Optical Constants of GaSe at the Fundamental Absorption Edge

The three‐dimensional mechanical character of GaSe single crystals makes it very difficult to study the optical properties along the c‐axis. A technique is developed based on the analysis of the interference pattern as a function of the wavelength. Using the envelope of maxima or minima it is possible to normalize the data in respect to the intensity of the incident light so to have an absolute measurement. The fit of the experimental normalized data gives the index of refraction n and the extinction coefficient k with an error of the order of 1\documentclass{article}\pagestyle{empty}$ {\raise0.7ex\hbox{$0$} \!\mathord{\left/ {\vphantom {0 {00}}}\right.} \!\lower0.7ex\hbox{${00}$}} $ for n ⟂ c and 3\documentclass{article}\pagestyle{empty}$ {\raise0.7ex\hbox{$0$} \!\mathord{\left/ {\vphantom {0 {00}}}\right.} \!\lower0.7ex\hbox{${00}$}} $ , for n ∥ c. An error ten times greater is obtained for k_⟂ and k_∥. It must be emphasized that in the presence of a variation of the refractive index as small as 1\documentclass{article}\pagestyle{empty}$ {\raise0.7ex\hbox{$0$} \!\mathord{\left/ {\vphantom {0 {00}}}\right.} \!\lower0.7ex\hbox{${00}$}} $ between two neighbouring maxima or minima the more traditional methods based only on the determination of the position of extrema are quite inconsistent. On the contrary the envelope method proposed can be used on a wider range. Moreover this method allows to determine k also in a region of low absorption, where 4πk/λ ≪ 1/z₀. The transmission between 8000 and 5900 Å is measured and n_⟂ land k_⟂ are obtained on the entire region while for n_∥ and k_∥ the determination is possible only in the exciton tail. In this region a ratio k_∥/k_⟂ ≈ 3 is found.