Comprehensive study on l-Proline Lithium Chloride Monohydrate single crystal: A semiorganic material for nonlinear optical applications

3.1 Single crystal and Powder X-ray diffraction (XRD)

The grown l-Proline Lithium Chloride Monohydrate (LPLCM) single crystal is subjected to single crystal XRD by using Bruker APEX CCD diffractometer. The source used in this analysis is graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The study reveals that the LPLCM single crystal belongs to monoclinic crystal system having lattice dimensions of a = 7.7641, b = 5.3757, c = 10.3254 with the space group of P2₁ which shows a suitable concurrence with the reported literature (Uma Devi et al., 2009). The space group indicates that the crystal is a noncentrosymmetric which is an elementary condition for SHG applications.

The diffracted planes present inside the crystal can be depicted out along the different angles (i.e. 2θ) by using powder X-ray diffraction. The single crystal was crushed into fine powder and then subjected to Rigaku X-ray diffractometer with Cu Kᾳ radiation (λ ∼ 1.54 Å) with scan speed of 4°/min over a range of 20–70°. The diffraction pattern was obtained along the 2θ values which are shown in Fig. 2(a). The strain calculations can be performed with respect to the data obtained from powder XRD by using Hall Williamson equation βCos θ = Kλ/τ + ηsin θ, where β, θ, K, λ and τ are full width at half maxima (FWHM) of diffraction peak, Bragg diffraction angle of the peak, Scherrer constant, wavelength of X-rays and crystallite size respectively. The value of the strain was measured with the slope drawn against the curve of βCos θ Vs Sin θ shown in Fig. 2(b). The slope value (η) is 0.004 with slight error of 0.00353 which suggests that the crystal carries compressive strain.

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Figure 2

Powder X-ray diffraction pattern along with βCos θ versus sin θ plot.

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3.2 Crystalline perfection

A PANalytical X’Pert PRO MRD high-resolution XRD system, with Cu Kα₁ radiation, was employed to assess the crystalline perfection of the grown crystal. The rocking curves of the crystal for the diffraction planes were recorded in symmetrical Bragg geometry using the natural facets by performing the ω scan (Bhagavannarayana et al., 2010) with double-axis geometry. The monochromated X-ray beam incident on the specimen was obtained using a high-resolution four-bounce Ge(220) monochromator. The diffracted beam from the specimen was detected using a scintillation detector without using any analyzer at the receiving stage (i.e. before the detector) to get all the possible information such as the individual peaks from structural grain boundaries, scattered intensity from the dislocations and other defects from the specimen crystal.

Fig. 3 shows the high-resolution diffraction curve (DC) recorded for a typical LPLCM single crystal specimen using (−2 1 1) diffracting planes in symmetrical Bragg geometry with Cu Kα₁ radiation. As seen in the figure, the DC contains a single peak and indicates that the specimen is free from structural grain boundaries. The full width at half maximum (FWHM) of this curve is 29 arc sec which is somewhat more than that expected from the plane wave theory of dynamical X-ray diffraction (Batterman and Cole, 1964) and reveals the presence of point defects and their aggregates. It is interesting to see the shape of the DC. The DC is asymmetric with respect to the Bragg peak position. For a particular angular deviation (Δθ) of glancing angle (θ) with respect to the Bragg peak position (taken as zero for the sake of convenience), the scattered intensity is much more in the negative direction in comparison with that of the positive direction. This feature clearly indicates that the crystal contains predominantly vacancy type of defects than that of interstitial defects. This can be well understood by the fact that due to vacancy defects, the lattice around these defects undergoes tensile stress (Bhagavannarayana et al., 2008) and the lattice parameter d (interplanar spacing) increases and leads to give more scattered (also known as diffuse X-ray scattering) intensity at slightly lower Bragg angles (θ_B) as d and sin θ_B are inversely proportional to each other in the Bragg equation (2d sin θ_B = nλ; n and λ being the order of reflection and wavelength respectively which are fixed). The inset in the curve shows the schematic to illustrate how the lattice around the defect core undergoes tensile stress. The converse explanation is true in case of interstitial defects which cause compressive stress in the lattice around the defect core leading to decrease of lattice spacing and in turn results in more scattered intensity at the higher Bragg angles. It may be mentioned here that the variation in lattice parameter is confined very close to the defect core which gives only the scattered intensity close to the Bragg peak. Long range order could not be expected and hence change in the lattice parameter is also not expected (Bhagavannarayana et al., 2010). It may be worth to mention here that the defects are more or less statistically distributed in the crystal. If the defects are not statistically distributed but distributed here and there as macroscopic clusters leading to mosaic, then the strain generated by such clusters is larger leading to cracks and structural grain boundaries which can be seen very clearly in HRXRD curves with additional peak(s) as observed in our recent study on urea-doped crystals in ZTS at various levels of doping (Bhagavannarayana and Kushwaha, 2010). However, in the present experiments the diffraction curve does not contain any additional peak and indicates the absence of clustering of point defects at macroscopic level. The single diffraction peak with reasonably low FWHM indicates that the crystalline perfection is quite good.

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Figure 3

Diffraction curve recorded for a typical LPLCM single crystal using (−2 1 1) diffracting planes with Cu Kα₁ radiation. Inset shows the schematic of an interstitial defect.

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3.3 Photoluminescence (PL) and its decay kinetics

The imperfection or defects present within the crystal can be figured out using photoluminescence (PL) technique for which the emission spectrum was registered at a specific excitation wavelength at room temperature (∼20 °C). In the current experiment, the PL decay kinetics have also been determined to explain the lifetime of exciton, which is one of the essential parameters for any device fabrication. This complete analysis of the sample was recorded using Edinburg luminescence spectrometer (Model: F900) equipped with microsecond xenon flash lamp as the source of excitation. Fig. 4(a) shows the PL emission spectrum of the sample registered at an excitation wavelength of 307 nm. The emission peak scrutinized at 469 nm suggests that it lies in the blue region in accordance with the electromagnetic spectrum.

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Figure 4

PL spectrum for LPLCM single crystal and (b) Time resolved PL decay on room temperature.

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The Time-Resolved PL (TRPL) spectroscopy explains the radiative recombination of the material along a particular transition. The efficiency of the recombination varies with respect to the quality of the grown crystal (Vijayan et al., 2014). The decay time was calculated in terms of microseconds. The decay plots are usually multiexponential in nature but in the current case it followed the trend of biexponential behavior that can be expressed as

3.4 Birefringence

Birefringence is one of the important optical analyses for nonlinear optical single crystal because it will give an idea about the homogeneity and defects in the grown specimen. In the present study, the modified channel spectrum (MCS) method has been employed for quantitative assessment of the optical quality of the grown specimens. Highly perfect and polished surface of LPLCM crystal have been used for the dispersion of birefringence experiment. When light passes through the LPLCM, the extraordinary and ordinary waves have a wavelength of λ/n_e and λ/n_o respectively, that are perpendicular to each other. The light wave exit from the flat surface of the LPLCM has 180° phase difference between two polarized components. The values of the birefringence have been calculated by finding the absolute fringe order for particular wavelength and computed using the relation: Δn = kλ/t, where λ is the wavelength in nm, t, the thickness of the crystal in mm and k, the fringe order (Bhoopathi et al., 2013). The LPLCM shows two beam interference color fringes in the visible wavelength range of 490–600 nm in Fig. 5(b). The graph drawn between birefringence (∇n) and wavelength (λ) is represented in Fig. 5(a). The birefringence value of the conventional grown crystals was found to be 0.053384 at the wavelength 600 nm for the thickness of 131 μm. It is necessary to have the large value of birefringence i.e. the crystal carries wide range of phase matching angles when a NLO crystal was subjected for harmonic generation or parametric oscillation device. However, the converting efficiency of the laser in harmonic generation is lowered as the birefringence of the crystal becomes larger. Therefore, if the crystal is used only in harmonic generation device with a particular wavelength range, only the minimum birefringence of the crystal which causes phase matching is required. The low birefringence value (Δn = 0.0573 at k = 532 nm) indicates that the crystal is suitable for harmonic generation device (Krishnan et al., 2014). The obtained values were found to be positive integer and decrease with increasing wavelength, which illustrated the grown LPLCM possess negative dispersion of birefringence and crystal belongs optically positive at room temperature respectively. In a case if the crystal carries the negative dispersion then it can be used to balance the positive dispersion of the laser medium (Fork et al., 1984). A slight dispersion shows in birefringence can highly be helpful in frequency conversion processes such as second and other high order harmonics generations (Yariv and Yeh, 1984). The plot Fig. 5(b) depicts some distortions from the linearity curve, which may be due to the short range defects in the specimen and is in tune with HRXRD (FWHM) value. The interference pattern of birefringence is an important tool to determine the optical homogeneity and defects in the crystals (Sangeetha et al., 2011). The dispersion pattern shows nonuniform fringes in LPLCM and it suggested signature of slight nonhomogeneity inside the crystal. In the pattern, the fringes are seen with slight disturbance which may be due to the slight defects or thermodynamical considerations during the growth of LPLCM from solution.

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Figure 5

(a) Birefringence interferogram and (b) plot drawn against Birefringence versus wavelength.

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3.5 Dielectric measurements

In case of nonconducting materials, the dielectric measurements are related to the electro-optic properties of the materials. The dielectric measurements have been carried out by using ‘Alfa-A’ high impedance analyzer (Nova Control technologies). The silver coating has been applied along both sides of plane (−2 1 1) that act as an electrode. Fig. 6a, 6b shows the dielectric constant ( ${\epsilon }_r)$ and dielectric loss (tan δ) with respect to the frequency over a range of 10 Hz–10 MHz at room temperature. It was observed that the ${\epsilon }_r$ values are higher at low frequency because of the presence of four types of polarization (i.e. space charge, dipolar, ionic and electronic polarizations) but at 1 kHz frequency its values become constant. The decrement in the values of ${\epsilon }_r$ at higher frequency is due to contribution of ionic and electronic polarization only (Riscob et al., 2011). As lower the value of the dielectric constant more the material will suitable for the enhancement of SHG property, with respect to Miller rule (Von Hundelshausen, 1971). The behavior of the dielectric loss curve at low frequency was completely depending upon the factors like defects and size of the crystal etc (Przesiawski et al., 1995). The low value of dielectric loss suggests that the crystal has less defects/impurities which were confirmed by HRXRD measurement.

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Figure 6

(a) Dielectric constant against frequency and (b) dielectric loss versus frequency.

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3.6 Laser threshold damage

Before going for any device fabrication it is necessary to analyze the bond strength or thermal stability of the single crystal by applying single shot and multiple shots of laser using laser damage threshold technique. The laser damage threshold of LPLCM was examined by high intensity Nd:YAG laser. In the present study, (−2 1 1) plane was exposed to Nd: YAG laser of wavelength 1024 nm to find out damage cause on surface of the crystal by applying single and multiple shots of the beam (Vanishri et al., 2006). It was seen that if the crystal possesses high specific heat then it carries high resistance toward laser exposure. And at the same time, it generates temperature which causes some damage on the surface of the specimen. Apart from the specific heat, crystalline perfection is also very important reason for laser threshold damage because if the crystal carries high defect density then they get maximum groves of energy absorption and results in the damage of the crystal surface. The values are observed for both single shot and multiple shots and it is given in Table 1. These values are further help in locating the tolerance power of single crystal against laser shots. It was found that the threshold value of multiple shots is less than single shot due to dominance of thermal effect in the single crystal.

3.7 Z-scan

The variation in the optical absorption properties of a material induced by an intense laser beam can be determined using the open aperture Z-scan technique. In a typical open aperture Z-scan experiment, the laser beam was focused using a Plano-convex lens of focal length 10.75 cm. The laser propagation direction is taken as Z and the sample is scanned along the Z direction through the focal region of the lens in small steps (200 μm) while its optical transmission is being measured. Sample dispersed in distilled water was taken in a1 mm cuvette which was mounted on a stepper motor controlled linear translational stage. The sample experiences different incident intensity (fluence) at each position ‘Z’ and the corresponding transmission is measured using a pyroelectric energy detector (Rj 735). A frequency doubled output (532 nm) from a Q-switched Nd:YAG laser was used for the Z-scan experiment. The nearly Gaussian laser pulse obtained from the laser has a pulsewidth (FWHM) of 5 ns and the average input laser pulse energy used was 150 μJ (Philip et al., 2012).

The intensity dependent nonlinear transmission measured in the present sample is shown in Fig. 7(a). The sample used has a linear transmittance of 80% at the excitation wavelength of 532 nm. The open aperture Z-scan curve exhibits a valley, which indicates a typical absorptive nonlinearity. The nonlinear transmission as a function of input intensity is plotted in Fig. 7(b). To understand the real mechanism behind this nonlinearity we have numerically fitted the experimental data to deferent nonlinear transmission equation and the obtained nonlinear transmission data are found fit well to a two photon absorption process. From the best fit the nonlinear absorption coefficient is calculated. The nonlinear transmission equation for a third order nonlinear process is given by,

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Figure 7

(a) Open aperture Z-scan curve measured for 532 nm, 5 ns laser pulse excitation at an average energy of 150 μJ. (b) Nonlinear transmission as a function of input intensity calculated from the Z-scan curve.

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3.8 PPE

Photopyroelectric (PPE) technique is a nondestructive technique which is used to measure the different thermal effects cause due to the periodical variation of temperature of the sample. The main aim of the technique is to determine the thermal parameters such as thermal diffusivity, thermal conductivity, thermal effusivity and specific heat of the crystal at ambient conditions (Menon and Phillip, 2000). The technique is highly beneficial to study the thermal properties of the title compound when it undergoes in phase transitions. Thermal parameters measured using this technique become very helpful when the material undergoes any phase transition such as ferroelectric transition, Para-ferroelectric phase transition and many more.

In the present study, one side of the sample was exposed to the irradiated 120 mW He–Cd laser (λ = 442 nm) and on the other side the temperature variation was recorded by using photopyroelectric detector (Mandelis et al., 1985). The detector was further attached with the thermally thick conductive backing to reduce the temperature fluctuation. The variation in temperature was arisen due to the energy absorption of the crystal which depends on quality of the grown crystal. The pyroelectric detector was 28 μm thick coated with Ni–Cr coated PVDF film, and further the detected PPE signals are amplified and processed with a lock-in-amplifier. For the device fabrication it is necessary to find the exact value of thermal conductivity because if in any case the thermal conductivity find low then it prevents the conduction which results in the accumulation of heat inside the crystal that causes the damage or cracks in the crystal. The phase and amplitude with respect to the frequency has been plotted in Fig. 8. From the curve thermal diffusivity and thermal effusivity were determined. Further these parameters can calculate thermal conductivity and heat capacity by using following relation

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Figure 8

PPE Amplitude Vs Frequency Curve and (b) Phase Vs Frequency Curve for LPLCM single crystal.

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3.9 Density measurement

The density of LPLCM was calculated experimentally by using floating method. It is the most precise and sensitive way to determine the density of crystal (Vijayan et al., 2006; Andreev and Hartmanoa, 1989). In the present experiment kerosene oil is used as reference. The standard volume of the sinker $V_s$ is 10 cm³. Initially the density of the kerosene was measured by using following expression