1 Introduction

Nonlinear optical activity has attained the pinnacle of a point in the current research owing to the requirement of the materials possessing this property as the application in the field of today’s frontier technology very much evolves and revolves around it. If nonlinear optical (NLO) activity is to exist in organic crystals, it would be crystallized in non-centrosymmetric space group and the electronic structure of the molecule should be very strongly coupled to the electric field of the applied electromagnetic waves [1]. In the past decades, organic crystals made of π conjugated molecules have attained much more focus in the field involved of organic optoelectronic materials than that of other materials [2,3,4]. Nonlinear optical susceptibilities of organic NLO materials are comparatively good, but their laser damage threshold values are low as we draw a parallel with inorganic materials. Quite a good number of materials of this sort have been reported over a period of time in the literature due to their potential applications in variety of fields [5]. Over the past few years, the centre of attraction towards benzil crystal has witnessed a consistent and continuous increase as it is a potential nonlinear material and thereby enabling much more possible applications in devices which are used for optical communications, optical computing, high-speed information process, electro-optical shutters and optical data storing. It is very much evident from the experimental results being found in the literature that benzil crystal has pretty good optical and dielectric properties [6, 7]. Considerably large values of nonlinear absorption coefficient (β) and nonlinear refractive index (n2) enable this material to be used in nonlinear optical devices such as ultrafast optical switches and power limiters. Moreover, benzil crystal possessing high durability and comparative easiness makes it a suitable material to fabricate waveguides and fibers. The ferroelectric behaviour of benzil is very much related to the second-order phase transition occurring at low temperature that paves the way for the transformation from uniaxial nature of the crystal at the ambient temperature to biaxial nature at lower temperature. As a result of this, the biaxial crystal enforces the phase matching process for an efficient NLO phenomenon and gives rise to essential electro-optic effect [8].

Benzil (C14H10O2) is an organic non-hygroscopic material possessing good nonlinear optical properties [9]. For the past years, many authors have reported the growth of benzil single crystal using various crystal growth methods and it has been characterized and the details could be found in the literature [10,11,12]. As far as slow evaporation method is concerned, its structural perfection and crystal quality are low and also it is not conducive for bulk growth of crystal because of its limited dimension boundaries. Furthermore, a huge quantity of saturated solution goes in vain and so it is much better to apt for the crystal growth of benzil adapting a specific orientation by Sankaranarayanan-Ramasamy (SR) method. Slow evaporation solution growth technique and SR method were already employed to grow benzil single crystal and the results are being found [12]. In the present work, unidirectional benzil crystal has been grown by SR method using the mixture of ethanol and benzene as the solvent. The grown benzil crystal was investigated by several characterization techniques and a few are not been found so far in the literature. Therefore, we report the characterizations such as High-resolution X-ray diffraction, Impedance analysis, P–E hysteresis analysis, and laser damage threshold and z-scan technique for benzil crystal grown by SR method.

2 Crystal growth

The commercially available benzil was dissolved in a mixed solution of benzene and ethanol in 1:1 volume ratio. The saturated solution was stirred continuously for 8 h so as to get a homogenous solution. After that, the solution was filtered using filter paper and then poured into a beaker which was thereafter kept undisturbed at ambient temperature for slow evaporation. Defect-free good quality benzil seed crystals were collected after a period of 8 days for the purpose of the growth of bulk crystal.

For uniaxial growth of benzil crystal, a good quality seed crystal was selected and mounted at the bottom of the ampoule in such a way that the selected orientation of the crystal facing upward. After that, the saturated benzil solution was poured into the glass ampoule without disturbing the position of the seed and kept in a water bath of SR method. A ring heater was placed at the top of the growth ampoule and controlled by a RTD temperature controller maintaining 40 °C for the controlled solvent evaporation. The formation of spurious nucleation at the surface region was also avoided by the ring heater as it was in proximity with the surface during the entire time period of the crystal growth. The solvent evaporation rate was minimised by the small holes made in the thin plastic paper covering the mouth of the ampoule and the controlled evaporation helped to achieve the uniform growth rate. The room temperature was maintained in the surrounding of the growth region of the ampoule throughout the growth process. The growth conditions were very carefully monitored in such a way that the set up was intact and consequently a slight dissolution was noticed at the surface of the seed crystal within 3 days time. As the experimental setup had the provision of transparent nature of the solution that could be viewed through the ampoule, a close up observation was possible which revealed that the solution–crystal interface was flat under optimized conditions and thus highly transparent crystal growth could be achieved. Crystal of 168 mm length and 14 mm diameter was successfully grown and harvested after 35 days. The growth rate of the crystal was found to be averaging about 4.8 mm/day. Figure 1a represents the as grown benzil single crystal at (100) plane in the ampoule. The grown crystal was carefully harvested from the glass ampoule not causing any mechanical damage with the help of a fine axis diamond cutter. The grown benzil crystal (Fig. 1b) was cut and polished and subjected to various characterization techniques.

Fig. 1
figure 1

a Photograph of benzil crystal along (100) plane as grown in the ampoule. b Cut and Polished benzil crystal sample

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3 Result and discussion

3.1 Powder XRD analysis

Powder X-ray diffraction study was performed by employing a Rigaku mini Flux II diffractometer with Cu Kα (λ = 1.54056 Å). Figure 2 shows the PXRD pattern of benzil crystal which was recorded by scanning the sample over the range of 10°–70°. The position of the peaks and cell parameters are very much in agreement with the JCPDS file no. 30-1539. The grown benzil crystal accommodates itself in hexagonal crystal family with noncentrosymmetric space group P3 1 21 [10].

Fig. 2
figure 2

Powder X-ray diffraction spectrum of grown benzil crystal

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3.2 High-resolution X-ray diffraction (HRXRD) studies

The cut and polished crystal along (100) plane was subjected to symmetrical Bragg geometry by employing the HRXRD analysis and the resultant finding is shown in Fig. 3. It is very much evident from the figure that the DC has witnessed a narrow single peak which indicates the nature of the specimen with no structural grain boundaries. Also the FWHM of the diffraction curve is 771 arc sec and the single diffraction curve asserts that the nature of crystalline perfection is considerably good. Furthermore, as compared with crystals grown by employing Bridgman technique along (110) plane (at which two peaks were observed with the FWHM of 24″ and 34″ arc sec) [10] benzil crystal grown by SR method exhibits single intense peak which confirms no grain boundary in the crystal affirming that the quality of the crystal has improved a lot. The higher value of FWHM may be due to the presence of point defect and their aggregates [13].

Fig. 3
figure 3

HRXRD curve for grown benzil crystal for (100) plane

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3.3 Optical transmission studies

NLO single crystals come across their prime and prominent use in optical applications in which the optical transmission and the transparency ranges establish a significant place. The optical transparency of the grown benzil crystal was analyzed using Varian carry 5E spectrometer in the range of 200–800 nm. It is found from the data that the crystal is transparent in the wavelength region between 460 and 800 nm so that it could be used for optical application. The cut-off value is found to be 437 nm as it could be seen from Fig. 4. As the required transmission window is observed for the grown benzil crystal in the visible region it renders itself with a medium for a fine optical transmission of signals possessing the second harmonic frequencies of Nd:YAG laser [12].

Fig. 4
figure 4

UV–visible transmission spectra of benzil crystal

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3.4 Impedance studies

The electro-optic properties of optical crystals are very much related to dielectric properties which have a direct impact on non-conducting crystals and piezoelectric materials and these materials are widely used in sensor applications [14]. To have dielectric measurements, the grown benzil crystal was precisely cut and effectively polished and then it was silver coated so as to be placed inside a dielectric cell. The experimental data for different temperature ranges, which were to be carried out, were fixed as 35, 55 and 75 °C and the frequency range was kept between 1 Hz and 2 MHz using a PSM 1735 Impedance meter. The bulk polarisability of a material has a specific impact on dielectric constant and the net value of the bulk polarisability is determined by the response of electronic, ionic, orientation and space charge polarizations occurring in the material when an alternating electric field is applied [11].

Figures 5 and 6 show the dielectric constant and dielectric loss versus frequency for different temperatures. It has been observed that the values of dielectric constant and dielectric loss decrease accordingly with the increase in the respective values of frequency and temperature. This effect can be explained on the basis in such a way that the mechanism of polarization is very much related to the conduction process. The electrical conduction in a dielectric is primarily a defect controlled process at low-temperature region. The defect concentration increases exponentially with varying temperature and accordingly the electrical conduction also increases and thereby the dielectric constant decreases. High density of these centres in the bulk crystal and the resulting high stresses can stimulate the development of block boundaries in a particular orientation. Due to this flaw, the value of dipole moment of the crystal undergoes a decreasing effect. Subsequently, the decrease in the dipole moment of the crystal imposes a gradual decrease on the dielectric constant [15]. The reason behind the dielectric constant to be high at low-frequency region is that it may be related to the rate of polarisability and the value of dielectric loss is low at higher frequency region that implies the crystal possess good optical quality with minimum defects and this particular parameter is of great significance for NLO materials to find their applications [10]. In general, two types of losses occur in all dielectric materials namely conduction loss and dielectric loss. The flow of actual charges through a material determines the conduction loss and the moment or rotation of atoms caused by the alternating field leads to the dielectric loss. The low values of dielectric loss show that the minimum defects present in the crystal [16].

Fig. 5
figure 5

Dielectric constant plot with frequency of applied field

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Fig. 6
figure 6

Dielectric loss versus frequency of applied field

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The dependence of frequency on the real part of impedance (Z′) and imaginary part of impedance (Z″) for various temperature ranges is shown in Figs. 7 and 8. It is observed that Z′ shows an exponential decay in the low-frequency region and plateau in the high-frequency region. Towards the terminal stage, all the curves close in with the value of zero approaching so as to represent Z′ is independent of frequency. The recombination is quicker, since the space charge has limited time to relax at higher frequency region which effectively reduces the space charge polarization and it leads to merger of all the curves showing that it is independent of temperature at higher frequency region [17]. It is seen from Fig. 8 that the magnitude of Z″ happens to be high at low-frequency region and it undergoes a gradual decrease in value as the frequency increases and subsequently all the curves merge in the high-frequency region. This effect is due to space charge polarization occurring dominantly in the high-frequency region and it is also asserted from the observed data that the space charge polarization is temperature independent at the high-frequency region [18].

Fig. 7
figure 7

Real part of impedance versus frequency at different temperatures

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Fig. 8
figure 8

Imaginary part of impedance versus frequency at different temperatures

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Cole–cole plot of the real and imaginary part of impedance are plotted and it can be related to Z = Z′+ iZ″ where Z′ is the real part of impedance that can be represented for a pure resistance R and iZ″ is the imaginary part and it can be represented for capacitance C, where Z = 1/jωC in which j is a imaginary number (√− 1). A semicircle centred on the real axis is the response of an ideal parallel circuit of resistance (R) and capacitance (C). The resistance can be determined from the diameter of the semicircle and capacitance can be measured from the frequency maximum of the semicircle. The spectrum shows reasonably large grain boundary which in turn gives rise to the total value of resistivity. Closer to the grain boundaries, imperfections are bound to occur in higher concentration and effectively control the transport behaviour of the material. This causes an added share to the value of grain boundary impedance. Figure 9 shows that the plot provides a semicircle arc for which the pattern of evolution changes upon increasing the temperature indicating the commencement of intergranular activities within the sample material of benzil crystal with definite contribution from grain interior [19].

Fig. 9
figure 9

Cole–cole plot of the benzil crystal at different temperatures

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The variation occurring in AC conductivity was analysed with respect to various frequencies and temperatures as shown in Fig. 10. The AC conductivity of benzil crystal is almost zero at the low-frequency region and it starts its incremental path with the increase in frequency. The electronic band structure is modified because of the decrease in mobility caused by the increasing ionic size which leads to the low magnitude of electrical conductivity at the region of low frequency. With the effect of increasing the temperature in steps of 55–75 °C, it has been observed that the value of AC conductivity also increases in the low-frequency region because of the occurrence of space charge polarization [20].

Fig. 10
figure 10

AC conductivity versus frequency of the grown crystal

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3.5 P–E hysteresis loop

A material to be used in photonics is predominantly decided by its ferroelectric properties which usually have a direct dependence on its crystallographic structure [21] and strongly influenced by the lattices imperfection. The occurrence of spontaneous electric polarization can be reversed by an applied electric field. The grown benzil crystal (100) was cut and polished and also silver coating was applied on either side so as to make a better electrical contact. After that, the crystal was mounted on a sample holder and an electric field was applied. The polarization (P–E) plot was traced applying an external field up to 38 kV/cm and the resultant curve is shown in Fig. 11. It is observed that the hysteresis loop obtained in the present investigation is elliptical, which confirms the ferroelectric behavior of the material. Area of the P–E loop decreases along with remnant polarization (Pr) and coercive field (Ec) with respect to the decreasing applied electrical field. The single domain materials possess large value of coercive field in comparison with multi-domain materials [22]. The values of coercive field and remnant polarization for benzil are obtained as 3.56 kV/cm and 0.00533 µC/cm2, respectively, and the maximum polarization is found to be 0.104 µC/cm2. Polarization reversal occurs in benzil at low coercive field if many domains are present.

Fig. 11
figure 11

Hysteresis loop of benzil crystal

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3.6 Laser damage threshold (LDT)

Damage induced by a laser is an observed physical process undergoing in a crystal when illuminated with very high intensity laser. Hence, it is imperative that at threshold intensities of radiation the crystal has to defy itself from not getting any damage [23]. In the present case, LDT measurement was performed for benzil crystal by a Q-switched Nd:YAG laser (λ = 1064 nm) with 10 Hz repetition rate. The laser beam (8 mm) was focused on the surface of the crystal and the laser pulse energy was gradually increased until an intense white visible spot was seen at the surface of the crystal. As the energy reached 22.1 mJ, a visible damage was seen on the surface where the laser beam was focused and this energy was referred as the damage threshold energy for this crystal. The radius of laser beam spot on the sample (r) is calculated to be 154 µm using the relation [24]

$$r=\frac{{1.27 \times \lambda \times f}}{d}\;{\text{cm.}}$$

Here, d is the thickness of the sample; f is the focal length of convex lens and λ is the wavelength of the laser. The energy density is calculated using the following formula:

$${\text{Energy}}\;{\text{density }}=\frac{E}{{\tau A}}\;{\text{GW/c}}{{\text{m}}^{\text{2}}}.$$

Here, E is the input energy (mJ), A is the area of the laser spot (cm2) and τ is the pulse width (ns). The energy density is calculated as 2.97 GW/cm2. The LDT of bulk materials relies on number of factors such as pulse width, focal spot geometry and growth technique used to crystallize the sample, wavelength, imperfections in the material, and so on. The LDT value of benzil crystal is drawn parallel with some of the well-known organic NLO crystals and the values are listed in Table 1. The good surface observed on the grown benzil crystal is because of its high laser damage threshold value. Henceforth, the achieved higher value of LDT for benzil crystal evidently makes it a potential material for NLO applications [25].

Table 1 Comparison of LDT values of benzil and few organic NLO crystals
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Here, VMST is 4-hydroxyl-3-methoxy-4′-N′-methylstilbazolium tosylate monohydrate, ICPNP is isonicotinamide P-nitrophrnol, DAST is 4-dimethylamino-N-methyl-4-stilbazolium tosylate.

3.7 z-Scan analysis

z-Scan technique has been employed to investigate and analyze the third-order nonlinear optical properties of the grown benzil crystal. Sheik Bahae et al. [29, 30] introduced this technique so as to determine accurately the sign and magnitude of nonlinear refractive index and nonlinear absorption coefficient. z-Scan technique has been very much recognised by NLO community as this method is easy to measure and very sensitive. 532 nm of Nd:YAG laser with an input intensity of 50 mW is used for this technique which excites the sample with the laser input and the propagation direction is maintained along z axis. The energy of the beam is found to be maximum at the focus and from thereon it is experiencing a gradual decrease symmetrically on either side of positive and negative values of z causing varying intensities at different positions of z. The z-scan values of open, closed apertures and the ratio of z scan curve of the recorded normalized transmittance for benzil crystal are shown in Figs. 12, 13 and 14.

Fig. 12
figure 12

Closed aperture curve of benzil crystal

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Fig. 13
figure 13

Open aperture curve of benzil crystal

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Fig. 14
figure 14

Ratio curve of benzil crystal

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A thin lens of variable focal length is formed by the sample and as the sample is brought near the focus, the irradiance of Gaussian beam may increase or decrease which depends on the absorption of the material and refractive index. The nonlinear absorption coefficient (β) can easily be calculated from open aperture method taking into consideration of maximum or minimum transmittance curves. As the input intensity of laser beam is increased, it could be noticed that the transmittance of the sample considerably increases that indicates the absence of reverse saturation absorption whereas the crystal is enhanced with strong saturation absorption.

The closed aperture curve enables to perform pre-focal peak to post-focal valley configuration that clearly suggests that benzil crystal has a negative value of third-order nonlinear refractive index which arises due to the self defocusing effect of the crystal and this might have resulted by the reduced transmittance and large beam divergence through the far field aperture so that an essential property of the crystal is identified for optical switching applications [31]. The magnitude of third-order nonlinear optical susceptibility (χ3), real and imaginary part of third-order susceptibility, nonlinear refractive index and nonlinear absorption coefficient values can be calculated using the Refs. [32,33,34].

The observed negative nonlinear absorption in the grown benzil crystal is because of the strong saturation absorption by the material. Third-order nonlinear refractive index and nonlinear absorption coefficient are found to be 7.62 × 10−8 and 0.03 × 10−4 cm/W, respectively, and also third-order nonlinear susceptibility of the grown crystal is calculated as 3.921 × 10−6 esu.

The obtained results of z-scan parameters for the grown benzil crystal are compared with few organic crystals and shown in Table 2. The obtained large value of (χ3) can be due to the electron density transfer (donor to acceptor) which is observed within the molecular system. Therefore, the polarization of π-conjugated electrons is high in the molecular system which contributes to the large value of (χ3) for the benzil crystal. The results are very convincing in such a way that the title crystal can be a potential candidate for third order NLO applications [35].

Table 2 Comparison of z-scan value of benzil with few organic crystals
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Here, VMST-4-hydroxy-3methoxy-4′-N′-methylstilbazolium tosylate monohydrate; MMST-4-methyl-4′-N′-methylstilbazolium tosylate.

4 Conclusion

A bulk single crystal of unidirectional benzil was grown by employing SR method. The lattice parameters were determined by powder X-ray diffraction and found to be in good agreement with the literature values. HRXRD study confirms the crystalline perfection and it is also known that the crystal is better than the crystal grown by Bridgman technique. UV–visible studies show that the grown crystal is transparent in the visible region suggesting that it can be utilized for NLO applications. The observed low values of dielectric constant and dielectric loss for high frequency region of the sample suggest that the sample is possessed with enhanced optical quality and with comparatively lesser defects. P–E hysteresis loop measurements confirm that the grown crystal is Ferro electric in nature. The LDT of benzil is measured as 2.97 GW/cm2 at 1064 nm proposing that the crystal can have favourable applications in laser frequency conversion. The third-order nonlinear optical study of benzil crystal makes it to be known that it has in it enhanced saturation absorption and negative nonlinear refractive index. These results assert that the grown benzil crystal can be used for third order NLO applications.