Linear/nonlinear optical properties and dispersion parameters of nanocrystalline indigo organic semiconductor films

Films of indigo dye as organic semiconductors have been prepared with different thicknesses (462–885 nm) through thermal evaporation. X-ray diffraction shows a nanocrystalline structure for the films with crystallite size 10.5 ± 0.1 nm. AFM confirms the nanocrystalline structure of the prepared films. The optical properties of the indigo films have been investigated using Swanepoel's method. These films have 2 optical energy gaps (1.67 and 2.61 eV) and Urbach's energy of 0.22 eV. The refractive index has normal behavior to decrease with increasing the wavelength following the Cauchy formula and the single oscillator model. The oscillator energy E_o and dispersion energy E_d are 2.75 and 4.81 eV, respectively. The lattice dielectric constant ε_L is 2.89 while the static dielectric constant ε_s is 2.75. The linear/nonlinear optical susceptibility and nonlinear refractive index have similar behavior to increase with increasing the photon energy.