Investigation on nonlinear optical and dielectric properties of _L-arginine doped ZTC crystal to explore photonic device applications

The chemical composition of LA-ZTC crystal has been qualitatively determined using a JEOL 6360 instrument to confirm the incorporation of LA in ZTC crystal. The compositional mass% of each element, namely carbon (C), nitrogen (N), zinc (Zn), chlorine (Cl) and oxygen (O), identified in the LA-ZTC crystal, are shown in EDS spectrum (Fig. 2a). As ZTC crystal does not possess oxygen, the presence of O with high mass% of 59.13 clearly evidences the presence of LA in ZTC crystal.

The surface of LA-ZTC crystal has been evaluated using the SEM analysis carried out with the JE0L-2300 instrument. The SEM micrograph (Fig. 2b) of selected area of LA-ZTC crystal sample has been recorded at a resolution of 100 μm which was achieved by detecting the signals received due to the electron-surface interaction. The major area of the image is covered by intense white color substance swhich evidences the presence of chlorine in the sample [9]. The large multinucleations were observed on the crystal surface which might have occurred due to organic dopant LA. The absence of cracks and voids confirm the perfection of LA-ZTC crystal.

The Vicker’s microhardness study has been done using a Shimadzu HMV-2T microhardness analyzer to assess the mechanical properties of LA-ZTC crystal. The Vicker’s hardness number (VHN) of LA-ZTC crystal has been determined at different indentation loads using the relation: H_v = 1.8544 P/d² kG/mm², where P is the applied load in kG and d is the average indentation diagonal length in mm [10]. The VHN for each load applied for a constant period of 5 sec is plotted in Fig. 3a. It reveals that the hardness of LA-ZTC crystal increases with the applied load which confirms that the stress required for homogeneous nucleation of lattice dislocations is larger for higher load. The work hardening coefficient (n) of material depends on the dislocation movement in the crystal system for which Meyer’s law, P = Kdⁿ can be employed [11]. The n of LA-ZTC crystal has been determined from the slope of the plot of logP vs. logd, shown in Fig. 3b. The magnitude of n has been found to be 2.97, and such materials falls to the category of soft materials obeying the Onitsch criterion [12]. In proper selection of materials for high-edge mechanical device fabrication the elastic stiffness constant (C11Hv7/4)$(\rm C_{11}=H_{v}^{7/4})$ and yield strength (σ_y = (0.1)^n-2 H_v/3) play a crucial role. the calculated hardness parameters are shown in Table 1. The increasing nature of C₁₁ and σ_y clearly evidences the high bond strength between the neighboring atoms of LA-ZTC crystal system [13]. The good mechanical stability of LA-ZTC crystal is beneficial for avoiding breakage and wastage of material while processing for device fabrication [10, 14].

Fig. 2

Fig. 3

The Kurtz-Perry powder SHG test is a decisive technique to determine the second order conversion efficiency of given material [15]. For present analysis, the single KDP and LA-ZTC crystals were powdered to micro-granules of even grain size and tightly packed in a quartz cavity. The prepared samples were multi-shot by a polarized Gaussian beam of Q-switched Nd:YAG laser operating at 1064 nm, having a pulse width of 6 ns, delivering the energy of 5.4 μJ/pulse with a repetition rate of 10 Hz. The output intensities of each sample were recorded through an array of photomultiplier tubes. The emission of bright green light at the output window confirmed the nonlinear behavior of the grown crystals. The SHG intensities of KDP and LA-ZTC crystal samples are plotted in Fig. 4. it reveals that the SHG efficiency of LA-ZTC crystal is 1.96 times higher than that of KDP crystal material. In the literature, the SHG efficiency of 1 mol.%, 3 mol.% and 5 mol.% of LA-ZTC crystal was reported to be 0.65, 0.71 and 0.75 times that of KDP crystal which is significantly lower than that of 1 wt.% LA-ZTC crystal [6]. The enhanced NLO response of LA-ZTC crystal is mainly attributed to the noncentrosymmetric complexity, wide hydrogen bonding network and enhanced photoinduced π-electron charge transfer through the donor-acceptor chromospheres [16]. The LA-ZTC with high SHG efficiency is highly advisable crystal for application in laser frequency conversion devices.

Fig. 4

The Z-scan is the most sensitive and effective technique to examine and measure the third order nonlinear optical (TONLO) parameters of material [17]. In order to determine the TONLO behavior of LA-ZTC crystal, closed and open aperture Z-scan technique (Table 2) was employed using the He-Ne laser operating at 632.8 nm. The crystal sample of 1mm thickness was placed at the focus (Z = 0) of a beam irradiated path and a polarized laser beam was concentrated on the sample through a converging lens. The crystal was translated to and fro about the focus along the Z-direction. The configuration in which the intensity transmitted by the crystal sample along the Z-directed translation is measured through a photodetector with closed aperture is defined as closed aperture Z-scan technique. The closed aperture Z-scan transmittance curve shown in Fig. 5a helps to identify the nature of nonlinear refraction (NLR) of the crystal. It reveals that the transmittance of LA-ZTC crystal offers a valley to peak phase transition about the focus, which confirms the self-focusing nature of the LA-ZTC crystal. The self-focusing nature identifies the LA-ZTC crystal as a positive NLR material [18]. The spatial distribution and localized absorption of highly repetitive optical energy along the crystal surface is the primary factor governing NLR in the crystal system [19]. The difference between the peak to valley transmittance (ΔT_p-v) in terms of on axis phase shift (ΔФ) is defined as [17]: ΔTp−v=0.406(1−S)0.25|Δϕ|$$\Delta {T_{p - v}} = 0.406{\left( {1 - S} \right)^{0.25}}\left| {\Delta \phi } \right|$$(1)

where S=[1−exp(−2ra2/ωa2)]${\text S} = \left[ {1 - \exp \left( { - 2r_a^2/\omega _a^2} \right)} \right]$ is the aperture linear transmittance, r_a is the aperture radius and ω_a is the beam radius at the aperture. The magnitude of NLR (n₂) was calculated using the equation [17]:

Fig. 5

where K = 2π/λ (λ is the laser wavelength), I₀ is the intensity of laser beam at the focus (Z = 0), L_eff = [1—exp (—αL)]/α, is the effective thickness of the sample depending on linear absorption coefficient (α) and L is the thickness of the sample. The NLR (n₂) of LA-ZTC crystal is found to be 4.45 × 10⁻⁹ cm²/W. The high magnitude of positive NLR defines the potential ability of material to inherit Kerr lens mode locking (KLM) which is the most desirable feature for designing photonic devices, such as laser alignment systems, shorter pulse generation accessories and optical switching devices [20, 21]. The open aperture (OA) Z-scan study determines the nature and magnitude of nonlinear absorption (NLA) behavior of material. The OA Z-scan curve of LA-ZTC crystal is shown in Fig. 5b. It is observed that the transmittance along the Z-direction falls at the focus which confirms the origin of reverse saturable absorption (RSA). The RSA effect is observed due to multiphoton absorption (MPA) effect facilitated by dominance of excited state absorption over the linear absorption coefficient of crystal [22]. The LA-ZTC crystal with RSA feature is a substantial material for photonic and biomedical devices [23].

The NLA coefficient (β) of crystal has been calculated from equation [17]: β=22ΔTI0Leff$$\beta = \frac{{2\sqrt {2\Delta T} }}{{{I_0}{L_{eff}}}}$$(3)

where ΔT is the one valley value at the open aperture Z-scan curve. The β of LA-ZTC crystal was found to be 8.23 × 10⁻⁴ cm/W. The TONLO susceptibility (χ³) of material determines the merit of photoinduced polarizing ability of material [24]. The χ³ was calculated using the following equations [17]: Reχ3esu=10−<!-- - -->4ε<!-- e -->0C2n02n2/π<!-- p -->cm2/W$${\mathop{Re}\nolimits} {\chi ^{\left( 3 \right)}}\left( {esu} \right) = {10^{ - 4}}\left( {{\varepsilon _0}{C^2}n_0^2{n_2}} \right)/\pi \left( {c{m^2}/W} \right)$$(4)Imχ3esu=10−2ε0C2n02λβ/4π2cm/W$${\mathop{Im}\nolimits} {\chi ^{\left( 3 \right)}}\left( {esu} \right) = {10^{ - 2}}\left( {{\varepsilon _0}{C^2}n_0^2\lambda \beta } \right)/4{\pi ^2}\left( {cm/W} \right)$$(5)χ3=Reχ!32+Imχ32$${\chi ^3} = \sqrt {{{\left( {{\mathop{Re}\nolimits} {\chi ^3}} \right)}^2} + {{\left( {{\mathop{Im}\nolimits} {\chi ^3}} \right)}^2}} $$(6)

where ε₀ is the vacuum permittivity, n₀ is the linear refractive index of the sample and c is the velocity of light in vacuum. The χ³ of LA-ZTC crystal was found to be of order 10⁻³ esu. The TONLO parameters of LA-ZTC crystal are comparatively collected in Table 3 [25, 26]. The figure of merit (T = βλ/n₂) of LA-ZTC crystal was found to be 21.86. This evidences the dominance of NLR over NLA which is a vital factor for designing photonic devices [27]. The promising TONLO parameters confirm the high suitability of LA-ZTC crystal for laser and photonic devices.

Fig. 6

The dielectric constant and dielectric loss of LA-ZTC crystal have been investigated in the range of 30 °C to 120 °C using a HIOKI-3532 LCR cube-meter. The sample crystal was polished and covered by silver paste on the surface to ensure good electrical contact with the LCR probe.

The dielectric properties of material are strongly influenced by the applied external frequency as well as temperature. The dielectric constants (ε_r = Cd/ε₀A, where C is the capacitance of the sample, d is the thickness, ε₀ is the permittivity of free space and A is the area of the crystal sample) of pure and LA-ZTC crystals are shown in Fig. 6a. The dielectric constant is attributed to the electronic, ionic, dipolar and space charge polarization which increases with the rise in temperature. The increase in dielectric constant is observed as the space charge polarization is dominant at lower frequency and higher temperature [10]. It is observed that the dielectric constant of LA-ZTC crystal is significantly lower than that of ZTC. As materials with lower dielectric constant consume less power, hence LA-ZTC crystal is superior to ZTC for designing broad-band electro-optic modulators, field detectors, photonics and microelectronics devices [28]. In Fig. 6b, the dielectric loss (tanδ = ε_r/ε₀) of LA-ZTC crystal is observed to be lower than that of ZTC crystal which confirms its improved quality and lower defect concentration [29].