Structural and Third-order Nonlinear Optical Properties of Lithium ...

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ScienceDirect Procedia Materials Science 6 (2014) 1566 – 1571

3rd International Conference on Materials Processing and Characterisation (ICMPC 2014)

Structural and Third-order Nonlinear Optical Properties of Lithium Hydrogen Phthalate Dihydrate Single Crystals D. Saravanan1, B. Sivakumar2, S. Gokul Raj3,*, G. Ramesh Kumar4, K. Thangaraj5 1 Department of Physics, A.R.Engineering College, Villupuram 605 109, India Department of Physics, University College of Engineering Kancheepuram, Anna Univesity Chennai Kancheepuram 631552. India 3 Department of Physics, Vel Tech Dr.RR & Dr.SR Technical University, Avadi, Chennai-600062. India 4 Department of Physics, University College of Engineering Arni, Anna University Chennai, Arni 632317. India

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Abstract Single crystals of Lithium Hydrogen phthalate dihydrate (LHP), a semi-organic nonlinear optical material have been successfully grown from aqueous solution, by slow evaporation solution growth technique. Single crystals in size 40h10h 5 mm3 were grown in a period of 2 weeks. The grown crystals were characterized by single crystal X-ray diffraction. LHP crystallizes in Pnm space group of Orthorhombic system, with the unit-cell dimensions at 293(2) K; a = 16.8356(10) Å; b = 6.8187(5) Å ; c = 8.1967(6) Å ; α = 90°, β = 90°, γ = 90°. Third order non-liner studies have also been studied by Z-scan techniques. Nonlinear absorption and nonlinear refractive index were found out and the third order bulk susceptibility of compound was also estimated. © The Authors. Published byaccess Elsevier Ltd.under the CC BY-NC-ND license © 2014 2014 Elsevier Ltd. This is an open article (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET). Selection and peer review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET) Keywords: solution Crystal growth, single crystal XRD, FTIR, thermal analysis, UV-Vis-NIR, Z-scan and nonlinear optical materials

1. Introduction: The semi-organic alkali hydrogen phthalate crystals are widely known for their application in the long-wave Xray spectrometers (A G. Boehm and K. Ulmer, 1971). Their optical, piezoelectric, NLO and elastic properties are investigated in detail (N. Kejalakshmy, K. Srinivasan 2004; S. Haussühl 1991; Andrzej Miniewicz, and Stanislaw Bartkiewicz 1993). Acid phthalate crystals were used as substrates for deposition of thin films of organic nonlinear materials (W. Sander et al. 2007) and standards in volumetric analysis (Sterling B. Smith 1931). * Corresponding author. Tel.: +91-0832-2580416 fax: +91-0832-2580416. E-mail address: Email:[email protected]

2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET) doi:10.1016/j.mspro.2014.07.138

D. Saravanan et al. / Procedia Materials Science 6 (2014) 1566 – 1571

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Lithium Acid Phthalate possesses piezo-electric, pyro-electric, elastic and non-linear optical properties (H. Kuppers, et al.1985; A.Senthil, et al. 2009; Shankar, M. V. and Varma, K. B. R. 1996). These crystals have excellent physical properties and have a good record for long term stability in devices (E.W. Vanstryland, et al. 1998). Tuning of band gap in semiconductor materials is an important tool in optoelectronic and photonic integration. The optical behavior of materials is an essential parameter to determine its usage in optoelectronic devices (Shahabuddin Khan M. D. and Narasimhamurty T. S., 1982. In the present work, single crystal of Lithium hydrogen phthalate dihydrate (LHP; also known as lithium acid phthalate), a semi-organic NLO, has been grown by slow evaporation technique. Though the Second order NLO property of LHP crystals was already reported, its Third Order NLO property has not been reported yet. The grown crystals were subjected to single-crystal X-ray diffraction, Fourier transform infrared (FTIR) analysis and thermal. In addition, third order NLO property of the LHP crystal was confirmed by the Z-scan studies. Also, here we reported the theoretical calculation for the determination of the nonlinear refractive index, in order to tune these factors for the requirements of the device and the results are discussed details.

2. Experimental procedure: Lithium hydrogen phthalate dihydrate (LHP.2H2O) was synthesized with high purity Lithium hydroxide (98% E-Merck) and phthalic acid (98% E-Merck) GR grade in the ratio1:1. The stoichiometric amounts of the reactants were dissolved in the de-ionized water and stirred well for about 4 hours (Temperature approximately at 55oc).This was then filtered and allowed to crystallize by slow evaporation technique (G. Adiwidjaja and H. Kupper 1978). The seed crystals with transparency were obtained by spontaneous nucleation. Among them, defect free seed crystal was suspended in the mother solution which was saturated at 34°C in constant temperature bath of ±0.05K accuracy. Optically good bulk crystals have dimension (4.5×1.0×0.7) cm has been grown within the period of 25 days and shown in Fig 1.

Fig 1. LHP single crystals grown from seed rotation technique

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3. Results and discussion: 3.1 Single crystal X-ray diffraction analysis: Three-dimensional intensity data of a transparent and good quality crystal were collected on an Enraf-Nonius CAD4 diffractometer equipped with MoKα radiation λ=0.71073Å. ω/2θ scan mode was employed for data collection. LHP crystallizes in an orthorhombic crystal system with the unit-cell dimensions at 293(2) K; a = 16.8356(10) Å; b = 6.8187(5) Å ; c = 8.1967(6) Å ; α = 90°, β = 90°, γ = 90°. 3.2 Microhardness measurements: From application point of view, hardness is an important solid state property of the single crystals as it plays a vital role in device fabrication. Hence Vicker’s Microhardness measurement was carried out for Lithium Hydrogen Phthalate crystals to assess their mechanical strength. To evaluate the vicker’s hardness number, as grown crystals of LHP was subjected to static indentation test at room temperature using Leitz wetzlar hardness tester fitted with vicker’s diamond pyramidal indentor. Several indentations were made on the (0 0 1) face of LHP single crystals. The vicker’s hardness number was calculated using the expression; ௉

‫ܪ‬௩ ൌ ͳǤͺͷͶͶ ቀ మቁ

(2.1)

ௗ

30

Vickerhardness number

25

20

15

10

20

30

40

50

60

70

80

90

100

Load(gram)

Fig 2. Microhardness Vs Load for LHP single crystals Surface pattern of indented area for 100 gram load along (0 0 1) plane of LHP, Crystals

where Hv is the vicker’s hardress number for a given load, P in gram and d is the average diagonal length of the indentation in mm. For loads ranging from 25 - 100 gram, the micro-hardness values of LHP was found to be in the increasing trend and it could be seen through Fig 2. When the indenter just touches the surface of the crystal, a dislocation is generated in the indenter region and thus causes the increases of Microhardness of the compounds initially. However, for the loads beyond 100 gram, cracks started developing around the indentation mark. The Hardness (HV) then decreases with load and saturates for higher loads which occur due to the rearrangement of dislocations and mutual interactions of dislocations. 3.3 Dielectric measurements Dielectric studies have been performed on (010) planes of lithium hydrogen phthalate single crystals at 30º C, 50º C and 75º C in the frequency range 50Hz – 5MHz using (LCR HIOKI-3532 LCR HITESTER) LCR meter. The sample has been coated with conductive silver paint for metallic contacts. A sinusoidal a.c. voltage was applied to the sample through the silver electrodes for various frequencies. Capacitance developed by the crystal was

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recorded and the dielectric constant has been calculated using the area and thickness of the sample.

10 o

20000

40 C o 50 C o 75 C

9

Dielectric Constant

Dielectric Loss

o

40 C o 50 C o 75 C

8

15000

10000

5000

7 6 5 4 3 2

0

1 1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

0

7.0

1.5

Log F (frequence in Hz)

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Log F (frequence in Hz)

Fig 3. Dielectric spectrum of LHP Dielectric Loss of LHP at various frequencies

The dielectric constant decreases with increase in frequency and after reaching a frequency of 1 MHz, the dielectric constant almost remains a constant Fig.3. The total dielectric polarization of materials is from the contribution of electronic, ionic, dipolar and space charge polarizations at lower frequencies and the value of ε r rises predominantly due to orientation of dipoles in the low frequency of range 1kHz – 5MHz. Since, the orientation polarization is highly dependent on temperature; the change in temperature of samples marginally affects the value of εr. Dielectric loss calculated at various frequencies reveals that the power loss of the sample on applying electrical energy was found to be negligible. 3.4 UV-Vis-NIR UV-Vis-NIR measurement was carried out for Lithium hydrogen phthalate dehydrate single crystals in the wavelength range 200-1200nm using a Varian Cary 5E UV and shown in Fig 4. 205

4.0

3.5

2.5 276

Abs%

3.0

799

344

2.0

1.5

1.0 500

1000

1500

2000

2500

wavelength(nm)

Fig 4. UV-Vis-NIR spectra of LHP

The maximum UV absorption occurs at 205nm. After this wavelength, absorption abruptly decreases to nearly 1.5-4%. The material possesses a very good optical transparency even up to 1500nm. This property would be much useful in field of an optical material. 3.5 Z-scan Measurement The Z-scan method has gained rapid acceptance by the nonlinear optics community as a standard technique for separately determining the nonlinear changes in refractive index and the change in nonlinear optical absorption. The nonlinear absorption and refractive index of LHP crystals (thickness ≈0.945x10-3m) were estimated using the

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single beam Z-scan method with laser beam intensity of 60mW and the wavelength of source used for the measurement was 632.8 nm. The study of nonlinear refraction by the Z-scan method depends on the position (Z) of the thin samples under the investigation along a focused Gaussian laser beam. The sample causes an additional focusing or defocusing, depending on whether nonlinear refraction is positive or negative. Such a scheme, referred to as an “Open aperture” Z-scan and it is suited for measuring nonlinear absorption in the sample. Results obtained from a typical closed aperture Z-scan study for the grown lithium hydrogen phthalate crystals are presented in Fig. 5. The nonlinear refractive index (n2) of the crystal was calculated using the standard relations given below :( M. Sheik-Bahae, et al. 1989; J.L. Bredas et al. 1994; J.J. Rodrigues et al.2002) ∆Tp-v = 0.406(1- S) 0.25/∆ϕo Where S= 1- exp (-ra2 /ωa2) is the aperture linear transmittance (0.01), ∆ϕo is the on-axis phase shift. The on-axis phase shift is related to the third-order nonlinear refractive index by ‫∆׀‬ϕo‫ = ׀‬kn2Leff Io Where k = 2π/λ, Leff = [1-exp(-αL)]/ α is the effective thickness of the sample, α is the linear absorption coefficient, L the thickness of the sample, Io is the on-axis irradiance at focus and n2 is the third-order nonlinear refractive index. 0.052 0.050

LiAP CA Normalised transmittance

Normalised transmittance

0.038

0.036

0.034

0.032

LiAP OA

0.048 0.046 0.044 0.042 0.040

0.030

0.038 -6

-4

-2

0

Z-Distance (mm)

2

4

6

-6

-4

-2

0

2

4

6

Z-Distance (mm)

Fig 5. Open and closed aperture Z-scan signature of LHP -11

Nonlinear refractive index (n2) of the LHP was calculated as 3.317x10 cm2/W the value of nonlinear -3 absorption coefficient has been found to be β ~ 5.789x10 cm2/W and nonlinear parameter are tabulated in table 3. 4. Conclusion Single crystals of Lithium hydrogen Phthalate dihydrate single crystals have been synthesized by the slow evaporation technique and bulk crystals were grown from slow cooling technique. Single crystal XRD measurement reveals that the grown crystal belongs to the orthorhombic system with a space group of Pmn. Optical absorption studies confirmed that the LHP.2H2O crystal is transparent in the wavelength region 210–1200nm with a UV transparency cut-off at 205nm. Third order nonlinear property was also investigated by the Z-scan technique. The n2 and β values thus obtained were 4.631×10−11 cm2/W & 2.1897x10-6 cm2/W respectively. The bulk third order susceptibility of the compound was also estimated from the above two entities. Acknowledgements One of the author B.Sivakumar wishes to thank to The Dean, University College of Engineering Kanchipuram Kanchipuram-632 552, for providing a platform to perform the research.


References A G. Boehm and K. Ulmer, 1971 Solution growth of organic analyzer crystals for ultrafast X-ray spectrocopy J. Crystal Growth 10(2) pp.175178. Andrzej Miniewicz, Stanislaw Bartkiewicz 1993 On the electro-optic properties of single crystals of sodium, potassium and rubidium acid phthalates Advanced Materials for Optics and Electronics V 2 Issue 4 pp.157–163. A.Senthil, P.Ramasamy, G.Bhagavannarayana 2009 Synthesis, growth, optical, dielectric and thermal studies of lithium hydrogen phthalate dihydrate crystals Journal of Crystal Growth. V 311 Issue 9 pp. 2696-2701. E.W. Vanstryland, M. Sheik-Bahae, in: M.G. Kuzyk, C.W. Dirk (Eds.), 1998 Characterisation Techniques and Tabulation for Organic Nonlinear Materials, Marcel Dekker Inc., pp. 655–692. G. Adiwidjaja and H. Kupper 1978 Lithium Hydrogen Phthalate-Methanol Acta Cryst. B34, pp.2003-2005. H. Kuppers, F. Takusagawa, T.F. Koetzle, 1985 Neutron diffraction study of lithium hydrogen phthalate monohydrate: A material with two very short intramolecular O‫ڄڄڄ‬H‫ڄڄڄ‬O hydrogen bonds J. Chem. Phys. 82 pp.5636-5647. J.J. Rodrigues Jr., Carlos H.T.P. Silva, S.C. Zilio, L. Misoguti, C.R. Mendonca 2002 Femtosecond Z-scan measurements of nonlinear refraction in amino acid solutions, Opt. Materials., Vol.20., pp.153–157. J.L. Bredas, C. Adant, P. Tackx, A. Persoons, 1994 Third order nonlinear optical response in organic materials: Theoretical and Experimental aspects, Chem. Rev. 94 pp.243–278. M. Sheik-Bahae, A.A. Said, E.W. VanStryland, 1989 High-sensitivity, single-beam (n2 ) measurements Optics Letters, Vol. 14, Issue 17, pp. 955957. N. Kejalakshmy, K. Srinivasan 2004 Growth, optical and electro-optical characterisations of potassium hydrogen phthalate crystals doped with Fe3+ and Cr3+ ions Optical Materials V 27 Issue 3 pp. 389–394. Shahabuddin Khan M. D. and Narasimhamurty T. S., 1982 Elasto-optic studies on potassium acid phthalate single crystal J. Mat. Sci. Lett. 1, pp.268-271. Shankar, M. V. and Varma, K. B. R.1996 Piezoelectric resonance in KAP single crystals,Ferroelectrics Letters Section 21(1),pp.55 – 59. Sterling B. Smith 1931 Equilibrium in the System, Phthalic acid- Potassium Phthalate-Water J. Am. Chem. Soc., 53 (10), pp.3711-3718. S. Haussühl 1991 Physical properties of phthalic acid and of eight salts of phthalic acid with monovalent cations Zeitschrift für Kristallographie: Vol. 196, No. 1-4, pp. 47-60. W. Sander Graswinckel, Fieke J. Van den Bruele, Willem J.P. Van Enckevort, Elias Vlieg, 2007 Epitaxy of Organic Crystal Films: Phenanthrene on Potassium Acid Phthalate Cryst. Growth Des., 7 (2), pp 243–249.

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