Growth and Characterisation of Non Linear Optical Single Crystal

The growth of single crystals and their characterisation towards device fabrication have gained great momentum due to their significant applications in the fields of semiconductors, solid state lasers, non linear optics, piezoelectric, photosensitive materials and crystalline thin films for microelectronics and computer industries. In particular the non linear optics plays a major role in the emerging areas of laser technology, optical communication, data storage technology, photonics and optoelectronics. Hence the nonlinear optical materials are important for future photonic technologies based on the fact that the photons are capable of processing information with the speed of light. Hence the growth of promising new non linear optical materials receive great attention and find its application in various fields of optical disk data storage and laser remote sensing.

Many organic and inorganic nonlinear optical materials have been reported in the literature with good optical and mechanical properties. In comparison with these crystals, the semi organic nonlinear optical crystal possess the advantage of both organic as well as inorganic materials. They have high damage threshold, wide transparency range, excellent nonlinear optical coefficient and superior mechanical properties.

Guanidinium based organic and inorganic compounds play a vital role in the field of nonlinear optical crystal growth. The guanidinum ion [C (NH2)3] + is an important functional group present in the amino acid and also the basic constituent of many biologically active molecule. Various derivatives of guanidinium ion are used in explosives and rocket repellent formulations. Guanidinium is a strong base which reacts with most organic acid resulting in the formation of guanidinium species. The 3 fold symmetry of the guanidinium ion with six equivalent hydrogen atoms provides excellent condition for hydrogen bonding and this property has made guanidinium compounds as potential materials in the field of nonlinear optical crystal growth and their applications. The crystal structure, vibrational spectroscopic studies and ferroelectric properties of some guanidinum metal sulphates have been reported in the literature. Many guanidinuim based nonlinear optical crystals were grown and reported from our laboratory. In this paper we discuss the growth and characterisation studies of the semi organic guanidinuim compound guanidinium tris cadmium sulphate octahydrate [GuCdS].

Synthesis and crystal growth

The guanidinium cadmium sulphate ocathydrate compounds were synthesised using AR grade reagents guanidinium carbonate, concentrated sulphuric acid and cadmium sulphate octahydrate and were taken in an equimolar stoichiometric ratio for the synthesis of the title compound. Distilled water was used as solvent and the crystallisation was carried out at room temperature. The solution was stirred well using magnetic stirrer for six hours to ensure the homogenous concentration and it was filtered using Whatmann filter paper and kept for slow evaporation of the solvent in a dust free atmosphere. The pH value of the solution was found to be at 1. After a few days the GuCdS compound was found to crystallise at the bottom of the beaker. The following equation explains the scheme of synthesis.[C (NH2)3]2CO3 + H2SO4 → [C (NH2)3]2 SO4+H2O+CO2 ↑[C (NH2)3]2 SO4 + 3CdSO4 8 H2O → [3Cd{C (NH2)3}2] (SO4)2. 8H2O

The purity of the synthesised compound was further improved by repeated recrystallization with the same solvent and was used for the growth of the bulk crystal. A saturated aqueous GuCdS solution of 100 mL was prepared from the recrystallised salt guanidinium cadmium sulphate and allowed to evaporate in a dust free atmosphere. After a period of thirteen days transparent, defect free single crystals of guanidinium cadmium sulphate were harvested and are shown in the Fig. 1.

Powder X-Ray Diffraction Analysis

The powder X-ray diffraction method is a decisive method for qualitative phase analysis. The powder pattern of a crystal is also important in determining the crystallinity and phase purity of the grown crystal. The powder X-ray diffraction analysis of the grown crystal was recorded using RICH SIEFERT powder X-ray diffractometer with Cu Kα (λ=1.5406Å) radiation. Grown crystals were ground using agate mortar and pestle and subjected to powder X-ray diffraction analysis. The sample was scanned in the range 10º-70º in steps of 0.04º. The powder X-ray diffraction spectrum of the grown crystal is shown in the Fig. 2. The intense and sharp peaks in the diffractogram indicate the good crystalline perfection of the grown crystals. The two theta values obtained from the powder X-ray analyses were used for indexing the powder pattern. The indexing of the peaks and the evaluation of the lattice cell parameters were carried out using the powder X software. From this it was found that the grown crystal GuCdS belongs to triclinic crystal system and the space group was found to be Pī which is a centrosymmetric crystal. The obtained cell parameters of the crystal are a = 6.444 Å, b = 6.456 Å, c = 10.020 Å, α = 90.16˚, β = 97.035˚ and γ = 110˚.

FTIR spectral analysis In order to identify various functional groups present in the grown guanidinium cadmium sulphate crystal, FTIR spectral analysis was carried out. The FTIR spectrum of the powdered sample was recorded using Perkin Elmer Spectrum-1 in the range 4000 to 450 cm-1.The assignment of the spectral bands were carried out in terms of the fundamental modes of vibration of the guanidinium ion [C(NH2)3]+, sulphate ion (SO42-) and water molecules [7]. The recorded FTIR spectrum of guanidinium cadmium sulphate is shown in Fig. 3.

Vibrations of Guanidinuim ions

The assignment of vibrational modes in guanidinium ion can be done in terms of CN3 and NH2 groups. In the IR spectrum of the GuCdS compound, a sharp strong band at 1624 cm-1 is due to asymmetric stretching vibrations of CN3 group.

Vibrations of the sulphate group

The sulphate group (SO42-) in its free ion state exhibit four fundamental modes of vibration. The modes are the non degenerate symmetric stretching mode (ν1), the doubly degenerate symmetric bending mode (ν2), the triply degenerate asymmetric stretching mode (ν3) and the triply degenerate asymmetric bending mode (ν4) with the wave numbers 981 cm-1, 451 cm-1, 1108 cm-1 and 613 cm-1 respectively. Among the four different modes of vibration only (ν3) and (ν4) are IR active. The triply degenerate asymmetric stretching (ν3) mode of sulphate ion has a strong band at 1117 cm-1 and the triply degenerate asymmetric bending mode (ν4) appears at 619 cm-1 in the FTIR spectrum.3.2.3 Vibrations of water moleculeA water molecule in general has three fundamental modes of vibration: (ν1) at 3652 cm-1, (ν2) at 1595 cm-1 and (ν3) at 3756 cm-1. The IR spectrum of GuCdS compound contains strong bands at 3451 and 3523 cm-1 which are assigned to the ν1 and ν3 vibrational modes of water molecule. The vibrational band assignments of FTIR spectrum of the grown crystal was found to be consistant with that of the values reported in the literature. The experimental vibrational frequencies of GuCdS are presented in Table 2.

UV -vis-NIR spectral study was performed for the grown crystal in the range between 200-1100 nm by PerkinElmer UV spectrophotometer and is shown in Fig. 4. The optical transmission range and transparency cut off are the most important optical parameters for laser frequency conversion applications [12]. For optical device fabrication the crystal should have good transmission in a wide range of wavelength. From the transmission spectrum it is evident that the as grown GuCdS crystal is optically transparent in the ultraviolet, entire visible and near infra red region. The transparency is about 80 % in the entire visible and IR regions and the lower cut off wavelength is found to be at of 200 nm. This transmission window (200 nm-1100 nm) is suitable for the generation of second harmonics (λ = 532 nm) as well as third harmonics (λ = 354 nm) of the Nd:YAG laser of wavelength 1064 nm [13].

Determination of optical constants

The values of optical constants such as optical band gap, extinction coefficient and refractive index are quite important in order to scrutinize the potential applications of the grown crystal in the field of optoelectronics. The optical transmission spectral data was used to determine the optical constants such as absorption coefficient (α), extinction coefficient (k) and the refractive index (n) of the grown GuCdS crystal. The optical absorption coefficient (α) was calculated from the transmission data using the following expression reported in the literature [14]: α=(2.303 log⁡〖 (1/T)〗)/t (1)where T is the transmittance and t is the thickness of the crystal. The optical band gap energy was estimated from the transmission spectrum using the following expression [14]: (αhν)2 =A (Eg - h ν) (2)where h is the Planck’s constant, Eg is the optical band gap energy of the crystal, α is the optical absorption coefficients near the absorption edge and A is a constant. The Tauc’s plot between photon energy (hν) and (αhν)2 was drawn. The band gap of the crystal was estimated by the extrapolation of the linear portion of the graph to the photon energy axis gives the value as 6.14 eV as shown in Fig. 5. The value indicates that the crystal is high band gap energy material and it can be suitable for UV tuneable laser.

The optical constants such as extinction coefficient (k) and the refractive index (n) were calculated for the GuCdS crystal using the expressions reported earlier [15]. The reflectance (R) of the grown crystal was calculated in terms of the optical absorption coefficient (α) using the following expression [15]: R= 1±√(1-e^((-αt))+e^((αt)) ) (3) 1+e^((-αt))The refractive index (n) of the grown crystal was calculated using the values of the reflectance (R) by the following relation: n = -(R+1)±√(3R^2+10R-3) (4) 2 (R-1)The extinction coefficient (k) is the fraction of incident light lost due to scattering and absorption per unit thickness in a particular medium which can be evaluated by the following expression (5): k = αλ/4π (5)The spectra of refractive index (n) and extinction coefficient (k) as the functions of wavelength are shown in the Fig. 6. From this spectra it is evident that the values of refractive index (n) and extinction coefficient (k) are strongly depend on the wavelength, particularly in the UV region. It is evident that the extinction coefficient increases with increase in wavelength. The values of the refractive index increase sharply in the UV region due to the absorption of photon by the crystal and the values remain almost constant in the visible region and IR region.

The optical conductivity (σ) is a measure of the frequency response of the material when irradiated with light and was calculated in terms of the optical absorption coefficient (α) using the following relation [16]: σ=αnc/4π (6)The calculated optical conductivity values are plotted against photon energy as shown in Fig. 7 and from the plot it is evident that the optical conductivity increases with increase of photon energy. The optical studies revealed that the GuCdS crystal possesses good optical behaviour for using it in optical device applications.