The crystals were subjected to single crystal X-ray diffraction (XRD) analysis using the Enraf Nonius CAD-4 crystal X-ray diffractometer. The pure and SMS doped ADP crystals were confirmed to have the tetragonal crystal structure and the slight changes were identified in the unit cell parameters of SMS-ADP crystal. The XRD data are discussed inTable 1 and the increase in volume of SMS-ADP crystal might have been attributed to the successful incorporation of SMS in lattice sites of ADP crystal.

Table 1

Table 1

Table 1 Crystallographic data Crystal Crystal system Cell parameters (nm) Volume (nm3) Space group ADP Tetragonal a = b = 0.751, c = 0.756 0.426 I-42d SMS-ADP Tetragonal a = b = 0.752, c = 0.761 0.430 I-42d

Table 1 Crystallographic data

The incorporation of SMS in ADP has been qualitatively confirmed by means of energy dispersive spectroscopic (EDS) technique using the HITACHI S4700 instrument. The grown SMS-ADP crystals were powdered and subjected to EDS analysis (energy range = 0 to 10 keV). The recorded EDS spectrum of SMS-ADP crystal is shown in Fig. 2(a). The peaks observed at 1 and 1.78 keV evidence the presence of sodium (Na) and silicon (Si) ions in the crystal lattice of ADP.

Fig. 2.

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	Fig. 2. (a) EDX spectrum of SMS-ADP, (b) FT-IR spectrum of SMS-ADP crystal.

The functional groups of grown crystal were identified by means of Fourier transform infrared (FT-IR) analysis using Bruker attenuated total reflection (ATR) spectrometer. The recorded FT-IR spectrum of SMS doped ADP crystal is shown in Fig. 2(b). The absorption peaks cited at wavenumber 3790 and 3232 cm^-1 correspond to Si-O-H stretching vibrations. The P-H stretching vibration is observed at 2362 cm^-1. The characteristic NO₂ asymmetric vibration is attributed at 1587 cm^-1. The sharp peak of N-H stretching vibrations occurs at 1417 cm^-1. The P=O stretching vibration is observed at 1253 cm^-1. The absorption peak at 1052 cm^-1 corresponds to the Si-O-Si asymmetric stretching vibrations. The NO₂ bending mode of vibration is attributed at 680 cm^-1.

3.4.1. UV-visible spectral analysis

The crystals with high optical homogeneity can be readily used for applications in wide range of optical devices. The UV-visible transmittance spectrum of pure and SMS doped ADP crystal was recorded in the range of 200-900 nm using the Shimadzu UV-2450 spectrophotometer. The analysis of spectrum (Fig. 3(a)) in visible region reveals that the transparency of ADP and SMS-ADP crystals is up to 79% and 86%, respectively. The enhanced transparency of SMS-ADP crystal by 7% justifies the assertive role of dopant in minimizing the defect centers and solvent inclusions which attributed the reduced optical scattering within the crystal system^[14]. The high optical transparency window of SMS-ADP crystal is of huge advantage for UV-tunable lasers, NLO and frequency conversion devices^[15]. The dependence of absorption coefficient (α ) on incident photon energy (hν ) is related as (α hν )² = A (hν - E_g), where E_g is the energy band gap of the material. The E_gof grown crystal has been determined from the Tauc's extrapolation plot shown inFig. 3(b). In the present study, the E_g of ADP and SMS-ADP crystal is found to be 5.1 and 5 eV, respectively, which is wide enough to exploit the SMS-ADP crystal for optoelectronics device applications^[15]. The optical density of the material medium can be understood from the nature of refractive index (n = 1/T + (1/T - 1), where T is the transmittance of the material), as shown in Fig. 4(a)^[16]. The SMS-ADP crystal possesses lower refractive index than ADP in visible region which makes it a more suitable material for calibrating the figure of merit of reflectors, resonators and filters which are vital optical components in large laser systems^[17]. The reflectance (R) offered by crystals in visible region is shown in Fig. 4(b). The electromagnetic energy is lost within material medium while propagation can be analyzed by observing the behavior of extinction coefficient (K = α λ /4π ), as shown inFig. 5. The high percent of transparency, lower refractive index, lower reflectance and extinction coefficient suggest the potential candidature of SMS-ADP crystal as antireflecting material demanded for coating solar thermal devices [15, 18].

Fig. 3.

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'>	Fig. 3. (a) UV-vVisible transmittance spectrum, (b) Tauc's plot.

Fig. 4.

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	Fig. 4. Plot of (a) refractive index and (b) reflectance as a function of wavelength.

Fig. 5.

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	Fig. 5. Plot of extinction coefficient as a function of wavelength.

3.4.2. Kurtz-Perry powder SHG test

In order to confirm the NLO behavior, the crystal samples were powdered and subjected to SHG efficiency test using the Kurtz-Perry powder technique. The Q-switched Nd:YAG laser beam (λ = 1064 nm, 6 ns, 10 Hz, 5.4 µ J/pulse) was made normally incident on the powdered sample of KDP, ADP and SMS-ADP crystals. The emergence of intense green signal confirmed the NLO behavior of each sample and the output SHG intensity of each sample is shown in Fig. 6. The SHG efficiency of SMS-ADP crystal is found to be 2.26 times that of KDP and 1.25 times that of ADP crystal material. The enhancement in SHG efficiency might have been observed due to the modified molecular orientation of ADP crystal due to the incorporation of ionic impurities of SMS. The SHG efficiency of SMS-ADP crystal is found as superior to oxalic acid^[7], potassium chloride^[10] and cobalt ion^[13]doped ADP crystals. The high SHG efficiency strongly advocates the prominence of SMS-ADP crystal for laser frequency conversion devices.

Fig. 6.

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	Fig. 6. Plot of SHG output intensity.

3.4.3. Z-scan analysis

The Z-scan is the most efficient and vital technique developed by Bahae et al. to explore the third order nonlinear behavior of the material at desired laser wavelength ^[19]. In the present Z-scan setup ( Table 2) the polarized beam of He-Ne laser is tightly focused on SMS-ADP crystal (1 mm) through a converging lens and the crystal was moved to and fro about the beam irradiated path focus (Z = 0). In closed aperture Z-scan configuration the position of crystal sample was changed along the Z direction and the transmittance was recorded through the closed slit of photo detector placed at far field. The close aperture Z-scan curve ( Fig. 7(a)) of SMS-ADP crystal shows the valley to peak phase shift which confirms the self focusing effect resulting to positive NLR response^[20]. The high magnitude of positive NLR (n₂) identifies the promising Kerr lens mode locking (KLM)^[21]potential of SMS-ADP crystal. The relation between axis phase shift (Δ Φ ) and peak to valley transmittance difference (Δ T_p-v ) is given as^[19],

Δ T_p-v=0.406(1-S)^0.25|Δ ϕ | (1)

where S = 1 - exp (-2r_a²/ω _a²) is the aperture linear transmittance, r_a is the aperture radius and ω _a is the beam radius at the aperture. The magnitude of nonlinear refractive index (n₂) is calculated using the relation^[19],

n2=∆∅KI2Leff(2)

where K = 2π /λ (λ is the laser wavelength), I₀ is the intensity of the laser beam at the focus (Z = 0), L_eff = (1 - exp(-α L))/α which is the effective thickness of the sample depending on linear absorption coefficient (α ) and the thickness of the sample (L). The magnitude of n₂ of SMS-ADP crystal is found to be 5.94 × 10^-10 cm²/W which is notably higher than potential technological crystals used in laser systems^[22]. The open aperture Z-scan curve ( Fig. 7(b)) of SMS-ADP crystal reveals the saturable absorption (SA) phenomenon which is attributed to higher ground state linear absorption over the excited state absorption^[23]. The nonlinear absorption coefficient (β ) value of crystal has been calculated from following equation^[19],

β=22∆TI0Leff(3)

where Δ T is the one valley value at the open aperture Z-scan curve. The β of SMS-ADP crystal is found to be 2.81 × 10^-5 cm/W. The cubic susceptibility (χ ³) defines the polarizing ability of the material which can be calculated using the following equations^[19],

Reχ ⁽³⁾(esu)=10^-4  (ε ₀C²n₀²n₂)/π (cm²/W) (4)

Imχ ⁽³⁾(esu)=10^-2  (ε ₀C²n₀²λ β )/4π ²  (cm/W) (5)

χ3=(Reχ3)2+(lmχ3)2(6)

where ε ₀ is the vacuum permittivity, n₀ is the linear refractive index of the sample and cis the velocity of light in vacuum. The larger χ ³ of crystal system confirms the high polarizing nature of SMS-ADP crystal and it is found to be 6.19 × 10^-5 esu. The larger value of χ ³ indicates the enhanced photoinduced intermolecular charge transfer through π -electron conjugate system^[24]. The SMS-ADP crystal with higher χ ³ could be a better replacement of several doped ADP crystals [4, 25, 26].

Table 2

Table 2

Table 2 Z-scan analysis data Parameters Magnitude Laser wavelength (λ ) 632.8 nm Laser power (P) 10 mW Lens focal length (f) 20 cm Optical path distance (Z) 113 cm Beam waist radius (ω a) 1 mm Aperture radius (ra) 15 mm Incident intensity at the focus (Io) 2.3375 kW/m2 Nonlinear refraction (n2) 5.94 × 10-10 cm2/W Nonlinear absorption (β ) 2.81 × 10-5 cm/W Cubic susceptibility (χ 3) 6.19 × 10-5 esu

Table 2 Z-scan analysis data

Fig. 7.

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	Fig. 7. (a) Close and (b) open aperture Z-scan transmittance curves.

The Vicker's microhardness studies were performed to unfold the influence of SMS on crystal strength and lattice perfection of ADP crystal. To study the hardness behavior, different loads (25 to 100 g) were applied for the period of 10 s on < 001> face of ADP and SMS-ADP crystal and the corresponding diagonal length was measured. The nearly square shaped indentations were observed on the indented surface which indicates the presence of uniform interatomic bonding and spacing in SMS-ADP crystal. The hardness number (H_v) of the crystals was calculated using relation H_v = 1.8544 × P/d², where P is the applied load in kg and d is the diagonal length in mm ^[27]. The variation of hardness number with increasing load is shown in Fig. 8(a). It reveals that the hardness of SMS-ADP crystal is superior to ADP crystal. The work hardening index n was derived using the Meyer's equation given as: logP = logK₁ + n logd. The plot between logd and logP is shown in Fig. 8(b). The slope of the line gives the value of n. The SMS-ADP crystal with large value of n (3.2) belongs to the category of soft materials satisfying the Onitsch results ^[28]. The hardness dependent yield strength (σ _y = (0.1)^n-2H_v/3) and elastic stiffness (C₁₁ = H_v^7/4) of crystals has also been evaluated at same loads. The higher magnitude of elastic stiffness of SMS-ADP crystal confirmed the strong interatomic bonding forces between the adjacent atoms^[29]. The hardness data of pure and doped ADP crystal are discussed in Table 3. The SMS-ADP crystal exhibits high hardness number, high yield strength and high elastic stiffness which are of huge importance while tuning and processing the material for fabricating NLO devices.

Fig. 8.

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	Fig. 8. (a) Load dependent hardness, (b) plot of logd vs logP.

Table 3

Table 3

Table 3 Microhardness parameters along < 001> face Crystal Hv (kg/mm2 ) n σ y (MPa) C11 (GPa) ADP 57.96 2.8 11.94 20.91 × 106 SMS-ADP 78.65 3.2 16.21 36.72 × 106

Table 3 Microhardness parameters along < 001> face

The dielectric properties of the grown crystal have been evaluated in the range of 30 to 120 ° C at a frequency of 100 kHz using the HIOKI-3532 LCR cubemeter. Before the analysis the crystal surfaces were smoothly applied with the silver paste so as to attain good electrical contact and measure the data with high accuracy. The dielectric constant of the materials is attributed to active electronic, ionic, space charge and dipolar polarization mechanisms^[30]. The dependence of dielectric constant (ε _r = Cd/ε ₀A) on temperature is depicted in Fig. 9(a). It is observed that the dielectric constant of the crystals increases with increasing temperature. It is contributed to the high space charge polarization of molecular dipoles in high temperature range^[31]. The lower dielectric constant of SMS-ADP crystal confirms the low electrical power consumption capability which is a vital parameter for designing photonics, microelectronics^[32], broad band electro-optic modulators (EOM)^[33], field detectors and THz wave generators^[34]. According to Miller's law, the lower dielectric constant facilitates the enhancement in SHG coefficient of the crystal^[28], which is also observed in the case of SMS-ADP crystal. The profile of dielectric loss (Fig. 9(b)) of material assists to evaluate the extent of loss of electrical energy in the form of heat in material medium. It is observed that the dielectric loss of SMS-ADP crystal is lower than ADP throughout the temperature range. The lower dielectric loss of SMS-ADP crystal defines the superior optical quality and less concentration of electrically active defects which is vital parameter for NLO applications^[35].

Fig. 9.

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	Fig. 9. Temperature dependent (a) dielectric constant and (b) dielectric loss.