© copyright FACULTY of ENGINEERING ‐ HUNEDOARA, ROMANIA 411 1. Simon JITIAN OBTAINING THE ABSORPTION SPECTRA OF SILICON FROM THE IR REFLECTANCE SPECTRA RECORDED AT TWO ANGLES 1. UNIVERSITY „POLITEHNICA” OF TIMIŞOARA, FACULTY OF ENGINEERING HUNEDOARA, ROMANIA ABSTRACT: This paper presents an analytical method for obtaining optical constants n and k , which define the complex refractive index k i n n ~ − = of solid absorbent materials. From specular reflectance, IR spectra recorded at two different incidence angles φ01 and φ02 the reflectances R are measured, using unpolarized radiation. We present an analytical method, using some approximations, to obtain reflectance spectra () ν = ~ f k and () ν = ~ f n from the reflectance spectra () ν = ~ f R recorded at two different incidence angles, using non-polarized radiation. To illustrate this method silicon was chosen. In order to obtain the optical constants n and k of silicon we used specular reflectance IR spectra. The spectra were recorded with a UR20 spectrograph, using non-polarized radiation. The specular external reflection spectra recorded at two or more different incidence angles can be used to determine the refractive index and absorption index spectra corresponding to solid materials. KEYWORDS: optical constants, two angles IR reflectance spectra, refractive index spectra INTRODUCTION The reflection of a plane polarized monochromatic radiation on the boundary of two different optical media is expressed by the Fresnel complex reflection coefficient ( ) δ ⋅ = i exp r r ~ . Two reflection coefficients s r ~ and p r ~ are defined for two components of plane polarized radiation with the electric field vector located perpendicular and parallel to the plane of incidence, respectively. The square modulus of the complex reflection coefficient is the reflectance (or the reflectivity) ∗ ⋅ = s s s r ~ r ~ R or ∗ ⋅ = p p p r ~ r ~ R . In the first approximation we can consider the reflectance for natural radiation to be the arithmetic mean of the two components s R and p R : 2 R R R p s + = (1) If we consider that the two components in incident radiation do not have equal weight, we can introduce a parameter S whose value is between S = 0 for the radiation polarized parallel to the incidence plane (R = R p ) and S = ∞ for the radiation polarized perpendicular to the incidence plane (R = R s ) [2]. S is defined as the ratio between the intensity of light polarized perpendicular to the plane of incidence and the parallel polarized one reaching the detector: 0 p 0 s R R S = (2) where: 0 s R and 0 p R are the perpendicular and parallel components that were measured. The reflectance can be expressed by: p s R 1 S 1 R 1 S S R + + + = (3) The reflection coefficients r ~ and the reflectance R depend on the relative complex refractive index of refractive and incidence medium, respectively: k i n n n ~ n ~ 0 1 − = = (4) according to relations: ( ) ϕ + ϕ ϕ − ϕ = θ = ~ cos n ~ cos ~ cos n ~ cos i exp r ~ r ~ 0 0 s s s (5)