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In this paper, we have performed a study on the optical properties of single crystals ZnGa₂S₄ by method of density functional theory (DFT) and ellipsometry measurements. The calculated results for the real ${\epsilon }_r$ and imaginary ${\epsilon }_i$ parts of the dielectric function are compared with the measured experimental spectroscopic ellipsometry (SE) spectra in the 0.7–6.5 eV spectral region. Absorption coefficient-photon energy dependency revealed the existence of direct bandgap transitions with energy 3.5 eV. This result is very close to bandgap obtained from our theoretical calculation.

Spinel ferrite ${\text{Ni}}_{0.08}$${\text{Mn}}_{0.90}$${\text{Zn}}_{0.02}$Fe₂O₄ was prepared by a conventional ceramic process followed by sintering at three different temperatures (1050${}^{\circ }$ C, 1100${}^{\circ }$ C and 1150${}^{\circ }$ C). X-ray diffraction (XRD) investigations stated the single-phase cubic spinel structure and the FTIR spectra revealed two prominent bands within the wavenumber region from 600 cm${}^{-1}$ to 400 cm${}^{-1}$. Surface morphology showed highly crystalline grain development with sizes ranging from 0.27 $\mu$m to 0.88 $\mu$m. The magnetic hysteresis curve at ambient temperature revealed a significant effect of sintering temperature on both coercivity ($H_c)$ and saturation magnetization ($M_s)$. Temperature caused a decrease in DC electrical resistivity, while the electron transport increased, suggesting the semiconducting nature of all samples and that they well followed the Arrhenius law from which their activation energies were determined. The values of Curie temperature ($T_c)$ and activation energy were influenced by the sintering temperature. Frequency-dependent dielectric behavior (100 Hz–1 MHz) was also analyzed, which may be interpreted by the Maxwell–Wagner-type polarization. The UV–vis–NIR reflectance curve was analyzed to calculate the bandgap of ferrites, which showed a decreasing trend with increasing sintering temperature.

TiO₂ is a highly potential, photoactive and dye-sensitive photocatalyst but its optical properties are further enhanced in the presence of different types of dyes. Dye-modified TiO₂ was prepared by the modified sol–gel method. The dyes are methyl orange (MO), potassium permanganate (KMnO₄) and beetroot. Analysis was conducted to study the physical, linear and nonlinear optical properties. XRD result suggests that the prepared TiO₂ is synthesized in the pure anatase phase. The bandgap is decreased in all the dye-modified TiO₂ samples. The dielectric constant, interband transition strength, energy loss functions, Urbach energy, optical conductivity and electrical conductivity were determined. Further, the dispersion parameter of refractive index was discussed as per the single-oscillator model proposed by Wemple and DiDomenico. Moreover, the analyses of nonlinear dispersions like first-order and third-order optical susceptibilities, electrical susceptibilities, nonlinear refractive index and reflection loss were performed. To further investigate the electronic properties, the plasma energy, Penn gap and Fermi energy were also determined.