Growth and characterization of proficient second order nonlinear optical material: L -asparaginium picrate (LASP)

Optics & Laser Technology 83 (2016) 67–75

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Growth and characterization of proficient second order nonlinear optical material: L-asparaginium picrate (LASP) M. Saravanan a,b,n, A. Senthil a, S. Abraham Rajasekar b,c a

Department of Physics, SRM University, Ramapuram Campus, Chennai, Tamilnadu, India Research and Development Centre, Bharathiar University, Coimbatore, Tamilnadu, India c Department of Physics, Sir Theagaraya College, Chennai, Tamilnadu, India b

art ic l e i nf o

a b s t r a c t

Article history: Received 19 August 2015 Received in revised form 18 December 2015 Accepted 22 March 2016

Good optical quality, potential second order nonlinear optical crystal L-asparaginium picrate (LASP) was grown by the slow cooling method. The solubility and metastable zone width of LASP specimen was studied. The LASP crystal belongs to monoclinic crystal system with noncentrosymmetric space group P21. UV–Visible-NIR transmittance spectrum determines the optical band gap of LASP. Excellence of the grown crystal is ascertained by the etching studies. Laser Damage Threshold and Photoluminescence studies designate that the grown crystal contains less imperfection. The mechanical behaviour of LASP sample was investigated at different temperatures. The piezoelectric nature, Photoconductive nature and the relative Second Harmonic Generation (for various particle sizes) of the material were also studied. Birefringence and ocular (optical) homogeneity of the crystal were assessed using modified channel spectrum method. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Solubility Single crystal growth Optical properties of material Piezoelectric studies SHG applications Nonlinear optical materials

1. Introduction Recently, organic non linear optical (NLO) materials are sketching added attention due to their soaring non linear coefficient, constancy in physical and chemical properties, and swift rejoinder to electro optic effect. They have already been used in different aspects of optical devices. [1]. A second harmonic analyzer has builded which can determine the space group ambiguities arising from Friedel's Law with a confidence level greater than 99% [2]. Organic materials have been of recognizable attention because the nonlinear optical rejoinders in this broad class of materials is infinitesimal in origin, providing an opportunity to utilize theoretical modeling united with synthetic suppleness to design and produce novel materials [3,4]. They are having a large deal of attention, as they contain soaring optical susceptibilities, inherent ultra swift response times and great optical thresholds for laser power compared with inorganic materials. Organic molecules with significant nonlinear optical activity generally consist of π-electron conjugated moiety substituted by an electron donor group on one end of the conjugated structure and an electron acceptor group on the other end. The conjugated π -electron n Corresponding author at: Department of Physics, SRM University, Ramapuram Campus, Chennai, Tamilnadu, India and Research and Development Centre, Bharathiar University, Coimbatore, Tamilnadu, India. E-mail addresses: [email protected], [email protected] (M. Saravanan).

http://dx.doi.org/10.1016/j.optlastec.2016.03.026 0030-3992/& 2016 Elsevier Ltd. All rights reserved.

moiety provides a pathway for the entire length of conjugation under the perturbation of an external electric field. The donor and acceptor groups proffer the ground state charge asymmetry of the molecule, which is obligatory for second-order nonlinearity. It is necessary to assess the excellence of the grown crystals and its mechanical strength, optical properties for device purpose. The chemical etching method to access the quality of the grown crystal [5]. Picric acid produces crystalline picrates of various organic molecules through ionic and hydrogen bonding and π-π interactions [6]. Moreover, picric acid acts not only as an acceptor to shape diverse π stacking complexes with other aromatic molecules but also as an acidic ligand to form salts through definite electrostatic or hydrogen bond interactions [7]. Bonding of electron donor/acceptor picric acid molecules sturdily depends on the character of the partners. The linkage could involve in electrostatic interactions and the formation of molecular complexes [8]. L-asparaginium picrate (LASP) is a π donor acceptor molecular compound in which L-asparagine acts as donor and the picric acid as electron acceptor. The earlier report dealt with the crystal structure and limited physicochemical properties of LASP [9,10]. Hence, we focus the bulk growth, UV–Visible-NIR spectral analysis, Chemical etching studies, Laser Damage Threshold studies,Mechanical, Piezoelectric studies and second harmonic generation (for various particle size) of the title compound (LASP) in the present investigation.

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2. Experimental

2.3. Material characterization techniques

2.1. Material synthesis and seed preparation

Single crystal XRD studies and morphology were carried out using Enraf Nonius CAD-4 single X-ray diffractometer to determine the lattice parameters and the space group. The optical properties of the grown crystals were studied using the Perkin-Elmer Lambda 35 UV–Vis spectrometer in the wavelength region from 200 to 1100 nm. Fluorescence spectra were recorded with the help of Perkin Elmer LS45UV fluorescence spectrophotometer. The chemical etching analysis was carried out using an OLYMPUS U-TV0.5XC-3 optical microscope in the reflection mode. The piezoelectric studies were made using the piezometer system. A precision force generator applied a calibrated force (0.25 N), which generated a charge on the piezoelectric material under test. Microhardness measurement was taken of LASP at different temperatures (303 K, 333 K, 373 K, 393 K, 423 K and 453 K) using shimadzu HMV-2000 fixed with Vikcer’s pyramidal indentor. Nonlinear optical properties were tested by Kurtz Perry powder technique. Laser damage threshold study was also carried out for the LASP crystal.

Single crystals of LASP were obtained using a blended solvent of acetone and water (1:1). The solubility of Picric acid is less in water, while the L-asparagine proves high attraction towards solubility in water. Equimolar magnitudes of the parent compounds picric acid and L-asparagine were liquefied in a concoction of acetone and water (1:1). The salt was obtained by crystallization at low temperature (25 °C) by cooling the solution. The material was decontaminated from aqueous solution by the recrystallization process. Thus, the solitary crystals of LASP have been grown from the saturated solution of synthesized salt by the slow evaporation technique at 30 °C using a constant temperature bath having a control accuracy of 7 0.01 °C. 2.2. Growth from slow cooling technique The decontaminated Seed crystals (gratis from macro defects) obtained by spontaneous nucleation from the saturated solution of LASP was used for bulk growth. The bulk growth of LASP single crystals were carried out by low temperature solution growth technique by slow cooling method. According to the solubility data of LASP in equimolar concentration of solute (Acetoneþdeionized water), the saturated solution of LASP at 40 °C was prepared from recrystallized material. The solution was sifted to eradicate any other insoluble impurities. We carefully ensured that the prepared solution was well within the metastable zone width region. A constant temperature bath controlled at an accuracy of 70.01 °C was used for the low temperature solution growth technique by slow cooling. Transparent and good quality seed crystals of size 6  4  5 mm3 obtained from slow evaporation technique were selected for the growth of bulk LASP single crystals by slow cooling method. The solution was maintained at 40 ºC in constant temperature bath for 2 days before seeding. The cooling rate of 0.3 °C/ d was employed throughout the growth period till 25 °C was reached. After the completion of the growth run, optically transparent crystal of size 10 mm  9 mm  5 mm was harvested in a period of 50 days [Fig. 1(a)]. The morphology of LASP ascertained that (0 0 1) plane is more prominent than other 8 developed faces as well as analogous friedel plane is too exist for each plane in the fully fledged crystal, which is shown in Fig. 1(b).

3. Results and discussions 3.1. Solubility and metastable stable zone width measurements Metastable zone width is an indispensable stricture for the augmentation of bulk size crystals from solution since it is the direct measure of the stability of the solution in its supersaturated region. The solubility of LASP in acetone and water (1:1) was measured as a function of temperature in the range 30–55 °C. The solubility was ascertained by dissolving the three epoch’s recrystallized solute in deionized water in an airtight container maintained at a constant temperature with continuous stirring. The solution was constantly stirred for 2 h using a magnetic stirrer for homogenization. The symmetry concentration of the solute was analysed gravimetrically after attaining the saturation. The solubility curve was thus obtained. Drenched solution of LASP has been prepared in harmony with the currently ascertained solubility curve for the nucleation experiments. The studies were carried out in a constant temperature bath controlled to an accuracy of 70.01 °C, provided with a cryostat for cooling below room temperature. Metastable zone width of LASP was measured using the polythermal method [11]. The acquired solubility and nucleation

Fig. 1. (a) Photographs of as grown single crystal of LASP by slow cooling method. (b) Morphology of as grown single LASP crystal.

69

70

10

Nucleation curve Solubility curve

LASP 60

9 50

Transmittance(%)

Concentration (g/100 ml) [Acetone + Water] (1:1)

M. Saravanan et al. / Optics & Laser Technology 83 (2016) 67–75

8

7

40

30

20

10

6 20

25

30

35

40

45

50

55

60

400

500

curve for LASP is shown in Fig. 2. It is seen from the figure that the LASP has a constructive gradient of solubility. It also shows that LASP has good solubility in the assorted solvent of acetone and water (1:1) and the solubility augments almost linearly with temperature. Hence low temperature solution growth technique by slow cooling method could be an enhanced method to grow good quality single crystals of LASP. The metastable zone width decreases with increasing temperature. 3.2. Single crystal X-ray diffraction study Single crystal X-ray diffraction analysis was carried out to identify the grown crystal and to determine the cell parameters and crystal structure. The obtained lattice parameter values are tabulated in Table 1, which corroborates that the grown crystal is L-asparaginium picrate in monoclinic crystal system with noncentrosymmetric space group P21, as it concurs with the reported values [9]. 3.3. Optical studies The optical transmission spectra of single crystal LASP is shown in Fig. 3. The optical transmission range, transparency cut-off and absorbance band are important optical parameters for laser frequency conversion applications. The LASP crystal is optically translucent in the complete visible region with 60% transmittance and cut-off wavelength of 532 nm corresponds to carboxylic group of the C ¼O moiety due to existence of π-π* transition, which can develop the second harmonic throughput. The percentage of Table 1 Single crystal X-ray diffraction data in comparison with literature. Crystal data (parameter)

Present study

[9]

Empirical formula Crystal shape, colour Crystal system Space group Unit cell dimension

C4H9N2O3 þ  C6H2N3O7  Block, yellow

C4H9N2O3 þ  C6H2N3O7  Block, yellow

Cell volume

Monoclinic Monoclinic P21 P21 a ¼ 10.342 (5) Å, a ¼10.367 (4) Å, b¼ 5.1312 (6) Å, b¼5.1611 (7) Å, c ¼ 13.113 (2) Å, c ¼13.120 (3) Å, α ¼ 90° , β¼ 93.18° ,γ ¼ 90° , α¼ 90° , β¼ 93.20° ,γ ¼90° , 1513.21(9) Å3 1513.19 (6) Å3

700

800

900

1000

1100

wavelength (nm)

o Temperature ( C) Fig. 2. Metastable zone width of LASP in Acetone þ Water (1:1).

600

Fig. 3. Transmission spectra for the grown crystal of LASP.

transmission (60%) may be accredited to abridge scattering from crystals point and line defects. The asymmetric unit of L-asparaginium picrates consist of an asparaginium cation and a picrate anion where the carboxyl group is protonated. Thus the π-π* transition arises in the carboxyl group, which provides the enhancement of NLO behaviour in this material [12]. The characteristic absorption band is experiential at 430 nm that may be attributed to amine substituent with N-H moiety, and there is no absorption band among 435 and 1100 nm [10]. Therefore, the crystal is anticipated to be translucent to all the UV–visible-NIR radiation flanked by these two wavelengths. This result formulates LASP crystal an excellent entrant for the second order NLO applications. The transmittance window in the visible region enables a good optical transmission of the second harmonic frequencies of Nd:YAG lasers. This transparent environment in the visible region is an attractive property for this material for NLO applications [13]. 3.4. Determination of optical band gap The optical absorption is an imperative tool to comprehend the band gap of optical materials [14]. The optical absorption coefficient (α) was calculated from the transmittance spectra using the relation, α ¼2.3036 log (1/T)/d, where T is the transmittance and d is the thickness of the crystal (2 mm). As a corollary of direct band gap, the crystal under swot has absorption co-efficient (α) obeying the relation for high photon energies (hυ), α ¼A(hυ  Eg)1/2/hυ, where Eg is the band gap and A is a constant. The optical band gap of LASP single crystal is appraised by extrapolating the linear portion of the Tauc’s graph [15] plot between (αhυ)2 and (hυ) is shown in Fig. 4. The band gap is found to be Eg ¼2.3 eV. As a corollary of wideband gap, the grown crystal has high transmittance in the complete visible and IR region. 3.5. Etching studies The etch pits patterns depend on the crystal faces, etchant and etching time. Etch pits are united with dislocations, dislocation bundles and hence bring out the crystal quality [16, 17]. The NLO behaviour of the crystal depends on the flawlessness of the grown crystal. Etching studies were performed using water as etchant at room temperature for 20 s on (3 0 1) plane of LASP single crystal. Fig. 5 shows the microscopic imagery of etch patterns (scale in 200 mm) produced on LASP single crystal which illustrates the series of ridges analogous to c-axis on its face. Estimated etch pit density (EPD) was found to be 48.4  102 cm  2. This is a low EPD,

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other organic NLO crystals which are shown in Tables 2 and 3. Superior value of LDT designated that a grown crystal contains low defect. Hence, the LASP crystal can be used for NLO and related high power laser applications due to its superior laser damage threshold values.

7000

6000

5000

3.7. Photoluminescence studies

3000

Defects in single crystals play a significant role in the evaluation of their optical properties and PL is extremely perceptive to the occurrence of such defects [19,20]. Aromatic molecules or the molecules which contains multiple conjugated double bonds with soaring degree of resonance stability shows the fluorescence effect [21]. The emission spectrum (Fig. 6) was measured in the range 400–900 nm. A peak at 546 nm was observed in the emission spectrum. These results designate that the LASP crystals have a green secretion under excitation. This green band emission at 2.27 eV maybe ascribed to radiative recombination between donors and acceptors [22]. The charge transfer and protonation of charge is extremely eloquent with second order NLO activity, where the crystal liberates green gesture for photon absorption. This sturdy emission designates there may be the presence of intrinsic defects in the forbidden band region [23]. This PL emission in LASP may be the existence of electron contributing group NH and electron withdrawing group COOH, which can develop the mobility of π electrons. The observed single maximum intensity peak at 546 nm in the inferior wavelength region is due to the existence of N-H moiety in L-asparagine to act as a proton acceptor. This also may be the protonation of amino group to the carboxyl group. However, the intensity is slowly reduced in the higher wavelength region. The inferior of photoluminescence intensity at higher wavelength region may be ascribed to a relatively low barrier for revolving of the carboxyl group around the central C-C bond. This analysis enumerates the intensity was decreased with the increasing wavelength and it attains nearly a zero value for the higher wavelengths. It also may be the formation of strong O-H…O and N-H…O hydrogen bonds by the essential role of picrate anion in hydrogen bonding (intermolecular hydrogen bonding) with the L-asparaginium residue [9]. Generally, the hydrogen bonds engross an interaction flanked by a hydrogen bond to sp3 nitrogen or oxygen through supplementary s-bond temperament in organic nonlinear optical materials. In the current system, the amino nitrogen performs as hydrogen bond acceptor and hence the hydrogen bond polarizes the electron cloud which enhances intramolecular nonlinear procedure. The above reports illustrate that the LASP comprises a feeble intramolecular and a sturdy intermolecular hydrogen bonding between picrate anion and L-asparaginium residue which is characterized by the same O-H…O. The donor and acceptor molecules of parent complex (L-asparagine and picric acid) are held together by the interactions of Vander Waals type, which results the organization of phenolate ion of picric acid into the horde milieu of LASP. Hence, the hyperpolarizability (β) value has been augmented by the effect of intermolecular hydrogen bonding between the phenolate ion of picric acid and the L-asparaginium residue, which is the requisite chattel for a system to show evidence of nonlinear optical process.

(α hν)

2

4000

2000

1000

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

hν(eV) Fig. 4. Plot of α vs photon energy for LASP crystal.

Fig. 5. Etch patterns observed on (3 0 1) plane of LASP single crystal with water as an etchant for a period of 20 s.

which designates that the grown crystal has smaller amount of dislocations. This breed of property might be useful for NLO applications. 3.6. Laser damage threshold The efficacy of NLO crystal depends not only on the linear and NLO properties, it also depends on its aptitude to with stand high power lasers [18]. The laser damage threshold value of LASP crystals has been determined using Q-switched Nd: YAG laser for 10 ns laser pulses at a wavelength of 1064 nm. The energy of the beam was augmented from 5 mJ and the speck dimension of the laser beam is 1 mm. The power density was premeditated by the expression, Power density (Pd)¼ E/τ πr2 , where E is the input energy (mJ), τ the pulse width and r the radius of the beam. The measured multi shot Laser Damage Threshold (LDT) value for grown LASP crystal is 8.4 GW/cm2. The LDT value of LASP is comparable with high excellence NLO material like KDP, urea and

Table 2 Comparisons of laser damage, energy gap and SHG efficiency of LASP crystal with high excellence NLO crystals. S. No.

High excellence NLO material

Multi shot LDT value in GW/cm2

Energy Gap in eV

SHG efficiency

1 2 3

Potassium dihydrogen phosphate (KDP) Urea L-asparaginium picrate (present study)

0.2 1.5 8.4

6.9 6.7 3.9

1 6.1 66.5 times more competent than KDP

M. Saravanan et al. / Optics & Laser Technology 83 (2016) 67–75

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Table 3 Comparisons of laser damage, energy gap and SHG efficiency of LASP crystal with other organic NLO crystals. S. No.

Organic NLO crystals

Multi shot LDT value in GW/cm2

Energy Gap in eV

SHG efficiency relative to KDP

1 2 3 4 5 6 7 8 9

Guanidinium L-Ascorbate (GuLA) N-Bromosuccinimide (NBS) Ammonium malate bis nicotinamidium bis D-tartrate 1.25-hydrate (BNBDT) L-arginine phosphate L-arginine 4-nitrophenolate 4-nitrophenol dihydrate Imidazole–imidazoliumpicratemonohydrate(IIP) l-Asparagine monohydrate L-asparaginium picrate (present study)

2.5 3.2 0.403 0.644 1.965 0.504 7.8 2.4 8.4

3.8 4.09 6.8 5.25 5.3 3.9 3.72 4.25 3.9

0.6 1.458 0.4 1.24 3 9.33 3.6 7 66.5

250

LASP

intensity (a.u.)

200

150

100

50

0 400

500

600

700

800

900

wavelength (nm) Fig. 6. PL Emission spectrum of LASP crystal.

3.9. Hardness

200

Room Temp. 333 K 373 K 393 K 423 K 453 K

180 160

2 Hardness (kg/mm )

140 120 100 80 60 40 20 0 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

Load (kg) Fig. 7. Load vs Hardness number for (3 0 1) Plane LASP at different temperatures.

3.8. Piezoelectric measurement The piezoelectric effect portrays the manifestation of an electrical polarization in a material when a strain is applied. Although this sounds a little like photoelasticity, it differs insofar as piezoelectricity occurs only in non-centrosymmetric crystals [24]. The piezoelectric coefficient measurement was carried out for the grown crystals without poling and 0.96 pCN  1 output was acquired directly from oscilloscope. This breed of property constructive for SHG applications.

Superior excellence single crystals are extremely needed for device fabrication. The second harmonic generation is always inferior from the flawed sectors compared to that from the more perfect sectors in single crystals. The good quality crystals are obligatory not only with excellent optical recital but with good mechanical behaviour [25, 26]. Microhardness testing is the method to understand the mechanical properties of materials such as yield strength, brittleness index, temperature of cracking and fracture behaviour [27, 28]. Vickers diamond pyramidal number, Hv was calculated from the following equation: Hv ¼[(1.8544 P)/d2] kg/mm2, where P is the applied load in kg and d is the diagonal length of indentation in millimetre [29]. The sample was subjected to heat treated at different temperatures such as room temperature (303 K), 333 K, 373 K, 393 K, 423 K and 453 K. Then the indentations were made on the (3 0 1) prominent face of the sample to gauge the microhardness (Hv ) for the applied loads varying from 0.01 to 0.1 kg for the reside time 15 s using Reichert Polyvar 2MET microscope. The sample was heated upto 180 °C, as LASP crystal starts melting at 223 °C [10]. Fig. 7 shows the variation of Hv as a role of applied load at different temperatures for LASP crystal. It is seen from the figure that, Hv increases with the increase of load and the same Hv decrease with increasing the temperature. The average interatomic distance becomes greater than that at room temperature due to lattice vibration while increasing the temperature. This escort to more and more lattice phonon interactions which origins the contravention of bond as well as reduce the hardness value [30]. Hence the LASP crystal can be used for NLO based device fabrication up to 223 °C.

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4. SHG study LASP

1.5

1.4

-2.5

-2.0

-1.5

-1.0

-0.5

log p Fig. 8. Variation of log P with log d of LASP crystal (n¼ 1.05).

10000

Room Temp. 333 K 373 K 393 K 423 K 453 K

Stiffness constant (C11 (x 10

14 Pa))

9000 8000 7000 6000 5000 4000 3000 2000 1000 0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

Load (kg) Fig. 9. Variation of load with Stiffness constant for LASP (3 0 1) Plane at different temperatures.

The relation involving the applied load P and diagonal length d of the indenter is given by Mayer’s law: P ¼adn , where ‘‘P’’ is the load (g), ‘‘d’’ is the diameter of recovered indentation (mm), ‘‘a’’ and ‘‘n’’ are constants for a given material. The plot of log P against log d shown in Fig. 8 is a straight line, which is in good harmony with Mayer’s law. The work hardening coefficient of grown LASP crystal was ascertained by the least-squares fit method. The n value of grown crystal at room temperature was found to be 1.05. According to Onitsch, 1.0rnr 1.6 for hard materials and n Z1.6 for soft materials [31]. Hence, LASP belongs to hard material category. Low value of work hardening coefficient (n) epitomizes lesser imperfections [32]. The yield strength can be calculated from hardness using [33]: sy ¼Hv/3(0.1)n  2 where sy is the yield strength, Hv is the hardness of the sample, and n is the logarithmic exponent. The yield strength is originated to be 59.10 MPa. The elastic stiffness constant (C11) for the plane (3 0 1) at different temperatures deliberate by Wooster’s [34] empirical formula (C11) ¼Hv7/4 is shown in Fig. 9. It is seen from the figure that the tendency of stiffness constant graph is analogous to hardness number graph, which bestows a scheme about tautness of bonding between adjoining atoms. The superior value of stiffness constant, hardness value and laser damage threshold illustrates that the title compound can be used for NLO modulators.

The SHG effectual nonlinearity of LASP fine particles were determined using Kurtz and Perry powder technique [35]. It enables to measure the SHG effective nonlinearity of materials relative to criterion potassium dihydrogen phosphate (KDP). The SHG is dependent on the polarization of the input and output doubled frequency signal. A Q-switched Nd:YAG laser operating at 1064 nm and 8 ns pulse width with an input repetition rate of 10 Hz and energy 1.9 mJ/pulse was used and the generated second harmonic signal in the crystalline specimen was inveterate from the emanation of green radiation of wavelength 532 nm from the crystalline powder. The second harmonic signal of 0.599 V was acquired for LASP crystal, while the standard potassium dihydrogen phosphate (KDP) crystal gives a SHG signal of 9 mV for the same input energy. The relative SHG efficiency of the material is 66.5 times more competent than KDP. It is clearly ascertained that LASP relatively shows very high competence compared to KDP counterparts. This powerfully recommends the title compound is a prospective aspirant for SHG applications. In adding up, the materials with noncentrosymmetric crystal structures, a screening technique can be performed to speck the materials with the capability for phase matching. The incessant amplify of SHG with increase of particle size and acquire the saturation which confirms the phase matching nature of the material [36]. The particle size addiction of SHG intensity was deliberated to authenticate the phase matching property. The SHG efficiency of those materials illustrate that it strongly depends on the particle size [37]. Solitary crystal of LASP was ground and sifted using Sieve shaker into dissimilar particle extent which ranges below 65, 65– 125, 125–250, 250–350, 350–500 and above 500 mm. The particle size addiction of SHG intensity in LASP is shown in Fig. 10. The SHG intensity increases almost linearly with increase in particle size until 250–350 mm and it digresses from the linearity and commences to accomplish the saturation above 250–350 mm. This sort of particle size reliance of SHG intensity was also observed in phase matchable crystals [38]. Hence, LASP crystal can be used as a competent frequency doubler and optical parametric oscillator. 4.1. Birefringence analysis Birefringence acting an imperative and fascinating character in nonlinear optical occurrence and is extensively employed in optical devices such as crystal filters, wave plates, liquid crystal displays and light modulators [39]. The modified channel spectrum 625

LASP

620

SHG output (mV)

log d

1.6

615

610

605

600

595 65-125

250-350

Particle Size (micro meter) Fig. 10. Plot of particle size vs SHG output.

>500

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0.033 t= 110 µm

- Linear fit

0.032

0.031

Birefringence

method has been utilized to assess the birefringence which relatively shows how it coupled with optical eminence and crystalline excellence of the grown LASP crystal [40]. Several remarkable inconsistencies were experiential while using this technique with the LASP crystal during the experimentation. In common, a 1000 W quartz iodine lamp (tungsten halogen lamp) was employed as the source to elucidate the white light for the dispersion of birefringence measurement. The CCD camera faced complications to detain the directly decipherable fringe moulds of LASP with the above source during the experiment. It is due to the yellow colour of LASP specimen, since this colour entirely soaks up inferior region of perceptible (visible) wavelength (the cut off wavelength of LASP is 532 nm in transmittance graph). The crystal faintly permits superior wavelength with reduced amount of intensity to CCD by means of analyzer and hence superior quality fringe mould not detain properly. This also inadequate for the precise dispersal of birefringence measurement. In order to obtain eminence fringe pattern, the quartz iodine lamp was reinstated with high power monochromatic He–Ne laser (wavelength (λ)¼ 632.8 nm) in the modified channel spectrum experimental setup for more dispersion of birefringence analysis. The birefringence values have been deliberate by primary finding tassel (fringe) order for meticulous wavelength using the relation: Δn ¼kλ/t , where ‘λ’ is the wavelength in nm, ‘k’ is the fringe order and ‘t’ is the width of the crystal in mm [41]. The LASP illustrated the two beam intrusion fringes and dispersal mould which is shown in Fig. 11. It is seen from the figure that the subsistence of non uniform regularity tassels in LASP. In view of the monoclinic crystal system of LASP, a solitary indicatrix axes constantly concur with the crystallographic b axis, the other two axes build an angle with crystallographic a and c axes. The feeble intramolecular and a sturdy intermolecular hydrogen bond flanked by picrate anion and L-asparaginium residue are robust in the b-axis, which is characterized by the O-H…O. The ocular (optical) linear birefringence values were soaring in this direction and also the symmetry dispersion mould was not owing to merely the donor–acceptor potency of the current system. It is due to the Z1 (zigzag) head-to-tail string of the L-asparagine. Fig. 12 exemplify the Birefringence vs wavelength plot of LASP solitary specimen. The birefringence assessment of the grown LASP crystals was found to be 0.0291 at the wavelength 632.8 nm for the thickness of 110 mm. It is obligatory

73

0.030

0.029

0.028

500

520

540

560

580

600

620

640

660

680

Wavelength (nm) Fig. 12. Birefringence vs wavelength conspire of LASP solitary specimen.

for the crystal to encompass fairly vast birefringence (which origins wide range phase matching) when a crystal was employed in a harmonic generation or parametric oscillation device. If birefringence of the crystal turns out to be larger, the converting competence of the laser in harmonic generation becomes lower. Hence, the minimum birefringence of the crystal is requisite which results phase matching, when the crystal is employed barely in harmonic generation device with a meticulous wavelength range. The squat birefringence value (Δn ¼0.0291 at λ ¼632.8 nm) designates that the LASP is appropriate for harmonic generation contrivance. The acquired birefringence values were found to be constructive (positive) integers and diminish with raising the wavelength. It also exemplify that the fully fledged LASP obsessed unconstructive (negative) dispersal of birefringence and optically positive at room temperature. The negative dispersal in materials can be employed for progress of a couple of prisms for the assembly of a net unconstructive dispersion, consequently offer a mode to assist in pondering the positive dispersion of the laser medium [42]. In most of the optical applications anticipate dispersion of birefringence value, which necessitate usage of crystals resembling in polarizers, formulate and progress of retardation plates etc. The lesser birefringence dispersal errands reasonable conversion competence to harmonic generation claim [43]. The conspire (Fig. 12) portrays few distortions from the linearity curve, which may be owing to the point imperfections in the crystal. The ocular homogeneity and defects in the crystals can be indomitable by birefringence interferogram [44]. The interferogram of LASP crystal demonstrates non uniform tassel widths thus presenting mark of slender non homogeneity inside the crystal which is due to the existence of interstitial type of defects as supported by Photoluminescence emission spectrum illustrated in preceding section. This besides inveterate from trivial riot in the visible region of transmittance curve. 4.2. Photoconductivity studies

Fig. 11. Birefringence interferogram of LASP solitary crystal.

In order to develop the crystal appliance for NLO and photonic contrivance fabrications, photoconductivity measurement was made on the slash and refined sample of LASP using KEITHLEY – 485 picoammeter in the occurrence of d.c electric field. The rectangular element of sample was set on a glass plate. Electrical contacts were made on the sample by silver painted copper wire as electrode with an electrode distance of 0.5 cm. The sample was then joined in sequence to a d.c power supply and a picoammeter. The sample was defended from all radiations then the applied field

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8.4 GW/cm2. Higher laser damage threshold values of LASP crystal shows that it can be used for NLO and related high power laser applications. The promising intensification characteristics of LASP crystal illustrates that, it is a potential Second Order Nonlinear Optical Material for NLO modulators. Squat birefringence of LASP acquired using modified channel spectrum method inveterate the handling of LASP crystal for harmonic generation applications. However, there was small disturbance in optical homogeneity as specified from interferogram. Photoconductivity measurement enumerates consummate of inducing dipoles due to strong incident radiation and also divulge the nonlinear behaviour of LASP crystal.

250

Photo current Dark current

Current (nA)

200

150

100

50

Acknowledgements 0 0

100

200

300

400

500

600

700

800

Applied field (V/cm) Fig. 13. Photoconductivity behaviour of LASP crystal.

was augmented from 50 to 700 V/cm and the analogous dark current (Id) in the picoammeter is recorded. The sample was subsequently elucidated with the emission from a halogen lantern (100 W) to record the photocurrent (Ip) due to the generation of carriers by photo excitation for the similar array of applied field. The disparity of dark current (Id) and the photo current (Ip) as a function of applied field is revealed in Fig. 13. It is experiential that the dark current (Id) and the photo current (Ip) illustrate linear response to the applied field. It is also establish that at every instantaneous, the dark current is greater than the photo current. Therefore, the above results concluded that LASP solitary crystal reveals unconstructive photo conductivity. This is due to the decrease in the number of charge carriers or their life span in the incidence of radiation. It also exemplify the higher energy level is situated between the fermi level and the conduction band whilst the other is positioned close to the valence band. The decreases in the number of charge carriers in the incidence of radiation also due to the soaring detain cross sections for electrons and holes. This status is able to detain electrons from the conduction band and holes from the valence band, due to which the number of mobile charge carriers diminished and it offers negative photoconductivity in the incidence of radiation.

5. Conclusions Optically good quality single crystals of LASP were successfully grown by the slow cooling method. The grown LASP crystal was confirmed by X-ray diffraction analysis and it is found that the crystal belongs to the monoclinic system with the space group P21. Transmittance spectrum shows that the crystal possesses high transparency in the intact visible and IR region with a wide band gap of 3.9 eV. Estimated etch pit density (EPD) was found to be 48.4  102 cm–2. The photoluminescence spectrum confirms a strong green emission, which indicates the high charge transfer and protonation. It also enumerates the subsistence of intrinsic defects in the forbidden energy gap of the LASP crystals. The grown LASP crystal has higher piezoelectric charge coefficient. Hardness number increases with increase of load. The hardness cram projects the crystal to be quite hard. The yield strength of LASP specimen is found to be 59.10 MPa. SHG study manifestly ascertained that LASP fairly shows very high competence compared to KDP counterparts. The particle size reliance of SHG intensity in LASP is shown that it strongly depends on the particle size. The laser damage threshold was deliberated and the value is

The authors are thankful to SAIF, IIT Chennai for scientific supports. The authors acknowledge Prof. P.K. Das, IISc Bangalore for SHG facility.

References [1] V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan, Hand Book of Nonlinear Optical Crystals, 2nd ed., Springer, NewYork, 1997. [2] J.P. Dougherty, S.K. Kurtz, A second harmonic analyzer for the detection of non-centrosymmetry, J. Appl. Cryst. 9 (1976) 145. [3] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers, Wiley, New York, 1991. [4] Nonlinear Optical Properties of Organic Molecules and Crystals, in: D. S. Chemla, J. Zyss (Eds.), Academic Press, New York, 1987. [5] Tanusri Pal, Tanusree Kar, Optical, mechanical and thermal studies of nonlinear optical crystal l-arginine acetate, Mater. Chem. Phys. 91 (2005) 343–347. [6] S. Yamaguchi, M. Goto, H. Takayanagi, H. Ogura, The crystal structure of phenanthrene: picric acid molecular complex, Bull. Chem. Soc. Jpn. 61 (No. 3) (1988) 1026–1028. [7] Y. In, H. Nagata, M. Doi, T. Ishida, A. Wakahara, Imidazole-4-acetic acid-picric acid (1/1) complex, Acta Crystallogr. C 53 (1997) 367–369. [8] P. Zaderenko, M.S. Gil, P. Lopez, P. Ballesteros, I. Fonseco, A. Albert, Diethyl 2benzimidazol-1-ylsuccinate-picric acid-(1 1) – an inclusion molecular complex, Acta Crystallogr. B 53 (1997) 961. [9] K. Anitha, S. Athimoolam, R.K. Rajaram, L-asparaginium picrate, Acta Crystallogr. E 61 (2005) o1463–o1465, ISSN 1600-5368. [10] P. Srinivasan, T. Kanagasekaran, R. Gopalakrishnan, G. Bhagavannarayana, P. Ramasamy, Studies on the growth and characterization of L-asparaginium picrate (LASP) – a novel nonlinear optical crystal, Cryst. Growth Des. 6 (7) (2006) 1663–1670. [11] K. Sangwal, On the estimation of surface entropy factor, interfacial tension, dissolution enthalpy and metastable zone-width for substances crystallizing from solution, J. Cryst. Growth 97 (1989) 393–405. [12] T. Pal, T. Kar, G. Boceli, L. Rigi, Morphology, crystal structure and thermal and spectral studies of semi organic nonlinear optical crystal LAHClBr, Cryst. Growth Des. 4 (2004) 743–747. [13] A. Senthil, P. Ramasamy, Synthesis, growth and characterization of strontium bis (hydrogen L-malate) hexahydrate bulk single crystal: a promising semiorganic nonlinear optical material, J. Cryst. Growth 312 (2010) 276–281. [14] F. Yakuphanoglu, M. Arslan, The fundamental absorption edge and optical constants of some charge transfer compounds, Opt. Mater. 27 (2004) 29–37. [15] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Physica Status Solidi 1966 (15) (1966) 627–637. [16] J. Bohm, R.B. Heimann, M. Hengst, R. Roewer, J. Schindler, Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN), and La3Ga5.5Ta0.5O14 (LGT): Part I, J. Cryst. Growth 204 (1999) 128. [17] I.H. Jung, K.B. Shim, K.H. Auh, T. Fukuda, The defect distribution and chemical etching of Langasite (La3Ga5SiO14) crystals grown by the Czochralski method, Mater. Lett. 46 (2000) 354. [18] G.C. Bhar, A.K. Chaudhary, P. Kumbhakar, Study of laser induced damage threshold and effects of inclusions in some nonlinear crystals, Appl. Surf. Sci. 161 (2000) 155–162. [19] G. Yue, J.D. Lorentzen, J. Lin, D. Han, Q. Wang, Photoluminescence and Raman studies in thin-film materials: transition from amorphous to microcrystalline silicon, Appl. Phys. Lett. 75 (1999) 492–494. [20] E. Tournie, C. Morhain, G. Neu, J.P. Faurie, R. Triboulet, J.O. Ndap, Photoluminescence study of ZnSe single crystals grown by solid-phase crystallization, Appl. Phys. Lett. 68 (1996) 1356–1358. [21] H.H. Willard, L.L. Merritt Jr., J.A. Dean, F.A. Settle Jr., Instrumental Methods of Analysis, sixth ed, Wadsworth Publishing Company, USA, 1986.

M. Saravanan et al. / Optics & Laser Technology 83 (2016) 67–75

[22] T. Kanagasekaran, P. Mythili, P. Srinivasan, N. Shailesh, Sharma, R. Gopalakrishnan, Synthesis, crystal growth and characterization of organic material N-Broumousuccinimide for NLO applications, Mater. Lett. 62 (2008) 2486–2489. [23] A. Meijerink, G. Blasse, M. Glasbeek, Photoluminescence and EPR studies on Zn4B6O13, J. Phys. Condens. Matter 2 (1990) 6303–6309. [24] Geoffrey New, Introduction to Nonlinear Optics, 2011, ISBN 978-0-521-877015, pg-96. [25] J.N. Sherwood, The growth, perfection and structural properties of organic electro-optic materials, Pure Appl. Opt. 7 (1998) 229–238. [26] I.V. Kityk, B. Marciniak, A. Mefleh, Photo induced second harmonic generation in molecular crystals caused by defects, J. Phys. D: Appl. Phys. 34 (2001) 1–4. [27] B.R. Lawn, E.R. Fuller, Equilibrium penny-like cracks in indentation fracture, J. Mater. Sci. 9 (1975) (2016). [28] J.H. Westbrook, Flow in rock salt structure, H. Report 58-RL-2033 of the G.E. Research laboratory, USA,1958. [29] H. Nakatani, W.R. Bosenberg, L.K. Cheng, C.L. Tang, Laser-induced damage in beta-barium metaborate, Appl. Phys. Lett. 53 (1988) 2587–2589. [30] S. Rogalski Mircea, Stuart B. Palmar, Solid State Physics, Gordon and Breach Science Publishers, New York 2005, p. 152. [31] E.M. Onitsch, U ber die Mikroharte der Metalle, Mikroscopia 2 (1947) 131. [32] S. Jerome Das, R. Gopinathan, Growth and characterization of single crystal of lead bromo chloride, Cryst. Res. Technol. 21 (1992) 17. [33] J.P. Cahoon, W.H. Broughton, A.R. Kutzuk, The determination of yield strength

75

from hardness measurements, Met. Trans. 2 (1971) 1979. [34] W.A. Wooster, Physical properties and atomic arrangements in crystals, Rep. Prog. Phys. 16 (1953) 62–82. [35] S.K. Kurtz, T.T. Perry, A powder technique for the evaluation of non linear optical materials, J. Appl. Phys. 39 (1968) 3798–3813. [36] Kang Min O.K. PorterY, N.S.P. Bhuvanesh, P.S. Halasyamani, Synthesis and Characterization of Te2SeO7: a powder second-harmonic-generating study of TeO2, Te2SeO7, Te2O5, and TeSeO4, Chem. Mater. 13 (2001) 1910–1915. [37] J.P. Dougherty, S.K. Kurtz, A second harmonic analyser for the detection of non-centro symmetry, J. Appl. Crystallogr. 9 (1976) 145–158. [38] L.N. Wang, X.Q. Wang, G.H. Zhang, X.T. Liu, Z.H. Sun, G.H. Sun, L. Wang, W. T. Yu, D. Xu, Single crystal growth, crystal structure and characterization of a novel crystal: l-arginine 4-nitrophenolate 4-nitrophenol dehydrate (LAPP), J. Cryst. Growth 327 (2011) 133. [39] P. Krishnan, K. Gayathri, P.R. Rajakumar, V. Jayaramakrishnan, S. Gunasekaran, G. Anbalagan, Spectrochim. Acta Part A 131 (2014) 114–124. [40] G. Bhoopathi, V. Jayaramakrishnan, K. Ravikumar, T. Prasanyaa, S. Karthikeyan, Mater. Sci. – Pol. 31 (2013) 1–5. [41] P. Krishnan, K. Gayathri, G. Bhagavanarayana, V. Jayaramakrishnan, S. Gunasekaran, G. Anbalagan, Spectrochim. Acta Part A 112 (2013) 152–160. [42] R.L. Fork, O.E. Martinez, J.P. Gordon, Opt. Lett. 9 (1984) 150–152. [43] A.L. Bajor, T. Piatkowski, M. Lesniewski, Meas. Sci. Technol. 17 (2006) 427–435. [44] K. Sangeetha, R. Ramesh Babu, G. Bhagavannarayana, K. Ramamurthi, Mater. Chem. Phys. 130 (2011) 487–492.