1 Introduction

Focussing on the growth of nonlinear optical (NLO) crystals helps in designing of materials in the domain of optoelectronics and photonics. NLO crystals exhibiting frequency conversion efficiency such as second harmonic generation (SHG) and third harmonic generation(THG) have a greater impact in photonic industry [1]. In distinct class of materials of non-polar benzyl group NLO chromophore, nonlinear optical effects exhibiting noncentrosymmetric phenomena have perceived at relatively large intensities [2]. Crystals of organic category possessing flexibility of molecules, and structural diversity allow fine tuning of the chemical structures and thus gaining optical nonlinearities and ultra-fast response times and tunable electro-optic filters [3, 4]. Enormous organic materials has been investigated in the previous times such as LAA, LPDMA, LAPA by Sangeetha et al., Jayaprakash et al., Jothimani et al. [5,6,7] declares the materials versatility in opto-electronics field occupying wide space signal processing and photonic switching devices, owing to their strong nonlinear optical absorption enabling optical limiting property. Frequency doubling phenomenon, a visibly dramatic nature was observed among the various nonlinear optical procedures such as frequency doublers, optical imaging, electro-optic modulators, high resolution printing and spectroscopy [8]. Also considering the organic molecular chiral level, the shift in the transfer of charge is supposed to control the output of SHG at the same time on the supramolecular level demands high output of SHG towards strong noncentrosymmetric molecular interaction and superior phase matching stability [9, 10].

The frequency tripling effect or THG examined using Z-scan technique greatly occupies future specific applications in optoelectronics and photonics [11]. Recent work on nonlinear optical crystals proved that organic molecules exposed wide range of third-order sensitivity paved a way to devise opto-electronic and photonics [5, 6, 10].

The role of organic crystals in nonlinear optics (NLO) applications due to their high nonlinear susceptibility [8] possessing delocalized electrons is of great interest. Amongst the organic crystals, aminoacids show specific features of interest such as molecular chirality, absence of strongly conjugated bonds, leading to wide transparency ranges in the visible and UV spectral regions, zwitterionic nature of molecules which creates hydrogen bond networks and chiral auxiliaries for nitro-aromatics and other donor–acceptor molecules having π electronic configuration with large hyperpolarizabilities and also as a basis for synthesizing organic–inorganic compounds [12,13,14].

Our work discloses the progress of growth and characterization of smallest aminoacid L-alanine and 5 wt % of boric acid doped L-alanine nonlinear optical single crystals grown from low temperature solution growth process ensure a new focus of science and development of techniques in photonics. Individually the study on L-alanine has been studied by many authors and also structure refinement has been reported [7, 8, 15,16,17,18]. L- Alanine (LA) exhibit noncentrosymmetric nature with a methyl group (-CH3) as side chain [8].

Many researchers have worked on borate-based crystals [19, 20] including Caroline et al. and Rathika et al. [21, 22] have reported metal doped with aminoacid L-alanine suitable for direct conversion process between electrons and photons such as optical computing, optical communication, optical data storage and electro-optic modulators. Boric acid B(OH)3 also named as boraic acid, orthoboric acid, borofax was structurally determined by Zachariason [23]from X-ray measurement. Later Bernal and Megaw studied the bonding of hydrogen atoms which was confirmed experimentally by Cowley [24]. Borate incorporated materials occupy a key place in frequency conversion as a tunable laser source in UV region [25]. The various studies on structure of boric acid B(OH)3 and infrared spectrum has been reported [24, 26]. Due to unique physical properties of borates like high damage threshold, high optical quality, showing wide range of transparency in UV region, large efficient nonlinear coefficient and luminescence comes under basic characteristics of borates leading to serve as unique materials in the domain of laser including short wavelength laser techniques [27,28,29,30]. Herein, we report synthesis of L-alanine and boric acid doped L-alanine utilizing the process of slow evaporation and obtained crystals were subjected to various characterizations namely dielectric, mechanical, thermal, fluorescence, Laser Damage Threshold, SEM and EDAX, second- and third-order nonlinearity in addition to the linear optical characteristics of the grown crystals.

2 Experimental process

2.1 Crystal growth

The pure L-alanine and 5 wt % Boric acid doped L-alanine crystals are grown using water as solvent by slow evaporation technique. From the knowledge of solubility of 19.0 g/100 ml for pure L-alanine at 45 °C in water and boric acid with a water solubility 50 g/litre at 50 °C [26], pure L-alanine and boric acid (B(OH)3) with 5 wt % doped with LA were taken in different vessels to prepare saturated solution. Both pure and boric acid doped LA was separately stirred for 7 h using magnetic stirrer at room temperature. The obtained homogenous state of saturated solutions were kept in petri dishes for slow evaporation to take place. The transparent crystals of LA and boron presence LA were harvested in growth duration of 15–20 days. The pictures of LA and boron element doped LA are presented in Fig. 1(a, b).

Fig. 1
figure 1

a. Picture of L-alanine (LA). b. Photograph of boric acid mixed LA

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3 Results and discussion

3.1 Single crystal XRD studies (SXRD)

The grown-up crystals were investigated by BRUKER APPA APEX II CCD diffractometer. The study discloses the orthorhombic crystal structure with unit cell parameters a = 6.032A°, b = 12.343 A°, c = 5.784 A°, V = 428.9A3, α = β = γ = 90° for LA and a = 6.01 A°, b = 12.3 A°, c = 5.77 A°, α = β = γ = 90° for boric acid doped L-alanine which attained the same crystal system of the parent LA with P212121 noncentrosymmetric space group [8].

3.2 FT-IR Vibrational Study

In the recorded FTIR spectroscopy, the characteristic absorption spectra for the various functional groups present in the parent LA (Fig. 2a) agree well with the FTIR spectra studied in the literature [8] and BA doped LA crystal FTIR profile (Fig. 2b), shows certain shift in the wavenumbers confirming the presence of boron and zwitterion in both crystals.

Fig. 2
figure 2

(a,b) FTIR Spectrum of LA and Boric Acid doped LA

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The occurrence of fundamental vibrational modes due to COO−, NH3+, CH2, CH groups are noticed in parent LA and doped LA. The carboxylic group is seen to exist as COO− in both the crystals with the characteristic vibrations seen in the range 1680–1540 cm−1 due to strong asymmetric stretching, i.e. at 1451 cm−1 (LA) and at 1441 cm−1 (BA doped LA), weak symmetric stretching observed at 1414 cm−1 (LA) and at 1410 cm−1 (BA doped LA) and scissoring mode is seen in the profiles pertaining to wavenumbers positioned at 644 cm−1 (LA) and 643 cm−1 (BA doped LA), respectively.

The ionization of the carboxyl group is confirmed from the IR bands observed at 539 cm−1, 530 cm−1 and 1414 cm−1, 1410 cm−1 in LA and doped LA spectra. The presence of NH3+ group in aminoacids, the stretching and bending vibrational modes are usually expected in the regions 3150–3000 cm−1,1660–1650 cm−1 and 1550–1480 cm−1.

In the recorded FTIR profiles (Fig. 2a and b), the IR bands existed at the positions 3081 cm−1 and 3048 cm−1, 1617 cm−1 and 1624 cm−1 is a clear indication for the presence of NH3+ group in the grown crystals, denoting the participation of three hydrogen atoms in the hydrogen bonding thereby weakening the N–H bond [8]. The vibrational modes observed just below 3000 cm−1 are weakly resolved due to methyl and methane groups. The absorption bands for C–N and C–C stretching vibrations are noticed in the wavenumber region 850–1150 cm−1 for the studied crystals. The wavenumber seen at around 840 cm−1 and at 838 cm−1 in the recorded FTIR profile for parent and doped crystals, are assigned to C–C–N symmetric stretching vibration.

The presence of Boron element in the doped LA crystal can be attributed to the wavenumbers between 800–600 cm−1 [25] The IR bands at 1097 cm−1 corresponds to C–C stretching. The shift in the wavenumbers observed in Fig. 2b compared to Fig. 2a clarifies the incorporation of boron in L-alanine, the same being ascertained from the wavelength at low frequency region [31] and hence supported from EDAX analysis. Henceforth in both crystals, the zwitterion structure of amino acids comprising of NH3+ group acts as a good hydrogen bond donor and carboxylic group as an excellent acceptor confirmed by strong hydrogen bonds between them in the crystal lattice of LA and BA doped LA. The characteristic vibrational frequencies of obtained pure LA and BA doped LA shows shift in the peaks is presented in Table 1.

Table 1 Assignments of vibrational frequencies observed in IR spectra of pure LA and BA doped LA
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3.3 UV–Visible analysis

In the domain of optical field, materials having excellent transmittance are needed [32]. The UV–Visible study of LA and boric acid doped LA crystals reveal high transmittance in the entire visible region. The spectra were recorded using LAMBDA-35 UV–Visible spectrophotometer Fig. 3(a, b). The lower cut off wavelength was observed at 245 nm (LA) and 238 nm (BA doped LA) reveal that they could occupy an important place in generation of blue/violet light using a diode laser [32]. The spectra aims to give structural information since absorption of UV and visible light involves rising of electrons in π and n orbital from ground state to excited state through the photon absorption process [33].

Fig. 3
figure 3

(a, b). UV–Vis spectra of LA and boric acid doped LA

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3.3.1 Calculation of Band gap

The optical energy gap (Eg) of the material provides useful information regarding the band structure of electronic states of crystals [34]. The band gap was calculated by,

$$\alpha h\nu \, = \,A\left( {h\nu - E_{{\text{g}}} } \right)^{1/2}$$
(1)

Here, h (Planck's constant), ν (frequency), A (constant). The energy gap of the material was estimated by Tauc's graph plotted between (αhν)2 versus hν (Fig. 4a, b. The calculated wide band gap of 5.34 eV (LA) and 5.31 eV (doped LA) suggest enormous transmittance in visible range creates the crystals suitability in the sector of nonlinear optics includes optoelectronic devices.

Fig. 4
figure 4

(a,b). Tauc's Plot of LA and boric acid doped LA

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3.3.2 Urbach's energy

The presence of absorption coefficient value in close proximity to the band edge shows an exponential part called Urbach tail. This is very low for good crystalline materials having fewer impurities [35].Closely near to band gap edge, the absorption coefficient is represented as,

$$\alpha \, = \,\alpha_{0} \exp \, (h\upsilon /E_{u} )$$
(2)

where α0—constant, Eu- energy of Urbach or band tail.

Taking log on both sides of the above expression,

$$\ln \alpha \, = \,\ln \alpha_{0} \, + \,(h\upsilon /E_{{\text{u}}} )$$
(3)

Figure 5(a, b) clearly illustrates the Urbach or band tail energy by taking the inverse of the slope from the straight linear line [36]. For LA and doped LA the Urbach (Eu) was identified to be 0.4163 eV and 0.5662 eV, respectively. Thus the lower magnitude confirms lesser structural defect with better purity and good crystalline nature [37].

Fig. 5
figure 5

(a,b). Plot ln(α) with hν for LA and boric acid doped LA

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3.4 Thermal studies

Thermal examination is an extremely essential procedure to ascertain thermal behaviour such as melting point, stability and decomposition point of the materials covering broad range of manufacturing and research fields. The thermal nature of pure L-alanine and boric acid doped L-alanine crystals were investigated by thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) by taking samples of 3.0060 mg pure L-alanine and 3.7760 mg boric acid doped L-alanine employing a SPT Q600 V20.9 thermal analyser. The powder samples were placed in temperature range 20–600 °C at a heating rate of 20 °C/min. From the TGA and DTA traces (Fig. 6 a and b), weight loss is not observed up to 110 °C in BA doped LA, approves absence of entrapped water. After that, weight loss occurred in a single step for pure L-alanine (Fig. 6a) and weight loss takes place in two stages for BA doped LA crystal (Fig. 6b) and several gaseous fractions like CO, CO2, NH2 etc. are liberated leading to bulk decomposition before 600 °C. DTA plot (Fig. 6a) provides sharp endothermic peak at 280 °C (pure L-alanine), two sharp endothermic peaks at 112.42 °C coincides with the first stage of decomposition in TGA trace and second endothermic peak 253 °C (boric acid doped L-alanine) (Fig. 6b)) corresponds to second stage of decomposition are due to volatilization of the materials and also may be due to the dehydration of boric acid above 150 °C (Fig. 6b)[38]. Hence we can conclude that pure LA and boric acid doped L-alanine crystals can withstand high temperature involving NLO applications up to 280 °C (pure L-alanine) and 253 °C (boric acid doped L-alanine) with good degree of crystallinity and purity of the crystals [21]. Also the thermal study shows that the BA doped LA crystal is thermally stable than certain metal based L-alanine crystals [39, 40].

Fig. 6
figure 6

a TG and DTA curves of pure L-alanine b TG and DTA curves of boric acid doped L-alanine

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3.5 Microhardness analysis

Hardness of the material can be interpreted as its capability to resist long-lasting damage and also one of the foremost characteristic of any device materials [41]. Since mechanical strength is one of the important properties of any device fabrication is represented by its hardness [15].

The analysis of microhardness can be done to identify their mechanical characteristics by changing the loads (25 to 100 g). The diagonal length which is represented as d for indentation impression was analysed. Vickers hardness number (Hv) is determined by [42, 43]:

$$H_{v} = 1.8544\frac{P}{{d^{2} }}\left( {kg/mm^{2} } \right)$$
(4)

Here 1.8544 is a geometrical factor, d indicates the length (diagonal). Figure 7(a, b) and Fig. 8(a, b) notices that hardness (Hv) of the crystals drastically rises with increase of load (applied) confirms their high mechanical potency. The correlation between load (applied) and indentation range can be constituted by, P = Kdn, K (constant) and n (Meyer’s index). According to Onitsch, 1 < n < 1.6 holds good for hard category and n > 1.6 for soft category material [44]. The slope value declares the value of work hardening coefficient (n = 3.14 (LA), 3.26 (BA doped LA)) is a clear indication of their softness showing RISE behaviour [45].

Fig. 7
figure 7

a Microhardness b Meyers plot of LA

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Fig. 8
figure 8

a Microhardness b Meyers plot of Boric acid doped LA

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The yield strength (σy) can be evaluated as,

$$\sigma_{y} = \frac{{H_{v} }}{3}(0.1)^{{n^{\prime} - 2}}$$
(5)

The elastic stiffness constant (C11) is determined using Wooster’s empirical formula [45],

$$C_{11} \, = \,\left( {H_{v} } \right)^{7/4}$$
(6)

C11 predicts an assessment of bonding to the closest atoms. The calculated mechanical parameters are shown in Table 2 (LA) and Table 3 (doped LA).

Table 2 Mechanical parameters of pure LA
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Table 3 Mechanical parameters of boric acid doped LA
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3.6 Laser damage threshold (LDT)

Optically grown materials are examined by LDT, as it is a remarkable parameter to understand the material’s capability in linear and nonlinear characteristics to withstand high power intensities of laser involving nonlinear optical application such as electro optic modulation and second harmonic generation. The pulse width of 6 ns with a repetition rate of 10 Hz and a fundamental wavelength of 1064 nm Gaussian beam (Quanta-Ray INDI, Spectra Physics) was employed to estimate the laser damage threshold. The damage threshold of laser was determined by the expression.

$${\text{Power density }}P_{{\text{d}}} \, = \,E \, / \, A\tau$$
(7)

Here, E, A and τ denotes the energy, area (circular spot) and pulse width (ns). The computed LDT for the crystals (LA and boric acid doped LA) is found as 2.52 and 2.64 GW/cm2 show higher damage threshold value than certain organic and semiorganic materials such as ASOX, BV, CDBA, 4MSTB, LATBF, LFMH, BTZA [46,47,48,49,50,51,52] and also compared to metal doped LA[21] and KDP(0.20 GW/cm2), Urea(1.50GW/cm2) [53, 54] is presented in Table 4 for comparison. The inclusion of dopants in the parent crystal reveal the laser withstanding limit from the enhanced LDT value could be made use of in the field of NLO applications involving optical switching devices [55]. Apart from thermal effect, multiphoton ionization is the important cause for laser-induced damage.

Table 4 Comparison of LDT of grown crystal with other reported NLO Crystals
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3.7 Dielectric analysis

The characteristics of dielectric nature of a crystal are a measure of electrical feature of the material promoted by the polarization configuration. The variance of dielectric parameters (dielectric constant (ε′) and dielectric loss (ε″)) with respect to frequency is displayed in Fig. 9a, b and Fig. 10 a, b. Because of space charge polarization, the dielectric constant possesses higher value at the region of low frequency. In the range of high frequencies, the value of dielectric loss is very low as the frequency of the electrical field applied does not get harmonized with the natural frequency. As the crystals show low measure of ε′, ε″ they could be utilized in electro optic modulations.

Fig. 9
figure 9

a, b Dielectric constant, Dielectric loss of LA

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Fig. 10
figure 10

a, b Dielectric constant, Dielectric loss of BA doped LA

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3.7.1 Activation energy

The graph drawn between log σac and inverse of temperature is represented in Fig. 11(a,b) using the relation σac = σo exp(−Ea/kBT), where, σac is the conduction at temperature T, Ea is the activation energy for the electrical process and kB is the Boltzmann constant (1.38 × 10–23 J/k). The profile shows almost a linear behaviour and the slope from this graph is used to evaluate the activation energy using the formula [25],

$$E_{{\text{a}}} \, = \, - {\text{Slope}}\, \times \,1000 \times k_{{\text{B}}}$$
(8)
Fig. 11
figure 11

a, b Plot of σac vs 1000/T for pure LA and BA doped LA

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The values obtained are 0.01019 eV, 0.01284 eV, 0.01345 eV and 0.03059 eV at frequencies 800 Hz, 4000 Hz, 30 kHz and 2 MHz for pure LA and 0.01142 eV, 0.01309 eV, 0.01357 eV, 0.01199 eV at 500 Hz, 000 Hz, 20 kHz and 1 MHz frequencies for BA doped LA respectively, proves that the changes caused in the crystals need greater activation energy to jump between the energy states. Hence crystals possess less number of defects owing to less activation energy suitable for device fabrications [25].

3.7.2 Jonscher’s power law of conduction mechanism

Frequency is closely connected with conductivity in a material which could be examined from Jonscher’s power law [56] who had studied the variation of electrical conductivity σac with frequency for different solids comprising of polymers, glasses and crystals [57], is plotted in Fig. 12 a, b for pure LA and BA doped LA crystals where conduction mechanism is understood. From Jonscher’s power law,

$$\sigma_{a.c} = \sigma_{d.c} + A\omega^{S}$$
(9)
Fig. 12
figure 12

a, b Plot of AC conductivity versus Frequency of pure LA and doped LA

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where σdc is the DC conductivity which predicts the frequency (ω)independent plateau in the low frequency region, A is temperature dependent constant rely on the strength of polarisation, and s is temperature dependent power law exponent lies in the range 0 < s < 1,varies material to material. The estimated values of s = 0.988 (pure LA) and s = 0.985 (BA doped LA) from the slope of the graph confirms that grown crystals obeys power law. From the plotted figures (Fig. 12 a, b), it is noted that AC conductivity increases comparatively at higher frequency due to hopping mechanism [58] observed for pure and doped LA crystals.

3.8 SEM and EDAX analysis

To examine the surface morphology of boric acid doped LA crystal, FE1 quanta high resolution SEM has been used. The pictures of SEM in the resolution range of 5 and 10 μm are shown in Fig. 13. The elements present in the grown crystal are represented with diverse colours (Fig. 14) which confirms the presence of boron in the material. The overlapping of the crystals at different magnifications occur due to the agglomeration affected by the temperature fluctuations during the growth process. Scanning electron microscope affirmed that the surface of the crystal is having smooth topographical nature.

Fig. 13
figure 13

SEM micrograph of boric acid doped LA for resolution 5 μm and 10 μm

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Fig. 14
figure 14

SEM micrograph for resolution 5 μm showing different elements in Boric acid doped LA crystal

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Energy dispersive X-ray Analysis (EDAX) was done to confirm the elements in the grown crystal of boric acid doped LA at diverse energies. The presence of boron is observed from the EDAX spectra (Fig. 15).

Fig. 15
figure 15

EDAX spectra of Boric acid doped LA

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3.9 Fluorescence studies

The emission spectrum of both the grown materials (pure and boric acid doped LA) is revealed in fluorescence spectrum (Fig. 16 a, b) by applying the excitation wavelength of 245 nm (LA) and 238 nm (BA doped LA). Crystals show a stronger photon energy absorption band in the UV region with a highest intensity violet emission radiation at 404 nm (pure LA) and at 392 nm (doped LA) is likely to be useful in optoelectronic device applications. The title materials exhibit a strong and broad emission peak 404 nm (LA) and 392 nm (doped LA) which is related to the p–p* localized electron between the donor and acceptor groups through organic chromophore (p-conjugated double bond), which lead to its luminescence characteristic nature [59]. The peak of emission having highest intensity at 404 nm (LA) and 392 nm (doped LA) falls in the visible region that corresponds to violet fluorescence spectra. The peak observed at 296 nm (LA) and 297 nm (boric acid doped LA) are assigned to the defects present inside the materials serves as a predominant parameter of optoelectronics device applications and tunable laser system [60].

Fig. 16
figure 16

a, b Emission spectrum of the LA and boric acid doped LA crystals

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4 Nonlinear Optical Studies

4.1 Second harmonic generation- SHG

Crystals of organic category acquires SHG efficiency because of their large band gap and lesser value of dielectric constant which plays a key role in photonic applications [61]. Kurtz Perry powder technique [62] was employed to figure out the frequency doubling mechanism utilizing Nd-YAG laser with pulse duration (6 ns) and pulse rate (10 Hz). The frequency doubling effect was found to be 0.33 (LA) and 0.38 (boric acid doped LA) times of KDP and found to be comparable. The second harmonic in LA and boric acid doped LA was confirmed by the green radiation emitted from the samples. This firmly approved that the grown materials are capable aspirants for NLO applications which includes very useful for optoelectronic and terahertz applications.

4.1.1 Third harmonic generation -THG

By the approach of Z-scan technique, frequency tripling feature of harvested materials have been studied utilising 532 nm diode pumped continuous wave Nd-YAG laser was used as source produces both the sign and magnitude of the phase change. Normalised transmittance in closed aperture, open aperture, ratio of closed to open aperture and transmittance versus input intensity are shown in Fig. 17 a–d and Fig. 18a–d for LA and boric acid doped LA. From the open aperture profile, the nonlinear absorption coefficient (β) is assessed for both samples [63]. The closed aperture determines nonlinear refraction. The difference in the transmittance change of peak and valley (∆TP−V) is given by,

$$\Delta T_{P - V\,} = \,0.406(1\, - \,S)^{0.25} \left| {\Delta \Phi_{0} } \right|$$
(10)
Fig. 17
figure 17

a Closed aperture spectrum b Open aperture spectrum c ratio of the closed to open Z scan traces (d) Normalised transmittance verses Input intensity of LA

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Fig. 18
figure 18

a Closed aperture spectrum b Open aperture spectrum c ratio of the closed to open Z scan traces d Normalised transmittance verses Input intensity of Boric acid doped LA

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Linear transmittance aperture(S) is assessed by,

$$S = 1 - \,\exp \left( {\frac{{ - 2r_{a}^{2} }}{{\omega_{a}^{2} }}} \right)$$
(11)

The nonlinear refractive index (n2) can be assessed from the formula

$$n_{2} = \frac{{\Delta \Phi_{0} }}{{KI_{0} L_{eff} }}$$
(12)

Nonlinear absorption coefficient can be found by,

$$\beta {\mkern 1mu} = {\mkern 1mu} \frac{{2\sqrt {2\Delta T} }}{{I_{0} L_{{eff}} }}$$
(13)

In the data of closed aperture, peak followed by valley is observed in both the samples, which indicates negative nonlinearity (self-defocussing). This defocussing feature of samples is owing to the (negative) refractive index which is utilized in night vision applications [59, 64,65,66]. From negative refractive index value, it could be seen that materials exhibits defocusing nature, which is an essential parameter for optical sensor applications [66]. This defocusing nature due to negative refractive index also suggests that the title crystal could exhibit higher laser damage threshold [66]. Night Vision Equipment utilizes the optoelectronic devices where active infrared night-vision combines infrared illumination of spectral range 700–1,000 nm [64, 65].

The third-order nonlinear optical susceptibility was given by

$${\text{Re}} \chi^{(3)} (esu) = \frac{{10^{ - 4} (\varepsilon_{0} C^{2} n_{o}^{2} n_{2} )}}{\pi }\left( {\frac{{cm^{2} }}{W}} \right)$$
(14)
$${\text{Im}} \chi^{(3)} (esu) = \frac{{10^{ - 2} (\varepsilon_{0} C^{2} n_{0} \lambda \beta )}}{{4\pi^{2} }}\left( {\frac{{cm^{2} }}{W}} \right)$$
(15)
$$\left| {\chi^{(3)} } \right| = \sqrt {({\text{Re}} (\chi^{(3)} ))^{2} + ({\text{Im}} (\chi^{(3)} ))^{2} }$$
(16)

Table 5 provides the experimental outcomes of Z-scan for LA and boric acid doped LA. The nonlinear absorption coefficient (β) is 0.0389 × 10–4(cm/W) and 0.0276 × 10–4 (cm/W) indicates two photon absorption. The intensity of the transmitted light is linearly increased at low input power and then it saturates beyond the 3.419 × 103 W/cm2 for pure L-alanine and 2.906 × 103 W/cm2. This result indicates the existence of noticeable optical limiting characteristics of the crystals. In general, the origin of the optical limiting is a complex process, however, it may be correlated with important factors such as nonlinear optical refraction (or thermal effect), nonlinear optical absorption and nonlinear optical scattering [65]. Hence, the obtained result satisfies the essential requirement of optical limiters create them as versatile candidates in optical limiting process [67]. Also third-order nonlinearity (χ(3) with other similar class of crystals reported earlier are compared and presented in Table 5.

Table 5 Z-scan measurements of LA and boric acid doped LA
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(χ(3)) values of grown materials are found to be higher than NLO materials reported [66, 68,69,70,71,72] also furnished for comparison with present pure LA and BA doped LA materials presented in Table 6.

Table 6 Comparison of χ3 value of present pure and boric acid doped LA with other NLO materials
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5 Conclusion

Single crystals of pure L-alanine and 5 wt % of boric acid doped L-alanine have been grown using slow evaporation technique with water as solvent and their structures analysed by single crystal XRD revealed noncentrosymmetric space group. The FT-IR frequencies observed confirms the zwitterion formation in the crystals and presence of doped element from the wavenumbers seen at lower frequencies. The UV–VIS study shows insignificant absorption in the visible region enabling the crystals implementation in opto-electronic field. The thermal studies discloses the materials stability up to 280 °C (LA) and 253 °C (BA doped LA). The fluorescence emission peak of violet is observed at 404 nm for LA and 392 nm for doped LA. The dielectric investigation indicates lower value of dielectric constant and dielectric loss in the grown materials suggests the materials advantage in microelectronics device applications. Also Jonscher’s power law; \(\sigma_{a.c} = \sigma_{d.c} + A\omega^{S}\), investigates that event of conduction predominantly increased at high frequency with temperature dependent exponent s < 1. Laser damage tolerance which is an imperative factor shows the crystals potential in withstanding high power lasers. SEM examination determines the surface topography at different magnifications and EDAX spectrum reveals the presence of element Boron in doped LA crystal. The analysis of microhardness infers that both crystals correspond to soft category. The third order nonlinear characteristics of the crystals suggest good focusing nature with nonlinear refractive index and nonlinear absorption coefficient and third-order susceptibility for pure LA and BA doped LA (χ3) = 9.087 × 10–8(esu), 6.207 × 10–8 (esu) suggest them in third-order nonlinear optical device applications. From the analyses carried out, L-alanine and boric acid doped L-alanine single crystals are proving to be promising contenders in frequency conversion process for optoelectronic applications together with optical switching, optical limiting and device fabrications.