Results in Physics 6 (2016) 645–650

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Results in Physics

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Optical and dielectric studies of KH2PO4 crystal influenced by organicligand of citric acid and L-valine: A single crystal growth andcomparative study

http://dx.doi.org/10.1016/j.rinp.2016.09.0012211-3797/� 2016 Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⇑ Corresponding authors.E-mail addresses: loganees@gmail.com (M. Anis), gajananggm@yahoo.co.in

(G.G. Muley).

Mohd Anis a,⇑, D.A. Hakeemb, G.G. Muley a,⇑aDepartment of Physics, Sant Gadge Baba Amravati University, Amravati 444602, Maharashtra, Indiab Faculty of Nanotechnology and Advanced Material Engineering, Sejong University, Seoul 143-147, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 August 2016Accepted 5 September 2016Available online 10 September 2016

Keywords:Crystal growthNonlinear optical materialsUV–visible studiesZ-scan analysisDielectric studies

In the present study pure, citric acid (CA) and L-valine (LV) doped potassium dihydrogen phosphate (KDP)crystals have been grown with the aim to investigate the nonlinear optical applications facilitated by UV–visible, third order nonlinear optical (TONLO) and dielectric properties. The structural parameters ofgrown crystals have been confirmed by single crystal X-ray diffraction analysis. The enhancement in opti-cal transparency of KDP crystal due to addition of CA and LV has been examined within 200–900 nm bymeans of UV–visible spectral analysis. In addition, the transmittance data have been used to evaluate theeffect of dopants on reflectance, refractive index and extinction coefficient of grown crystals in the visibleregion. The Z-scan analysis has been performed at 632.8 nm to identify the nature of photoinduced non-linear refraction and nonlinear absorption in doped KDP crystals. The influence of p-bonded ligand ofdopant CA and LV on TONLO susceptibility (v3), refractive index (n2) and absorption coefficient (b) ofKDP crystals has been evaluated to discuss laser assisted device applications. The decrease in dielectricconstant and dielectric loss of KDP crystal due to addition of CA and LV has been explored using the tem-perature dependent dielectric studies.� 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

In a wide range of materials available for industrial applications,potassium dihydrogen phosphate (KDP) single crystals seek enor-mous attention owing to optically active accentric structural sym-metry, high optical homogeneity, enhanced charge mobility, shortphotonic response time and large nonlinear coefficient. Inhabitingthe foresaid features the KDP single crystals are readily demandedin hi-tech fields for designing optical switching, data storage, fre-quency conversion, laser assisted remote sensing, high speed opti-cal information processing and photonic devices [1,2]. Atechnologically vital potassium dihydrogen phosphate (KDP) isan accentric NLO crystal with tetragonal structure belonging toI42d space group. The second harmonic generation (SHG) effi-ciency in KDP is majorly contributed by the phosphate group inaddition to that the outstanding third order nonlinear optical(TONLO) properties vitalize its importance for optical switchingdevices [3,4]. Since last few decades the properties of KDP have

been constantly investigated by doping Li (I), Ca (II), Ce (IV), V(V), Au+, amaranth-, rhodamine-, methyl orange-dye, urea phos-phate, L-arginine, L-histidine, L-glycine, L-alanine, L-lysine and L-glutamine to achieve the enhanced SHG efficiency, UV–visible,thermal, mechanical and dielectric properties [5–11]. At the sametime very less literature is available on studies exploring theTONLO properties of doped KDP crystal. Research interest of ourgroup has been focused on investigating the linear-nonlinear opti-cal properties of KDP crystals by doping different organic additivessuch as carboxylic acids and amino acids. The organic additivesplay a crucial role in the development of optical, second harmonicgeneration, dielectric and TONLO properties of KDP crystals whichare most essential for optical switching and laser assisted NLOapplications. Significant enhancement in NLO properties of KDPcrystal due to doping of formic acid [12], oxalic acid, maleic acid[13] and L-cysteine [14] has been reported. In the present study,following the same analogy citric acid and L-valine have beenselected for doping KDP crystals. Citric acid possesses p-electronbonding network and high electron delocalization tendency whileamino acid L-valine offers wide hydrogen bonding network, zwitterionic nature and strong polarizing nature which may facilitatelarge improvement in optical and dielectric quality of KDP crystals.

646 M. Anis et al. / Results in Physics 6 (2016) 645–650

Hence the current investigation is focused to explore the influenceof citric acid (CA) and L-valine (LV) on structural, UV–visible,TONLO and dielectric properties of KDP crystals to discuss theextended utility of doped KDP crystals for advanced NLOapplications.

Experimental details

The Merck made high purity KDP salt was gradually dissolved indouble distilled deionized water to obtain the supersaturated solu-tion of the KDP material at a temperature of 38 �C. The supersatu-rated KDP solution was filtered in two beakers. One beakercontaining the supersaturated solution of KDP was added by 1 wt% of CA and the other beaker was added by 1 wt% of LV. The bea-kers were allowed to agitate on a magnetic stirrer for six hoursat a constant speed to achieve the homogeneous doping through-out the supersaturated solution of KDP. The CA and LV dopedKDP solutions were filtered in rinsed beakers and covered by theperforated film to allow slow solution evaporation in a constanttemperature bath maintained at 38 �C with an accuracy of±0.01 �C. The recrystallization method was adopted to achieve highgrowth in crystal materials. The growth of bulk crystal at lowertemperature causes active trapping of solvent inclusions anddefects as a result of which several inclusions appear at the bound-aries of LV doped KDP crystal. The pure, CA and LV doped KDP sin-gle crystals harvested within 3–4 weeks are shown in Fig. 1.

Results and discussion

Single crystal XRD analysis

The pure and doped KDP single crystals were subjected to XRDanalysis using the Enraf Nonious CAD4 single crystal X-ray diffrac-tometer with Mo Ka (k = 0.717 Å) radiation. It is confirmed that theKDP crystal retains its tetragonal structure after doping with CAand LV. However, the slight change observed in lattice parametersof doped KDP crystals might have occurred due to the latticestrains inhabited by the presence of dopants on lattice sites ofKDP crystals. The measured crystal dimensions are detailed intable 1.

UV–visible spectral analysis

The optical spectrum is an imprint of electronic transitions tak-ing place due to absorption of optical energy by specific orbitals offunctional chromophores associated with the material. Thereby toensure the practical utility of the crystal for optical device applica-tions the crystal must exhibit high transmittance in the visibleregion. In order to determine the optical transparency, the growncrystals of 2 mm thickness have been scanned in the range of200–900 nm using the Shimadzu UV-2450 spectrophotometer

Fig. 1. Single crystal of (a) KDP, (b) L

(scan speed = medium, slit width = 2 nm) and the recorded trans-mittance spectrum are shown in Fig. 2. In the present analysis,the transmittance of CA and LV doped KDP crystals is found to berelatively higher than KDP crystal. The highest transmittance is78% for KDP, 82% for LV doped KDP and 84% for CA doped KDP crys-tal. This indicates that the presence of organic dopant in KDP crys-tals has favored less optical scattering and absorption favoring theincrease in transmittance of doped KDP crystals [15]. It is notice-able that the dopants have significantly modified the high trans-mittance cut-off wavelength of KDP crystal and it is observed tobe 362 nm for KDP, 355 nm for CA doped KDP and 381 nm for LVdoped KDP crystal. The doped KDP crystals with high transmit-tance and modified cut-off wavelength may find huge advantagein UV-tunable lasers and NLO device applications [16]. The studyof optical constants play a decisive role in identifying the crystalsapplication for several optoelectronics applications [17]. Thus thenature of refractive index (n), reflectance (R) and extinction coeffi-cient (K) of pure and doped KDP crystals has been comparativelyevaluated using the standard formulae available in the literature[18]. Refractive index is an optical constant that governs the devi-ation of light in a medium. Ideally, the refractive index should belower for crystals. The photonic response of the refractive indexand reflectance of grown crystals is shown in Fig. 3a and b respec-tively. It is observed that the refractive index and reflectance ofKDP crystal has been modified due to addition of CA and LV whichmakes the doped KDP crystals most desirable photorefractivematerials useful for holographic data storage devices [19], alsothe low refractive index crystals are highly demanded for coatingsolar thermal devices [20] and calibrating the merit of photonicdevice components [21]. The extinction coefficient of the materialdetermines the optical loss in a material mediumwhich is the mostessential parameter to control the strength of the optical signal. Forgrown crystals the response of extinction coefficient is shown inFig. 3c. It is observed that the extinction coefficient of KDP crystalhas been brought to a lower magnitude due to the addition ofdopants. The low extinction coefficient tendency of LV and CAdoped KDP crystals pronounce their credibility for high speed opti-cal data processing and signaling devices [22].

Z-scan analysis

The close and open aperture Z-scan technique is the crucial tooldeveloped by Bahae et al. to determine the magnitude and studythe nature of nonlinear refraction (NLR) as well as nonlinearabsorption (NLA) [23]. The TONLO properties of crystals (1 mmthickness) have been evaluated by means of laser assisted Z-scansetup (table 2). In closed aperture Z-scan analysis the highly repet-itive gaussian beam of He-Ne laser is focused on the crystal sampleof doped KDP material through a converging lens and the sample istranslated back and forth about the beam irradiated path focus(Z = 0). The transmitted intensity from the respective crystal sam-ple was collected via closed aperture photo detector placed at the

V doped KDP, (c) CA doped KDP.

Table 1Structural analysis data.

Crystal Crystal system Space group Lattice parameters (Å) Volume (Å)3

KDP Tetragonal I42d a = b = 7.44, c = 6.94 384.15KDP + LV Tetragonal I42d a = b = 7.49, c = 6.92 388.21KDP + CA Tetragonal I42d a = b = 7.48, c = 6.98 390.53

Fig. 2. Transmittance spectrum.

Fig. 3. Optical response of (a) Refractive index, (b) Reflectance, (c) Extinctioncoefficient.

M. Anis et al. / Results in Physics 6 (2016) 645–650 647

far field. The path dependent closed aperture Z-scan transmittancecurve of LV and CA doped KDP crystal is shown in Fig. 4a and brespectively. In LV doped KDP crystal the transmittance trails thepre-focus valley and post-focus peak indicating the existence ofpositive NLR response which is the characteristic feature of mate-rial having self-focusing tendency [24]. The positive NLR of order of10�16 esu signifies the strong kerr lens modelocking (KLM) abilityof LV doped KDP crystal pronouncing its suitability for shorterpulse generation systems [25]. On the other hand, the peak to val-ley phase shift about the focus confirms the self-defocusing ten-dency or negative NLR behavior of CA doped KDP crystal. Thematerials with negative NLR nature have huge demand for applica-tions in night vision sensor devices [26]. The relation between ofon axis phase shift (DU) and peak to valley transmittance differ-ence (DTp-v) is given by [23],

DTp�v ¼ 0:406ð1� SÞ0:25jD/j ð1Þwhere S = [1�exp(�2ra2/xa

2)] is the aperture linear transmittance,ra is the aperture radius and xa is the beam radius at the aperture.The third order NLR (n2) of doped KDP crystal was calculated usingthe relation [23],

n2 ¼ D/KI0Leff

ð2Þ

where K = 2p/k (k is the laser wavelength), I0 is the intensity of thelaser beam at the focus (Z = 0), Leff = [1�exp (�aL)]/a, is the effectivethickness of the sample depending on linear absorption coefficient(a) and L thickness of the sample. The path dependent open aper-ture Z-scan transmittance curves of LV and CA doped KDP crystalshave been analyzed to reveal the occurrence of NLA in crystal mate-rial. The LV doped KDP crystal offers saturable absorption (Fig. 5a),

Table 2Optical resolution of Z-scan setup.

Parameters Magnitude

Laser wavelength (k) 632.8 nmLaser power (P) 10 mWLens focal length (f) 20 cmOptical path distance (Z) 115 cmBeam waist radius (xa) 1 mmAperture radius (ra) 15 mmIncident intensity at the focus (Io) 2.3375 KW/m2

Fig. 4. Close aperture Z-scan curve of (a) LV and (b) CA doped KDP crystal.

Fig. 5. Open aperture Z-scan curve of (a) LV and (b) CA doped KDP crystal.

648 M. Anis et al. / Results in Physics 6 (2016) 645–650

originating due to dominance of linear absorption coefficient overthe excited state absorption [8,27]. Fig. 5b evidences the reversesaturable absorption (RSA) phenomenon in CA doped KDP crystal.The additional multi photon absorption by the excited state overtwo photon absorption in the CA doped KDP crystal leads to theRSA effect vitalizing its prominence for biomedical, photonics,

optical limiting and laser stabilization applications [28,29]. Themagnitude of nonlinear absorption coefficient (b) of grown crystalshas been calculated using the equation [23],

b ¼ 2ffiffiffi2

pDT

I0Leffð3Þ

where DT is the one valley value at the open aperture Z-scan curve.For LV and CA doped KDP crystals the magnitude of b is found to be1.84 � 10�4 cm/W and 2.4 � 10�4 cm/W respectively. The measure-ment of TONLO susceptibility (v3) measures the polarizing ability ofgrown crystals. The v3 of crystals has been calculated using belowequations [23],

Revð3ÞðesuÞ ¼ 10�4ðe0C2n20n2Þ=p ðcm2=WÞ ð4Þ

Imvð3ÞðesuÞ ¼ 10�2ðe0C2n20kbÞ=4p2 ðcm=WÞ ð5Þ

Table 3Nonlinear optical parameters.

Dopants in KDP n2 (esu) v3 (esu)

Pure KDP [4] 2.34 � 10�13 3.72 � 10�14

CA⁄ �6.1 � 10�16 7.39 � 10�4

LV⁄ 3.34 � 10�16 5.66 � 10�4

FA [13] �1.14 � 10�5 3.81 � 10�7

OA [14] 2.25 � 10�5 1.90 � 10�7

MA [14] 7.92 � 10�5 2.13 � 10�7

Fig. 6. Temperature dependent (a) Dielectric constant and (b) Dielectric loss.

M. Anis et al. / Results in Physics 6 (2016) 645–650 649

v3 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðRev3Þ2 þ ðImv3Þ2

qð6Þ

where e0 is the vacuum permittivity, n0 is the linear refractive indexof the sample and C is the velocity of light in vacuum. The suscep-tibility of LV and CA doped KDP crystal is of order 10�4 esu which issufficiently higher than several doped KDP crystals (Table 3)[8,13,14]. The large enhancement in TONLO susceptibility of dopedKDP crystals might have been facilitated by the photoinduced delo-calization of charge through the p-bonding network [30].

Dielectric studies

The dielectric measurements of pure, CA and LV doped KDPcrystals (2.5 mm) have been carried out in the temperature rangeof 40–120 �C at a frequency of 100 kHz using the HIOKI-3532LCR cubemeter. In order to obtain accurate results the parallelfaced quality single crystals were applied by the silver paste andconnected to the electrical probes. The external electric field andtemperature majorly influence the dielectric constant of materialas shown in Fig. 6a. It is evident that the increase in temperatureleads to instability in polarization activity (ionic, electronic, dipolarand space charge) of the crystal as a consequence of which thedielectric constant of crystals increases with the increase in tem-perature [31]. The analysis confirms that the dielectric constantof KDP crystal has been reduced to a lower level after the additionof LV and CA respectively. The lower dielectric constant favors lesspower consumption and enhancement in SHG coefficient of mate-rial which are advantageous parameters for designing electro-opticmodulators, photonics, NLO and microelectronics devices [32]. Themeasurement of dielectric loss enables to investigate the dissipa-tion of electromagnetic energy through defects (solvent impurities,macro and micro crackd, grain boundaries, porosity and randomcrystallite orientation) in crystal medium [33]. The variation ofdielectric loss with temperature is shown in Fig. 6b and it revealsthat the dopants LV and CA effectively reduce the dielectric lossof KDP crystal demonstrating the improved optical quality andminimized electrically active defects in doped KDP crystals [34].The lower dielectric constant and dielectric loss of doped KDP crys-tals is a significant and decisive parameter for optoelectronics andNLO applications [35].

Conclusion

A successful attempt has been made to improve the optical anddielectric properties of KDP crystal by doping organic (CA and LV)additives and explore the utility for distinct NLO device applica-tions. The XRD analysis confirmed the tetragonal structure andslight variation in lattice parameters of doped KDP crystals. UV–visible spectral analysis revealed that the optical transmittance ofKDP crystal has been enhanced by 4% and 8% due to inclusion ofCA and LV respectively. The transmittance cut-off, extinction coef-ficient, refractive index and reflectance of KDP crystal were effec-tively tuned in the visible region by CA and LV which is vital forUV-tunable lasers, holographic data storage and photonic devices.The Z-scan analysis confirmed the promising TONLO nature ofdoped KDP crystals. In close aperture Z-scan analysis, the phaseshift in transmittance about the focus assisted by localized absorp-tion and highly repetitive optical beam confirmed the nonlinearrefraction in CA and LV doped KDP crystals. The positive NLR(3.34 � 10�16 esu) confirms the potential kerr lensing ability ofLV doped KDP crystal which is vital for laser stabilization andshorter pulse generation systems. The negative NLR(�6.1 � 10�16 esu) of CA doped KDP crystal is essential for nightvision sensor applications. The LV and CA doped KDP crystal exhib-ited saturable absorption and reverse saturable absorption respec-tively. The enhanced TONLO susceptibility of CA and LV doped KDPcrystal is found to be 7.39 � 10�4 esu and 5.66 � 10�4 esu which issufficiently higher than the KDP crystal. The dielectric studiesinferred the decrease in dielectric constant and dielectric loss ofKDP crystal due to addition of LV and CA respectively. The lowerdielectrics of doped KDP crystals might play a vital role in design-ing microelectronics and NLO devices. The high transparency win-dow, improved TONLO quality and lower dielectric parameterssuggest the superiority of doped KDP crystals over KDP crystal tobe used for advanced nonlinear optical applications.

650 M. Anis et al. / Results in Physics 6 (2016) 645–650

Acknowledgement

Author Mohd Anis is thankful to the UGC, New Delhi, India forawarding the Maulana Azad National Fellowship (F1-17.1/2015-16/MANF-2015-17-MAH-68193).

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