto induce sufficient nonlinear absorption for optical limiting. Forsample B1, the nonlinear medium behaves linearly till the inputpower is below 24.9 mW and is then clamped at the output fluenceof 419 lW. The limiting threshold value gradually increases to28.3 mW for samples B2 and B3 with the clamping output of 516and 582 lW respectively. Hence, the order of clamping outputvalue is pure LTB < B1 < B2 < B3. Fig. 6d clearly shows that thereis no further increase in the output power even though the inputfluence is raised further. The clamping and the nonlinear absorp-tion coefficient (b) values increase with dopant concentration.The direct or reflected laser beam causes severe and permanentdamage to the naked human eye. Visible and near infra-red regionis proven to be the retinal hazardous region whereas cornea getsaffected by UV and far infrared region. Therefore, photosensitivereactions and skin burns will be caused due to the photochemicaland thermal retinal injury respectively when human eye getsexposure to the lasers in visible regime (400–780 nm) [20]. Hence,a wide choice for optical power limiting at 532 nm cw Nd:YAGlaser could be provided by varying the doping level, for the safeusage of laser with proper eye and sensor protections.

Fig. 6. Z-scan pattern for pure and LTB:Co (a) closed (b) open (c) ratio (d) low power optical limiting behavior.

Table 2Comparison of nonlinear optical parameters of pure and LTB:Co.

Laser Diode pumped Nd:YAG laser (50 mW)

Radius of the laser beam, xL 0.16 mmRadius of the aperture, ra 2 mmBeam rad at the aperture, xa 15 mmPower of the O/P beam of laser, P 34.3 mWZ 700 mm

Sample Pure LTB B1 B2 B3Parameter

Nonlinear refractive index, n2 (�10�8 cm2/W) �2.862 �5.583 �5.85 �5.95Nonlinear absorption coefficienct, b (�10�2 cm/W) �6.70 �1.519 �2.117 �6.611Real part of nonlinear optical susceptibility, Re(v3) (�10�6 esu) 5.780 7.095 8.660 10.696Imaginary part of nonlinear optical susceptibility, Im(v3) (�10�5 esu) �5.738 �0.8179 �1.325 �5.033Third order nonlinear optical susceptibility, v3 (�10�5 esu) 5.767 1.082 1.583 5.146Optical limiting threshold 24.9 mW 24.9 mW 28.3 mW 28.3 mW

(143 lW) (419 lW) (516 lW) (582 lW)

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4. Conclusion

Cobalt doped lithium tetraborate nanoparticles of different con-centrations were synthesized by sol–gel method. No other second-ary phases were found in the XRD pattern even at maximumdoping level (0.05 M) and the lattice constants varies slightly dueto the incorporation of dopant in the host matrix. UV transmittancespectrum shows that the samples are completely transparent inthe visible region. The presence of lithium–vacancy pair and cobaltin the prepared nanoparticles were identified from the

luminescence spectrum. Since, SHG efficiency of pure and dopedsamples are equal to that of KDP, LTB:Co nanoparticles can be agood candidate for frequency doubling, especially for the genera-tion of short wavelength lasers. In the Z-scan pattern, a pre-focuspeak followed by a post-focus valley indicates that the nonlinearproperty of the sample is negative and the lensing effect is defocus-ing. The high magnitude of nonlinearity is primarily of thermal ori-gin. The nonlinear refractive index, nonlinear absorptioncoefficient and third order nonlinear optical susceptibility of thesamples are in the order of 10�8 cm2/W, 10�2 cm/W and 10�5 esu

S. Dhanuskodi et al. / Optical Materials 36 (2014) 1598–1603 1603

respectively. (0.01 M) LTB:Co shows a good low optical power lim-iting threshold in the range of 24.9 mW. Hence it can be effectivelyused as an efficient optical limiter by utilizing its high nonlinearrefractive index, in low power cw regime.

Acknowledgement

The authors sincerely thank the University Grants Commission,New Delhi (Project Grant F. No. 34-11/2008 (SR)) for the financialsupport.

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