Optical Properties of Alkali Halides in Ultraviolet Spectral Regions

Abstract

The optical reflectance spectra of alkali halide crystals KI and RbI were measured over the energy range of 4.14 to 6.91 eV. Both single crystal and poly-crystal samples were used to accomplish this task. The phase θ(ω) was computed using the Kramers-Kronig relation between the real and imaginary parts of the complex function, ln r = ln |r| + iθ(ω). Subsequently, the optical constants n and κ were determined from the Fresnel reflectivity equation. The real and imaginary parts of dielectric constants ε 1 and ε 2 were then calculated using n and κ. The optical absorption spectra of the crystal have also been measured in these spectral regions. The spectra agree reasonably well with the current theory concerning exciton peaks. In addition, a shoulder was found in the spectra similar to those previously seen and associated with the band-to-band transition in the alkali iodides.Optics 2020, 1 19 were limited in terms of reflectivity studies and band structures. Before 1970, Baldini et al. studied the reflectivity data of many single-crystal alkali halides such as KI, KCl, KBr, RbI, RbCl, and RbBr in 5 eV to 10 eV [23,24]. Similarly, single-crystal alkali halides CsI [25], , KBr [30], and KCl [31] were also studied to calculate their optical constants. In contrast, studies of thin films of many alkali halides in the UV spectral region were limited in absorption measurements [32][33][34]. In addition, a literature review reveals a few more analyses of alkali halide crystals, such as a study on the influence of surface condition [35], on the strain effects on fundamental absorption [36], and on the effect of the layer thickness or uniaxial stress on the shape/location of exciton bands in RbI [37,38]. The literature on alkali halides has demonstrated that studies on the optical properties of RbI are comparatively limited. Cardona and Lynch studied the optical properties of a thin film of RbI, but their work was concentrated in the extreme ultraviolet region, i.e., from 50 eV to 250 eV [39]. S. Hashimoto and N. Momi-ie studied the interference effect and dispersion in thin RbI crystals [40]. The authors calculated the refractive indices in the spectral range of 4.8 eV to 5.7 eV, which is only part of the spectral range of present study. Similarly, the dielectric properties of single crystal RbI were discussed by Peimann and Skibowski in photon energies between 10 to 30 eV [41]. Finally, Roessler and Walker undertook research which was very similar to the present work for both KI and RbI single crystals [42]. They measured reflectance spectra in the range of 4-11 eV at 300 K and 77 K, respectively. Even though their first exciton peak matched with the present work for single crystal KI, the reflectance spectra for polycrystalline thin films of KI and RbI were not provided. Also, the authors used reflectance data measured at 77 K to calculate the real and imaginary parts of dielectric constants, instead of data measured at room temperature. They also did not calculate the refractive indices and absorption coefficient of th...

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