Wednesday, September 17, 2014

Structural and Third-order Nonlinear Optical Properties of Lithium Hydrogen Phthalate Dihydrate Single Crystals

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 40×10× 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.

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). 

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 
55°c).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. 

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 CAD-4 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;
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 recorded and the dielectric constant has been calculated using the area and thickness of the sample.  

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 εr
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.

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
-3 m) were estimated using the 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)  
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 
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.
Nonlinear refractive index (n2) of the LHP  was calculated as 3.317x10
-11cm2/W the value of nonlinear absorption coefficient has been found to be β ~ 5.789x10-3cm2/W and nonlinear parameter are tabulated in table 3. 

  • solution Crystal growth
  • single crystal XRD
  • FTIR
  • thermal analysis
  • UV-Vis-NIR
  • Z-scan and nonlinear optical materials