Dielectric, etching and Z-scan studies of glycine doped potassium thiourea chloride crystal

The dielectric studies are vital to understand the electro-optic properties of crystalline material [10] for which the dielectric constant and dielectric loss of pure and glycine doped potassium thiourea chloride crystals have been evaluated at different frequencies using the Gwinstek-819 LCR cube meter at room temperature. To conduct good measurements, the parallel-faced single crystals were selected and covered by silver paste to avoid loss of data due to external factors. The active electronic, ionic, dipolar and space charge polarization mechanism at lower frequencies strongly influences the relaxation time of molecular dipoles in a material causing an increase of dielectric constant [11].

The frequency response of dielectric constant of the crystals is shown in Fig. 2a which evidences the decreasing behavior of dielectric constant of pure and glycine doped potassium thiourea chloride crystal with increasing frequency. The high polarization activity in lower frequency domain leads to higher dielectric constant while the low polarization activity at higher frequencies results in reduction of dielectric constant of crystals [12]. As organic groups favor lower dielectric activity, substantial reduction in magnitude of dielectric constant of glycine doped potassium thiourea chloride crystal is observed in comparison to PTC crystal. The materials with lower dielectric constant consume less power which is the decisive factor for designing NLO, photonics and electro-optic modulation devices [13]. The dissipation factor mainly depends on defect centres in crystalline material [14]. The profile of dielectric loss of the crystals is plotted in Fig. 2b. The less dielectric loss indicates that the glycine doped potassium thiourea chloride crystal possesses less defect centres than the PTC crystal [15]. The lower dielectric constant and dielectric loss of glycine doped potassium thiourea chloride crystal suggest its usefulness for optoelectronics device applications.

The etching studies were employed to examine surface purity, growth habit and structural defects associated with glycine doped potassium thiourea chloride crystal. A selected surface of glycine doped potassium thiourea chloride crystal was etched with water and the etch micrographs of the crystal surface were recorded after a regular interval of 10 s. The micrograph (Fig. 3a) of the grown crystal surface shows irregular micropits and intermixed patterns which might have occurred due to variation in temperature of surface during the growth. After 10 s of etching (Fig. 3b) the surface of glycine doped potassium thiourea chloride crystal reveals a symmetrically stacked step growth trend with the absence of pits and line defects. The stacked growth habit of glycine doped potassium thiourea chloride crystal is clearly observed after 20 s of etching as seen in Fig. 3c. Further etching of the surface for 40 s (Fig. 3d) confirms the step growth nucleation along the studied plane. The absence of micropits, structural defects and symmetrical step nucleation along the plane confirms the good crystalline nature of the glycine doped potassium thiourea chloride crystal.

The TONLO properties of glycine doped potassium thiourea chloride crystal have been evaluated at 632.8 nm using the Z-scan technique developed by Bahae et al. [16]. The optical resolution of experimental Z-scan setup is given in Table 1. The closed aperture Z-scan technique provides the information about the nature and magnitude of path dependent third order refraction nonlinearity in a crystal. The tightly focused gaussian beam of He-Ne laser was irradiated through a converging lens on a crystal sample and the sample was gradually translated about the beam irradiated path focus (Z = 0). The shift in transmittance of the crystal about the focus along the Z path was observed. The distribution of optical energy along the crystal surface, due to localized absorption of highly repetitive optical beam, causes the phase shift in transmittance about the focus (Z = 0) [17]. The peak to valley phase shift about the focus confirms the negative nonlinear refraction (NLR) tendency in glycine doped potassium thiourea chloride crystal as shown in Fig. 4a. The negative NLR is characteristic of self-defocusing dominant material which is highly demanded for protective optical night vision sensor devices [18].

Table 1

Z-scan setup and TONLO parameters of the grown crystal.

The on axis phase shift (ΔΦ) and peak to valley transmittance difference (ΔT_p-v) are determined using equation [16]: ΔTp−<!-- - -->v = 0.406(1−<!-- - -->S)0.25 Δϕ $$\Delta {T_{p - v}}{\text{ = 0}}.406(1 -S{{\text{)}}^{{\text{0}}{\text{.25 }}}}\left| {\Delta \phi } \right| $$ (1)

where[ S=1−<!-- - -->exp(−<!-- - -->2ra2/ωa2) $ S = \left[ {1\; - \;\exp \;(\; - \;2{{\rm r}_{\rm a}^2}/ \omega_{\rm a}^2)} \right] $ ] is the aperture linear transmittance, r_a is the aperture radius and ω_ais the beam radius at the aperture. The third order NLR (n₂) of glycine doped potassium thiourea chloride crystal was calculated using the relation [16]: n2=ΔϕKI0Leff $${n_2} = \frac{{\Delta \phi }}{{K{I_0}{L_{eff}}}} $$ (2)

where K = 2π/λ (λ is the laser wavelength), I₀ = 5 MW/cm² is the intensity of the 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 n₂ of glycine doped potassium thiourea chloride crystal is found to be 7.27 × 10⁻¹² cm²/W. The origin of nonlinear absorption β in glycine doped potassium thiourea chloride crystal has been investigated by means of open aperture Z-scan configuration. The open aperture Z-scan transmittance shows a significant fall at the beam irradiated path focus (Z = 0) confirming the presence of reverse saturable absorption (RSA) effect in glycine doped potassium thiourea chloride crystal as shown in Fig. 4b. The origin of RSA effect is assisted by multi photon absorption in excited states [19]. The crystals with origin of RSA phenomenon are highly desirable for biomedical and optical limiting photonic devices [20]. The nonlinear absorption coefficient β of glycine doped potassium thiourea chloride crystal is calculated using equation [16]: 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 glycine doped potassium thiourea chloride crystal exhibits the RSA effect with the β coefficient of a magnitude 1.61 × 10⁻⁶ cm/W. The measure of TONLO susceptibility (χ³) determines the polarizing nature of the material determined using the equations below [16]: Reχ(3)(esu)=10−<!-- - -->4(ε<!-- e -->0C2n02n2/π<!-- p -->(cm2/W) $$ Re{\chi ^{(3)}}\;(\text {esu})\; = \;{10^{ - 4}}\;({\varepsilon _0}{C^2}n_0^2{n_2}/\pi ( {\text {cm}^2/\text{W}}) $$ (4)

Imχ(3)(esu)=10−<!-- - -->2(ε<!-- e -->0C2n02λβ/4π<!-- p -->2(cm/W) $$Im {\chi ^{(3)}}\;(\text {esu})\; = \;{10^{-2}}\;({\varepsilon _0}{C^2}n_0^2\lambda \beta /4{\pi ^2}(\text {cm}/\text {W})$$ (5)

χ3=(Reχ3)2+(Imχ3)2 $${\chi ^3} = \;\sqrt {{{({Re} {\chi ^3})}^2} + {{({Im} {\chi ^3})}^2}} $$ (6)

where ∈₀ 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 glycine doped potassium thiourea chloride crystal is found to be of order of 6.99 × 10⁻⁶ esu. The photo induced mobility of charge transfer through the π-electron dominated chromophores leads to large enhancement in nonlinearity of glycine doped potassium thiourea chloride crystal [21, 22].